Scientific Output

Over 10.000 scientific papers have been published by members of the Materials Chain since the foundation of the University Alliance Ruhr in 2010. This tremendous output is proof of the excellent environment the Ruhr Area provides for research in the field of materials science and technology.

Below, you can either scroll through the complete list of our annually published material, or search for a specific author or term via the free text search to get to know our research strengths. You can also review the publication record of every Materials Chain member via his or her personal member’s page.

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  • 2022 • 231 A CutFEM based framework for numerical simulations of machine driven tunnels with arbitrary alignments
    Bui, H.-G. and Schillinger, D. and Zendaki, Y. and Meschke, G.
    Computers and Geotechnics 144 (2022)
    We describe a novel computational framework based on the cut finite element method (CutFEM) for process-oriented simulation in mechanized tunneling. The framework incorporates all relevant components required for the simulation of the tunnel advance process, namely the ground, the staged installation of the lining support, the tail void grouting and the tunnel boring machine. We demonstrate that CutFEM concepts can significantly facilitate the modeling, discretization and coupling of the different components, while maintaining the same accuracy as the standard boundary-fitted finite element method. The proposed CutFEM technology, which is being applied and investigated in the context of advancement simulations in mechanized tunneling for the first time, enables the seamless analysis of an arbitrary number of different tunnel alignment variants on the same structured background mesh without the need to set up a new model for each variant. This is a shift of paradigm in simulation-supported tunnel design, as the CutFEM based framework considerably facilitates a direct integration of geometric, building information and simulation models in early stages of a tunnel project. The simulation model allows for the damage assessment of the buildings during tunnel advancement with regards to different excavation scenarios, as shown in the numerical examples in this paper. © 2022 Elsevier Ltd
    view abstractdoi: 10.1016/j.compgeo.2022.104637
  • 2022 • 230 Ensuring the Reliability of Virtual Sensors Based on Artificial Intelligence within Vehicle Dynamics Control Systems
    Sieberg, P.M. and Schramm, D.
    Sensors 22 (2022)
    The use of virtual sensors in vehicles represents a cost-effective alternative to the installation of physical hardware. In addition to physical models resulting from theoretical modeling, artificial intelligence and machine learning approaches are increasingly used, which incorporate experimental modeling. Due to the resulting black-box characteristics, virtual sensors based on artificial intelligence are not fully reliable, which can have fatal consequences in safety-critical applications. Therefore, a hybrid method is presented that safeguards the reliability of artificial intelligence-based estimations. The application example is the state estimation of the vehicle roll angle. The state estimation is coupled with a central predictive vehicle dynamics control. The implementation and validation is performed by a co-simulation between IPG CarMaker and MATLAB/Simulink. By using the hybrid method, unreliable estimations by the artificial intelligence-based model resulting from erroneous input signals are detected and handled. Thus, a valid and reliable state estimate is available throughout. © 2022 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/s22093513
  • 2022 • 229 In Situ Monitoring of Palladium-Catalyzed Chemical Reactions by Nanogap-Enhanced Raman Scattering using Single Pd Cube Dimers
    Wang, D. and Shi, F. and Jose, J. and Hu, Y. and Zhang, C. and Zhu, A. and Grzeschik, R. and Schlücker, S. and Xie, W.
    Journal of the American Chemical Society 144 5003-5009 (2022)
    The central dilemma in label-free in situ surface-enhanced Raman scattering (SERS) for monitoring of heterogeneously catalyzed reactions is the need of plasmonically active nanostructures for signal enhancement. Here, we show that the assembly of catalytically active transition-metal nanoparticles into dimers boosts their intrinsically insufficient plasmonic activity at the monomer level by several orders of magnitude, thereby enabling the in situ SERS monitoring of various important heterogeneously catalyzed reactions at the single-dimer level. Specifically, we demonstrate that Pd nanocubes (NCs), which alone are not sufficiently plasmonically active as monomers, can act as a monometallic yet bifunctional platform with both catalytic and satisfactory plasmonic activity via controlled assembly into single dimers with an ∼1 nm gap. Computer simulations reveal that the highest enhancement factors (EFs) occur at the corners of the gap, which has important implications for the SERS-based detection of catalytic conversions: it is sufficient for molecules to come in contact with the "hot spot corners", and it is not required that they diffuse deeply into the gap. For the widely employed Pd-catalyzed Suzuki-Miyaura cross-coupling reaction, we demonstrate that such Pd NC dimers can be employed for in situ kinetic SERS monitoring, using a whole series of aryl halides as educts. Our generic approach based on the controlled assembly into dimers can easily be extended to other transition-metal nanostructures. © 2022 American Chemical Society.
    view abstractdoi: 10.1021/jacs.1c13240
  • 2022 • 228 Magnetic Resonance Imaging–based biomechanical simulation of cartilage: A systematic review
    Seyedpour, S.M. and Nafisi, S. and Nabati, M. and Pierce, D.M. and Reichenbach, J.R. and Ricken, T.
    Journal of the Mechanical Behavior of Biomedical Materials 126 (2022)
    MRI-based mathematical and computational modeling studies can contribute to a better understanding of the mechanisms governing cartilage's mechanical performance and cartilage disease. In addition, distinct modeling of cartilage is needed to optimize artificial cartilage production. These studies have opened up the prospect of further deepening our understanding of cartilage function. Furthermore, these studies reveal the initiation of an engineering-level approach to how cartilage disease affects material properties and cartilage function. Aimed at researchers in the field of MRI-based cartilage simulation, research articles pertinent to MRI-based cartilage modeling were identified, reviewed, and summarized systematically. Various MRI applications for cartilage modeling are highlighted, and the limitations of different constitutive models used are addressed. In addition, the clinical application of simulations and studied diseases are discussed. The paper's quality, based on the developed questionnaire, was assessed, and out of 79 reviewed papers, 34 papers were determined as high-quality. Due to the lack of the best constitutive models for various clinical conditions, researchers may consider the effect of constitutive material models on the cartilage disease simulation. In the future, research groups may incorporate various aspects of machine learning into constitutive models and MRI data extraction to further refine the study methodology. Moreover, researchers should strive for further reproducibility and rigorous model validation and verification, such as gait analysis. © 2021
    view abstractdoi: 10.1016/j.jmbbm.2021.104963
  • 2020 • 227 A combined experimental and modelling approach for the Weimberg pathway optimisation
    Shen, L. and Kohlhaas, M. and Enoki, J. and Meier, R. and Schönenberger, B. and Wohlgemuth, R. and Kourist, R. and Niemeyer, F. and van Niekerk, D. and Bräsen, C. and Niemeyer, J. and Snoep, J. and Siebers, B.
    Nature Communications 11 (2020)
    The oxidative Weimberg pathway for the five-step pentose degradation to α-ketoglutarate is a key route for sustainable bioconversion of lignocellulosic biomass to added-value products and biofuels. The oxidative pathway from Caulobacter crescentus has been employed in in-vivo metabolic engineering with intact cells and in in-vitro enzyme cascades. The performance of such engineering approaches is often hampered by systems complexity, caused by non-linear kinetics and allosteric regulatory mechanisms. Here we report an iterative approach to construct and validate a quantitative model for the Weimberg pathway. Two sensitive points in pathway performance have been identified as follows: (1) product inhibition of the dehydrogenases (particularly in the absence of an efficient NAD+ recycling mechanism) and (2) balancing the activities of the dehydratases. The resulting model is utilized to design enzyme cascades for optimized conversion and to analyse pathway performance in C. cresensus cell-free extracts. © 2020, The Author(s).
    view abstractdoi: 10.1038/s41467-020-14830-y
  • 2020 • 226 A parallelization strategy for hydro-mechanically coupled mechanized tunneling simulations
    Bui, H.-G. and Meschke, G.
    Computers and Geotechnics 120 (2020)
    3D computational simulations of the tunnel advancement in mechanized tunneling usually demand high computational resources. This paper addresses strategies to enable large scale simulations of the advancement process by means of iterative, parallel solvers, considering frictional contact between the tunnel boring machine and the soil, fully saturated soil with different permeabilities and a number of additional features inherently connected with tunnel simulations, which require special attention in regards to robustness. To improve the computational performance of mechanized tunnel simulations a solution strategy based on domain decomposition is proposed to solve the resulting discretized linear system in parallel using an appropriately accelerated iterative solver. The interaction between the TBM and the ground is modeled by means of robust contact algorithm based on penalty method. The domain decomposition scheme accounts for contact treatment in parallel, which becomes relevant when the TBM traverses through different domains in the mesh. Different variants of block preconditioners are investigated in regards to their performance to speed-up the convergence of the iterative solver for the block linear system arising from simulations of tunnel advancement in saturated soft soil, considering different permeabilities. A validation of the computational model and selected numerical examples to showcase the efficiency of the proposed method and to verify the applicability of this strategy to high performance tunneling simulations are presented. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.compgeo.2019.103378
  • 2020 • 225 Arduino-based slider setup for gas–liquid mass transfer investigations: Experiments and CFD simulations
    Krieger, W. and Bayraktar, E. and Mierka, O. and Kaiser, L. and Dinter, R. and Hennekes, J. and Turek, S. and Kockmann, N.
    AIChE Journal 66 (2020)
    The implementation of traditional sensors is a drawback when investigating mass transfer phenomena within microstructured devices, since they disturb the flow and reactor characteristics. An Arduino based slider setup is developed, which is equipped with a computer-vision system to track gas–liquid slug flow. This setup is combined with an optical analytical method allowing to compare experimental results against CFD simulations and investigate the entire lifetime of a single liquid slug with high spatial and temporal resolution. Volumetric mass transfer coefficients are measured and compared with data from literature and the mass transfer contribution of the liquid film is discussed. © 2020 The Authors. AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers.
    view abstractdoi: 10.1002/aic.16953
  • 2020 • 224 Effects of size and elasticity on the relation between flow velocity and wall shear stress in side-wall aneurysms: A lattice Boltzmann-based computer simulation study
    Wang, H. and Krüger, T. and Varnik, F.
    PLoS ONE 15 (2020)
    Blood flow in an artery is a fluid-structure interaction problem. It is widely accepted that aneurysm formation, enlargement and failure are associated with wall shear stress (WSS) which is exerted by flowing blood on the aneurysmal wall. To date, the combined effect of aneurysm size and wall elasticity on intra-aneurysm (IA) flow characteristics, particularly in the case of side-wall aneurysms, is poorly understood. Here we propose a model of three-dimensional viscous flow in a compliant artery containing an aneurysm by employing the immersed boundary-lattice Boltzmann-finite element method. This model allows to adequately account for the elastic deformation of both the blood vessel and aneurysm walls. Using this model, we perform a detailed investigation of the flow through aneurysm under different conditions with a focus on the parameters which may influence the wall shear stress. Most importantly, it is shown in this work that the use of flow velocity as a proxy for wall shear stress is well justified only in those sections of the vessel which are close to the ideal cylindrical geometry. Within the aneurysm domain, however, the correlation between wall shear stress and flow velocity is largely lost due to the complexity of the geometry and the resulting flow pattern. Moreover, the correlations weaken further with the phase shift between flow velocity and transmural pressure. These findings have important implications for medical applications since wall shear stress is believed to play a crucial role in aneurysm rupture. © 2020 Wang et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
    view abstractdoi: 10.1371/journal.pone.0227770
  • 2020 • 223 Integrated BIM-to-FEM approach in mechanised tunnelling
    Alsahly, A. and Hegemann, F. and König, M. and Meschke, G.
    Geomechanik und Tunnelbau 13 212-220 (2020)
    In current tunnelling practice, Finite Element (FE) simulations form an integral element in the planning and the design phase of mechanised tunnelling projects. The generation of adequate computational models is often time consuming and requires data from many different sources, in particular, when manually generated using 2D-CAD drawings. Incorporating Building Information Modelling (BIM) concepts offers opportunities to simplify this process by using geometrical BIM sub-models as a basis for structural analyses. This paper presents a Tunnel Information Model (TIM) as a BIM specifically tailored to fit the needs of mechanised tunnelling projects and a ”BIM-to-FEM“ technology, that automatically extracts relevant information (geology, alignment, lining, material and process parameters) needed for FE simulations from BIM sub-models and subsequently performs FE analysis of the tunnel drive. The results of the analysis are stored centrally on a data server to which the user has continuous access. A case study from the Wehrhahn-Metro line project in Düsseldorf, Germany, is presented and discussed to demonstrate the efficiency and the applicability of the proposed BIM-to-FEM workflow. © 2020, Ernst und Sohn. All rights reserved.
    view abstractdoi: 10.1002/geot.202000002
  • 2020 • 222 Interface tracking characteristics of color-gradient lattice Boltzmann model for immiscible fluids
    Subhedar, A. and Reiter, A. and Selzer, M. and Varnik, F. and Nestler, B.
    Physical Review E 101 (2020)
    We study the interface tracking characteristics of a color-gradient-based lattice Boltzmann model for immiscible flows. Investigation of the local density change in one of the fluid phases, via a Taylor series expansion of the recursive lattice Boltzmann equation, leads to the evolution equation of the order parameter that differentiates the fluids. It turns out that this interface evolution follows a conservative Allen-Cahn equation with a mobility which is independent of the fluid viscosities and surface tension. The mobility of the interface, which solely depends upon lattice speed of sound, can have a crucial effect on the physical dynamics of the interface. Further, we find that, when the equivalent lattice weights inside the segregation operator are modified, the resulting differential operators have a discretization error that is anisotropic to the leading order. As a consequence, the discretization errors in the segregation operator, which ensures a finite interface width, can act as a source of the spurious currents. These findings are supported with the help of numerical simulations. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevE.101.013313
  • 2020 • 221 Thin interface limit of the double-sided phase-field model with convection
    Subhedar, A. and Galenko, P.K. and Varnik, F.
    Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 378 (2020)
    The thin interface limit of the phase-field model is extended to include transport via melt convection. A double-sided model (equal diffusivity in liquid and solid phases) is considered for the present analysis. For the coupling between phase-field and Navier-Stokes equations, two commonly used schemes are investigated using a matched asymptotic analysis: (i) variable viscosity and (ii) drag force model. While for the variable viscosity model, the existence of a thin interface limit can be shown up to the second order in the expansion parameter, difficulties arise in satisfying no-slip boundary condition at this order for the drag force model. Nevertheless, detailed numerical simulations in two dimensions show practically no difference in dendritic growth profiles in the presence of forced melt flow obtained for the two coupling schemes. This suggests that both approaches can be used for the purpose of numerical simulations. Simulation results are also compared to analytic theory, showing excellent agreement for weak flow. Deviations at higher fluid velocities are discussed in terms of the underlying theoretical assumptions. © 2020 The Author(s) Published by the Royal Society. All rights reserved.
    view abstractdoi: 10.1098/rsta.2019.0540
  • 2019 • 220 Cannula position and Bernoulli effect contribute to leg malperfusion during extracorporeal life support with femoral arterial cannulation-an in silico simulation study
    Bongert, M. and Gehron, J. and Geller, M. and Böning, A. and Grieshaber, P.
    Interactive Cardiovascular and Thoracic Surgery 29 312-319 (2019)
    OBJECTIVES: Limb ischaemia during extracorporeal life support (ECLS) using femoral artery cannulation is frequently observed even in patients with regular vessel diameters and without peripheral arterial occlusive disease. We investigated underlying pathomechanisms using a virtual fluid-mechanical simulation of the human circulation. METHODS: A life-sized model of the human aorta and major vascular branches was virtualized using 3-dimensional segmentation software (Mimics, Materialise). Steady-state simulation of different grades of cardiac output (0-100%) was performed using Computational Fluid Dynamics (CFX, ANSYS). A straight cannula [virtualized 16 Fr (5.3 mm)] was inserted into the model via the left common femoral artery. The ECLS flow was varied between 1 and 5 l/min. The pressure boundary conditions at the arterial outlets were selected to demonstrate the downstream vascular system. Qualitative and quantitative analyses concerning flow velocity and direction were carried out in various regions of the model. RESULTS: During all simulated stages of reduced cardiac output and subsequently adapted ECLS support, retrograde blood flow originating from the ECLS cannula was observed from the cannulation site up to the aortic bifurcation. Analysis of pressure showed induction of zones of negative pressure close to the cannula tip, consistent with the Bernoulli principle. Depending on cannula position and ECLS flow rate, this resulted in negative flow from the ipsilateral superficial femoral artery or the contralateral internal iliac artery. The antegrade flow to the non-cannulated side was generally greater than that to the cannulated side. CONCLUSIONS: The cannula position and ECLS flow rate both influence lower limb perfusion during femoral ECLS. Therefore, efforts to optimize the cannula position and to avoid limb malperfusion, including placement of a distal perfusion cannula, should be undertaken in patients treated with ECLS. © 2019 The Author(s) 2019. Published by Oxford University Press on behalf of the European Association for Cardio-Thoracic Surgery. All rights reserved.
    view abstractdoi: 10.1093/icvts/ivz048
  • 2019 • 219 Coating damage localization of naval vessels using artificial neural networks
    Thiel, C. and Neumann, K. and Ludwar, F. and Rennings, A. and Doose, J. and Erni, D.
    Ocean Engineering 192 (2019)
    For the localization of coating damages of naval vessels numerical simulation using the FEM software COMSOL Multiphysics were carried out to calculate the corresponding underwater electric potential (UEP) signature. Therefore, we defined said damages at random hull surface positions and used the information provided by the impressed current cathodic protection (ICCP) system, more exactly the cathodic current itself, and the calculated UEP signatures as input parameters to train an artificial neural network (ANN) for predicting the coating damage location. With this deep learning approach, more than 90% of all coating damages are predicted correctly, considering a generic ship model with 50m length, whose hull is divided into 12 different sectors. Even the mere use of ICCP currents as highly aggregated input parameters for the ANN lead to a satisfactory prediction rate over 80% within the predefined sectors, thus providing quite accurate results using minimal amount of data. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.oceaneng.2019.106560
  • 2019 • 218 Dissociation of the Signaling Protein K-Ras4B from Lipid Membranes Induced by a Molecular Tweezer
    Li, L. and Erwin, N. and Möbitz, S. and Niemeyer, F. and Schrader, T. and Winter, R.H.A.
    Chemistry - A European Journal 25 9827-9833 (2019)
    Oncogenic Ras mutations occur in more than 30 % of human cancers. K-Ras4B is the most frequently mutated isoform of Ras proteins. Development of effective K-Ras4B inhibitors has been challenging, hence new approaches to inhibit this oncogenic protein are urgently required. The polybasic domain of K-Ras4B with its stretch of lysine residues is essential for its plasma membrane targeting and localization. Employing CD and fluorescence spectroscopy, confocal fluorescence, and atomic force microscopy we show that the molecular tweezer CLR01 is able to efficiently bind to the lysine stretch in the polybasic domain of K-Ras4B, resulting in dissociation of the K-Ras4B protein from the lipid membrane and disintegration of K-Ras4B nanoclusters in the lipid bilayer. These results suggest that targeting of the polybasic domain of K-Ras4B by properly designed tweezers might represent an effective strategy for inactivation of K-Ras4B signaling. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201901861
  • 2019 • 217 Femtosecond x-ray diffraction reveals a liquid–liquid phase transition in phase-change materials
    Zalden, P. and Quirin, F. and Schumacher, M. and Siegel, J. and Wei, S. and Koc, A. and Nicoul, M. and Trigo, M. and Andreasson, P. and Enquist, H. and Shu, M.J. and Pardini, T. and Chollet, M. and Zhu, D. and Lemke, H. and Ronneb...
    Science 364 1062-1067 (2019)
    In phase-change memory devices, a material is cycled between glassy and crystalline states. The highly temperature-dependent kinetics of its crystallization process enables application in memory technology, but the transition has not been resolved on an atomic scale. Using femtosecond x-ray diffraction and ab initio computer simulations, we determined the time-dependent pair-correlation function of phase-change materials throughout the melt-quenching and crystallization process. We found a liquid–liquid phase transition in the phase-change materials Ag4In3Sb67Te26 and Ge15Sb85 at 660 and 610 kelvin, respectively. The transition is predominantly caused by the onset of Peierls distortions, the amplitude of which correlates with an increase of the apparent activation energy of diffusivity. This reveals a relationship between atomic structure and kinetics, enabling a systematic optimization of the memory-switching kinetics. 2017 © The Authors, some rights reserved
    view abstractdoi: 10.1126/science.aaw1773
  • 2019 • 216 Geometrically nonlinear simulation of textile membrane structures based on orthotropic hyperelastic energy functions
    Motevalli, M. and Uhlemann, J. and Stranghöner, N. and Balzani, D.
    Composite Structures 223 (2019)
    New hyperelastic orthotropic models are proposed for the simulation of textile membranes used in civil engineering applications. In contrast to published models, part of the new models is polyconvex and ensures thereby a physically meaningful and mathematically sound formulation. The models are adjusted to uniaxial tension tests performed in warp and fill direction, where not only the stress-strain response in tension direction is accounted for but also the lateral contraction. Thereby, the crosswise interaction between the warp and fill direction is captured. In a series of different boundary value problems the new models as well as a competitive formulation given in literature are compared with respect to the accuracy to represent the experimental data, the mathematical properties as well as the numerical robustness. As it turns out, most formulations including the model from the literature show a loss of material stability and non-converging Newton iterations in structural simulations. Only one of the proposed polyconvex formulations works robustly in numerical simulations of realistic structural engineering problems. Thereby, this new orthotropic model enables realistic simulations of textile membranes in a fully geometrically nonlinear setting, which does not require simplifications based on linearized strains, which are currently used as standard in engineering practice. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.compstruct.2019.110908
  • 2019 • 215 Material defects localization in concrete plate-like structures – Experimental and numerical study
    Stojić, D. and Nestorović, T. and Marković, N. and Cvetković, R.
    Mechanics Research Communications 98 9-15 (2019)
    In this paper, the hybrid algorithm for localization of damage and defects is implemented on the concrete plate-like structures for localizing the clay and gypsum inclusions. The hybrid approach employs fast discrete wavelet decomposition of sensor output signals, as well as energy and time of flight criteria. The applied localization algorithm is verified both experimentally and numerically on the concrete plates with one and two inclusions. The experiment is conducted in controlled laboratory conditions, using a piezoelectric actuator for excitation of the wave propagation in the structure, while the ultrasonic laser is used for measuring vibrations at the sensor locations. Numerical simulation of wave propagation is done using the explicit finite element method on 3D models. The numerically obtained results are in full correspondence with the experimental results. The images of material defects positions obtained by the hybrid approach show a good agreement with the actual positions, which indicates a good potential of the used approach in localization of various types of material defects in plate-like concrete structures. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.mechrescom.2019.05.002
  • 2019 • 214 Robust segmental lining design – Potentials of advanced numerical simulations for the design of TBM driven tunnels
    Meschke, G. and Neu, G.E. and Marwan, A.
    Geomechanik und Tunnelbau 12 484-490 (2019)
    Loading assumptions used for the structural design of segmental linings often improperly reflect the complex load combinations that develop during the construction of a bored tunnel. Therefore segment designs used in practice tend to be on the safe side and often rely on conventional reinforcement methods instead of including other reinforcement concepts, such as steel fibres. In this contribution, a multi-scale computational modelling framework is proposed to investigate the response of steel-fibre reinforced, traditionally reinforced, and hybrid-reinforced lining segments to radial loadings with an emphasis on the longitudinal joints. This modelling approach offers an opportunity to directly investigate the influence of type and content of steel fibres on the performance of segmented linings at the structural scale. Using this framework, a method for robust optimization is applied in order to generate damage-tolerant hybrid segment designs. © 2019 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin
    view abstractdoi: 10.1002/geot.201900032
  • 2019 • 213 Simulation of personalised haemodynamics by various mounting positions of a prosthetic valve using computational fluid dynamics
    Bongert, M. and Geller, M. and Pennekamp, W. and Nicolas, V.
    Biomedizinische Technik 64 147-156 (2019)
    Diseases of the cardiovascular system account for nearly 42% of all deaths in the European Union. In Germany, approximately 12,000 patients receive surgical replacement of the aortic valve due to heart valve disease alone each year. A three-dimensional (3D) numerical model based on patient-specific anatomy derived from four-dimensional (4D) magnetic resonance imaging (MRI) data was developed to investigate preoperatively the flow-induced impact of mounting positions of aortic prosthetic valves to select the best orientation for individual patients. Systematic steady-state analysis of blood flow for different rotational mounting positions of the valve is only possible using a virtual patient model. A maximum velocity of 1 m/s was used as an inlet boundary condition, because the opening angle of the valve is at its largest at this velocity. For a comparative serial examination, it is important to define the standardised general requirements to avoid impacts other than the rotated implantation of the prosthetic aortic valve. In this study, a uniform velocity profile at the inlet for the inflow of the aortic valve and the real aortic anatomy were chosen for all simulations. An iterative process, with the weighted parameters flow resistance (1), shear stress (2) and velocity (3), was necessary to determine the best rotated orientation. Blood flow was optimal at a 45° rotation from the standard implantation orientation, which will offer a supply to the coronary arteries. © 2019 Walter de Gruyter GmbH, Berlin/Boston.
    view abstractdoi: 10.1515/bmt-2017-0092
  • 2018 • 212 Experimental and numerical research on damage localization in plate-like concrete structures using hybrid approach
    Stojić, D. and Nestorović, T. and Marković, N. and Marjanović, M.
    Structural Control and Health Monitoring 25 (2018)
    This paper presents an experimental–numerical analysis of damage localization of concrete plate-like elements on the basis of hybrid approach. The proposed hybrid approach uses the fast discrete wavelet transform, energy approach, and time of flight criterion for the purpose of localization of single and multidamage problems inside or on the periphery of concrete elements. Verification of the proposed damage localization approach has been performed under laboratory conditions using a laser scanning-based system with piezoelectric excitation of the wave propagation. Numerical simulation of the wave propagation is performed using the explicit finite element method using 3D models with linear-elastic material model of concrete with Rayleigh damping. The Rayleigh damping coefficients are determined on the basis of experimental data and implemented in numerical models. Validation of the numerical model is conducted, based on the comparison with sensor output signals obtained through experimental measuring and a very good agreement of results is obtained. The proposed hybrid approach to damage localization is verified using 15 different models/specimens, varying the number, shape (circular or notched), and position of damage, as well as the number and placement of actuators/sensors. For all the analyzed scenarios, the hybrid approach successfully localized the damage even for the least number of used sensor positions. In the models with the circular damage, the damage image created on the basis of the hybrid approach is almost identical to the actual shape of the damage, indicating a good potential of the method for damage localization. © 2018 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/stc.2214
  • 2017 • 211 Investigation of local heat transfer in random particle packings by a fully resolved LBM-approach
    Kravets, B. and Kruggel-Emden, H.
    Powder Technology 318 293-305 (2017)
    Static particle/fluid systems occur in a wide range of technical processes in chemical industry and in energy technology. Direct numerical simulations (DNS) can be applied to model the fluid flow in particle packings and thus to examine heat and momentum transfer in those systems. Besides traditional CFD simulations, the Lattice-Boltzmann method (LBM) has been established as an alternative and efficient approach. Thermal LBM (TLBM) relies on a set of two distribution functions which represent the fluid flow and its internal energy and cover convection-diffusion problems. In the present study a D3Q19 model with a multiple-relaxation-time (MRT) collision model for density distributions and a Bhatnagar–Gross–Krook (BGK) collision model for energy distributions is used. The fluid-solid boundaries are represented by interpolated bounce-back methods. Numerical investigations are performed for single sphere and random particle packings. Particle averaged and local heat transfer coefficients are considered. Obtained results are compared against available state of the art correlations and recent numerical results from the literature. The results demonstrate the accuracy of the derived LBM approach. Particle averaged and local heat transfer coefficients in sphere packings are presented in a range of Rep = 20 − 100 and ε = 0.6 − 1. A correlation of local heat transfer coefficients in random sphere packings is derived. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.powtec.2017.05.039
  • 2017 • 210 Micromagnetic Simulations for Coercivity Improvement Through Nano-Structuring of Rare-Earth-Free L10-FeNi Magnets
    Kovacs, A. and Fischbacher, J. and Oezelt, H. and Schrefl, T. and Kaidatzis, A. and Salikhov, R. and Farle, M. and Giannopoulos, G. and Niarchos, D.
    IEEE Transactions on Magnetics 53 (2017)
    In this paper, we investigate the potential of tetragonal L10-ordered FeNi as the candidate phase for rare-earth-free permanent magnets considering anisotropy values from recently synthesized, partially ordered FeNi thin films. In particular, we estimate the maximum energy product (BH)max of L10-FeNi nanostructures using micromagnetic simulations. The maximum energy product is limited due to the small coercive field of partially ordered L10-FeNi. Nano-structured magnets consisting of 128 equi-axed, platelet-like, and columnar-shaped grains show a theoretical maximum energy product of 228, 208, and 252 kJm $^{-3}$ , respectively. © 1965-2012 IEEE.
    view abstractdoi: 10.1109/TMAG.2017.2701418
  • 2017 • 209 Mineral in skeletal elements of the terrestrial crustacean Porcellio scaber: SRμCT of function related distribution and changes during the moult cycle
    Ziegler, A. and Neues, F. and Janáček, J. and Beckmann, F. and Epple, M.
    Arthropod Structure and Development 46 63-76 (2017)
    Terrestrial isopods moult first the posterior and then the anterior half of the body, allowing for storage and recycling of CaCO3. We used synchrotron-radiation microtomography to estimate mineral content within skeletal segments in sequential moulting stages of Porcellio scaber. The results suggest that all examined cuticular segments contribute to storage and recycling, however, to varying extents. The mineral within the hepatopancreas after moult suggests an uptake of mineral from the ingested exuviae. The total maximum loss of mineral was 46% for the anterior and 43% for the posterior cuticle. The time course of resorption of mineral and mineralisation of the new cuticle suggests storage and recycling of mineral in the posterior and anterior cuticle. The mineral in the anterior pereiopods decreases by 25% only. P. scaber has long legs and can run fast; therefore, a less mineralised and thus lightweight cuticle in pereiopods likely serves to lower energy consumption during escape behaviour. Differential demineralisation occurs in the head cuticle, in which the cornea of the complex eyes remains completely mineralised. The partes incisivae of the mandibles are mineralised before the old cuticle is demineralised and shed. Probably, this enables the animal to ingest the old exuviae after each half moult. © 2016 Elsevier Ltd.
    view abstractdoi: 10.1016/j.asd.2016.05.004
  • 2017 • 208 Non-universal transmission phase behaviour of a large quantum dot
    Edlbauer, H. and Takada, S. and Roussely, G. and Yamamoto, M. and Tarucha, S. and Ludwig, Ar. and Wieck, A.D. and Meunier, T. and Baüerle, C.
    Nature Communications 8 (2017)
    The electron wave function experiences a phase modification at coherent transmission through a quantum dot. This transmission phase undergoes a characteristic shift of π when scanning through a Coulomb blockade resonance. Between successive resonances either a transmission phase lapse of π or a phase plateau is theoretically expected to occur depending on the parity of quantum dot states. Despite considerable experimental effort, this transmission phase behaviour has remained elusive for a large quantum dot. Here we report on transmission phase measurements across such a large quantum dot hosting hundreds of electrons. Scanning the transmission phase along 14 successive resonances with an original two-path interferometer, we observe both phase lapses and plateaus. We demonstrate that quantum dot deformation alters the sequence of phase lapses and plateaus via parity modifications of the involved quantum dot states. Our findings set a milestone towards an comprehensive understanding of the transmission phase of quantum dots. © 2017 The Author(s).
    view abstractdoi: 10.1038/s41467-017-01685-z
  • 2017 • 207 Polymer conformations in ionic microgels in the presence of salt: Theoretical and mesoscale simulation results
    Kobayashi, H. and Halver, R. and Sutmann, G. and Winkler, R.G.
    Polymers 9 (2017)
    We investigate the conformational properties of polymers in ionic microgels in the presence of salt ions by molecular dynamics simulations and analytical theory. A microgel particle consists of coarse-grained linear polymers, which are tetra-functionally crosslinked. Counterions and salt ions are taken into account explicitly, and charge-charge interactions are described by the Coulomb potential. By varying the charge interaction strength and salt concentration, we characterize the swelling of the polyelectrolytes and the charge distribution. In particular, we determine the amount of trapped mobile charges inside the microgel and the Debye screening length. Moreover, we analyze the polymer extension theoretically in terms of the tension blob model taking into account counterions and salt ions implicitly by the Debye-Hückel model. Our studies reveal a strong dependence of the amount of ions absorbed in the interior of the microgel on the electrostatic interaction strength, which is related to the degree of the gel swelling. This implies a dependence of the inverse Debye screening length k on the ion concentration; we find a power-law increase of k with the Coulomb interaction strength with the exponent 3/5 for a salt-free microgel and an exponent 1/2 for moderate salt concentrations. Additionally, the radial dependence of polymer conformations and ion distributions is addressed. © 2017 by the authors.
    view abstractdoi: 10.3390/polym9010015
  • 2017 • 206 Vibrational Density Matrix Renormalization Group
    Baiardi, A. and Stein, C.J. and Barone, V. and Reiher, M.
    Journal of Chemical Theory and Computation 13 3764-3777 (2017)
    Variational approaches for the calculation of vibrational wave functions and energies are a natural route to obtain highly accurate results with controllable errors. Here, we demonstrate how the density matrix renormalization group (DMRG) can be exploited to optimize vibrational wave functions (vDMRG) expressed as matrix product states. We study the convergence of these calculations with respect to the size of the local basis of each mode, the number of renormalized block states, and the number of DMRG sweeps required. We demonstrate the high accuracy achieved by vDMRG for small molecules that were intensively studied in the literature. We then proceed to show that the complete fingerprint region of the sarcosyn-glycin dipeptide can be calculated with vDMRG. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.jctc.7b00329
  • 2016 • 205 A microstructurally based continuum model of cartilage viscoelasticity and permeability incorporating measured statistical fiber orientations
    Pierce, D.M. and Unterberger, M.J. and Trobin, W. and Ricken, T. and Holzapfel, G.A.
    Biomechanics and Modeling in Mechanobiology 15 229-244 (2016)
    The remarkable mechanical properties of cartilage derive from an interplay of isotropically distributed, densely packed and negatively charged proteoglycans; a highly anisotropic and inhomogeneously oriented fiber network of collagens; and an interstitial electrolytic fluid. We propose a new 3D finite strain constitutive model capable of simultaneously addressing both solid (reinforcement) and fluid (permeability) dependence of the tissue’s mechanical response on the patient-specific collagen fiber network. To represent fiber reinforcement, we integrate the strain energies of single collagen fibers—weighted by an orientation distribution function (ODF) defined over a unit sphere—over the distributed fiber orientations in 3D. We define the anisotropic intrinsic permeability of the tissue with a structure tensor based again on the integration of the local ODF over all spatial fiber orientations. By design, our modeling formulation accepts structural data on patient-specific collagen fiber networks as determined via diffusion tensor MRI. We implement our new model in 3D large strain finite elements and study the distributions of interstitial fluid pressure, fluid pressure load support and shear stress within a cartilage sample under indentation. Results show that the fiber network dramatically increases interstitial fluid pressure and focuses it near the surface. Inhomogeneity in the tissue’s composition also increases fluid pressure and reduces shear stress in the solid. Finally, a biphasic neo-Hookean material model, as is available in commercial finite element codes, does not capture important features of the intra-tissue response, e.g., distributions of interstitial fluid pressure and principal shear stress. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s10237-015-0685-x
  • 2016 • 204 Following the steps of a reaction by direct imaging of many individual molecules
    Van Vörden, D. and Wortmann, B. and Schmidt, N. and Lange, M. and Robles, R. and Brendel, L. and Bobisch, C.A. and Möller, R.
    Chemical Communications 52 7711-7714 (2016)
    The dehydrogenation and dechlorination of FeOEP-Cl on Cu(111) has been studied in detail by scanning tunneling microscopy. Although, it is not possible to follow the reaction of an individual molecule, the complete pathway of the reaction with 22 inequivalent intermediate states and the rates of the involved processes are revealed. This is achieved by combining the analysis of a large data set showing thousands of molecules in the different stages of the reaction with numerical simulations. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6cc02959k
  • 2016 • 203 Numerical modeling of fluid–structure interaction in arteries with anisotropic polyconvex hyperelastic and anisotropic viscoelastic material models at finite strains
    Balzani, D. and Deparis, S. and Fausten, S. and Forti, D. and Heinlein, A. and Klawonn, A. and Quarteroni, A. and Rheinbach, O. and Schröder, J.
    International Journal for Numerical Methods in Biomedical Engineering 32 (2016)
    The accurate prediction of transmural stresses in arterial walls requires on the one hand robust and efficient numerical schemes for the solution of boundary value problems including fluid–structure interactions and on the other hand the use of a material model for the vessel wall that is able to capture the relevant features of the material behavior. One of the main contributions of this paper is the application of a highly nonlinear, polyconvex anisotropic structural model for the solid in the context of fluid–structure interaction, together with a suitable discretization. Additionally, the influence of viscoelasticity is investigated. The fluid–structure interaction problem is solved using a monolithic approach; that is, the nonlinear system is solved (after time and space discretizations) as a whole without splitting among its components. The linearized block systems are solved iteratively using parallel domain decomposition preconditioners. A simple – but nonsymmetric – curved geometry is proposed that is demonstrated to be suitable as a benchmark testbed for fluid–structure interaction simulations in biomechanics where nonlinear structural models are used. Based on the curved benchmark geometry, the influence of different material models, spatial discretizations, and meshes of varying refinement is investigated. It turns out that often-used standard displacement elements with linear shape functions are not sufficient to provide good approximations of the arterial wall stresses, whereas for standard displacement elements or F-bar formulations with quadratic shape functions, suitable results are obtained. For the time discretization, a second-order backward differentiation formula scheme is used. It is shown that the curved geometry enables the analysis of non-rotationally symmetric distributions of the mechanical fields. For instance, the maximal shear stresses in the fluid–structure interface are found to be higher in the inner curve that corresponds to clinical observations indicating a high plaque nucleation probability at such locations. Copyright © 2015 John Wiley & Sons, Ltd. Copyright © 2015 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/cnm.2756
  • 2015 • 202 A numerical model of the combustion of single lithium particles with CO2
    Fischer, P. and Schiemann, M. and Scherer, V. and Maas, P. and Schmid, G. and Taroata, D.
    Fuel 160 87-99 (2015)
    Abstract A numerical model for the ignition and combustion of lithium particles (dp = 20-250 μm) in pure CO2 atmosphere was developed and implemented in ANSYS Fluent's "discrete phase model". The combustion model is based on experimental findings gained in a laminar flow reactor: the experiments indicate two reaction mechanisms: An initial high temperature above gas-phase combustion (>2500 K) with a reaction zone apart from the particle surface ("stand-of flame") followed by a surface reaction at lower temperature (1500-1800 K). As reaction kinetics is only available for the surface reaction, a theoretical approach was established to calculate duration and mass conversion of the gas-phase reaction. The complete model includes inert heating, lithium melting and the reaction steps described above and enables the complete calculation of single particle or droplet combustion of lithium. The results of the numerical simulation were compared to experiments conducted in a laminar flow reactor. As the numerical results show, the predicted combustion behavior is in good agreement with the experimental results. © 2015 Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.fuel.2015.07.033
  • 2015 • 201 Analyzing the BBOB results by means of benchmarking concepts
    Mersmann, O. and Preuss, M. and Trautmann, H. and Bischl, B. and Weihs, C.
    Evolutionary Computation 23 161-185 (2015)
    We presentmethods to answer two basic questions that arise when benchmarking optimization algorithms. The first one is: which algorithm is the “best” one? and the second one is: which algorithm should I use for my real-world problem? Both are connected and neither is easy to answer. We present a theoretical framework for designing and analyzing the raw data of such benchmark experiments. This represents a first step in answering the aforementioned questions. The 2009 and 2010 BBOB benchmark results are analyzed by means of this framework and we derive insight regarding the answers to the two questions. Furthermore, we discuss how to properly aggregate rankings from algorithm evaluations on individual problems into a consensus, its theoretical background and which common pitfalls should be avoided. Finally, we address the grouping of test problems into sets with similar optimizer rankings and investigate whether these are reflected by already proposed test problem characteristics, finding that this is not always the case. © 2015 by the Massachusetts Institute of Technology.
    view abstractdoi: 10.1162/EVCO_a_00134
  • 2015 • 200 Benchmark Thermochemistry for Biologically Relevant Adenine and Cytosine. A Combined Experimental and Theoretical Study
    Emel'yanenko, V.N. and Zaitsau, D.H. and Shoifet, E. and Meurer, F. and Verevkin, S.P. and Schick, C. and Held, C.
    Journal of Physical Chemistry A 119 9680-9691 (2015)
    The thermochemical properties available in the literature for adenine and cytosine are in disarray. A new condensed phase standard (p° = 0.1 MPa) molar enthalpy of formation at T = 298.15 K was measured by using combustion calorimetry. New molar enthalpies of sublimation were derived from the temperature dependence of vapor pressure measured by transpiration and by the quarz-crystal microbalance technique. The heat capacities of crystalline adenine and cytosine were measured by temperature-modulated DSC. Thermodynamic data on adenine and cytosine available in the literature were collected, evaluated, and combined with our experimental results. Thus, the evaluated collection of data together with the new experimental results reported here has helped to resolve contradictions in the available enthalpies of formation. A set of reliable thermochemical data is recommended for adenine and cytosine for further thermochemical calculations. Quantum-chemical calculations of the gas phase molar enthalpies of formation of adenine and cytosine have been performed by using the G4 method and results were in excellent agreement with the recommended experimental data. The standard molar entropies of formation and the standard molar Gibbs functions of formation in crystal and gas state have been calculated. Experimental vapor-pressure data measured in this work were used to estimate pure-component PC-SAFT parameters. This allowed modeling solubility of adenine and cytosine in water over the temperature interval 278-310 K. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpca.5b04753
  • 2015 • 199 Development of a simulation-software for a hydrogen production process on a solar tower
    Säck, J.-P. and Roeb, M. and Sattler, C. and Pitz-Paal, R. and Heinzel, A.
    Solar Energy 112 205-217 (2015)
    A simulation and control model for a two-step thermo-chemical water splitting cycle using metal oxids for the generation of hydrogen with a solar tower system as heat source has been developed. The simulation and control model consists of three main parts, the simulation of the solar flux distribution on the receiver, of the temperatures in the driven reactor modules and the produced hydrogen in the metal oxide.The results of the three parts of the simulation model have been evaluated by comparing and validating them with experimental data from the Hydrosol 100kWth pilot plant at the Plataforma Solar de Almería (PSA) in Spain.With the overall model of the hydrogen production plant that was created, an evaluation of the two-step thermochemical cycle process in combination with a solar tower system was performed. The model was used to perform parametric studies for the development of the plant and the operation strategies. For this purpose, a provision in the overall model was integrated. The simulation helps to reduce the frequency of using the flux measurement system and can be used for the heliostat field control, in particular for the temperature control in the solar chemical reactor modules. Because of these promising results the overall system model is being extended to enable a use as a control model with controller for the temperature control of the two core reactions in the process.The central control variable of the process control was the operating temperatures for the hydrogen production and the regeneration of the two modules. The process control with its PI controller turned out suitable to compensate diurnal changes of solar input power as well as certain statistical fluctuation due to cloud passage. At the same time the limits of the operability and controllability of the process became clear in terms of the minimum of solar power needed and maximum acceptable gradients.With this experience an operating strategy, the basic parameters of the system in operation, especially the starting up and shutdown procedures, regular operation and the response to disturbances were selected and optimized. With this operation/control strategy such a complex system can be operated in the future on a commercial scale automatically. The obtained results can also be adapted for other solar chemical processes. © 2014 Elsevier Ltd.
    view abstractdoi: 10.1016/j.solener.2014.11.026
  • 2015 • 198 Fluctuating multicomponent lattice Boltzmann model
    Belardinelli, D. and Sbragaglia, M. and Biferale, L. and Gross, M. and Varnik, F.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 91 (2015)
    Current implementations of fluctuating lattice Boltzmann equations (FLBEs) describe single component fluids. In this paper, a model based on the continuum kinetic Boltzmann equation for describing multicomponent fluids is extended to incorporate the effects of thermal fluctuations. The thus obtained fluctuating Boltzmann equation is first linearized to apply the theory of linear fluctuations, and expressions for the noise covariances are determined by invoking the fluctuation-dissipation theorem directly at the kinetic level. Crucial for our analysis is the projection of the Boltzmann equation onto the orthonormal Hermite basis. By integrating in space and time the fluctuating Boltzmann equation with a discrete number of velocities, the FLBE is obtained for both ideal and nonideal multicomponent fluids. Numerical simulations are specialized to the case where mean-field interactions are introduced on the lattice, indicating a proper thermalization of the system. © 2015 American Physical Society.
    view abstractdoi: 10.1103/PhysRevE.91.023313
  • 2015 • 197 Heat transfer in indirect heated rotary drums filled with monodisperse spheres: Comparison of experiments with DEM simulations
    Komossa, H. and Wirtz, S. and Scherer, V. and Herz, F. and Specht, E.
    Powder Technology 286 722-731 (2015)
    Numerical simulations with the discrete element method (DEM) and corresponding experimental investigations were carried out to understand and to quantify the heat transfer in indirect heated rotating drums. Monodisperse glass spheres (diameter 2. mm) were used and the bulk movement was kept within the rolling motion mode (rotational speed between 1 and 9. rpm). The focus is on the heat transfer between the covered wall and the particles in contact with this wall, as well as between the particles on the free bed surface and the adjacent fluid. Radiative heat transfer has been neglected due to the low maximum temperature within the system (474. K). Effective heat transfer coefficients for the heat fluxes mentioned were derived from the DEM simulations, considering the actual particle velocities on the free bed surface and on the wall.The particle movement and the heat transfer resulting from the simulations show good agreement with the experiments in general and thus allow the calculation of the effective heat transfer coefficients for the range of parameters considered in the current study. © 2015 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.powtec.2015.07.022
  • 2015 • 196 ICME for Crashworthiness of TWIP Steels: From Ab Initio to the Crash Performance
    Güvenç, O. and Roters, F. and Hickel, T. and Bambach, M.
    JOM 67 120-128 (2015)
    During the last decade, integrated computational materials engineering (ICME) emerged as a field which aims to promote synergetic usage of formerly isolated simulation models, data and knowledge in materials science and engineering, in order to solve complex engineering problems. In our work, we applied the ICME approach to a crash box, a common automobile component crucial to passenger safety. A newly developed high manganese steel was selected as the material of the component and its crashworthiness was assessed by simulated and real drop tower tests. The crashworthiness of twinning-induced plasticity (TWIP) steel is intrinsically related to the strain hardening behavior caused by the combination of dislocation glide and deformation twinning. The relative contributions of those to the overall hardening behavior depend on the stacking fault energy (SFE) of the selected material. Both the deformation twinning mechanism and the stacking fault energy are individually well-researched topics, but especially for high-manganese steels, the determination of the stacking-fault energy and the occurrence of deformation twinning as a function of the SFE are crucial to understand the strain hardening behavior. We applied ab initio methods to calculate the stacking fault energy of the selected steel composition as an input to a recently developed strain hardening model which models deformation twinning based on the SFE-dependent dislocation mechanisms. This physically based material model is then applied to simulate a drop tower test in order to calculate the energy absorption capacity of the designed component. The results are in good agreement with experiments. The model chain links the crash performance to the SFE and hence to the chemical composition, which paves the way for computational materials design for crashworthiness. © 2014, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/s11837-014-1192-8
  • 2015 • 195 Investigation of the sampling nozzle effect on laminar flat flames
    Deng, L. and Kempf, A. and Hasemann, O. and Korobeinichev, O.P. and Wlokas, I.
    Combustion and Flame 162 1737-1747 (2015)
    Sampling probes used for the mass spectrometric sampling of a flame can affect the flame's flow field. Although this effect is already compensated for by heuristic correction functions, state of the art 3-D simulations may permit an even better consideration of this effect. This work has investigated the perturbations induced by sampling probes in burner-stabilized, laminar, flat flames using numerical simulations. Any deviations in the flow and temperature fields from the ideal, one-dimensional flat flame were generated here by a perforated burner plate; they are also examined. Corresponding mass spectrometric measurements were performed in flames of CH4/O2/Ar and H2/O2/N2, burning under atmospheric conditions. In the present study, heat transfer from the flame to the sampling nozzle was studied with a conjugate heat transfer model. Combustion was described using a finite rate chemistry model, employing a detailed reaction mechanism for a H2/O2/N2 flame and a reduced mechanism for a CH4/O2/Ar flame. Compared to the ideal, one-dimensional, and unperturbed flame, the probe was found to affect the measurements of the concentrations of some species by up to 50%. The results highlight the value of supporting numerical simulations of both the flow and combustion for such measurements with invasive probing. © 2014 The Combustion Institute.
    view abstractdoi: 10.1016/j.combustflame.2014.11.035
  • 2015 • 194 Modeling of self-healing effects in polymeric composites
    Bluhm, J. and Specht, S. and Schröder, J.
    Archive of Applied Mechanics 85 1469-1481 (2015)
    Polymers and polymer composites are used in many engineering applications, but they can loose a high rate of stiffness and strength due to internal micro cracks/damages during their lifetime cycle. These damages are very hard to detect and nearly impossible to repair. To avoid failure due to such damages, a self-healing system is considered where microencapsulated healing agents and catalysts are embedded in the polymer matrix. For the numerical simulation of such a self-healing material, a thermodynamically consistent multiphase model, based on the Theory of Porous Media, is developed in this contribution. The different phases of the model are the solid matrix material with embedded catalysts, the liquid healing agents, the solid healed material and the gas phase, which represents the volume fraction of the micro cracks in the model. For the description of the healing mechanism, a mass exchange between the liquid healing agents and the solid healed material, in consideration of the change of the aggregate state, is introduced, which depends on the local concentration of catalysts in the polymer matrix. The applicability of the developed model is shown by means of numerical test simulations of a tapered double cantilever beam. © 2014, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00419-014-0946-7
  • 2015 • 193 Numerical simulation of polymer film stretching
    Damanik, H. and Ouazzi, A. and Turek, S.
    Lecture Notes in Computational Science and Engineering 103 709-716 (2015)
    We present numerical simulations of a film stretching process between two rolls of different temperature and rotational velocity. Film stretching is part of the industrial production of sheets of plastics which takes place after the extrusion process. The goal of the stretching of the sheet material is to rearrange the orientation of the polymer chains. Thus, the final products have more smooth surfaces and homogeneous properties. In numerical simulation, the plastic sheet is modelled geometrically as a membrane and rheologically as a polymer melt. The thickness of the membrane is not assumed to be constant but rather depends on the rheology of the polymer and the heat transfer. The rheology of the sheet material is governed by a viscoelastic fluid and is coupled to the flow model. An A-stable time integrator is applied to the systems in which the continuous spatial system is discretized within the FEM framework at each time step. The resulting discrete systems are solved via Newton-multigrid techniques. Moreover, a level set method is used to capture the free surface. We obtain similar results for test configurations with available results from literature and present “neck-in” as well as “dog-bone” effects. © Springer International Publishing Switzerland 2015.
    view abstractdoi: 10.1007/978-3-319-10705-9_70
  • 2014 • 192 3D-CFD-simulation of melting processes in a high-speed-extruder with solid-melt-separation
    Karrenberg, G. and Wortberg, J.
    AIP Conference Proceedings 1593 623-627 (2014)
    This paper deals with the development of the so called High-Speed-S-Truder. The alternative extrusion concept uses a special plastification sleeve with hundreds of bores surrounding the screw to separate the emerging melt from solid material in the screw channel. To analyze and improve the complex fluid flow in this process CFD-simulations are used. However, the ability of simulating the flow as well as the plastification process is yet not given in any CFD-software. Thus an approach for 3D-CFDsimulations of melting polymeric materials in extrusion processes has been developed. A new material model enables to differ between solid phase and fluid phase in dependence of temperature in just one set of property descriptions. Hence it becomes possible to simulate melting in a single fluid domain without presupposing any melting mechanism. Therefore the model is universally applicable and can be used for the simulation of ordinary extrusion processes under high speed conditions as well as for the investigation and improvement of the melting mechanism in the High-Speed-S-Truder. © 2014 American Institute of Physics.
    view abstractdoi: 10.1063/1.4873857
  • 2014 • 191 A molecular fragment cheminformatics roadmap for mesoscopic simulation
    Truszkowski, A. and Daniel, M. and Kuhn, H. and Neumann, S. and Steinbeck, C. and Zielesny, A. and Epple, M.
    Journal of Cheminformatics 6 (2014)
    Background: Mesoscopic simulation studies the structure, dynamics and properties of large molecular ensembles with millions of atoms: Its basic interacting units (beads) are no longer the nuclei and electrons of quantum chemical ab-initio calculations or the atom types of molecular mechanics but molecular fragments, molecules or even larger molecular entities. For its simulation setup and output a mesoscopic simulation kernel software uses abstract matrix (array) representations for bead topology and connectivity. Therefore a pure kernel-based mesoscopic simulation task is a tedious, time-consuming and error-prone venture that limits its practical use and application. A consequent cheminformatics approach tackles these problems and provides solutions for a considerably enhanced accessibility. This study aims at outlining a complete cheminformatics roadmap that frames a mesoscopic Molecular Fragment Dynamics (MFD) simulation kernel to allow its efficient use and practical application. Results: The molecular fragment cheminformatics roadmap consists of four consecutive building blocks: An adequate fragment structure representation (1), defined operations on these fragment structures (2), the description of compartments with defined compositions and structural alignments (3), and the graphical setup and analysis of a whole simulation box (4). The basis of the cheminformatics approach (i.e. building block 1) is a SMILES-like line notation (denoted fSMILES) with connected molecular fragments to represent a molecular structure. The fSMILES notation and the following concepts and methods for building blocks 2-4 are outlined with examples and practical usage scenarios. It is shown that the requirements of the roadmap may be partly covered by already existing open-source cheminformatics software. Conclusions: Mesoscopic simulation techniques like MFD may be considerably alleviated and broadened for practical use with a consequent cheminformatics layer that successfully tackles its setup subtleties and conceptual usage hurdles. Molecular Fragment Cheminformatics may be regarded as a crucial accelerator to propagate MFD and similar mesoscopic simulation techniques in the molecular sciences. [Figure not available: see fulltext.]. © 2014 Truszkowski et al.; licensee Springer.
    view abstractdoi: 10.1186/s13321-014-0045-3
  • 2014 • 190 A novel approach to representative orientation distribution functions for modeling and simulation of polycrystalline shape memory alloys
    Junker, P.
    International Journal for Numerical Methods in Engineering 98 799-818 (2014)
    SUMMARY: A micromechanical model for polycrystalline shape memory alloys (SMAs) was introduced in a series of papers by Hackl and coauthors. In order to model the polycrystalline aspect, they assumed a specific set of orientation distribution functions that had to be resolved with high numerical effort. Although this model displays interesting aspects, its use to simulate macroscopic specimens is problematic due to the long calculation time.In this paper, we present a new approach to modeling and simulation of polycrystalline SMAs that is based on parameterization of a class of orientation distribution functions by using only a few parameters. A variational concept is applied to derive evolution equations for these parameters. The resultant material model drastically reduces the calculation time and may thus provide an approach to efficient micromechanical simulation of specimens that are of engineering interest.This study presents a variety of different numerical examples, such as pseudoelastic and pseudoplastic material behavior for CuAlNi and NiTi SMAs, to demonstrate the broad applicability of the material model. The numerical benefit of the presented modeling approach is demonstrated by comparative calculations of the new model versus the previous model. © 2014 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/nme.4655
  • 2014 • 189 A novel scheme for the approximation of residual stresses in arterial walls
    Schröder, J. and Brinkhues, S.
    Archive of Applied Mechanics 84 881-898 (2014)
    In this contribution, a novel approach for the modeling of residual stresses in human arteries is proposed. The starting point in a variety of contributions is the opening angle of the section of an artery as a consequence of a longitudinal cut. In contrast to this, we focus directly on the current stress state within the arterial wall. To be more precise, we analyze the gradients of suitable invariant stress measures in thickness direction of the arterial wall. As an underlying optimization criterion, we assume that these gradients have to be smoothed between their inner and outer margins of the individual layers in an appropriate way. In order to do this, we define suitable radial sections for the media and adventitia, where this condition has to be enforced independently. The efficiency of the proposed model is demonstrated by means of a patient-specific cross section of a diseased artery in a two-dimensional simulation. © 2014 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00419-014-0838-x
  • 2014 • 188 A stacked sensor concept for industry compatible plasma diagnostics
    Schulz, C. and Will, B. and Rolfes, I.
    Proceedings - 2014 International Conference on Electromagnetics in Advanced Applications, ICEAA 2014 683-686 (2014)
    This paper presents a stacked sensor concept, which is producible entirely on standard printed circuit boards and applicable for the control of industrial plasma processes. Starting with the so-called multipole resonance probe (MRP) the prospects and the development towards the stacked MRP (sMRP) are discussed. The conversion of the MRP into a discretized spherical setup, using printed half discs of varying radii, is investigated. 3D electromagnetic field simulations yield an optimized assembly of 25 PCBs for a replication of the probe head. Measurements of a first prototype in a double inductive coupled plasma with an excited argon-hydrogen plasma have proven the expected resonance behaviour. In comparison to the MRP and the simulations, the applicability of the stacked sensor concept is demonstrated. © 2014 IEEE.
    view abstractdoi: 10.1109/ICEAA.2014.6903945
  • 2014 • 187 An invariant formulation for phase field models in ferroelectrics
    Schrade, D. and Müller, R. and Gross, D. and Keip, M.-A. and Thai, H. and Schröder, J.
    International Journal of Solids and Structures 51 2144-2156 (2014)
    This paper introduces an electro-mechanically coupled phase field model for ferroelectric domain evolution based on an invariant formulation for transversely isotropic piezoelectric material behavior. The thermodynamic framework rests upon Gurtin's notion of a micro-force system in conjunction with an associated micro-force balance. This leads to a formulation of the second law, from which a generalized Ginzburg-Landau evolution equation is derived. The invariant formulation of the thermodynamic potential provides a consistent way to obtain the order parameter dependent elastic stiffness, piezoelectric, and dielectric tensor. The model is reduced to 2d and implemented into a finite element framework. The material constants used in the simulations are adapted to meet the thermodynamic condition of a vanishing micro-force. It is found that the thermodynamic potential taken from the literature has to be extended in order to avoid a loss of positive definiteness of the stiffness and the dielectric tensor. The small-signal response is investigated in the presence and in the absence of the additional regularizing terms in the potential. The simulations show the pathological behavior of the model in case these terms are not taken into account. The paper closes with microstructure simulations concerning a ferroelectric nanodot subjected to an electric field, a cracked single crystal, and a ferroelectric bi-crystal. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijsolstr.2014.02.021
  • 2014 • 186 Analytical model for the pullout behavior of straight and hooked-end steel fibers
    Zhan, Y. and Meschke, G.
    Journal of Engineering Mechanics 140 (2014)
    In the context of multiscale-oriented computational analyses of fiber-RC (FRC) structures, the modeling of single fiber pullout behavior represents the basic constituent to provide traction-displacement relations to be used for the modeling of FRC on a macroscopic scale. This essential ingredient needs to be formulated such that it only requires minimal computational effort. To this end, an analytical model for the pullout behavior of single fibers embedded in a concrete matrix for various configurations of fiber type, matrix strength, and embedment condition is proposed. An interface law is developed for the frictional behavior between the fiber and matrix. In the case of inclined fibers, the plastic deformation of the fiber and the local damage of concrete are also considered. For hooked-end fibers, the anchorage effect due to the deformed topology of the fiber ends is taken into account in the formulation. By combining these submodels, the pullout response of single fibers embedded in a concrete matrix is predicted. In addition, numerical simulations of pullout tests are performed to obtain insight into the local fiber-concrete interactions and provide supporting information for analytical modeling. The model is successfully validated by means of representative experimental results. © 2014 American Society of Civil Engineers.
    view abstractdoi: 10.1061/(ASCE)EM.1943-7889.0000800
  • 2014 • 185 Bloch-wave homogenization on large time scales and dispersive effective wave equations
    Dohnal, T. and Lamacz, A. and Schweizer, B.
    Multiscale Modeling and Simulation 12 488-513 (2014)
    We investigate second order linear wave equations in periodic media, aiming at the derivation of effective equations in Rn, n € {1, 2, 3}. Standard homogenization theory provides, for the limit of a small periodicity length ε > 0, an effective second order wave equation that describes solutions on time intervals [0, T]. In order to approximate solutions on large time intervals [0, Tε-2], one has to use a dispersive, higher order wave equation. In this work, we provide a well-posed, weakly dispersive effective equation and an estimate for errors between the solution of the original heterogeneous problem and the solution of the dispersive wave equation. We use Bloch-wave analysis to identify a family of relevant limit models and introduce an approach to select a well-posed effective model under symmetry assumptions on the periodic structure. The analytical results are confirmed and illustrated by numerical tests. © 2014 Society for Industrial and Applied Mathematics.
    view abstractdoi: 10.1137/130935033
  • 2014 • 184 Coherent switching of polarization oscillations in vertical-cavity surface-emitting lasers
    Höpfner, H. and Lindemann, M. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 9001 (2014)
    Spin polarized lasers, especially spin polarized vertical-cavity surface-emitting lasers (VCSEL) provide improved performance when compared to conventional, purely charge-based lasers. Advantages of these spin-enhanced devices lie in their reduced laser threshold, increased emission intensity, amplification of spin information, chirp control and possibilities for ultrafast modulation due to their faster dynamics. Utilizing a commercially available conventional VCSEL and additional spin polarized optical pumping we are able to enhance the modulation dynamics of a conventional VCSEL with new spin effects. Our experiments show polarization oscillations in the spin-photon system that result in oscillations of the circular polarization of the VCSEL emission. The resulting polarization oscillations are of significantly higher frequency than the direct modulation bandwidth of the VCSEL and persist for durations longer than the spin lifetime in the active region. Simulations based on a rate-equation model show that with an improved VCSEL layout it should be possible to reach oscillation frequencies well above 100 GHz. Here, we show that with multiple optical spin polarized pulses these oscillations can be coherently excited, amplified and also stopped. Using this excitation scheme, polarization oscillations faster than the purely charge-based dynamics can be achieved with half-cycle to multi-cycle duration. Various influences of unpolarized electrical bias, optical excitation power and delay between pulses will be discussed both theoretically and experimentally. Additionally, we analyze the qualification of this new concept for ultrafast optical communication. © 2014 SPIE.
    view abstractdoi: 10.1117/12.2039196
  • 2014 • 183 Computation of dispersion curves for embedded waveguides using a dashpot boundary condition
    Gravenkamp, H. and Birk, C. and Song, C.
    Journal of the Acoustical Society of America 135 1127-1138 (2014)
    In this paper a numerical approach is presented to compute dispersion curves for solid waveguides coupled to an infinite medium. The derivation is based on the scaled boundary finite element method that has been developed previously for waveguides with stress-free surfaces. The effect of the surrounding medium is accounted for by introducing a dashpot boundary condition at the interface between the waveguide and the adjoining medium. The damping coefficients are derived from the acoustic impedances of the surrounding medium. Results are validated using an improved implementation of an absorbing region. Since no discretization of the surrounding medium is required for the dashpot approach, the required number of degrees of freedom is typically 10 to 50 times smaller compared to the absorbing region. When compared to other finite element based results presented in the literature, the number of degrees of freedom can be reduced by as much as a factor of 4000. © 2014 Acoustical Society of America.
    view abstractdoi: 10.1121/1.4864303
  • 2014 • 182 Correlations of plasticity in sheared glasses
    Varnik, F. and Mandal, S. and Chikkadi, V. and Denisov, D. and Olsson, P. and Vågberg, D. and Raabe, D. and Schall, P.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 89 (2014)
    In a recent paper [Mandal, Phys. Rev. E 88, 022129 (2013)PLEEE81539-375510. 1103/PhysRevE.88.022129], the nature of spatial correlations of plasticity in hard-sphere glasses was addressed both via computer simulations and in experiments. It was found that the experimentally obtained correlations obey a power law, whereas the correlations from simulations are better fitted by an exponential decay. We here provide direct evidence - via simulations of a hard-sphere glass in two dimensions (2D) - that this discrepancy is a consequence of the finite system size in the 3D simulations. By extending the study to a 2D soft disk model at zero temperature [Durian, Phys. Rev. Lett. 75, 4780 (1995)PRLTAO0031-900710.1103/PhysRevLett.75.4780], the robustness of the power-law decay in sheared amorphous solids is underlined. Deviations from a power law occur when either reducing the packing fraction towards the supercooled regime in the case of hard spheres or changing the dissipation mechanism from contact dissipation to a mean-field-type drag in the case of soft disks. © 2014 American Physical Society.
    view abstractdoi: 10.1103/PhysRevE.89.040301
  • 2014 • 181 Cutting edge geometries
    Denkena, B. and Biermann, D.
    CIRP Annals - Manufacturing Technology 63 631-653 (2014)
    Tool life and performance are decisively determined by cutting edge geometry. An appropriate shape of the cutting edge improves wear resistance, tool life and process reliability. This paper reviews major developments in cutting edge preparation technologies and methods of cutting edge characterization. Moreover, the influences of cutting edge geometry on chip formation, material flow, as well as mechanical and thermal loads on the tool are discussed. The essential modeling and simulation approaches are presented. Effects on surface integrity are described. Finally, an overview of important perceptions for prospective research and development in this field is provided. © 2014 CIRP.
    view abstractdoi: 10.1016/j.cirp.2014.05.009
  • 2014 • 180 Cyclic plasticity and lifetime of the nickel-based Alloy C-263: Experiments, models and component simulation
    Maier, G. and Hübsch, O. and Riedel, H. and Somsen, C. and Klöwer, J. and Mohrmann, R.
    MATEC Web of Conferences 14 (2014)
    The present work deals with the thermomechanical fatigue and low-cycle fatigue behavior of C-263 in two different material conditions. Microstructural characteristics and fracture modes are investigated with light and electron microscopy. The experimental results indicate that viscoplastic deformations depend on the heat treatment or rather on the current state of the microstructure. The measured data are used to adjust the parameters of a Chaboche type model and a fracture-mechanics based model for fatigue lifetime prediction. The Chaboche model is able to describe the essential phenomena of time and temperature dependent cyclic plasticity including the complex cyclic hardening during thermo-cyclic loading of both material conditions with a unique set of material parameters. This could be achieved by including an additional internal variable into the Chaboche model which accounts for changes in the precipitation microstructure during high temperature loading. Furthermore, the proposed lifetime model is well suited for a common fatigue life prediction of both investigated heats. The deformation and lifetime models are implemented into a user defined material routine. In this work, the material routine is applied for the lifetime prediction of a critical power plant component using the finite element method. © 2014 Owned by the authors, published by EDP Sciences.
    view abstractdoi: 10.1051/matecconf/20141416006
  • 2014 • 179 Effects of microtubule mechanics on hydrolysis and catastrophes
    Müller, N. and Kierfeld, J.
    Physical Biology 11 (2014)
    We introduce a model for microtubule (MT) mechanics containing lateral bonds between dimmers in neighboring protofilaments, bending rigidity of dimers, and repulsive interactions between protofilaments modeling steric constraints to investigate the influence of mechanical forces on hydrolysis and catastrophes. We use the allosteric dimer model, where tubulin dimers are characterized by an equilibrium bending angle, which changes from 0°to 22°by hydrolysis of a dimer. This also affects the lateral interaction and bending energies and, thus, the mechanical equilibrium state of the MT. As hydrolysis gives rise to conformational changes in dimers, mechanical forces also influence the hydrolysis rates by mechanical energy changes modulating the hydrolysis rate. The interaction via the MT mechanics then gives rise to correlation effects in the hydrolysis dynamics, which have not been taken into account before. Assuming a dominant influence of mechanical energies on hydrolysis rates, we investigate the most probable hydrolysis pathways both for vectorial and random hydrolysis. Investigating the stability with respect to lateral bond rupture, we identify initiation configurations for catastrophes along the hydrolysis pathways and values for a lateral bond rupture force. If we allow for rupturing of lateral bonds between dimers in neighboring protofilaments above this threshold force, our model exhibits avalanche-like catastrophe events. © 2014 IOP Publishing Ltd Printed in the UK.
    view abstractdoi: 10.1088/1478-3975/11/4/046001
  • 2014 • 178 Efficient Large-Scale Coating Microstructure Formation Using Realistic CFD Models
    Wiederkehr, T. and Müller, H.
    Journal of Thermal Spray Technology 24 283-295 (2014)
    For the understanding of physical effects during the formation of thermally sprayed coating layers and the deduction of the macroscopic properties of a coating, microstructure modeling and simulation techniques play an important role. In this contribution, a coupled simulation framework consisting of a detailed, CFD-based single splat simulation, and a large-scale coating build-up simulation is presented that is capable to compute large-scale, three-dimensional, porous microstructures by sequential drop impingement of more than 10,000 individual particles on multicore workstation hardware. Due to the geometry-based coupling of the two simulations, the deformation, cooling, and solidification of every particle is sensitive to the hit surface area and thereby pores develop naturally in the model. The single splat simulation employs the highly parallel Lattice-Boltzmann method, which is well suited for GPU-acceleration. In order to save splat calculations, the coating simulation includes a database-driven approach that re-uses already computed splats for similar underground shapes at the randomly chosen impact sites. For a fast database search, three different methods of efficient pre-selection of candidates are described and compared against each other. © 2014, ASM International.
    view abstractdoi: 10.1007/s11666-014-0194-y
  • 2014 • 177 Experimental and computational studies on the femoral fracture risk for advanced core decompression
    Tran, T.N. and Warwas, S. and Haversath, M. and Classen, T. and Hohn, H.P. and Jäger, M. and Kowalczyk, W. and Landgraeber, S.
    Clinical Biomechanics 29 412-417 (2014)
    Background Two questions are often addressed by orthopedists relating to core decompression procedure: 1) Is the core decompression procedure associated with a considerable lack of structural support of the bone? and 2) Is there an optimal region for the surgical entrance point for which the fracture risk would be lowest? As bioresorbable bone substitutes become more and more common and core decompression has been described in combination with them, the current study takes this into account. Methods Finite element model of a femur treated by core decompression with bone substitute was simulated and analyzed. In-vitro compression testing of femora was used to confirm finite element results. Findings The results showed that for core decompression with standard drilling in combination with artificial bone substitute refilling, daily activities (normal walking and walking downstairs) are not risky for femoral fracture. The femoral fracture risk increased successively when the entrance point is located further distal. The critical value of the deviation of the entrance point to a more distal part is about 20 mm. Interpretation The study findings demonstrate that optimal entrance point should locate on the proximal subtrochanteric region in order to reduce the subtrochanteric fracture risk. Furthermore the consistent results of finite element and in-vitro testing imply that the simulations are sufficient. © 2014 Elsevier Ltd.
    view abstractdoi: 10.1016/j.clinbiomech.2014.02.001
  • 2014 • 176 FSI-simulation of Liquid supported Stretch Blow Molding (LBO): Model validation and study of series production scenario
    Zimmer, J. and Stommel, M.
    Key Engineering Materials 611-612 892-900 (2014)
    Liquid-Driven Stretch Blow Molding is a new and innovative method to produce PET bottles [1]. In the well-established Stretch Blow Molding (SBM) process, preforms are biaxially deformed by pressurized air into a cavity. The resulting bottles are transferred to a separate machine, where the desired product is filled in. In contrast to that, Liquid-Driven Stretch Blow Molding is characterized by employing the liquid product to deform the material. The former separated blowing and filling steps are thus combined to a single forming stage leading to numerous advantages in energy consumption, cycle time and machine footprint. In this paper, a numerical simulation of the new process is presented. An additional challenge compared to SBM simulations is thereby the consideration of the interaction between liquid and preform. The load application cannot be solely represented by the pressure because the influx behavior as well as gravity and inertia forces influence the preform deformation. A smoothed particle hydrodynamics (SPH) approach is applied to the simulation to incorporate the additional effects. The process model is evaluated by prototype experiments. In addition, a feasibility study shows the applicability of a rotary forming system to the new process. © 2014 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2014 • 175 FT-IR and FT-Raman spectra of 5-fluoroorotic acid with solid state simulation by DFT methods
    Cuellar, A. and Alcolea Palafox, M. and Rastogi, V.K. and Kiefer, W. and Schlücker, S. and Rathor, S.K.
    Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy 132 430-445 (2014)
    FT-Raman and FT-IR studies of the biomolecule 5-fluoroorotic acid in the solid state were carried out. The unit cell found in the crystal was simulated as a tetramer form by density functional calculations. They were performed to clarify wavenumber assignments of the experimental observed bands in the spectra. Correlations with the molecule of uracil were made, and specific scale equations were employed to scale the wavenumbers of 5-fluoroorotic acid. Good reproduction of the experimental wavenumbers is obtained and the % error is very small in the majority of the bands. This fact confirms our simplified solid state model. The molecular structure was fully optimized using DFT and MP2 methods. The relative stability of both the syn and anti conformations was investigated, and the anti-form was found to be slightly more stable, by 7.49 kJ/mol at the MP2 level. The structures of all possible tautomeric forms were determined. The keto-form appeared as the most stable one. The NBO atomic charges and several thermodynamic parameters were also calculated. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.saa.2014.04.107
  • 2014 • 174 Grinding process simulation of free-formed WC-Co hard material coated surfaces on machining centers using poisson-disk sampled dexel representations
    Siebrecht, T. and Rausch, S. and Kersting, P. and Biermann, D.
    CIRP Journal of Manufacturing Science and Technology 7 168-175 (2014)
    Deep drawing tools are used in various production processes. In order to increase the life cycle of these tools, thermally sprayed abrasive-wear resistant WC-Co hard material coatings can be applied. With respect to the shape accuracy and surface quality of the forming tools, the coated surfaces have to be finished. A suitable machining process to meet these conditions is grinding on machining centers. In this paper, a geometric simulation model for this grinding process based on the modeling of individual grains with constructive solid geometry techniques is presented. The workpiece is represented by poisson-disk sampled dexels. Validation experiments show a good match of the simulated and measured process forces in different engagement situations. © 2014 CIRP.
    view abstractdoi: 10.1016/j.cirpj.2014.01.001
  • 2014 • 173 Investigations on the manufacturability of thin press hardened steel components
    Georgiadis, G. and Tekkaya, A.E. and Weigert, P. and Weiher, J. and Kurz, H.
    Procedia CIRP 18 74-79 (2014)
    In the recent years, the automotive industry is focusing on the reduction of the vehicles weight, so as to minimize the CO2emissions, while improving the crashworthiness levels. In order to achieve a further body-in-white weight reduction and exploit the potential for lightweight construction of the hot-dip aluminized press hardening manganese-boron steel, further research on the design and development of a process chain for the production of thin hot stamped components is carried out. For this purpose the impact of different blank thickness-dependent process parameters on the component properties is determined through both simulation and experimental analysis. The temperature profile of the heating process is determined for different blank thicknesses and the development of the diffusion layer between the AlSi-coating and the base material is examined. With respect to the transfer process of the thin austenitized blanks from the roller hearth furnace into the forming tool, the time slot is determined via the analysis of the corresponding time-temperature curves. In addition, different simulation models are developed, aiming at the validation and optimization of the transfer process. In order to further investigate the manufacturability of thin press hardened components, the simulation of a hot stamping process is carried out. The developed models are verified by an optical 3D forming analysis of the hot stamped components. As a conclusion of the current investigations, thin hot stamped components can be manufactured only under the precondition that the process is optimally designed and the process chain properly adjusted. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.procir.2014.06.110
  • 2014 • 172 LES of flow processes in an SI engine using two approaches: Openfoam and psiphi
    Nguyen, T. and Janas, P. and Lucchini, T. and D'Errico, G. and Kaiser, S. and Kempf, A.
    SAE Technical Papers 1 (2014)
    In this study two different simulation approaches to large eddy simulation of spark-ignition engines are compared. Additionally, some of the simulation results are compared to experimentally obtained in-cylinder velocity measurements. The first approach applies unstructured grids with an automated meshing procedure, using OpenFoam and Lib-ICE with a mapping approach. The second approach applies the efficient in-house code PsiPhi on equidistant, Cartesian grids, representing walls by immersed boundaries, where the moving piston and valves are described as topologically connected groups of Lagrangian particles. In the experiments, two-dimensional two-component particle image velocimetry is applied in the central tumble plane of the cylinder of an optically accessible engine. Good agreement between numerical results and experiment are obtained by both approaches. Copyright © 2014 SAE International.
    view abstractdoi: 10.4271/2014-01-1121
  • 2014 • 171 Linear and nonlinear sectional loads with potential and field methods
    von Graefe, A. and el Moctar, O. and Oberhagemann, J. and Shigunov, V.
    Journal of Offshore Mechanics and Arctic Engineering 136 (2014)
    A Rankine source method is applied to predict linear and weakly nonlinear sectional loads of a modern container ship. The method uses solution in the frequency domain, linearized with respect to wave amplitude about the nonlinear steady flow due to forward speed, which accounts for the nonlinear free-surface conditions, ship wave, and dynamic trim and sinkage. Weak nonlinearity of the sectional loads in waves (e.g., hoggingsagging asymmetry) is taken into account by pressure extrapolation and integration up to the estimated actual water line. The sectional forces obtained with this method are compared with the results of other methods, including (1) linear Rankine panel method, where flow due to waves is linearized about the double-body flow, (2) linear zero-speed Green function method with correction for forward speed, (3) fully nonlinear simulations based on field-based solution of Reynolds-averaged Navier-Stokes (RANS) equations, and (4) model tests. Comparison with RANS solution and model tests shows, that the proposed method can accurately predict sectional loads for small to moderate wave heights. © 2014 by ASME.
    view abstractdoi: 10.1115/1.4026885
  • 2014 • 170 Magnetic vortices induced by a monopole tip
    Magiera, M.P. and Schulz, S.
    IEEE Transactions on Magnetics 50 (2014)
    A ferromagnetic monolayer with an easy-plane anisotropy scanned by a magnetic tip that is moved with constant velocity \({v}\) is studied using atomistic computer simulations. The spin dynamics are treated using the Landau-Lifshitz-Gilbert equation. To study the influence of the tip's field, it is modeled by a monopole field instead of a dipole field, which is a common near-field approximation of a scanning probe microscopy cantilever. The magnetic structures induced by the moving tip are analyzed with respect to the strength of the coupling as well as the scanning velocity, and the energy dissipation is calculated. The results agree with calculations in a continuum model using Thiele's equation, as well as with earlier results obtained from simulations using a dipolar tip. The quantitative influence of the field is illustrated using energetic arguments. © 2014 IEEE.
    view abstractdoi: 10.1109/TMAG.2014.2317457
  • 2014 • 169 Modelling of dendritic growth during alloy solidification under natural convection
    Zhu, M. and Sun, D. and Pan, S. and Zhang, Q. and Raabe, D.
    Modelling and Simulation in Materials Science and Engineering 22 (2014)
    A two-dimensional (2D) lattice Boltzmann method (LBM)-cellular automaton model is presented to investigate the dendritic growth of binary alloys in the presence of natural convection. The kinetic-based LBM is adopted to calculate the transport phenomena by the evolution of distribution functions of moving pseudo-particles. To numerically solve natural convection thermal and solute transport simultaneously, three sets of distribution functions are employed in conjunction with the lattice Bhatnagar-Gross-Krook scheme. Based on the LBM calculated local temperature and concentration at the solid/liquid interface, the kinetics of dendritic growth is determined according to a local solute equilibrium approach. Thus, the physics of a complete time-dependent interaction of natural convection, thermal and solutal transport, and dendritic growth during alloy solidification is embedded in the model. Model validation is performed by comparing the simulated results with literature data and analytical predictions. The model is applied to simulate dendritic growth in binary alloys under the influence of natural convection. The effects of Rayleigh numbers and initial undercooling on dendrite growth are investigated. The results show that natural buoyancy flow, induced by thermal and solutal gradients under gravity, transports the heat and solute from the lower region to the upper region. The dendritic growth is thus accelerated in the downward direction, whereas it is inhibited in the upward direction, yielding asymmetrical dendrite patterns. Increasing the Rayleigh number and undercooling will enhance and reduce, respectively, the influence of natural flow on the dendritic growth. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/22/3/034006
  • 2014 • 168 Multiple reentrant glass transitions in confined hard-sphere glasses
    Mandal, S. and Lang, S. and Gross, M. and Oettel, M. and Raabe, D. and Franosch, T. and Varnik, F.
    Nature Communications 5 (2014)
    Glass-forming liquids exhibit a rich phenomenology upon confinement. This is often related to the effects arising from wall-fluid interactions. Here we focus on the interesting limit where the separation of the confining walls becomes of the order of a few particle diameters. For a moderately polydisperse, densely packed hard-sphere fluid confined between two smooth hard walls, we show via event-driven molecular dynamics simulations the emergence of a multiple reentrant glass transition scenario upon a variation of the wall separation. Using thermodynamic relations, this reentrant phenomenon is shown to persist also under constant chemical potential. This allows straightforward experimental investigation and opens the way to a variety of applications in micro-and nanotechnology, where channel dimensions are comparable to the size of the contained particles. The results are in line with theoretical predictions obtained by a combination of density functional theory and the mode-coupling theory of the glass transition. © 2014 Macmillan Publishers Limited.
    view abstractdoi: 10.1038/ncomms5435
  • 2014 • 167 Nanoparticle impacts reveal magnetic field induced agglomeration and reduced dissolution rates
    Tschulik, K. and Compton, R.G.
    Physical Chemistry Chemical Physics 16 13909-13913 (2014)
    Superparamagnetic nanoparticles (NPs) are used in a variety of magnetic field-assisted chemical and medical applications, yet little of their fate during magnetic field interrogation is known. Here, fundamental and new insights in this are gained by cathodic particle coulometry. This methodology is used to study individual Fe3O4 NPs in the presence and absence of a magnetic field. It is first noticed that no major NP agglomeration occurs in the absence of a magnetic field even in a suspension of high ionic strength. In contrast, a significant magnetic field-induced agglomeration of NPs is observed in a magnetic field. A second new finding is that the dissolution of Fe 3O4 NPs is strongly inhibited in a magnetic field. This is explained as a result of the magnetic field gradient force trapping the released Fe2+ ions near the surface of a magnetized Fe 3O4 NP and thus hindering the mass-transport controlled NP dissolution. Consequently, fundamental magnetic field effects are measured and quantified on both the single NP scale and in suspension and two novel effects are discovered. This journal is © the Partner Organisations 2014.
    view abstractdoi: 10.1039/c4cp01618a
  • 2014 • 166 Optimization of thermomechanical processes for the functional gradation of polymers by means of advanced empirical modeling techniques
    Biermann, D. and Hess, S. and Ries, A. and Wagner, T. and Wibbeke, A.
    AIP Conference Proceedings 1593 766-770 (2014)
    I In this paper, an optimization procedure for complex manufacturing processes is presented. The procedure is based on advanced empirical modeling techniques and will be presented in two parts. The first part comprises the selection and generation of the empirical surrogate models. The process organization and the design of experiments are taken into account. In order to analyze and optimize the processes based on the empirical models, advanced methods and tools are presented in the second part. These tools include visualization methods and a sensitivity and robustness analysis. Moreover, the obtained surrogate models are used for a model-based multi-objective optimization in order to explore the gradation potential of the processes. The procedure is applied to two thermo-mechanical processes for the functional gradation of polymers - a monoxiale stretching of polycarbonate films and a compression moulding process for polypropylene sheets. © 2014 American Institute of Physics.
    view abstractdoi: 10.1063/1.4873888
  • 2014 • 165 Optimizing the magnetocaloric effect in Ni-Mn-Sn by substitution: A first-principles study
    Grünebohm, A. and Comtesse, D. and Hucht, A. and Gruner, M.E. and Maslovskaya, A. and Entel, P.
    IEEE Transactions on Magnetics 50 (2014)
    We optimize the magnetic and structural properties of Ni(Co,Cu)MnSn Heusler alloys for the magnetocaloric effect (MCE) by means of density functional theory combined with Monte Carlo simulations of a classical Heisenberg model. NiMnSn alloys show a drop of magnetization at the martensitic phase transition, which leads to the inverse MCE. We find either disordered or frustrated magnetic configurations directly below the martensitic transition temperature. However, the jump of magnetization at the magnetostructural transition is small as the austenite is in a ferrimagnetic state and not fully magnetized. For Co and Cu substitution, the structural phase transition temperature shifts to lower temperatures. In particular, Co substitution is promising, as the magnetization of the austenite increases by additional ferromagnetic interactions, which enhances the jump of magnetization. © 2014 IEEE.
    view abstractdoi: 10.1109/TMAG.2014.2330845
  • 2014 • 164 Parameter study on the adsorptive drying of isopropanol in a fixed bed adsorber
    Burrichter, B. and Pasel, C. and Luckas, M. and Bathen, D.
    Separation and Purification Technology 132 736-743 (2014)
    This work focuses on the influence of process parameters on the dynamics of adsorptive water removal from polar organic solvents in a fixed bed adsorber. As a model solvent isopropanol with water concentrations between 5 and 4000 ppmw was used. In a first step equilibrium loadings on 3A and 4A zeolites were determined by shaker bottle experiments. The results were fitted to the Langmuir equation. In a second step fixed bed experiments were carried out in order to characterize the dynamic behavior of the adsorption process. In these experiments 3A zeolite shows a better drying performance than 4A zeolite. The breakthrough curves (BTC) could be well described by dynamic simulations using a set of differential equations for the mass balances and a linear driving force approach (LDF) for the kinetics. Pore diffusivities in the order of 10-12 m2/s were obtained by the simulation, indicating that surface diffusion in the pores of the zeolites is the predominant mechanism of mass transfer. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.seppur.2014.06.030
  • 2014 • 163 Parameterized electronic description of carbon cohesion in iron grain boundaries
    Hatcher, N. and Madsen, G.K.H. and Drautz, R.
    Journal of Physics Condensed Matter 26 (2014)
    We employ a recently developed iron-carbon orthogonal tight-binding model in calculations of carbon in iron grain boundaries. We use the model to evaluate the properties of carbon near and on the Σ5 (3 1 0)[0 0 1] symmetric tilt grain boundary (GB) in iron, and calculations show that a carbon atom lowers the GB energy by 0.29 eV/atom in accordance with DFT. Carbon segregation to the GB is analyzed, and we find an energy barrier of 0.92 eV for carbon to segregate to the carbon-free interface while segregation to a fully filled interface is disfavored. Local volume (via Voronoi tessellation), magnetic, and electronic effects are correlated with atomic energy changes, and we isolate two different mechanisms governing carbon's behavior in iron: a volumetric strain which increases the energy of carbon in interstitial α iron and a non-strained local bonding which stabilizes carbon at the GB. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/26/14/145502
  • 2014 • 162 Product properties of a two-phase magneto-electric composite: Synthesis and numerical modeling
    Labusch, M. and Etier, M. and Lupascu, D.C. and Schröder, J. and Keip, M.-A.
    Computational Mechanics 54 71-83 (2014)
    Magneto-electric (ME) materials are of high interest for a variety of advanced applications like in data storage and sensor technology. Due to the low ME coupling in natural materials, composite structures become relevant which generate the effective ME coupling as a strain-mediated product property. In this framework, it seems to be possible to achieve effective ME coefficients that can be exploited technologically. The present contribution investigates the realization of particulate ME composites with a focus on their experimental and computational characterization. We will show that different states of pre-polarizations of the ferroelectric material have a decisive influence on the overall obtainable ME coefficient. Details on the synthesis of two-phase composite microstructures consisting of a barium titanate matrix and cobalt ferrite inclusions will be discussed. Subsequently we will employ computational homogenization in order to determine the effective properties of the experimental composite numerically. We investigate the influence of different states of pre-polarization on the resulting ME-coefficients. For the numerical incorporation of the pre-polarization we use a heuristic method. © 2014 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00466-014-1031-3
  • 2014 • 161 Rheological properties of colloidal systems
    Rehage, H. and Willenbacher, N.
    Current Opinion in Colloid and Interface Science 19 501-502 (2014)
    doi: 10.1016/j.cocis.2014.10.005
  • 2014 • 160 Scale bridging between atomistic and mesoscale modelling: Applications of amplitude equation descriptions
    Hüter, C. and Nguyen, C.-D. and Spatschek, R. and Neugebauer, J.
    Modelling and Simulation in Materials Science and Engineering 22 (2014)
    Amplitude equations are discussed as an extension of phase field models, which contain atomic resolution and allow one to describe polycrystalline structures, lattice deformations and defects. The interaction of adjacent grains, which are separated by a thin melt layer, leads to structural interactions if the grains are slightly misplaced, similar to the concept of γ-surfaces. We are able to predict these interactions essentially analytically, leading to a superposition of short-ranged interaction terms related to the individual density waves. Deviations from the analytical predictions are found only at short distances between the grains and are most pronounced in situations with different ranges of the contributions. Furthermore, we demonstrate the ability of the amplitude equation model to predict dislocation pairing transitions at high temperatures, which supports earlier findings using molecular dynamics and phase field crystal simulations. To effectively perform the numerical simulations, we present a way to implement the model on graphics cards. An enormous acceleration of the code in comparison to a single CPU code by up to two orders of magnitude is reached. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/22/3/034001
  • 2014 • 159 Simulation of a cable-driven actuation concept for a humanoid robot prototype
    Feldmann, S. and Bruckmann, T. and Schramm, D.
    MESA 2014 - 10th IEEE/ASME International Conference on Mechatronic and Embedded Systems and Applications, Conference Proceedings (2014)
    This paper explores the feasibility of cable-driven actuators in combination with the lightweight skeleton structure of the humanoid robot HUMECH. At the beginning the setup of the robot prototype is described in detail followed by a Dymola® simulation model of the cable-driven actuators. However, the load and dynamic motion behavior of the Dyneema® cable-fibers are examined in order to obtain an evaluation of the developed model and its dynamic behavior. Finally the simulation results being presented and discussed in accordance to the goal of following a human motion trajectory of the right shoulder. © 2014 IEEE.
    view abstractdoi: 10.1109/MESA.2014.6935588
  • 2014 • 158 Smaller is less stable: Size effects on twinning vs. transformation of reverted austenite in TRIP-maraging steels
    Wang, M.-M. and Tasan, C.C. and Ponge, D. and Kostka, A. and Raabe, D.
    Acta Materialia 79 268-281 (2014)
    Steels containing reverted nanoscale austenite (γRN) islands or films dispersed in a martensitic matrix show excellent strength, ductility and toughness. The underlying microstructural mechanisms responsible for these improvements are not yet understood, but are observed to be strongly connected to the γRN island or film size. Two main micromechanical effects are conceivable in this context, namely: (i) interaction of γRN with microcracks from the matrix (crack blunting or arresting); and (ii) deformation-induced phase transformation of γRN to martensite (TRIP effect). The focus here is on the latter phenomenon. To investigate size effects on γRN transformation independent of other factors that can influence austenite stability (composition, crystallographic orientation, defect density, surrounding phase, etc.), a model (TRIP-maraging steel) microstructure is designed with support from diffusion simulations (using DICTRA software) to have the same, homogeneous chemical composition in all γRN grains. Characterization is conducted by in-situ tension and bending experiments in conjunction with high-resolution electron backscatter diffraction mapping and scanning electron microscopy imaging, as well as post-mortem transmission electron microscopy and synchrotron X-ray diffraction analysis. Results reveal an unexpected "smaller is less stable" effect due to the size-dependent competition between mechanical twinning and deformation-induced phase transformation. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.07.020
  • 2014 • 157 The amorphous silica-liquid water interface studied by ab initio molecular dynamics (AIMD): Local organization in global disorder
    Cimas, Á. and Tielens, F. and Sulpizi, M. and Gaigeot, M.-P. and Costa, D.
    Journal of Physics Condensed Matter 26 (2014)
    The structural organization of water at a model of amorphous silica-liquid water interface is investigated by ab initio molecular dynamics (AIMD) simulations at room temperature. The amorphous surface is constructed with isolated, H-bonded vicinal and geminal silanols. In the absence of water, the silanols have orientations that depend on the local surface topology (i.e. presence of concave and convex zones). However, in the presence of liquid water, only the strong inter-silanol H-bonds are maintained, whereas the weaker ones are replaced by H-bonds formed with interfacial water molecules. All silanols are found to act as H-bond donors to water. The vicinal silanols are simultaneously found to be H-bond acceptors from water. The geminal pairs are also characterized by the formation of water H-bonded rings, which could provide special pathways for proton transfer(s) at the interface. The first water layer above the surface is overall rather disordered, with three main domains of orientations of the water molecules. We discuss the similarities and differences in the structural organization of the interfacial water layer at the surface of the amorphous silica and at the surface of the crystalline (0 0 0 1) quartz surface. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/26/24/244106
  • 2014 • 156 The calibration of numerically simulated color and material change processes
    Szöke, L. and Wortberg, J.
    AIP Conference Proceedings 1593 628-631 (2014)
    As shown in the past, the first steps of product changing processes within extrusion dies can be observed through numerical simulations using transient calculations and the volume of fluid (VOF) approach. However, in the later part of the changing process, influences from the system, such as surface properties of the die channel or particle types in the polymer, govern the progress of the melt flow directly at the wall. Recently an approach allows these effects to be implemented into the numerical simulation despite the complexity of the whole flow system. In general it is common to assume a zero velocity at the channel wall in fluid dynamic calculations. In reality the exchange of materials during the extrusion process can be observed. Therefore, a finite velocity at the wall has to exist. Although this velocity is very low, it cannot be ignored for product changes as for most other calculations, because the velocity of the source material at the wall dominates the time or the amount of target material needed to complete the change. To calibrate the numerical calculation considering the effects near the channel wall a correcting function based on the experimental data is used. Therefore a wall velocity is calculated analytically and implemented as boundary condition in the numerical computation. This function is based on experimental data from color and material changes of several low density polyethylene (LDPE) types. © 2014 American Institute of Physics.
    view abstractdoi: 10.1063/1.4873858
  • 2014 • 155 The multipole resonance probe: Progression and evaluation of a process compatible plasma sensor
    Schulz, C. and Styrnoll, T. and Storch, R. and Awakowicz, P. and Musch, T. and Rolfes, I.
    IEEE Sensors Journal 14 3408-3417 (2014)
    A robust and sensitive plasma sensor, the multipole resonance probe (MRP), and its process compatibility are presented and discussed in this paper. Based on its innovative concept and simple model describing the system 'probe-plasma', three steps of development are introduced. 3D electromagnetic field simulations are applied as an indispensable tool for an economical and efficient investigation and optimization of different sensor layouts. Independent of the chosen sensor design, a developed pulse-based measurement device yields an economical signal generation and evaluation. Electron density profiles, determined with the MRP and the pulse-based system utilized in a capacitive coupled plasma, confirm and demonstrate the simulation results and the measurement concept, respectively. © 2014 IEEE.
    view abstractdoi: 10.1109/JSEN.2014.2333659
  • 2014 • 154 Thermal loads of working coils in electromagnetic sheet metal forming
    Gies, S. and Löbbe, C. and Weddeling, C. and Tekkaya, A.E.
    Journal of Materials Processing Technology 214 2537-2544 (2014)
    One basic problem of electromagnetic forming is the limited tool life. Besides the mechanical loads especially thermal loads acting on the tool coil affect its lifetime. In electromagnetic forming, about 50% of the deployed electrical energy is lost because of joule heating in the working coil. In case of high volume production, an accumulation of this heat promotes failure of the coil and reduces the coil lifetime. Despite this importance of the thermal loads only insufficient information about the coil temperature and its influencing parameters is available. Focus of this paper is on the determination of the temperature distribution in case of long-term discharge sequences. Experimental investigations using an infrared camera were performed to measure the coil surface temperature. Numerical process simulation is used to gather information about the temperature inside the working coil. The results prove that the coil reaches an equilibrium temperature after several discharges. For the analyzed range of input power the maximum coil surface temperature and the maximum coil winding temperature reached values of 92 °C and 178 °C, respectively. These temperatures exceed the weakening temperature of most reinforcement and insulation materials. The derived knowledge about the parameters influencing the coil temperature can be used for an improved process design to avoid thermal overstressing of the coil. A comparison of experiments with and without workpiece deformation revealed that the temperature in case of prevented deformation is always higher, and thus, represents an upper bound for the coil temperature. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2014.05.005
  • 2014 • 153 UWB chipless RFID system performance based on real world 3D-deterministic channel model and ZF equalization
    Fawky, A. and Mohammed, M. and El-Hadidy, M. and Kaiser, T.
    8th European Conference on Antennas and Propagation, EuCAP 2014 1765-1768 (2014)
    In this paper a 3D-deterministic channel model for UWB chipless RFID system is presented. The tag in this work will be modeled as a 3D object, with frequency dependent Radar Cross Section (RCS), embedded in the channel which has several incident and reflected multipath at various directions both from the reader-tag and the reader-reader environment effect. In this model all channel effects as multipath components, fading, frequency dependence, reflections, diffractions and polarization dependence will be considered. Moreover, a novel chipless RFID reader system model with Zero Forcing (ZF) based equalizer is proposed. The equalizer based system will increase the chipless tag identification distance by mitigating the aforementioned channel effect. The concept is tested using a deterministic channel from a ray-tracing model with a 8-bit frequency coded concept of a compact printable orientation independent chipless RFID tag. Furthermore, a 3D-Electromagnetic (EM) simulation tool is used to compute the Frequency Coding Response (FCR) and the RCS of the CFC-RFID tag. Simulation results show constructive guidelines for designing multi-tag UWB RFID communications and enhancing the channel estimation process of the chipless RFID tag systems. © 2014 European Association on Antennas and Propagation.
    view abstractdoi: 10.1109/EuCAP.2014.6902135
  • 2013 • 152 A comparison of low cost structure-borne sound measurement and acceleration measurement for detection of workpiece vibrations in 5-axis simultaneous machining
    Biermann, D. and Zabel, A. and Brüggemann, T. and Barthelmey, A.
    Procedia CIRP 12 91-96 (2013)
    In the field of machining technology the vibration of the system machine, tool and workpiece during processing is the limiting factor of productivity. Therefore the process monitoring of vibration today plays an important role for the real time monitoring of machining processes, as well as for the optimization of simulation models. For monitoring workpiece vibrations, different kinds of strategies are in use. As the piezoelectric acceleration sensors were already field-tested at the department of machining technology (ISF), the use of Contact Emission-Sensors, which are customarily used for stringed instrument tuning, could pose an effective alternative strategy. Proving the possibility of using those sensors, could simplify the future choice of the process monitoring strategy for different machining operations and has cost saving potentials, due to an abbreviated measurement chain. Within this work the workpiece vibration during a 5-axis milling process were detected simultaneously by two monitoring strategies. In these experiments, the detection of vibration by a piezoelectric acceleration sensor was chosen as the reference strategy. The second technique deployed was the measurement of the contact-emission with a common contact microphone. On a milling machine for five-axis simultaneous machining, aluminium shafts were fixed in a three-jaw chuck one-sided and processed with an end mill. The geometry of the milling pockets was varied in the different processing sessions, as well as the feed parameters, in order to obtain both stable and unstable processing. The vibration measurements resulting from the two monitoring strategies were compared in form of time signal and frequency spectrum as well as in the combined form of a 3-dimensional waterfall diagram. Monitoring structure borne noise is an easy, cost-efficient alternative to measures with an acceleration sensor. Delivering reliable results concerning the vibration frequencies, which are multiples of the spindle rotation frequency, this method could be applied for process monitoring. The experiments have shown that the location of the Contact Emission-Sensors has major impact on the quality of results. To implement the Contact Emission-Sensors for this kind of measurements, the sensor must be positioned close to the place of vibration origin taking into account both damping characteristics of all components involved and the way of workpiece clamping.
    view abstractdoi: 10.1016/j.procir.2013.09.017
  • 2013 • 151 A simple finite strain non-linear visco-plastic model for thermoplastics and its application to the simulation of incremental cold forming of polyvinylchloride (PVC)
    Alkas Yonan, S. and Soyarslan, C. and Haupt, P. and Kwiatkowski, L. and Tekkaya, A.E.
    International Journal of Mechanical Sciences 66 192-201 (2013)
    This paper introduces a finite strain extension of a non-linear visco-plastic material model, previously proposed by the authors, and its application to the finite element simulation of incremental cold forming processes of thermoplastics, demonstrated on PVC. Preserving the original structure of the model, its finite strain extension does not rely on any presumed kinematic split, either multiplicative or additive, among elastic and inelastic parts. It uses a systematic replacement of the strain and stress tensors and their rates by their respective spatial counterparts. A deviatoric Oldroyd rate is introduced to preserve the objectivity as well as the deviatoricity of the integration of the rate forms of deviatoric tensors. To cope with the incremental loading paths within the process, where through-thickness variations of the variables gain importance, the material model is posed in 3D formulation. The developed model is implemented as an ABAQUS®/UMAT subroutine and used in the simulations following parameter identification studies. The numerical results are compared with analogous experimental ones to evaluate the performance of the material model where PVC sheets of three different thicknesses are formed incrementally with path controlled tool force monitoring. The investigations have the following consequences: the deformation-limited homogeneous stress-strain portion at uni-axial tensile tests, which is generally used in parameter identification of the constitutive model, is not able to reflect the post necking regime and its extrapolation ends up with a stiffer response with much less retained strains. Once a semi-inverse parameter identification is followed by taking into account the overall experimental outputs, one ends up with a considerable improvement in the tool force, geometry and the wall thickness predictions. Nevertheless, these improvements are inversely proportional with the sheet thickness where the local indentation effects (strains and stresses) become larger. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijmecsci.2012.11.007
  • 2013 • 150 Ab Initio Based conformational study of the crystalline α-chitin
    Petrov, M. and Lymperakis, L. and Friák, M. and Neugebauer, J.
    Biopolymers 99 22-34 (2013)
    The equilibrium structure including the network of hydrogen bonds of an α-chitin crystal is determined combining density-functional theory (DFT), self-consistent DFT-based tight-binding (SCC-DFTB), and empirical forcefield molecular dynamics (MD) simulations. Based on the equilibrium geometry several possible crystal conformations (local energy minima) have been identified and related to hydrogen bond patterns. Our results provide new insight and allow to resolve the contradicting α-chitin structural models proposed by various experiments. © 2012 Wiley Periodicals, Inc. Copyright © 2012 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/bip.22131
  • 2013 • 149 Ab initio prediction of the critical thickness of a precipitate
    Sampath, S. and Janisch, R.
    Journal of Physics Condensed Matter 25 (2013)
    Segregation and precipitation of second phases in metals and metallic alloys is an important phenomenon that has a strong influence on the mechanical properties of the material. Models exist that describe the growth of coherent, semi-coherent and incoherent precipitates. One important parameter of these models is the energy of the interface between matrix and precipitate. In this work we apply ab initio density functional theory calculations to obtain this parameter and to understand how it depends on chemical composition and mechanical strain at the interface. Our example is a metastable Mo-C phase, the body-centred tetragonal structure, which exists as a semi-coherent precipitate in body-centred cubic molybdenum. The interface of this precipitate is supposed to change from coherent to semi-coherent during the growth of the precipitate. We predict the critical thickness of the precipitate by calculating the different contributions to a semi-coherent interface energy by means of ab initio density functional theory calculations. The parameters in our model include the elastic strain energy stored in the precipitate, as well as a misfit dislocation energy that depends on the dislocation core width and the dislocation spacing. Our predicted critical thickness agrees well with experimental observations. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/25/35/355005
  • 2013 • 148 Ab initio study of single-crystalline and polycrystalline elastic properties of Mg-substituted calcite crystals
    Zhu, L.-F. and Friák, M. and Lymperakis, L. and Titrian, H. and Aydin, U. and Janus, A.M. and Fabritius, H.-O. and Ziegler, A. and Nikolov, S. and Hemzalová, P. and Raabe, D. and Neugebauer, J.
    Journal of the Mechanical Behavior of Biomedical Materials 20 296-304 (2013)
    We employ ab initio calculations and investigate the single-crystalline elastic properties of (Ca,Mg)CO3 crystals covering the whole range of concentrations from pure calcite CaCO3 to pure magnesite MgCO3. Studying different distributions of Ca and Mg atoms within 30-atom supercells, our theoretical results show that the energetically most favorable configurations are characterized by elastic constants that nearly monotonously increase with the Mg content. Based on the first principles-derived single-crystalline elastic anisotropy, the integral elastic response of (Ca,Mg)CO3 polycrystals is determined employing a mean-field self-consistent homogenization method. As in case of single-crystalline elastic properties, the computed polycrystalline elastic parameters sensitively depend on the chemical composition and show a significant stiffening impact of Mg atoms on calcite crystals in agreement with the experimental findings. Our analysis also shows that it is not advantageous to use a higher-scale two-phase mix of stoichiometric calcite and magnesite instead of substituting Ca atoms by Mg ones on the atomic scale. Such two-phase composites are not significantly thermodynamically favorable and do not provide any strong additional stiffening effect. © 2013 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmbbm.2013.01.030
  • 2013 • 147 Augmented Lagrange methods for quasi-incompressible materials-Applications to soft biological tissue
    Brinkhues, S. and Klawonn, A. and Rheinbach, O. and Schröder, J.
    International Journal for Numerical Methods in Biomedical Engineering 29 332-350 (2013)
    Arterial walls in the healthy physiological regime are characterized by quasi-incompressible, anisotropic, hyperelastic material behavior. Polyconvex material functions representing such materials typically incorporate a penalty function to account for the incompressibility. Unfortunately, the penalty will affect the conditioning of the stiffness matrices. For high penalty parameters, the performance of iterative solvers will degrade, and when direct solvers are used, the quality of the solutions will deteriorate. In this paper, an augmented Lagrange approach is used to cope with the quasi-incompressibility condition. Here, the penalty parameter can be chosen much smaller, and as a consequence, the arising linear systems of equations have better properties. An improved convergence is then observed for the finite element tearing and interconnecting-dual primal domain decomposition method, which is used as an iterative solver. Numerical results for an arterial geometry obtained from ultrasound imaging are presented. © 2012 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/cnm.2504
  • 2013 • 146 Computational model for the cell-mechanical response of the osteocyte cytoskeleton based on self-stabilizing tensegrity structures
    Kardas, D. and Nackenhorst, U. and Balzani, D.
    Biomechanics and Modeling in Mechanobiology 12 167-183 (2013)
    The mechanism by which mechanical stimulation on osteocytes results in biochemical signals that initiate the remodeling process inside living bone tissue is largely unknown. Even the type of stimulation acting on these cells is not yet clearly identified. However, the cytoskeleton of osteocytes is suggested to play a major role in the mechanosensory process due to the direct connection to the nucleus. In this paper, a computational approach to model and simulate the cell structure of osteocytes based on self-stabilizing tensegrity structures is suggested. The computational model of the cell consists of the major components with respect to mechanical aspects: the integrins that connect the cell with the extracellular bone matrix, and different types of protein fibers (microtubules and intermediate filaments) that form the cytoskeleton, the membrane-cytoskeleton (microfilaments), the nucleus and the centrosome. The proposed geometrical cell models represent the cell in its physiological environment which is necessary in order to give a statement on the cell behavior in vivo. Studies on the mechanical response of osteocytes after physiological loading and in particular the mechanical response of the nucleus show that the load acting on the nucleus is rising with increasing deformation applied to the integrins. © 2012 Springer-Verlag.
    view abstractdoi: 10.1007/s10237-012-0390-y
  • 2013 • 145 Continuum and kinetic simulations of the neutral gas flow in an industrial physical vapor deposition reactor
    Bobzin, K. and Brinkmann, R.P. and Mussenbrock, T. and Bagcivan, N. and Brugnara, R.H. and Schäfer, M. and Trieschmann, J.
    Surface and Coatings Technology 237 176-181 (2013)
    Magnetron sputtering used for physical vapor deposition processes often requires gas pressures well below 1. Pa. Under these conditions the gas flow in the reactor is usually determined by a Knudsen number of about one, i.e., a transition regime between the hydrodynamic and the rarefied gas regime. In the first, the gas flow is well described by the Navier-Stokes equations, while in the second a kinetic approach via the Boltzmann equation is necessary. In this paper the neutral gas flow of argon and molecular nitrogen gas inside an industrial scale plasma reactor was simulated using both a fluid model and a fully kinetic Direct Simulation Monte Carlo model.By comparing both model results the validity of the fluid model was checked. Although in both models a Maxwell-Boltzmann energy distribution of the neutral particles is the natural outcome, the results of the gas flow differ significantly. The fluid model description breaks down, due to the inappropriate assumption of a fluid continuum. This is due to exclusion of non-local effects in the multi dimensional velocity space, as well as invalid gas/wall interactions. Only the kinetic model is able to provide an accurate physical description of the gas flow in the transition regime. Our analysis is completed with a brief investigation of different definitions of the local Knudsen number. We conclude that the most decisive parameter - the spatial length scale L - has to be very careful chosen in order to obtain a reasonable estimate of the gas flow regime. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2013.08.018
  • 2013 • 144 Cooperative dynamics of microtubule ensembles: Polymerization forces and rescue-induced oscillations
    Zelinski, B. and Kierfeld, J.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 87 (2013)
    We investigate the cooperative dynamics of an ensemble of N microtubules growing against an elastic barrier. Microtubules undergo so-called catastrophes, which are abrupt stochastic transitions from a growing to a shrinking state, and rescues, which are transitions back to the growing state. Microtubules can exert pushing or polymerization forces on an obstacle, such as an elastic barrier, if the growing end is in contact with the obstacle. We use dynamical mean-field theory and stochastic simulations to analyze a model where each microtubule undergoes catastrophes and rescues and where microtubules interact by force sharing. For zero rescue rate, cooperative growth terminates in a collective catastrophe. The maximal polymerization force before catastrophes grows linearly with N for small N or a stiff elastic barrier, in agreement with available experimental results, whereas it crosses over to a logarithmic dependence for larger N or a soft elastic barrier. For a nonzero rescue rate and a soft elastic barrier, the dynamics becomes oscillatory with both collective catastrophe and rescue events, which are part of a robust limit cycle. Both the average and maximal polymerization forces then grow linearly with N, and we investigate their dependence on tubulin on-rates and rescue rates, which can be involved in cellular regulation mechanisms. We further investigate the robustness of the collective catastrophe and rescue oscillations with respect to different catastrophe models. © 2013 American Physical Society.
    view abstractdoi: 10.1103/PhysRevE.87.012703
  • 2013 • 143 Crossover from tumbling to tank-treading-like motion in dense simulated suspensions of red blood cells
    Krüger, T. and Gross, M. and Raabe, D. and Varnik, F.
    Soft Matter 9 9008-9015 (2013)
    Via computer simulations, we provide evidence that the shear rate induced red blood cell tumbling-to-tank-treading transition also occurs at quite high volume fractions, where collective effects are important. The transition takes place as the ratio of effective suspension stress to the characteristic cell membrane stress exceeds a certain value and does not explicitly depend on volume fraction or cell deformability. This value coincides with that for a transition from an orientationally less ordered to a highly ordered phase. The average cell deformation does not show any signature of transition, but rather follows a simple scaling law independent of volume fraction. © 2013 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c3sm51645h
  • 2013 • 142 De Novo design of protein kinase inhibitors by in silico identification of hinge region-binding fragments
    Urich, R. and Wishart, G. and Kiczun, M. and Richters, A. and Tidten-Luksch, N. and Rauh, D. and Sherborne, B. and Wyatt, P.G. and Brenk, R.
    ACS Chemical Biology 8 1044-1052 (2013)
    Protein kinases constitute an attractive family of enzyme targets with high relevance to cell and disease biology. Small molecule inhibitors are powerful tools to dissect and elucidate the function of kinases in chemical biology research and to serve as potential starting points for drug discovery. However, the discovery and development of novel inhibitors remains challenging. Here, we describe a structure-based de novo design approach that generates novel, hinge-binding fragments that are synthetically feasible and can be elaborated to small molecule libraries. Starting from commercially available compounds, core fragments were extracted, filtered for pharmacophoric properties compatible with hinge-region binding, and docked into a panel of protein kinases. Fragments with a high consensus score were subsequently short-listed for synthesis. Application of this strategy led to a number of core fragments with no previously reported activity against kinases. Small libraries around the core fragments were synthesized, and representative compounds were tested against a large panel of protein kinases and subjected to co-crystallization experiments. Each of the tested compounds was active against at least one kinase, but not all kinases in the panel were inhibited. A number of compounds showed high ligand efficiencies for therapeutically relevant kinases; among them were MAPKAP-K3, SRPK1, SGK1, TAK1, and GCK for which only few inhibitors are reported in the literature. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/cb300729y
  • 2013 • 141 Determination of the geometry of the RVE for cancellous bone by using the effective complex shear modulus
    Klinge, S.
    Biomechanics and Modeling in Mechanobiology 12 401-412 (2013)
    This contribution deals with the application of the inverse homogenization method to the determination of geometrical properties of cancellous bone. The approach represents a combination of an extended version of the Marquardt-Levenberg method with the multiscale finite element method. The former belongs to the group of gradient-based optimization strategies, while the latter is a numerical homogenization method, suitable for the modeling of materials with a highly heterogeneous microstructure. The extension of the Marquardt-Levenberg method is concerned with the selection strategy for distinguishing the global minimum from the plethora of local minima. Within the numerical examples, the bone is modeled as a biphasic viscoelastic medium and three different representative volume elements are taken into consideration. Different models enable the simulation of the bone either as a purely isotropic or as a transversally anisotropic medium. Main geometrical properties of trabeculae are determined from data on effective shear modulus but alternative schemes are also possible. © 2012 Springer-Verlag.
    view abstractdoi: 10.1007/s10237-012-0408-5
  • 2013 • 140 Electrochemical micromachining of passive electrodes
    Sueptitz, R. and Dunne, P. and Tschulik, K. and Uhlemann, M. and Eckert, J. and Gebert, A.
    Electrochimica Acta 109 562-569 (2013)
    The electronic model describing the electrochemical micromachining (ECMM) of passive electrodes uti-lizing the transpassive dissolution is discussed. Numerical simulations are performed on a machiningmodel circuit using measured electrochemical properties of the model system which consisted of a tung-sten tool electrode, a 1 M H2SO4electrolyte and a stainless steel work piece electrode. The results of thesesimulations were verified by performing machining experiments applying the same model system. For apassive stainless steel electrode it is shown that it can be treated like an actively dissolving electrode withhigh reaction overpotential. The efficiency of the machining process can be enhanced by polarizing thesteel work piece electrode close to the transpassive potential region. Three different ways of achievingthis polarization are discussed: by polarizing the work piece electrode only, by polarizing both electrodesand by adding oxidizing species to the electrolyte solution. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2013.07.139
  • 2013 • 139 Evaluation method for stretch blow moulding simulations with process-oriented experiments
    Zimmer, J. and Detrois, C. and Stommel, M.
    Key Engineering Materials 554-557 1658-1668 (2013)
    In the Stretch Blow Moulding (SBM) process, Poly (ethylene terephthalate) (PET)- preforms are biaxially deformed to produce thin walled bottles. Finite-Element (FE)-Simulations are an important tool to optimise this process in terms of material usage and product performance. Thereby, the implementation of the thermo-mechanical material behaviour of PET plays an important role to achieve realistic simulation results. A common approach for this purpose is to calibrate a material model with stress-strain curves from biaxial stretching experiments. Thin PETsheets are stretched under defined temperatures and strain rates. However, these experiments include process simplifications regarding geometry, heating and deformation parameters. This paper presents a method for extracting temperature dependent stress-strain-curves from experiments close to the production process. PET-Preforms receive thermal treatment with Infrared (IR)-heaters from an SBM-machine and are subsequently inflated in free air (free-blow-trial). A IR-camera is used to image the axial and radial temperature distribution on the preform immediately before blowing. The deformation process is recorded via two high speed cameras with a frame rate of 2000/s. The cameras are synchronised with a pressure sensor to consequently calculate reliable stress-strain curves at any points on the preform. In addition FE-simulations of the free blow trials are conducted using a material model calibrated with the simplified stretching experiments of thin PET sheets. Resulting stress-strain-curves from simulations and free-blow-trials are finally compared to evaluate the quality of the material model as well as the underlying testing procedure. Copyright © 2013 Trans Tech Publications Ltd.
    view abstractdoi: 10.4028/
  • 2013 • 138 High-pressure shock-tube investigation of the impact of 3-pentanone on the ignition properties of primary reference fuels
    Fikri, M. and Cancino, L.R. and Hartmann, M. and Schulz, C.
    Proceedings of the Combustion Institute 34 393-400 (2013)
    Ignition-delay times for pure 3-pentanone, 3-pentanone/iso-octane (10/90% by volume) and 3-pentanone/n-Heptane mixtures (10/90% by volume) have been determined in a high-pressure shock tube under engine-relevant conditions (p5 = 10, 20, and 40 bar) for equivalence ratios φ = 0.5 and 1.0 and over a wide temperature range 690 K < T5 < 1270 K. The results were compared to ignition delay times of primary reference fuels under identical conditions. A detailed kinetics model is proposed for the ignition of all fuel mixtures. The model predicts well the ignition delay times for pure 3-pentanone for a wide range of pressure and temperature and equivalence ratios in argon dilution as well as in air. Ignition delay times for 3-pentanone-doped mixtures, especially in the low-temperature range are overpredicted by approx. a factor of 0.5 (at 800 K, 40 bar, φ = 1.0) by the calculation but the model still reproduces the overall trend of the experimental data. For lean conditions, 10% 3-pentanone reduces the reactivity of n-Heptane below 1000 K while for stoichiometric conditions it does not alter the ignition delay by more than 11% at 850 K and 20 bar. In iso-octane the effect is inverse, leading to acceleration of the main ignition. Based on the model, the influence of 3-pentanone on the main heat release in a n-Heptane-fueled HCCI engine cycle is simulated. © 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.proci.2012.05.101
  • 2013 • 137 Homogenization of the degenerate two-phase flow equations
    Henning, P. and Ohlberger, M. and Schweizer, B.
    Mathematical Models and Methods in Applied Sciences 23 2323-2352 (2013)
    We analyze two-phase flow in highly heterogeneous media. Problems related to the degeneracy of the permeability coefficient functions are treated with a new concept of weighted solutions. Instead of the pressure variables we formulate the problem with the weighted pressure function ψ, which is obtained as the product of permeability and pressure. We perform the homogenization limit and obtain effective equations in the form of a two-scale limit system. The nonlinear effective system is of the classical form in the non-degenerate case. In the degenerate case, the two-scale system uses again a weighted pressure variable. Our approach allows to work without the global pressure function. Even though internal interfaces are included, our approach provides the homogenization limit without any smallness assumptions on permeabilities or capillary pressures. © 2013 World Scientific Publishing Company.
    view abstractdoi: 10.1142/S0218202513500334
  • 2013 • 136 Interfacial roughening in nonideal fluids: Dynamic scaling in the weak- and strong-damping regime
    Gross, M. and Varnik, F.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 87 (2013)
    Interfacial roughening denotes the nonequilibrium process by which an initially flat interface reaches its equilibrium state, characterized by the presence of thermally excited capillary waves. Roughening of fluid interfaces has been first analyzed by Flekkoy and Rothman, where the dynamic scaling exponents in the weakly damped case in two dimensions were found to agree with the Kardar-Parisi-Zhang universality class. We extend this work by taking into account also the strong-damping regime and perform extensive fluctuating hydrodynamics simulations in two dimensions using the Lattice Boltzmann method. We show that the dynamic scaling behavior is different in the weakly and strongly damped case. © 2013 American Physical Society.
    view abstractdoi: 10.1103/PhysRevE.87.022407
  • 2013 • 135 Inverse-motion-based form finding for quasi-incompressible finite electroelasticity
    Ask, A. and Denzer, R. and Menzel, A. and Ristinmaa, M.
    International Journal for Numerical Methods in Engineering 94 554-572 (2013)
    This work deals with inverse-motion-based form finding for electroelasticity. The inverse motion problem is formulated for the electroelastic case, and the resulting equations are implemented within a finite element framework. A four-field variational approach is adopted, taking into consideration the typically incompressible behavior of the elastomer materials commonly used in electromechanical applications. By means of numerical simulations, the inverse-motion-based form finding makes it possible to design the referential configuration so that a given set of loads and boundary conditions results in a prespecified deformed configuration. The computational finite element framework established in this work allows for such numerical simulations and testing and thereby the possibility to improve the design and accuracy in electroelastic applications such as grippers, sensors, and seals. © 2013 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/nme.4462
  • 2013 • 134 Investigation of the influence of reflection on the attenuation of cancellous bone
    Klinge, S. and Hackl, K. and Gilbert, R.P.
    Biomechanics and Modeling in Mechanobiology 12 185-199 (2013)
    The model proposed in this paper is based on the fact that the reflection might have a significant contribution to the attenuation of the acoustic waves propagating through the cancellous bone. The numerical implementation of the mentioned effect is realized by the development of a new representative volume element that includes an infinitesimally thin 'transient' layer on the contact surface of the bone and the marrow. This layer serves to model the amplitude transformation of the incident wave by the transition through media with different acoustic impedances and to take into account the energy loss due to the reflection. The proposed representative volume element together with the multiscale finite element is used to simulate the wave propagation and to evaluate the attenuation coefficient for samples with different effective densities in the dependence of the applied excitation frequency. The obtained numerical values show a very good agreement with the experimental results. Moreover, the model enables the determination of the upper and the lower bound for the attenuation coefficient. © 2012 Springer-Verlag.
    view abstractdoi: 10.1007/s10237-012-0391-x
  • 2013 • 133 Investigations on the impact of different electric vehicle traction systems in urban traffic
    Jeschke, S. and Hirsch, H. and Koppers, M. and Schramm, D.
    2013 9th IEEE Vehicle Power and Propulsion Conference, IEEE VPPC 2013 185-190 (2013)
    Currently electric vehicles are introduced in e.g. public transport and individual traffic in order to reduce i.a. The green house gas emissions. The main disadvantage of electric vehicles compared to vehicles with conventional drive is the shorter operating distance. In contrast this disadvantage is partially negligible in urban usage scenarios, like e.g. taxi or delivery services. This paper focuses on the simulation of electric vehicle propulsion systems using a Hardware in the Loop (HiL) model. The model consisting of components used in actual electric vehicles is scaled using Buckingham's Pi-Theorem in order to analyze the impact of different electric traction systems on the vehicle's energy consumption. Thus the available operating distance of such vehicles can be optimized in urban traffic. © 2013 IEEE.
    view abstractdoi: 10.1109/VPPC.2013.6671687
  • 2013 • 132 Mechanism of iron oxide formation from iron pentacarbonyl-doped low-pressure hydrogen/oxygen flames
    Wlokas, I. and Faccinetto, A. and Tribalet, B. and Schulz, C. and Kempf, A.
    International Journal of Chemical Kinetics 45 487-498 (2013)
    A chemical reaction mechanism was developed for the formation of iron oxide (Fe2O3) from iron pentacarbonyl (Fe(CO)5) in a low-pressure hydrogen-oxygen flame reactor. In this paper, we describe an extensive approach for the flame-precursor chemistry and the development of a novel model for the formation of Fe2O3 from the gas phase. The detailed reaction mechanism is reduced for the implementation in two-dimensional, reacting flow simulations. The comprehensive simulation approach is completed by a model for the formation and growth of the iron oxide nanoparticles. The exhaustive and compact reaction mechanism is validated using experimental data from iron-atom laser-induced fluorescence imaging. The particle formation and growth model are verified with new measurements from particle mass spectrometry. © 2013 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/kin.20786
  • 2013 • 131 Method for the evaluation of stretch blow molding simulations with free blow trials
    Zimmer, J. and Stommel, M.
    IOP Conference Series: Materials Science and Engineering 48 (2013)
    Finite-Element (FE) simulations are a valuable tool to support the analysis and optimization of production processes. In order to achieve realistic simulation results, a consistent simulation set-up followed by an evaluation through experiments is crucial. Stretch Blow Molding (SBM) is a commonly applied forming method to produce thin walled bottles. Polyethylene terephthalate (PET) preforms are biaxially stretched into a closed cavity to form a bottle. In this process the thermo-mechanical material behavior during forming greatly influences the performance of the end product and consequently plays a key role for a reliable process simulation. To ensure a realistic material representation in the simulation model, an adequate material model is calibrated with stress-strain curves from biaxial tests. Thin PET-sheets are stretched under defined temperatures and strain rates. These representative experiments include process simplifications regarding geometry, heating and deformation parameters. Therefore, an evaluation step subsequent to the simulation set-up is inevitable. This paper presents a method for extracting temperature dependent stress-strain-curves from experiments close to the production process which enables the crucial evaluation of a process simulation. In the SBM process, the wall thickness distribution of the bottle refers to the preform deformation over time but does not fully define the thermo-mechanical material behavior. In the presented method, PET-preforms receive thermal treatment with Infrared (IR)-heaters from an SBM-machine and are subsequently inflated into free air (free-blow-trial). An IR-camera is used to obtain the temperature distribution on the preform immediately before blowing. Two high speed cameras are synchronized with a pressure sensor to consequently calculate reliable stress-strain curves at any point on the preform surface. These data is finally compared to results from FE-simulations of the free blow trials.
    view abstractdoi: 10.1088/1757-899X/48/1/012004
  • 2013 • 130 Modeling and simulation of heat input in deep-hole drilling with twist drills and MQL
    Biermann, D. and Iovkov, I.
    Procedia CIRP 8 88-93 (2013)
    Former investigations on deep-hole drilling using twist drills and MQL indicated that the heat input into the workpiece results not only from the primary thermal load within the machining zone but also from the secondary heating at the borehole wall. In order to determine the primary heat flow into the machined part an experimental setup for drilling devoid of the influence of the borehole wall has been developed. The results show that the machining time is the major factor: the lower the machining time the lower the measured temperature within the workpiece. A finite-element-(FE)-based simulation in consideration of the material removal has been implemented and studied, regarding the process, the FE-mesh and the time discretisation. Based on the reliable parameter, obtained by the discretisation study, a simulation control loop is presented which allows the calculation of the rate of heat flow into the machined part. It is remarkable that due to the material removal the heat flow into the workpiece increases when machining with higher cutting speed and feed values, while the measured and simulated temperature decreases. Copyright © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.procir.2013.06.070
  • 2013 • 129 Modeling approach for the determination of material flow and welding conditions in porthole die extrusion with gas pocket formation
    Schwane, M. and Gagliardi, F. and Jäger, A. and Ben Khalifa, N. and Tekkaya, A.E.
    Key Engineering Materials 554-557 787-793 (2013)
    The material flow in porthole dies is of crucial importance with regard to the seam weld quality in aluminum extrusion. Thus, experimental as well as numerical investigations on the effect of die geometry on the material flow were conducted. The experimental tests were performed on a 10 MN laboratory extrusion press. During the experimental trials, the extrusion ratio was varied by means of exchangeable die plates. Since the modular die allows removal of the aluminum in the welding chamber as well as in the feeders after the process, the material flow could be inspected in detail. The experimental results were used to improve the accuracy of FEA simulations, which were also conducted by commercial software. An attempt was made to improve the result quality of Eulerian FEA model regarding the simulation of an extrusion process with a gas pocket in the welding chamber. The influence of the modeling approach on the predicted material flow and on the contact pressure was analyzed and finally linked to the seam weld quality. Copyright © 2013 Trans Tech Publications Ltd.
    view abstractdoi: 10.4028/
  • 2013 • 128 Modeling sample/patient-specific structural and diffusional responses of cartilage using DT-MRI
    Pierce, D.M. and Ricken, T. and Holzapfel, G.A.
    International Journal for Numerical Methods in Biomedical Engineering 29 807-821 (2013)
    We propose a new 3D biphasic constitutive model designed to incorporate structural data on the sample/patient-specific collagen fiber network. The finite strain model focuses on the load-bearing morphology, that is, an incompressible, poroelastic solid matrix, reinforced by an inhomogeneous, dispersed fiber fabric, saturated with an incompressible fluid at constant electrolytic conditions residing in strain-dependent pores of the collagen-proteoglycan solid matrix. In addition, the fiber network of the solid influences the fluid permeability and an intrafibrillar portion that cannot be 'squeezed out' from the tissue. We implement the model into a finite element code. To demonstrate the utility of our proposed modeling approach, we test two hypotheses by simulating an indentation experiment for a human tissue sample. The simulations use ultra-high field diffusion tensor magnetic resonance imaging that was performed on the tissue sample. We test the following hypotheses: (i) the through-thickness structural arrangement of the collagen fiber network adjusts fluid permeation to maintain fluid pressure (Biomech. Model. Mechanobiol. 7:367-378, 2008); and (ii) the inhomogeneity of mechanical properties through the cartilage thickness acts to maintain fluid pressure at the articular surface (J. Biomech. Eng. 125:569-577, 2003). For the tissue sample investigated, both through-thickness inhomogeneities of the collagen fiber distribution and of the material properties serve to influence the interstitial fluid pressure distribution and maintain fluid pressure underneath the indenter at the cartilage surface. Tissue inhomogeneity appears to have a larger effect on fluid pressure retention in this tissue sample and on the advantageous pressure distribution. © 2012 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/cnm.2524
  • 2013 • 127 Modelling the lattice dynamics in SixGe1-x alloys
    Katre, A. and Drautz, R. and Madsen, G.K.H.
    Journal of Physics Condensed Matter 25 (2013)
    The development of simplified models for the simulation of thermodynamic and thermal transport properties in random alloys is of great importance. In this paper we show how a simple second nearest neighbour model can reliably capture the lattice dynamics of SixGe1-x alloys. The model parameters are extracted from DFT-calculated force constant matrices for pure Si, pure Ge and the Si0.5Ge0.5 ordered alloy. We extract the nearest neighbour contributions directly from density functional theory, whereas effective interactions are obtained for the second nearest neighbour contributions. We demonstrate how the thermal properties, including the expansion coefficient, can be reliably reproduced and that the model is transferable to random SixGe1-x alloys. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/25/36/365403
  • 2013 • 126 Modelling, simulation and experimental investigation of chip formation in internal traverse grinding
    Holtermann, R. and Schumann, S. and Menzel, A. and Biermann, D.
    Production Engineering 7 251-263 (2013)
    We present recent developments in modelling and simulation of internal traverse grinding, a high speed machining process which enables both a large material removal rate and high surface quality. We invoke a hybrid modelling framework, including a process scale model, simulations on a mesoscale capturing the proximity of a single cBN grain and an analysis framework to investigate the grinding wheel topography. Moreover, we perform experiments to verify our simulations. Focus in this context is the influence of the cutting speed variation on the grain specific heat generation. © 2013 German Academic Society for Production Engineering (WGP).
    view abstractdoi: 10.1007/s11740-013-0449-3
  • 2013 • 125 Molecular fragment dynamics study on the water-air interface behavior of non-ionic polyoxyethylene alkyl ether surfactants
    Truszkowski, A. and Epple, M. and Fiethen, A. and Zielesny, A. and Kuhn, H.
    Journal of Colloid and Interface Science 410 140-145 (2013)
    Molecular fragment dynamics (MFD) is a mesoscopic simulation technique based on dissipative particle dynamics (DPD). MFD simulations of the self-aggregation of the polyoxyethylene alkyl ether surfactants C6E6, C10E6, C12E6 and C16E6 at the water-air surface lead to equilibrium nanoscale structures and computationally determined surface tensions which are in agreement with experimental data for different surfactant concentrations. Thus, molecular fragment dynamics is a well-suited predictive technique to study the behavior of new surfactant systems. © 2013 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcis.2013.07.069
  • 2013 • 124 Nanoscale thermomechanics of wear-resilient polymeric bilayer systems
    Kaule, T. and Zhang, Y. and Emmerling, S. and Pihan, S. and Foerch, R. and Gutmann, J. and Butt, H.-J. and Berger, R. and Duerig, U. and Knoll, A.W.
    ACS Nano 7 748-759 (2013)
    We explore the effect of an ultrathin elastic coating to optimize the mechanical stability of an underlying polymer film for nanoscale applications. The coating consists of a several nanometer thin plasma-polymerized norbornene layer. Scanning probes are used to characterize the system in terms of shear-force-induced wear and thermally assisted indentation. The layer transforms a weakly performing polystyrene film into a highly wear-resistive system, ideal for high-density and low-power data storage applications. The result can be understood from the indentation characteristics with a hot and sharp indenter tip. The latter gives rise to a deformation mode in the fully plastic regime, enabling a simple interpretation of the results. The softening transition and the yield stress of the system on a microsecond time scale and a nanometer size scale were obtained. We show that the plastic deformation is governed by yielding in the polystyrene sublayer, which renders the overall system soft for plastic deformation. The ultrathin protection layer contributes as an elastic skin, which shields part of the temperature and pressure and enables the high wear resistance against lateral forces. Moreover, the method of probing polymers at microsecond and nanometer size scales opens up new opportunities for studying polymer physics in a largely unexplored regime. Thus, we find softening temperatures of more than 100 °C above the polystyrene glass transition, which implies that for the short interaction time scales the glassy state of the polymer is preserved up to this temperature. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/nn305047m
  • 2013 • 123 Numerical simulation of particle distribution in capillary membrane during backwash
    Mansour, H. and Keller, A. and Gimbel, R. and Kowalczyk, W.
    Membranes 3 249-265 (2013)
    The membrane filtration with inside-out dead-end driven UF-/MF-capillary membranes is an effective process for particle removal in water treatment. Its industrial application increased in the last decade exponentially. To date, the research activities in this field were aimed first of all at the analysis of filtration phenomena disregarding the influence of backwash on the operation parameters of filtration plants. However, following the main hypothesis of this paper, backwash has great potential to increase the efficiency of filtration. In this paper, a numerical approach for a detailed study of fluid dynamic processes in capillary membranes during backwash is presented. The effect of particle size and inlet flux on the backwash process are investigated. The evaluation of these data concentrates on the analysis of particle behavior in the cross sectional plane and the appearance of eventually formed particle plugs inside the membrane capillary. Simulations are conducted in dead-end filtration mode and with two configurations. The first configuration includes a particle concentration of 10% homogeneously distributed within the capillary and the second configuration demonstrates a cake layer on the membrane surface with a packing density of 0.6. Analyzing the hydrodynamic forces acting on the particles shows that the lift force plays the main role in defining the particle enrichment areas. The operation parameters contribute in enhancing the lift force and the heterogeneity to anticipate the clogging of the membrane. © 2013 by the authors; licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/membranes3040249
  • 2013 • 122 On the crystallographic anisotropy of nanoindentation in pseudoelastic NiTi
    Pfetzing-Micklich, J. and Somsen, C. and Dlouhy, A. and Begau, C. and Hartmaier, A. and Wagner, M.F.-X. and Eggeler, G.
    Acta Materialia 61 602-616 (2013)
    We use a nanoindenter with a Berkovich tip to study local mechanical properties of two polycrystalline intermetallics with a B2 crystal structure, NiAl and NiTi. We use orientation imaging scanning electron microscopy to select a relevant number of grains with appropriate sizes and surface normals parallel to 〈0 0 1〉, 〈1 0 1〉 and 〈1 1 1〉. As a striking new result, we find a strong crystallographic orientation dependence for NiTi. This anisotropy is less pronounced in the case of NiAl. For NiTi, the indentation force required to impose a specific indentation depth is highest for indentation experiments performed in the 〈0 0 1〉 direction and lowest along the 〈1 1 1〉 direction. We consider transmission electron microscopy results from cross-sections below the indents and use molecular dynamics simulations and resolved shear stress calculations to discuss how this difference can be accounted for in terms of elementary deformation and transformation processes, related to dislocation plasticity (NiAl and NiTi), and in terms of the stress-induced formation and growth of martensite (NiTi). Our results show that the crystallographic anisotropy during nanoindentation of NiTi is governed by the orientation dependence of the martensitic transformation; dislocation plasticity appears to be less important. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.09.081
  • 2013 • 121 Parallel Brownian dynamics simulations with the message-passing and PGAS programming models
    Teijeiro, C. and Sutmann, G. and Taboada, G.L. and Touriño, J.
    Computer Physics Communications 184 1191-1202 (2013)
    The simulation of particle dynamics is among the most important mechanisms to study the behavior of molecules in a medium under specific conditions of temperature and density. Several models can be used to compute efficiently the forces that act on each particle, and also the interactions between them. This work presents the design and implementation of a parallel simulation code for the Brownian motion of particles in a fluid. Two different parallelization approaches have been followed: (1) using traditional distributed memory message-passing programming with MPI, and (2) using the Partitioned Global Address Space (PGAS) programming model, oriented towards hybrid shared/distributed memory systems, with the Unified Parallel C (UPC) language. Different techniques for domain decomposition and work distribution are analyzed in terms of efficiency and programmability, in order to select the most suitable strategy. Performance results on a supercomputer using up to 2048 cores are also presented for both MPI and UPC codes. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cpc.2012.12.015
  • 2013 • 120 Parallel simulation of brownian dynamics on shared memory systems with OpenMP and unified parallel C
    Teijeiro, C. and Sutmann, G. and Taboada, G.L. and Touriño, J.
    Journal of Supercomputing 65 1050-1062 (2013)
    The simulation of particle dynamics is an essential method to analyze and predict the behavior of molecules in a given medium. This work presents the design and implementation of a parallel simulation of Brownian dynamics with hydrodynamic interactions for shared memory systems using two approaches: (1) OpenMP directives and (2) the Partitioned Global Address Space (PGAS) paradigm with the Unified Parallel C (UPC) language. The structure of the code is described, and different techniques for work distribution are analyzed in terms of efficiency, in order to select the most suitable strategy for each part of the simulation. Additionally, performance results have been collected from two representative NUMA systems, and they are studied and compared against the original sequential code. © 2012 Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s11227-012-0843-1
  • 2013 • 119 Parallelized computational modeling of pile-soil interactions in mechanized tunneling
    Meschke, G. and Ninić, J. and Stascheit, J. and Alsahly, A.
    Engineering Structures 47 35-44 (2013)
    The construction of tunnels in soft ground causes short and long term ground deformations resulting from the disturbance of the virgin stress state of the soil and the changing pore water conditions. In particular in urban tunneling, in each stage of the construction process, interactions between the construction process, the soil and existing building infrastructure need to be evaluated to limit the risk of damage on existing buildings and to decide on appropriate mitigation measures. Besides conventional tunneling, mechanized tunneling is a well established and flexible technology in particular in urban areas, which allows for tunnel advances in a wide range of soils and difficult conditions. The paper presents a finite element model for the simulation of interactions between mechanized tunnel construction, the surrounding soil and existing buildings resting on pile foundations in the framework of a process-oriented simulation model for mechanized tunneling. The performance of the model is demonstrated by means of selected prototype analyses. As a consequence of the high computational demand connected with this type of spatio-temporal simulations, problem specific parallelization techniques are investigated to increase the numerical efficiency of the numerical analyses. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.engstruct.2012.07.001
  • 2013 • 118 Simulation of electromagnetic forming of a cross-shaped cup by means of a viscoplasticity model coupled with damage at finite strains
    Kiliclar, Y. and Demir, O.K. and Vladimirov, I.N. and Kwiatkowski, L. and Reese, S. and Tekkaya, A.E.
    Key Engineering Materials 554-557 2363-2368 (2013)
    In the field of sheet metal forming conventional forming processes are well established. However, a quasi-static forming process combined with a high speed forming process can enhance the forming limits of a single one. In this paper, the investigation of the process chain quasi-static deep drawing - electromagnetic forming by means of a new coupled damage-viscoplasticity model for large deformations is performed. The finite strain constitutive model, used in the finite element simulation combines nonlinear kinematic and isotropic hardening and is derived in a thermodynamically consistent setting. This anisotropic viscoplastic model is based on the multiplicative decomposition of the deformation gradient in the context of hyperelasticity. The kinematic hardening component represents a continuum extension of the classical rheological model of Armstrong-Frederick kinematic hardening. The coupling of damage and plasticity is carried out in a constitutive manner according to the effective stress concept. The constitutive equations of the material model are integrated in an explicit manner and implemented as a user material subroutine in the commercial finite element package LS-DYNA with the electromagnetical module. The aim of the work is to show the increasing formability of the sheet by combining quasistatic deep drawing processes with high speed electromagnetic forming. Copyright © 2013 Trans Tech Publications Ltd.
    view abstractdoi: 10.4028/
  • 2013 • 117 Simulation of viscous sintering using the lattice Boltzmann method
    Varnik, F. and Rios, A. and Gross, M. and Steinbach, I.
    Modelling and Simulation in Materials Science and Engineering 21 (2013)
    The viscous flow is one of the important mass transport mechanisms in sintering powder materials such as porous glass. The main underlying driving force here is the tendency of the system to minimize the overall surface free energy. In this work, we first address the fundamental problem of the coalescence of two suspending drops and provide an alternative derivation of Frenkel's formula by explicitly taking account of driving forces which arise from the curvature of the contact zone. The result thus obtained is compared with non-ideal (two phase) fluid lattice Boltzmann simulations. Furthermore, we use this simulation tool to address the more complex case of the sintering of a compact of viscous spherical particles. We observe a significantly faster densification dynamics than in experiments but obtain qualitative agreement upon rescaling the unit of time. We attribute this faster dynamics in simulations to the absence of intergranular friction and discuss a possible improvement of the model. We also propose, for the present LB model, a simple way of imposing shear flow to determine the time dependence of the effective viscosity during the sintering process. The method is benchmarked via a simple analytic case and first simulation results are presented for situations for which no analytic theory exists. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/21/2/025003
  • 2013 • 116 Spray roasting of iron chloride FeCl2: Numerical modelling of industrial scale reactors
    Schiemann, M. and Wirtz, S. and Scherer, V. and Bärhold, F.
    Powder Technology 245 70-79 (2013)
    Iron chloride solutions are a waste product in steel pickling plants. A technique to recover the spent solutions is a spray roasting process, where the spent solution is sprayed into a hot reaction atmosphere and solid iron oxide particles are formed. The particle history in spray roasting reactors has an important influence on the efficiency of the recovery process and on the quality of the by-product Fe2O3. The iron oxide underlays strong quality demands for further processing. The particle quality is influenced by the plant design and operation parameters. To investigate the influence of those properties on the iron oxide produced in the spray roasting process, a model for CFD simulations has been developed. It describes the particle formation and chemical reaction of the iron chloride solution in the spray roasting reactor in a simplified way suitable for CFD simulations. Simulations of two different industrial reactor configurations show the capability of the model to predict the influence of geometric variations on the composition of the resulting iron oxide. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.powtec.2013.04.034
  • 2013 • 115 Towards comprehensive coal combustion modelling for les
    Stein, O.T. and Olenik, G. and Kronenburg, A. and Cavallo Marincola, F. and Franchetti, B.M. and Kempf, A.M. and Ghiani, M. and Vascellari, M. and Hasse, C.
    Flow, Turbulence and Combustion 90 859-884 (2013)
    Large eddy simulations of pulverised coal combustion (PCC-LES) stabilised on a laboratory-scale piloted jet burner are carried out. The joint simulation effort of three research groups at Freiberg University (FG), Imperial College (IC) and Stuttgart University (ST) is presented, and the details of the comprehensive coal combustion models and their numerical implementation in three different computer programs are discussed. The (standard) coal sub-models and parameters used by the different groups are unified wherever possible. Differences amongst the groups are a different code basis and an Eulerian treatment of the coal particles by IC, while FG and ST use the Lagrangian framework for particle transport. The flow modelling is first validated for the corresponding non-reacting case and all LES calculations accurately capture the experimental trends. Velocity field statistics for the PCC case are in good accordance with the experimental evidence, but scalar statistics illustrate the complexity of coal combustion modelling. The results show notable differences amongst the groups that cannot only be attributed to the different treatment of the particle phase, and they highlight the difficulty to assess and interpret the quality of specific modelling approaches, and a need for further work by the research community. The present study is the first to compare three originally independent transient coal simulations and a step towards comprehensive PCC-LES. © 2012 Springer Science+Business Media Dordrecht.
    view abstractdoi: 10.1007/s10494-012-9423-y
  • 2013 • 114 Universal frozen spectra after time-dependent symmetry restoring phase transitions
    Queisser, F. and Navez, P. and Schützhold, R.
    Journal of Physics Condensed Matter 25 (2013)
    For a general O(N) model, we study the time-dependent phase transition from a state with broken symmetry to the symmetric phase . During this non-equilibrium process, the primordial quantum (or thermal) fluctuations of the initial Goldstone modes are frozen and result in a deviation from the final ground (or thermal) state. For very slow transitions, we find that these fluctuations display a universal scaling behaviour. Their spectra are universal functions of a single parameter, which combines the initial frequency of the Goldstone modes and the sweep rate. As a result, the final two-point function is not exponentially suppressed at large distances Δr = r - r′ (as it would be in the ground state) but decays polynomially in 1/|Δr|. Finally, we exemplify this universal behaviour for the transition from the super-fluid phase to the Mott state in the Bose-Hubbard model. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/25/40/404215
  • 2013 • 113 Validation of a heat input model for the prediction of thermomechanical deformations during NC milling
    Joliet, R. and Byfut, A. and Kersting, P. and Schröder, A. and Zabel, A.
    Procedia CIRP 8 403-408 (2013)
    During roughing in NC milling, heat is introduced into the workpiece. For the manufacturing of large structural components, a constantly changing temperature field is created due to the rapid movement and the varying contact conditions between tool and workpiece. Therefore, significant deformations can cause form errors that lead to rejects in the production process. In this paper, a simulation system for the prediction of transient workpiece temperatures is presented. In order to calibrate the system, simple experiments have been conducted, and a model for the introduction of energy into the workpiece via cutting has been developed. The newly developed cutting-energy input model makes it possible to perform fast simulations. Therefore, it can be used to perform simulations of the thermoelastic workpiece deformations during milling of complex shaped parts. Copyright © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.procir.2013.06.124
  • 2012 • 112 A general spectral method for the numerical simulation of one-dimensional interacting fermions
    Clason, C. and von Winckel, G.
    Computer Physics Communications 183 405--417 (2012)
    This work introduces a general framework for the direct numerical simulation of systems of interacting fermions in one spatial dimension. The approach is based on a specially adapted nodal spectral Galerkin method, where the basis functions are constructed to obey the antisymmetry relations of fermionic wave functions. An efficient MATLAB program for the assembly of the stiffness and potential matrices is presented, which exploits the combinatorial structure of the sparsity pattern arising from this discretization to achieve optimal run-time complexity. This program allows the accurate cliscretization of systems with multiple fermions subject to arbitrary potentials, e.g., for verifying the accuracy of multi-particle approximations such as Hartree-Fock in the few-particle limit. It can be used for eigenvalue computations or numerical solutions of the time-dependent Schrodinger equation. Program summary Program title: assembleFermiMatrix Catalogue identifier: AEKO_v1_0 Program summary URL: Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, No. of lines in distributed program, including test data, etc.: 102 No. of bytes in distributed program, including test data, etc.: 2294 Distribution format: tar.gz Programming language: MATLAB Computer: Any architecture supported by MATLAB Operating system: Any supported by MATLAB; tested under Linux (x86-64) and Mac OS X (10.6) RAM: Depends on the data Classification: 4.3, 2.2 Nature of problem: The direct numerical solution of the multi-particle one-dimensional Schrodinger equation in a quantum well is challenging due to the exponential growth in the number of degrees of freedom with increasing particles. Solution method: A nodal spectral Galerkin scheme is used where the basis functions are constructed to obey the antisymmetry relations of the fermionic wave function. The assembly of these matrices is performed efficiently by exploiting the combinatorial structure of the sparsity patterns. Restrictions: Only one-dimensional computational domains with homogeneous Dirichlet or periodic boundary conditions are supported. Running time: Seconds to minutes (C) 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cpc.2011.10.005
  • 2012 • 111 A model for the devolatilisation of large thermoplastic particles under co-firing conditions
    Bluhm-Drenhaus, T. and Becker, A. and Wirtz, S. and Scherer, V.
    Fuel 101 161-170 (2012)
    Thermoplastic materials are utilised in large quantities for packaging purposes and thus make up a significant share of municipal solid waste. Due to their high calorific value, plastic solid waste (PSW) is suitable for co-firing as a refuse derived fuel in high temperature processes such as industrial furnaces and boilers. In order to achieve a high substitution ratio of fossil fuels and a high degree of burnout, a characterisation of the PSW properties and a prediction of the decomposition behaviour are crucial for process optimisation. However, for PSW particles e.g. polyethylene in the size range of several millimetres combustion models are scarcely published nor are they available in commercial simulation packages. Therefore a simplified devolatilisation model is presented in this paper, transferring findings from the fire protection literature to combustion modelling. The model is capable of calculating the conversion rate of thermoplastic particles under co-firing conditions, considering aspects like particle size, particle shape and related heat and mass transfer phenomena. The modelling results are compared to laboratory experiments in a bench scale reactor and an extrapolation to co-firing conditions is carried out for particles of generic shape. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.fuel.2012.05.041
  • 2012 • 110 Absolute acidity of clay edge sites from ab-initio simulations
    Tazi, S. and Rotenberg, B. and Salanne, M. and Sprik, M. and Sulpizi, M.
    Geochimica et Cosmochimica Acta 94 1-11 (2012)
    We provide a microscopic understanding of the solvation structure and reactivity of the edges of neutral clays. In particular we address the tendency to deprotonation of the different reactive groups on the (010) face of pyrophyllite. Such information cannot be inferred directly from titration experiments, which do not discriminate between different sites and whose interpretation resorts to macroscopic models. The determination of the corresponding pK a then usually relies on bond valence models, sometimes improved by incorporating some structural information from ab-initio simulations. Here we use density functional theory based molecular dynamics simulations, combined with thermodynamic integration, to compute the free energy of the reactions of water with the different surface groups, leading to a deprotonated site and an aqueous hydronium ion. Our approach consistently describes the clay and water sides of the interface and includes naturally electronic polarization effects. It also allows to investigate the structure and solvation of all sites separately. We find that the most acidic group is SiOH, due to its ability to establish strong hydrogen bonds with adsorbed water, as it also happens on the quartz and amorphous silica surfaces. The acidity constant of AlOH 2 is only 1 pK a unit larger. Finally, the pK a of AlOH is outside the possible range in water and this site should not deprotonate in aqueous solution. We show that the solvation of surface sites and hence their acidity is strongly affected by the proximity of other sites, in particular for AlOH and AlOH 2 which share the same Al. We discuss the implications of our findings on the applicability of bond valence models to predict the acidity of edge sites of clays. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.gca.2012.07.010
  • 2012 • 109 Activity coefficients of complex molecules by molecular simulation and Gibbs-Duhem integration
    Hempel, S. and Fischer, J. and Paschek, D. and Sadowski, G.
    Soft Materials 10 26-41 (2012)
    Activity coefficients of solvents and solutes in different aqueous solutions of alcohols and polymers are determined by molecular dynamic simulations. These data are often not accessible by simulation due to unacceptably high computational demands. Therefore, we applied a combination of two methods: water activity coefficients were determined directly via Overlapping Distribution Method, while counter-component activity coefficients were calculated indirectly by Gibbs-Duhem integration of the respective water activities. Results are in good agreement with experimental data. The method can easily be applied to determine activity coefficients of very complex components in water or other simple solvents. © 2012 Copyright Taylor and Francis Group, LLC.
    view abstractdoi: 10.1080/1539445X.2011.599698
  • 2012 • 108 An improved electrical and thermal model of a microbolometer for electronic circuit simulation
    Würfel, D. and Vogt, H.
    Advances in Radio Science 10 183-186 (2012)
    The need for uncooled infrared focal plane arrays (IRFPA) for imaging systems has increased since the beginning of the nineties. Examples for the application of IRFPAs are thermography, pedestrian detection for automotives, fire fighting, and infrared spectroscopy. It is very important to have a correct electro-optical model for the simulation of the microbolometer during the development of the readout integrated circuit (ROIC) used for IRFPAs. The microbolometer as the sensing element absorbs infrared radiation which leads to a change of its temperature due to a very good thermal insulation. In conjunction with a high temperature coefficient of resistance (TCR) of the sensing material (typical vanadium oxide or amorphous silicon) this temperature change results in a change of the electrical resistance. During readout, electrical power is dissipated in the microbolometer, which increases the temperature continuously. The standard model for the electro-optical simulation of a microbolometer includes the radiation emitted by an observed blackbody, radiation emitted by the substrate, radiation emitted by the microbolometer itself to the surrounding, a heat loss through the legs which connect the microbolometer electrically and mechanically to the substrate, and the electrical power dissipation during readout of the microbolometer (Wood, 1997). The improved model presented in this paper takes a closer look on additional radiation effects in a real IR camera system, for example the radiation emitted by the casing and the lens. The proposed model will consider that some parts of the radiation that is reflected from the casing and the substrate is also absorbed by the microbolometer. Finally, the proposed model will include that some fraction of the radiation is transmitted through the microbolometer at first and then absorbed after the reflection at the surface of the substrate. Compared to the standard model temperature and resistance of the microbolometer can be modelled more realistically when these higher order effects are taken into account. A Verilog-A model for electronic circuit simulations is developed based on the improved thermal model of the microbolometer. Finally, a simulation result of a simple circuit is presented. © 2012 Author(s).
    view abstractdoi: 10.5194/ars-10-183-2012
  • 2012 • 107 Bond order potentials for fracture, wear, and plasticity
    Pastewka, L. and Mrovec, M. and Moseler, M. and Gumbsch, P.
    MRS Bulletin 37 493-503 (2012)
    Coulson's bond order is a chemically intuitive quantity that measures the difference in the occupation of bonding and anti-bonding orbitals. Both empirical and rigorously derived bond order expressions have evolved in the course of time and proven very useful for atomistic modeling of materials. The latest generation of empirical formulations has recently been augmented by screening-function approaches. Using friction and wear of diamond and diamond-like carbon as examples, we demonstrate that such a screened bond order scheme allows for a faithful description of dynamical bond-breaking processes in materials far from equilibrium. The rigorous bond order expansions are obtained by systematic coarse-graining of the tight binding approximation and form a bridge between the electronic structure and the atomistic modeling hierarchies. They have enabled bottom-up derivations of bond order potentials for covalently bonded semiconductors, transition metals, and multicomponent intermetallics. The recently developed magnetic bond order potential gives a correct description of both directional covalent bonds and magnetic interactions in iron and is able to correctly predict the stability of bulk Fe polymorphs as well as the intricate properties of dislocation cores. The bond order schemes hence represent a family of reliable and powerful models that can be applied in large-scale simulations of complex processes involving fracture, wear, and plasticity. © 2012 Materials Research Society.
    view abstractdoi: 10.1557/mrs.2012.94
  • 2012 • 106 Bulk forming of sheet metal
    Merklein, M. and Allwood, J.M. and Behrens, B.-A. and Brosius, A. and Hagenah, H. and Kuzman, K. and Mori, K. and Tekkaya, A.E. and Weckenmann, A.
    CIRP Annals - Manufacturing Technology 61 725-745 (2012)
    Ever increasing demands on functional integration of high strength light weight products leads to the development of a new class of manufacturing processes. The application of bulk forming processes to sheet or plate semi-finished products, sometimes in combination with conventional sheet forming processes creates new products with the requested properties. The paper defines this new class of sheet-bulk metal forming processes, gives an overview of the existing processes belonging to this class, highlights the tooling aspects as well as the resulting product properties and presents a short summary of the relevant work that has been done towards modeling and simulation. © 2012 CIRP.
    view abstractdoi: 10.1016/j.cirp.2012.05.007
  • 2012 • 105 Characterization of anisotropy of sheet metals employing inhomogeneous strain fields for Yld2000-2D yield function
    Güner, A. and Soyarslan, C. and Brosius, A. and Tekkaya, A.E.
    International Journal of Solids and Structures 49 3517-3527 (2012)
    A method to include the distribution of strains in the identification of the planar anisotropy of sheet metals is proposed. The method includes the optical measurement of strains on a flat specimen with a varying cross-section and an inverse parameter identification scheme which minimizes the differences between the numerical simulation results and the experimental measurements by using Levenberg-Marquardt algorithm. The main advantage is the reduction of the needed number of material tests especially for complex material models, under the assumption of negligible kinematic hardening. The utilized specimen geometry covers a deformation state between uniaxial tension and plane strain tension cases. In order to supply additional information to the inverse scheme, the equi-biaxial stress state obtained from layer compression test is also included in the definition of the objective function. The anisotropy of the sheet is modeled with the Yld2000-2D model which is implemented as a VUMAT subroutine for ABAQUS-Explicit. Numerical tests point out that the orientation of the specimen defines the quality of the found yield loci. The proposed method is applied to characterize the commercial aluminum alloy AA6016-T4 and the obtained material parameters are used to analyze a deep drawn car hood geometry. The results show that the use of the strain distribution is a crucial point in identification of the planar anisotropy. The yield loci obtained with the proposed method are in accordance with the conventionally obtained yield stresses and r-values. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijsolstr.2012.05.001
  • 2012 • 104 Coarsening phenomena of metal nanoparticles and the influence of the support pre-treatment: Pt/TiO 2(110)
    Behafarid, F. and Roldan Cuenya, B.
    Surface Science 606 908-918 (2012)
    One of the technologically most important requirements for the application of oxide-supported metal nanoparticles (NPs) in the fields of molecular electronics, plasmonics, and catalysis is the achievement of thermally stable systems. For this purpose, a thorough understanding of the different pathways underlying thermally-driven coarsening phenomena, and the effect of the nanoparticle synthesis method, support morphology, and degree of support reduction on NP sintering is needed. In this study, the sintering of supported metal NPs has been monitored via scanning tunneling microscopy combined with simulations following the Ostwald ripening and diffusion-coalescence models. Modifications were introduced to the diffusion-coalescence model to incorporate the correct temperature dependence and energetics. Such methods were applied to describe coarsening phenomena of physical-vapor deposited (PVD) and micellar Pt NPs supported on TiO 2(110). The TiO 2(110) substrates were exposed to different pre-treatments, leading to reduced, oxidized and polymer-modified TiO 2 surfaces. Such pre-treatments were found to affect the coarsening behavior of the NPs. No coarsening was observed for the micellar Pt NPs, maintaining their as-prepared size of ~ 3 nm after annealing in UHV at 1060 °C. Regardless of the initial substrate pre-treatment, the average size of the PVD-grown NPs was found to increase after identical thermal cycles, namely, from 0.5 ± 0.2 nm to 1.0 ± 0.3 nm for pristine TiO 2, and from 0.8 ± 0.3 nm to 1.3 ± 0.6 nm for polymer-coated TiO 2 after identical thermal treatments. Although no direct real-time in situ microscopic evidence is available to determine the dominant coarsening mechanism of the PVD NPs unequivocally, our simulations following the diffusion-coalescence coarsening route were in significantly better agreement with the experimental data as compared to those based on the Ostwald-ripening model. The enhanced thermal stability of the micellar NPs as compared to the PVD clusters might be related to their initial larger NP size, narrower size distribution, and larger interparticle distances. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.susc.2012.01.022
  • 2012 • 103 Communication: An exact bound on the bridge function in integral equation theories
    Kast, S.M. and Tomazic, D.
    Journal of Chemical Physics 137 (2012)
    We show that the formal solution of the general closure relation occurring in Ornstein-Zernike-type integral equation theories in terms of the Lambert W function leads to an exact relation between the bridge function and correlation functions, most notably to an inequality that bounds possible bridge values. The analytical results are illustrated on the example of the Lennard-Jones fluid for which the exact bridge function is known from computer simulations under various conditions. The inequality has consequences for the development of bridge function models and rationalizes numerical convergence issues. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.4766465
  • 2012 • 102 Comparison of device models for organic solar cells: Band-to-band vs. tail states recombination
    Soldera, M. and Taretto, K. and Kirchartz, T.
    Physica Status Solidi (A) Applications and Materials Science 209 207-215 (2012)
    The efficiency-limiting recombination mechanism in bulk-heterojunction (BHJ) solar cells is a current topic of investigation and debate in organic photovoltaics. In this work, we simulate state-of-the-art BHJ solar cells using two different models. The first model takes into account band-to-band recombination and field dependent carrier generation. The second model assumes a Shockley-Read-Hall (SRH) recombination mechanism via tail states and field independent carrier generation. Additionally, we include in both cases optical modelling and, thus, position-dependent exciton generation and non-ideal exciton collection. We explore both recombination mechanisms by fitting light and dark current-voltage (JV) characteristics of BHJ cells of five materials: P3HT, MDMO-PPV, MEH-PPV, PCDTBT and PF10TBT, all blended with fullerene derivatives. We show that although main device parameters such as short circuit current, open circuit voltage, fill factor and ideality factor are accurately reproduced by both Langevin and tail recombination, only tail recombination reproduces also the ideality factor of dark characteristics accurately. Nevertheless, the model with SRH recombination via tail states needs the inclusion of external circuitry to account for the heavy shunt present in all the blends, except P3HT:PCBM, when illuminated. Finally, we propose a means to find analytical expressions for the short circuit current by assuming a linear relation between the recombination rate and the concentration of free minority carriers. The model reproduces experimental data of P3HT cells at various thickness values using realistic parameters for this material. Dark JV measurement (circles) of a PCDTBT:PC 70BM solar cell (Park et al., Nature Photon. 3, 297 (2009) [1]), the fit with the model including recombination via tail states (solid line) and the fit with the model reported by (Koster et al., Phys. Rev. B 72, 085205 (2005) [2]) that includes bimolecular band-to-band recombination and charge transfer state (CTS) dissociation. The inset shows the JV curves under white light. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201127264
  • 2012 • 101 Computational fluid dynamics based stochastic aerosol modeling: Combination of a cell-based weighted random walk method and a constant-number Monte-Carlo method for aerosol dynamics
    Kruis, F.E. and Wei, J. and van der Zwaag, T. and Haep, S.
    Chemical Engineering Science 70 109-120 (2012)
    No method is currently available to combine stochastic, particle-based PBE modeling by means of Monte-Carlo simulation of individual particles and CFD. CFD is based on solving numerically partial differential equations, whereas Monte-Carlo simulation of the PBE bases on converting kinetic rate equations into probabilities and selecting the relevant events by means of random numbers. A joint mathematical framework is thus missing. The goal of this work is to develop a method which allows combining Monte-Carlo based PBE modeling with a CFD model. As a first step towards this goal, a Weighted Random Walk (WRW) method to simulate the particle transport due to convection and diffusion is developed. The simulation particles have no exact position as in Lagrangian particle tracking methods but belong to a CFD cell. The movement of the simulation particles in space is performed by calculation of the transition probability into the neighboring cells and the use of random numbers to simulate the particle transport into these cells. As the particle number concentration can be very different in different regions of the simulated reactor volume, we introduce here also a weighting method which allows fixing the number of simulation particles per cell. The WRW method is combined with a relatively simple constant-number MC method allowing to simulate stochastically the dynamic evolution of the particle population. Four different validative case studies of increasing complexity are performed, comparing the simulation results with those of a CFD-based moment model. © 2011 Elsevier Ltd.
    view abstractdoi: 10.1016/j.ces.2011.10.040
  • 2012 • 100 Computational simulation of mechanized tunneling as part of an integrated decision support platform
    Meschke, G. and Nagel, F. and Stascheit, J.
    International Journal of Geomechanics 11 519-528 (2012)
    Mechanized tunneling is characterized by a staged procedure of excavation and lining erection and continuous support of the soil by means of supporting fluids (or compressed air) at the tunnel face and pressurized grouting of the tail gap. The interactions between the tunnel boring machine (TBM), the support measures, and the soil, including the groundwater, determine the efficiency, safety, and effects on the existing infrastructure. In this paper, a process-oriented numerical simulation model for mechanized tunneling and its integration in the context of an integrated optimization platform for tunneling (IOPT) is addressed. The simulation model is based upon the finite-element method and considers the transient excavation process and all relevant components, support measures, and processes, along with their interactions during tunnel advance. In particular, the model allows the investigation of the effects of drilling and stand-still periods upon the generation of a filter cake at the tunnel face. This is demonstrated by the numerical analysis of a straight tunnel advance by means of a hydroshield machine in water-saturated soft soil. © 2011 American Society of Civil Engineers.
    view abstractdoi: 10.1061/(ASCE)GM.1943-5622.0000044
  • 2012 • 99 Computer simulation of two-step atomization in graphite furnaces for analytical atomic spectrometry
    Zakharov, Yu.A. and Kokorina, O.B. and Lysogorskii, Yu.V. and Staroverov, A.E.
    Journal of Analytical Chemistry 67 714-721 (2012)
    The processes of sample fractionation by two-step atomization with the intermediate condensation of the analyte on a cold surface in graphite furnaces were theoretically studied. The transfer equation was solved for the atoms, molecules, and condensed particles of the sample from a flow of argon directed along this surface. The spatial distributions of vapor and the condensate formed were calculated depending on the composition and flow rate. It was found that a cold surface section with a length of 6 mm is sufficient for the complete trapping of atomic analyte vapor from an argon layer having a velocity of about 1 m/sec and a thickness of 5 mm. In this case, the molecules and clusters condensation coefficients smaller than unity were deposited insignificantly; that is, they were fractionally separated. The results of the shadow spectral visualization of the process of sample fractionation on a cold probe surface of in commercial HGA and THGA atomizers were interpreted. The advantages of analytical signals upon the evaporation of a sample condensate from the probe in these atomizers and inductively coupled plasma were demonstrated. © 2012 Pleiades Publishing, Inc.
    view abstractdoi: 10.1134/S1061934812060214
  • 2012 • 98 Construction of high-dimensional neural network potentials using environment-dependent atom pairs
    Jose, K.V.J. and Artrith, N. and Behler, J.
    Journal of Chemical Physics 136 (2012)
    An accurate determination of the potential energy is the crucial step in computer simulations of chemical processes, but using electronic structure methods on-the-fly in molecular dynamics (MD) is computationally too demanding for many systems. Constructing more efficient interatomic potentials becomes intricate with increasing dimensionality of the potential-energy surface (PES), and for numerous systems the accuracy that can be achieved is still not satisfying and far from the reliability of first-principles calculations. Feed-forward neural networks (NNs) have a very flexible functional form, and in recent years they have been shown to be an accurate tool to construct efficient PESs. High-dimensional NN potentials based on environment-dependent atomic energy contributions have been presented for a number of materials. Still, these potentials may be improved by a more detailed structural description, e.g., in form of atom pairs, which directly reflect the atomic interactions and take the chemical environment into account. We present an implementation of an NN method based on atom pairs, and its accuracy and performance are compared to the atom-based NN approach using two very different systems, the methanol molecule and metallic copper. We find that both types of NN potentials provide an excellent description of both PESs, with the pair-based method yielding a slightly higher accuracy making it a competitive alternative for addressing complex systems in MD simulations. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.4712397
  • 2012 • 97 Critical dynamics of an isothermal compressible nonideal fluid
    Gross, M. and Varnik, F.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 86 (2012)
    A pure fluid at its critical point shows a dramatic slow-down in its dynamics, due to a divergence of the order-parameter susceptibility and the coefficient of heat transport. Under isothermal conditions, however, sound waves provide the only possible relaxation mechanism for order-parameter fluctuations. Here we study the critical dynamics of an isothermal, compressible nonideal fluid via scaling arguments and computer simulations of the corresponding fluctuating hydrodynamics equations. We show that, below a critical dimension of 4, the order-parameter dynamics of an isothermal fluid effectively reduces to "model A," characterized by overdamped sound waves and a divergent bulk viscosity. In contrast, the shear viscosity remains finite above two dimensions. Possible applications of the model are discussed. © 2012 American Physical Society.
    view abstractdoi: 10.1103/PhysRevE.86.061119
  • 2012 • 96 Development of a system model for a hydrogen production process on a solar tower
    Säck, J.-P. and Roeb, M. and Sattler, C. and Pitz-Paal, R. and Heinzel, A.
    Solar Energy 86 99-111 (2012)
    An attractive path to the production of hydrogen from water is a two-step thermo chemical cycle powered by concentrated sunlight from a solar tower system. In the first process step the redox system, a ferrite coated on a monolithic honeycomb absorber, is present in its reduced form while the concentrated solar energy hits the ceramic absorber. When water vapour is fed to the honeycomb at 800. °C, oxygen is abstracted from the water molecules, bond in the redox system and hydrogen is produced. When the metal oxide system is completely oxidised it is heated up for regeneration at 1100-1200. °C in an oxygen-lean atmosphere. Under those conditions and in the second process step, oxygen is set free from the redox system, so the metal oxide is being reduced and after completion of the reaction again capable for water splitting. Since the overall process consists of two core reaction steps, which need to be carried out sequentially in a reactor unit at two different temperature steps, a special process and plant concept had to be developed enabling the continuous supply of product regardless of the alternating nature of the solar reactor operation. The challenge of the process control is to keep the two core reaction temperatures constant and to ensure regular temperature switches after completion of the individual process steps, independent of the weather conditions, like DNI fluctuation, clouds and wind speed. Also start-up, the fast switching after completion of half-cycles and the shutdown must be controlled. State of the art is the manual switching of heliostats to fulfil those control tasks. This paper describes the development and use of a system model of this process. The model consists of three main parts: the simulation of the solar flux distribution at the receiver aperture, the simulation of the temperatures in the reactor modules and the simulation of the hydrogen generation. It can be used for the analysis of the operational behaviour. The model is intended to be used in the future for the control of the whole process. © 2011 Elsevier Ltd.
    view abstractdoi: 10.1016/j.solener.2011.09.010
  • 2012 • 95 Direct determination of minority carrier diffusion lengths at axial GaAs nanowire p-n junctions
    Gutsche, C. and Niepelt, R. and Gnauck, M. and Lysov, A. and Prost, W. and Ronning, C. and Tegude, F.-J.
    Nano Letters 12 1453-1458 (2012)
    Axial GaAs nanowire p-n diodes, possibly one of the core elements of future nanowire solar cells and light emitters, were grown via the Au-assisted vapor-liquid-solid mode, contacted by electron beam lithography, and investigated using electron beam induced current measurements. The minority carrier diffusion lengths and dynamics of both, electrons and holes, were determined directly at the vicinity of the p-n junction. The generated photocurrent shows an exponential decay on both sides of the junction and the extracted diffusion lengths are about 1 order of magnitude lower compared to bulk material due to surface recombination. Moreover, the observed strong diameter-dependence is well in line with the surface-to-volume ratio of semiconductor nanowires. Estimating the surface recombination velocities clearly indicates a nonabrupt p-n junction, which is in essential agreement with the model of delayed dopant incorporation in the Au-assisted vapor-liquid-solid mechanism. Surface passivation using ammonium sulfide effectively reduces the surface recombination and thus leads to higher minority carrier diffusion lengths. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/nl204126n
  • 2012 • 94 Electrical control of a solid-state flying qubit
    Yamamoto, M. and Takada, S. and Bäuerle, C. and Watanabe, K. and Wieck, A.D. and Tarucha, S.
    Nature Nanotechnology 7 247-251 (2012)
    Solid-state approaches to quantum information technology are attractive because they are scalable. The coherent transport of quantum information over large distances is a requirement for any practical quantum computer and has been demonstrated by coupling super-conducting qubits to photons. Single electrons have also been transferred between distant quantum dots in times shorter than their spin coherence time. However, until now, there have been no demonstrations of scalable 'flying qubit' architectures - systems in which it is possible to perform quantum operations on qubits while they are being coherently transferred - in solid-state systems. These architectures allow for control over qubit separation and for non-local entanglement, which makes them more amenable to integration and scaling than static qubit approaches. Here, we report the transport and manipulation of qubits over distances of 6 μm within 40 ps, in an Aharonov - Bohm ring connected to two-channel wires that have a tunable tunnel coupling between channels. The flying qubit state is defined by the presence of a travelling electron in either channel of the wire, and can be controlled without a magnetic field. Our device has shorter quantum gates (< μm), longer coherence lengths (∼86 μm at 70 mK) and higher operating frequencies (∼100 GHz) than other solid-state implementations of flying qubits. © 2012 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/nnano.2012.28
  • 2012 • 93 Experimental and numerical analysis of material flow in porthole die extrusion
    Kloppenborg, T. and Schwane, M. and Khalifa, N.B. and Tekkaya, A.E. and Brosius, A.
    Key Engineering Materials 491 97-104 (2012)
    The design of porthole dies for aluminum extrusion processes is very complex. For the accurate design, fundamental knowledge about material flow is of major importance. To gain these information, numerical methods are increasingly utilized. The accuracy of the simulation results depends mainly on the precision of the used boundary conditions in the model. Therefore, visioplastic analyses of the material flow inside a porthole die are presented in this paper. A special modular tool concept was developed to prepare and visualize the material flow inside the process. The results of the experimental analysis were used for the verification of numerical results which were calculated with the commercial software codes Deform3D and HyperXtrude. © (2012) Trans Tech Publications.
    view abstractdoi: 10.4028/
  • 2012 • 92 Experimental and numerical lifetime assessment of Al 2024 sheet
    Khan, S. and Kintzel, O. and Mosler, J.
    International Journal of Fatigue 37 112-122 (2012)
    In the present paper, a thorough analysis of the low-cycle fatigue behavior of flat sheets of aluminum Al 2024-T351 is given. For that purpose, material characterization is combined with material modeling. The experimental analyses include monotonic and cyclic loading tests at high stress levels. For the assessment of microstructural characteristics, advanced imaging technology is used to reveal, e.g. crack initiation loci and particle sizing. The numerical simulation is done using a novel ductile-brittle damage model. Thereby, the model parameters are optimized by means of an inverse parameter identification strategy which, overall, leads to a very good agreement between experimentally observed and computationally predicted data. For demonstrating the prediction capability of the novel coupled model also for complex engineering problems, a certain stringer assembly, as used in fuselage parts of airplanes, is analyzed. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijfatigue.2011.09.010
  • 2012 • 91 Experimental model identification and vibration control of a smart cantilever beam using piezoelectric actuators and sensors
    Nestorović, T. and Durrani, N. and Trajkov, M.
    Journal of Electroceramics 29 42-55 (2012)
    Mechanical lightweight structures often tend to unwanted vibrations due to disturbances. Passive methods for increasing the structural damping are often inadequate for the vibration suppression, since they include additional mass in the form of damping materials, additional stiffening designs or mass damper. In this paper the concept of an active vibration control for piezoelectric light weight structures is introduced and presented through several subsequent steps: model identification, controller design, simulation, experimental verification and implementation on a particular object-piezoelectric smart cantilever beam. Special attention is paid to experimental testing and verification of the results obtained through simulations. The efficiency of the modeling procedure through the subspace-based system identification along with the efficiency of the designed optimal controller are proven based on the experimental verification, which results in vibration suppression to a very high extent not only in comparison with the uncontrolled case, but also in comparison with previously achieved results. The experimental work demonstrates a very good agreement between simulations and experimental results. © 2012 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s10832-012-9736-1
  • 2012 • 90 How to choose the simulation model for computer experiments: A local approach
    Mühlenstädt, T. and Gösling, M. and Kuhnt, S.
    Applied Stochastic Models in Business and Industry 28 354-361 (2012)
    In many scientific areas, non-stochastic simulation models such as finite element simulations replace real experiments. A common approach is to fit a meta-model, for example a Gaussian process model, a radial basis function interpolation, or a kernel interpolation, to computer experiments conducted with the simulation model. This article deals with situations where more than one simulation model is available for the same real experiment, with none being the best over all possible input combinations. From fitted models for a real experiment as well as for computer experiments using the different simulation models, a criterion is derived to identify the locally best one. Applying this criterion to a number of design points allows the design space to be split into areas where the individual simulation models are locally superior. An example from sheet metal forming is analyzed, where three different simulation models are available. In this application and many similar problems, the new approach provides valuable assistance with the choice of the simulation model to be used. © 2011 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/asmb.909
  • 2012 • 89 Improving the simulation accuracy in NC milling by using a global CSG workpiece model
    Odendahl, S. and Peuker, A. and Zabel, A.
    Procedia CIRP 1 657-662 (2012)
    The highest geometric precision in a milling simulation can be achieved by using a CSG-based workpiece model. This allows the prediction of cutting forces, process dynamics and surface structures. To further improve the accuracy of the predictions, a new global CSG model is proposed to replace the previously used local CSG model without any negative impact on the runtime of the simulation. In addition, the runout of the tool has to be included in the modeling process to obtain correct results. In this paper, the new modeling techniques and their experimental validation are presented. © 2012 The Authors.
    view abstractdoi: 10.1016/j.procir.2012.04.118
  • 2012 • 88 Mechanical characterization and constitutive modeling of Mg alloy sheets
    Mekonen, M.N. and Steglich, D. and Bohlen, J. and Letzig, D. and Mosler, J.
    Materials Science and Engineering A 540 174-186 (2012)
    In this paper, an experimental mechanical characterization of the magnesium alloys ZE10 and AZ31 is performed and a suitable constitutive model is established. The mechanical characterization is based on uniaxial tensile tests. In order to avoid poor formability at room temperature, the tests were conducted at elevated temperature (200. °C). The uniaxial tensile tests reveal sufficient ductility allowing sheet forming processes at this temperature. The differences in yield stresses and plastic strain ratios (r-values) confirm the anisotropic response of the materials under study. The constitutive model is established so that the characteristic mechanical features observed in magnesium alloys such as anisotropy and compression-tension asymmetry can be accommodated. This model is thermodynamically consistent, incorporates rate effect, is formulated based on finite strain plasticity theory and is applicable in sheet forming simulations of magnesium alloys. More precisely, a model originally proposed by Cazacu and Barlat in 2004 and later modified to account for the evolution of the material anisotropy is rewritten in a thermodynamically consistent framework. The calibrated constitutive model is shown to capture the characteristic mechanical features observed in magnesium alloy sheets. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2012.01.122
  • 2012 • 87 Modelling of dendritic growth and bubble formation
    Wu, W. and Zhu, M.F. and Sun, D.K. and Dai, T. and Han, Q.Y. and Raabe, D.
    IOP Conference Series: Materials Science and Engineering 33 (2012)
    A two-dimensional lattice Boltzmann method (LBM)-cellular automaton (CA) model is developed for the simulation of dendritic growth and bubble formation during alloy solidification. In the model, a kinetic LBM, which describes flow dynamics through the evolution of distribution functions of moving pseudo-particles, is adopted to numerically solve the gas-liquid two-phase flow based on the Shan-Chen multiphase scheme. The kinetics of dendritic growth is determined according to a local solute equilibrium approach. The present model takes into account the effect of liquid-solid phase transformation on the nucleation and growth of bubbles. The interaction mechanism between dendrites and bubbles is also embedded in the model. The wettability of a bubble on a smooth solid surface is simulated. The simulated contact angles with various interaction coefficients agree well with the data calculated from an empirical formula derived from the Young's equation. The proposed model is applied to simulate dendritic growth and bubble formation under directional solidification conditions. The simulated results are compared with those observed experimentally during solidification of a transparent organic material. The simulation results reveal some dynamic features of bubble nucleation, growth, and motion, as well as the interaction between the dendritic growth and bubble formation during solidification. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1757-899X/33/1/012103
  • 2012 • 86 Numerical simulation of dynamic strain-induced austenite-ferrite transformation and post-dynamic kinetics in a low carbon steel
    Zheng, C. and Raabe, D. and Li, D.
    Materials Science Forum 706-709 1592-1597 (2012)
    2-D cellular automaton model was developed to simulate the dynamic strain-induced transformation (DSIT) from austenite (?) to ferrite (a) and the post-dynamic kinetic behavior in a low carbon steel with the purpose of developing a methodology of mesoscopic computer simulation for an improved understanding of the formation of ultra-fine ferrite (UFF) in DSIT and the conservation of this microstructure during the post-deformation period. The predicted microstructure obtained after DSIT was compared with a quenched dual-phase steel. Its microstructure, consisting of fine-grained ferrite and fine islands of retained austenite dispersed in the matrix, were found to be in good agreement with the predictions. The simulated results indicate that the refinement of ferrite grains produced via DSIT can be interpreted in terms of unsaturated nucleation and limited growth mechanisms. It is also revealed that continuing transformation from retained austenite to ferrite and the reverse transformation both could take place simultaneously during the post-deformation isothermal holding. A competition between them exists at the early stage of the post-dynamic transformation. © 2012 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2012 • 85 Numerical simulation of interactions between the shield-supported tunnel construction process and the response of soft water-saturated soils
    Nagel, F. and Stascheit, J. and Meschke, G.
    International Journal of Geomechanics 12 689-696 (2012)
    During the design and construction of shield-driven tunnels, a reliable analysis of the construction process is required for the prognosis of the process-induced surface settlements, changes in soil stresses, and changes in groundwater conditions, as well as for the determination of the loads acting on the tunnel tube and on the tunnel-boring machine. In this context, numerical simulation methods like the finite-element method allow for a realistic description of the construction process and its impact on the surrounding underground. The investigated problem is governed by the interactions between the tunneling process and the surrounding underground and its constituents-soil grains, groundwater, and pore air. The tunnel-construction process interacts with the surrounding underground via the heading face support, by frictional contact between shield skin and soil, and because of grouting of the annular gap. Considering these interactions, a holistic simulation model is presented for the process-oriented simulation of shield-supported tunnel advance and its interactions with fully saturated, partially saturated, or nonsaturated soft soil. Its applicability is demonstrated by selected simulations of real-scale examples. Parametric studies are performed to investigate the influence of soil conditions and of process parameters on the time-variant settlements and groundwater conditions, showing its capabilities with respect to the simulation of the soil-process interactions in front, above, and behind the tunnel-boring machine. © 2012 American Society of Civil Engineers.
    view abstractdoi: 10.1061/(ASCE)GM.1943-5622.0000174
  • 2012 • 84 Performances analysis of Vehicle-to-X Communication Systems
    Nguyen, H. and Nguyen, V.D. and Nguyen, T.H. and Ha, D.T. and Kaiser, T.
    2012 IEEE International Symposium on Signal Processing and Information Technology, ISSPIT 2012 326-330 (2012)
    This paper investigates the influence of the inter-carrier interference on a vehicle to infrastructure (V2X) communication system, which has been defined in the IEEE 802.11p standard. The frequency shift due to high speed of cars destroys the orthogonality between subcarriers in OFDM signals and rises the inter-carrier interference (ICI). The main contributions of this paper are proposed analytic formulas and simulation results of the signal to interference power ratio (SIR) by considering the statistical effects of ICI and evaluation of symbol error rate (SER) for several types of fading and Doppler spread in time domain approach. The results show that for the Rayleigh fading model, SIR decreases when the maximum Doppler spread increases. On the other hand, in the Rician channel, SIR depends not only on maximum Doppler spread but also on the angle of arrival (AoA) and Rician factor of the LOS component. Moreover, the simulation result in terms of SER is obtained for different channel models. It shows an excellent agreement between the theoretical calculation of SIR and the simulation results of SER. © 2012 IEEE.
    view abstractdoi: 10.1109/ISSPIT.2012.6621309
  • 2012 • 83 Phenomenological modeling of viscous electrostrictive polymers
    Ask, A. and Menzel, A. and Ristinmaa, M.
    International Journal of Non-Linear Mechanics 47 156-165 (2012)
    A common usage for electroactive polymers (EAPs) is in different types of actuators, where advantage is taken of the deformation of the polymer due to an electric field. It turns out that time-dependent effects are present in these applications. One of these effects is the viscoelastic behavior of the polymer material. In view of the modeling and simulation of applications for EAP within a continuum mechanics setting, a phenomenological framework for an electro-viscoelastic material model is elaborated in this work. The different specific models are fitted to experimental data available in the literature. While the experimental data used for inherent electrostriction is restricted to small strains, a large strain setting is used for the model in order to account for possible applications where the polymers undergo large deformations, such as in pre-strained actuators. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijnonlinmec.2011.03.020
  • 2012 • 82 Polycrystal model of the mechanical behavior of a Mo-TiC 30 vol.% metal-ceramic composite using a three-dimensional microstructure map obtained by dual beam focused ion beam scanning electron microscopy
    Cédat, D. and Fandeur, O. and Rey, C. and Raabe, D.
    Acta Materialia 60 1623-1632 (2012)
    The mechanical behavior of a Mo-TiC 30 vol.% ceramic-metal composite was investigated over a wide temperature range (25-700 °C). High-energy X-ray tomography was used to reveal percolation of the hard titanium carbide phase through the composite. Using a polycrystal approach for a two-phase material, finite-element simulations were performed on a real three-dimensional (3-D) aggregate of the material. The 3-D microstructure, used as the starting configuration for the predictions, was obtained by serial sectioning in a dual beam focused ion beam scanning electron microscope coupled to an electron backscattered diffraction system. The 3-D aggregate consists of a molybdenum matrix and a percolating TiC skeleton. As for most body-centered cubic (bcc) metals, the molybdenum matrix phase is characterized by a change in plasticity mechanism with temperature. We used a polycrystal model for bcc materials which was extended to two phases (TiC and Mo). The model parameters of the matrix were determined from experiments on pure molydenum. For all temperatures investigated the TiC particles were considered to be brittle. Gradual damage to the TiC particles was treated, based on an accumulative failure law that is approximated by evolution of the apparent particle elastic stiffness. The model enabled us to determine the evolution of the local mechanical fields with deformation and temperature. We showed that a 3-D aggregate representing the actual microstructure of the composite is required to understand the local and global mechanical properties of the composite studied. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.11.055
  • 2012 • 81 Quantitative characterization of a dielectric barrier discharge in air applying non-calibrated spectrometer, current measurement and numerical simulation
    Rajasekaran, P. and Bibinov, N. and Awakowicz, P.
    Measurement Science and Technology 23 (2012)
    A non-calibrated spectrometer is used for quantitative characterization of a dielectric barrier discharge (DBD) in air wherein optical emission spectroscopy (OES) is completed by current measurement and numerical simulation. This diagnostic method is applicable when the cross-sectional area of the active plasma volume and the current density can be determined. The nitrogen emission in the spectral range of 330-406nm is used for OES diagnostics. The electric field in the active plasma volume is determined by applying the measured spectrum, well-known Franck-Condon factors for nitrogen transitions and numerically simulated electron distribution functions. The measured electric current density is used for the determination of electron density in plasma. Using the determined plasma parameters, the dissociation rates of nitrogen and oxygen in active plasma volume are calculated, which can be used for the simulation of chemical kinetics. © 2012 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-0233/23/8/085605
  • 2012 • 80 Rankine source method for seakeeping predictions
    Söding, H. and Von Graefe, A. and El Moctar, O. and Shigunov, V.
    Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE 4 449-460 (2012)
    Model tests are usually used for the traditional seakeeping predictions (transfer functions of ship motions and loads in regular waves). Experience shows that numerical solution of Reynolds-averaged Navier-Stokes equations (RANSE) can provide accurate results for this task, however, such computations require too much computational time for the required large number of the loading conditions, ship speeds and wave directions and periods. Traditionally, potential flow methods are used for such computations at early design stages. Although potential flow methods can produce results very quickly for large number of conditions, viscosity effects (most important for the roll motion) have to be taken into account using measurements or RANSE computations. Rankine source method, applied to seakeeping problems perhaps for the first time by Yeung [1] to oscillating ship sections, is increasingly used in practical seakeeping analysis. This paper presents a three-dimensional Rankine source code GL Rankine. Patch method is used instead of the usual collocation method to satisfy boundary conditions on the solid body surface. Periodic flow due to waves is linearized with respect to wave and motion amplitude, taking into account interactions between the nonlinear steady flow and periodic flow due to waves and ship motions. The steady flow solution accounts for the nonlinear free-surface conditions, ship wave and dynamic squat. The paper shows results of the method for ship motions in waves in comparison with model measurements and RANSE simulations. Copyright © 2012 by ASMEp.
    view abstractdoi: 10.1115/OMAE2012-83450
  • 2012 • 79 Simulation of the long term behaviour of plastics components
    Stommel, M. and Naumann, T.
    Macromolecular Symposia 311 92-97 (2012)
    This paper presents a method to model the mechanical behavior of polymers over a wide time- and load-range by means of finite element analyses. The method includes a material model as well as the determination of material parameters to calibrate the material model. As a special feature of this method the model is calibrated only by using creep data that are commonly available in material data bases. So the procedure improves the simulation of the long time behavior of plastic-components without an additional experimental effort. In combination with time-temperature-superposition principle, the temperature dependency of the long term behavior is represented, too. The simulation results are validated by creep experiments on an example part. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/masy.201000099
  • 2012 • 78 Simulation-based prediction of process forces for grinding free-formed surfaces on machining centers
    Rausch, S. and Odendahl, S. and Kersting, P. and Biermann, D. and Zabel, A.
    Procedia CIRP 4 161-165 (2012)
    During the grinding of hard materials using cylindrically and spherically shaped mounted points - like for the machining of complex forming tools with abrasive-wear-resistant coatings - the process force is an important factor influencing the accuracy of the machining outcome. A simulation-based prediction of these forces could be used to adapt the tool path and, thereby, to keep the grinding forces at a low level. In this paper, a simulation system based on the modeling of each grain of the grinding tool and the validation of this simulation model are presented. © 2012 The Authors.
    view abstractdoi: 10.1016/j.procir.2012.10.029
  • 2012 • 77 Spray roasting of iron chloride FeCl 2: Laboratory scale experiments and a model for numerical simulation
    Schiemann, M. and Wirtz, S. and Scherer, V. and Bärhold, F.
    Powder Technology 228 301-308 (2012)
    Iron chloride solutions are a waste product in steel pickling plants. A technique to recover the spent solutions is the so-called spray roasting process, where the spent solution is sprayed into a hot reaction atmosphere and solid iron oxide particles are formed. The particle formation in spray roasting reactors has important influence on the efficiency of the recovery process and on the quality of the desired by-product Fe 2O 3. A laboratory reactor was designed to investigate the particle formation. Experiments were carried out covering the predominant conditions in spray roasting reactors. The results offer valuable insight into the particle formation process, providing data on the surface structure of the Fe 2O 3 particles formed and on the progress of chemical conversion. Based on these results, a simplified model applicable to CFD-modelling of spray roasting reactors has been developed. Simulations of particle trajectories in the laboratory reactor are presented to show the capabilities of the model. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.powtec.2012.05.037
  • 2012 • 76 The effect of Peltier heat during current activated densification
    Becker, A. and Angst, S. and Schmitz, A. and Engenhorst, M. and Stoetzel, J. and Gautam, D. and Wiggers, H. and Wolf, D.E. and Schierning, G. and Schmechel, R.
    Applied Physics Letters 101 (2012)
    It is shown that current-activated pressure-assisted densification (CAPAD) is sensitive to the Peltier effect. Under CAPAD, the Peltier effect leads to a significant redistribution of heat within the sample during the densification. The densification of highly p-doped silicon nanoparticles during CAPAD and the properties of the obtained samples are investigated experimentally and by computer simulation. Both, simulation and experiments, indicate clearly a higher temperature on the cathode side and a decreasing temperature from the center to the outer shell. Furthermore, computer simulations provide additional insights into the temperature profile which explain the anisotropic properties of the measured sample. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.4731272
  • 2012 • 75 Turing instabilities in a mathematical model for signaling networks
    Rätz, A. and Röger, M.
    Journal of Mathematical Biology 65 1215-1244 (2012)
    GTPase molecules are important regulators in cells that continuously run through an activation/deactivation and membrane-attachment/membrane-detachment cycle. Activated GTPase is able to localize in parts of the membranes and to induce cell polarity. As feedback loops contribute to the GTPase cycle and as the coupling between membrane-bound and cytoplasmic processes introduces different diffusion coefficients a Turing mechanism is a natural candidate for this symmetry breaking. We formulate a mathematical model that couples a reaction-diffusion system in the inner volume to a reaction-diffusion system on the membrane via a flux condition and an attachment/detachment law at the membrane. We present a reduction to a simpler non-local reaction-diffusion model and perform a stability analysis and numerical simulations for this reduction. Our model in principle does support Turing instabilities but only if the lateral diffusion of inactivated GTPase is much faster than the diffusion of activated GTPase. © 2011 Springer-Verlag.
    view abstractdoi: 10.1007/s00285-011-0495-4
  • 2012 • 74 Vortex shedding from a bilge keel in a transient turbulent flow
    Piehl, H. and El Moctar, B.O.
    Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE 5 907-914 (2012)
    The hydrodynamic damping of a bilge keel during the roll motion of a ship is fairly well understood and its basic principle can be summarized as follows: The larger the bilge keel attached to the hull, the stronger is its roll damping effect. The geometric limitations regarding the size of the bilge keel are set by class regulations mainly dependent on the ship's length, breadth and draft. Most bilge keel shapes are simple flat bars attached to the center of the bilge radius. While the added resistance of the bilge keel has to be kept as low as possible, the effective area for the cross flow generated by the roll motion should be maximized. Therefore the bilge keel's cross section has to be kept small and its camber line parallel to the streamlines. In more sophisticated designs L-shaped bilge keels are applied in order to increase the damping effect on the roll motion. The aspects above need to be considered when defining the geometric limits of a bilge keel, however to further optimize the design of bilge keels numerical simulations are needed. Even with today's computing power, the costs of simulating a full ship hull with a sufficiently high mesh resolution to capture viscous vortex shedding effects would be prohibitive. To address and overcome this restriction a numerical test setup was developed that simulates the flow only in the near vicinity of the bilge keel. By further neglecting the influence of the free surface, it was possible to use a standard single-phase, incompressible, turbulent, transient solver. The open source FVM code Open FOAM was used for all three stages of the simulation: mesh generation, solution process and post-processing. With this simplified simulation model a systematic investigation of the turbulence model, the temporal and spacial discretization, as well as the principles of vortex shedding was carried out. The damping efficiency of the bilge keels was evaluated on basis of the mechanical work - by moving the hull through a viscous fluid - and the kinetic energy transported within the vortices. The findings from these flow simulations provide insights into the principles of bilge keel vortex shedding and their interaction with the hull and enable the development of bilge keel design guidelines. Copyright © 2012 by ASME.
    view abstractdoi: 10.1115/OMAE2012-84013
  • 2011 • 73 A flexible, plane-wave-based formulation of continuum elasticity and multiband k·p models
    Marquardt, O. and Schulz, S. and O'Reilly, E.P. and Freysoldt, C. and Boeck, S. and Hickel, T. and Neugebauer, J.
    Proceedings of the International Conference on Numerical Simulation of Optoelectronic Devices, NUSOD 111-112 (2011)
    We present a highly flexible, plane-wave based formulation of continuum elasticity and multiband k·p-formalism to study the elastic and electronic properties of semiconductor nanostructures. This approach has been implemented in the framework of the density functional theory (DFT) software library S/Phi/nX [1] and allows the investigation of arbitrary-shaped nanostructures such as quantum wells, wires and dots consisting of various materials. Moreover, our approach grants the flexibility to employ user-generated k·p Hamiltonians suited to the requirements of the study regarding accuracy and computational costs. © 2011 IEEE.
    view abstractdoi: 10.1109/NUSOD.2011.6041165
  • 2011 • 72 Ab-initio modeling of Fe-Mn based alloys and nanoclusters
    Entel, P. and Comtesse, D. and Herper, H.C. and Gruner, M.E. and Siewert, M. and Sahoo, S. and Hucht, A.
    Materials Research Society Symposium Proceedings 1296 80-91 (2011)
    New methods in steel design and basic understanding of the novel materials require large scale ab initio calculations of ground state and finite temperature properties of transition metal alloys. In this contribution we present ab initio modeling of the structural and magnetic properties of XYZ compounds and alloys where X, Y = Mn, Fe, Co Ni and Z = C, Si with emphasis on the Fe-Mn steels. The optimization of structural and magnetic properties is performed by using different simulation tools. In particular, the finite-temperature magnetic properties are simulated using a Heisenberg model with magnetic exchange interactions from first-principles calculations. Part of the calculations are extended to the nanoparticle range showing how ferromagnetic and antiferromagnetic trends influence the nucleation, morphologies and growth of Fe-Mn-based nanoparticles. © 2011 Materials Research Society.
    view abstractdoi: 10.1557/opl.2011.1449
  • 2011 • 71 Accurate deep drawing simulation by combining analytical approaches
    Cwiekala, T. and Brosius, A. and Tekkaya, A.E.
    International Journal of Mechanical Sciences 53 374-386 (2011)
    The basic contribution of this work is the description of the development of an analytical simulation method for deep drawing processes. By considering multiple deformation steps, this method takes time dependent process parameters and non-linear deformation paths into account. Contrary to existing analytical approaches, this method allows an accurate strain prediction and, thus, a prediction of formability. Compared to numerical onestep solvers, the developed method is much faster, and due to a better consideration of deformation paths, also a higher accuracy is reached in simulating axisymmetric and prismatic parts. Due to its efficient combination of computation speed and accuracy, this method allows an application in fast process optimizations or online process control systems, where existing approaches are either too slow in case of numerical simulation or too inaccurate in case of analytical simulation. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijmecsci.2011.02.007
  • 2011 • 70 An analytical study of the static state of multi-junctions in a multi-phase field model
    Guo, W. and Spatschek, R. and Steinbach, I.
    Physica D: Nonlinear Phenomena 240 382-388 (2011)
    We investigate the properties of the multi-order parameter phase field model of Steinbach and Pezzolla [I. Steinbach, F. Pezzolla, A generalized field method for multi-phase transformations using interface fields, Physica D 134 (1999) 385393] with respect to the behavior in triple and higher order junctions. From the structure of this model, it was speculated that "dynamical" solutions may exist in the triple junction, which could lead to a violation of Young's law. Here we confirm analytically recent numerical simulations showing that such dynamical states do not exist, and that an equilibrium solution therefore does indeed correspond to a minimum of the free energy; this implies that Young's law must be satisfied in the framework of the model. We show that Young's law is a consequence of the interface kinetic equilibrium and not due to a mechanical force balance, in agreement with earlier predictions [C. Caroli, C. Misbah, On static and dynamical Young's condition at a trijunction, J. Phys. I France 7 (1997) 12591265]. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.physd.2010.09.014
  • 2011 • 69 An experimental and numerical assessment of sheet-bulk formability of mild steel DC04
    Soyarslan, C. and Fassmann, D.P.F. and Plugge, B. and Isik, K. and Kwiatkowski, L. and Schaper, M. and Brosius, A. and Tekkaya, A.E.
    Journal of Manufacturing Science and Engineering, Transactions of the ASME 133 (2011)
    This paper presents investigations on development of a new way of teeth-forming, which is related to sheet-bulk metal forming, with application of incremental bulk forming process to sheets. For this purpose, a combined experimental-numerical study on damage assessment in sheet-bulk forming of DC04 is presented. Using scanning electron microscope (SEM) and glow discharge optical emission spectrometry (GDOS), a combined quantitativequalitative metallurgical survey is carried out on undeformed specimens to illuminate microstructural aspects in the context of nonmetallic inclusion content, distribution and size which act as prime failure factors. These surveys are extended to monitor ductile damage accumulation with cavitation at different stages of the incremental sheet indentation process over certain sections. An anticipated failure mode is captured where formability is limited by severe macro-cracking preceded by localization with void sheeting. To this end, using a developed VUMAT subroutine for the micromechanically based Gurson damage model which is recently enhanced for shear fracture, the processes are simulated in ABAQUSExplicit and comparisons with experiments are provided. The results support the requirement of integrating powerful coupled accumulative damage models in the virtual process design procedure for sheet-bulk metal forming. This requirement also arises from distinct features of these class of processes from conventional sheet metal forming processes which preclude use of forming limit curves. © 2011 American Society of Mechanical Engineers.
    view abstractdoi: 10.1115/1.4004852
  • 2011 • 68 Antenna impact on the gauging accuracy of industrial radar level measurements
    Armbrecht, G. and Zietz, C. and Denicke, E. and Rolfes, I.
    IEEE Transactions on Microwave Theory and Techniques 59 2554-2562 (2011)
    This paper deals with a systematic evaluation of the antenna impact on the gauging accuracy of industrial radar level measurements for process automation. By this means, a framework is provided by which the antenna parameters of major influence on the gauging accuracy can be identified. Guidelines are given to what extent an improvement of these parameters results in a measurable accuracy increase. Firstly, the radar system theory for monostatic level gauging and the emulation of such radar systems by software and hardware are briefly reviewed. Secondly, an analytical model of traveling-wave endfire antennas is introduced, allowing to separately study the influence of individual antenna parameters on the distance measurement accuracy. Thus, a direct relationship between characteristic antenna properties, such as the level of the pattern attenuation in the direction of a parasitic scatterer, and the gauging performance is obtained. The investigations are conducted for one specific pulse-based barycentric signal-processing scheme, which is predestined for industrial level gauging due to its low complexity and reliability. Finally, the results are verified by measurements within a compact radar test range. © 1963-2012 IEEE.
    view abstractdoi: 10.1109/TMTT.2011.2163075
  • 2011 • 67 Applications of an energy-dispersive pnCCD for X-ray reflectivity: Investigation of interdiffusion in Fe-Pt multilayers
    Abboud, A. and Send, S. and Hartmann, R. and Strüder, L. and Savan, A. and Ludwig, Al. and Zotov, N. and Pietsch, U.
    Physica Status Solidi (A) Applications and Materials Science 208 2601-2607 (2011)
    A frame store pn-junction CCD (pnCCD) detector was applied to study thermally induced interdiffusion in Fe/Pt thin film multilayers (MLs) in a temperature range between 300 and 585K. Based on the energy resolution of the detector the reflectivity was measured simultaneously in a spectral range between 8keV< E< 20keV including the Pt L-edge energies close to 11.5keV. Above T=533K we find a strong drop of intensities at 1st and 2nd order ML Bragg peak interpreted by mutual interdiffusion. Considering a simulated model of interdiffusion it has been found that the concentration of iron that diffuses into the platinum sub layers is higher than that of platinum into iron. The time dependence of inter diffusion was also calculated in the range of 533-568K and was described by the Arrhenius equation D(T)=D 0exp(-H a/k BT). The activation energy for the MLs used [Fe 1.7nm/Pt 2nm] 50 was found to be 0.94±0.22eV. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201184268
  • 2011 • 66 Atom-centered symmetry functions for constructing high-dimensional neural network potentials
    Behler, J.
    Journal of Chemical Physics 134 (2011)
    Neural networks offer an unbiased and numerically very accurate approach to represent high-dimensional ab initio potential-energy surfaces. Once constructed, neural network potentials can provide the energies and forces many orders of magnitude faster than electronic structure calculations, and thus enable molecular dynamics simulations of large systems. However, Cartesian coordinates are not a good choice to represent the atomic positions, and a transformation to symmetry functions is required. Using simple benchmark systems, the properties of several types of symmetry functions suitable for the construction of high-dimensional neural network potential-energy surfaces are discussed in detail. The symmetry functions are general and can be applied to all types of systems such as molecules, crystalline and amorphous solids, and liquids. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3553717
  • 2011 • 65 Designing shape-memory Heusler alloys from first-principles
    Siewert, M. and Gruner, M.E. and Dannenberg, A. and Chakrabarti, A. and Herper, H.C. and Wuttig, M. and Barman, S.R. and Singh, S. and Al-Zubi, A. and Hickel, T. and Neugebauer, J. and Gillessen, M. and Dronskowski, R. and Entel, P.
    Applied Physics Letters 99 (2011)
    The phase diagrams of magnetic shape-memory Heusler alloys, in particular, ternary Ni-Mn-Z and quarternary (Pt, Ni)-Mn-Z alloys with Z = Ga, Sn, have been addressed by density functional theory and Monte Carlo simulations. Finite temperature free energy calculations show that the phonon contribution stabilizes the high-temperature austenite structure while at low temperatures magnetism and the band Jahn-Teller effect favor the modulated monoclinic 14M or the nonmodulated tetragonal structure. The substitution of Ni by Pt leads to a series of magnetic shape-memory alloys with very similar properties to Ni-Mn-Ga but with a maximal eigenstrain of 14. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3655905
  • 2011 • 64 Efficient and accurate simulations of deformable particles immersed in a fluid using a combined immersed boundary lattice Boltzmann finite element method
    Krüger, T. and Varnik, F. and Raabe, D.
    Computers and Mathematics with Applications 61 3485-3505 (2011)
    The deformation of an initially spherical capsule, freely suspended in simple shear flow, can be computed analytically in the limit of small deformations [D. Barths-Biesel, J.M. Rallison, The time-dependent deformation of a capsule freely suspended in a linear shear flow, J. Fluid Mech. 113 (1981) 251267]. Those analytic approximations are used to study the influence of the mesh tessellation method, the spatial resolution, and the discrete delta function of the immersed boundary method on the numerical results obtained by a coupled immersed boundary lattice Boltzmann finite element method. For the description of the capsule membrane, a finite element method and the Skalak constitutive model [R. Skalak, A. Tozeren, R.P. Zarda, S. Chien, Strain energy function of red blood cell membranes, Biophys. J. 13 (1973) 245264] have been employed. Our primary goal is the investigation of the presented model for small resolutions to provide a sound basis for efficient but accurate simulations of multiple deformable particles immersed in a fluid. We come to the conclusion that details of the membrane mesh, as tessellation method and resolution, play only a minor role. The hydrodynamic resolution, i.e., the width of the discrete delta function, can significantly influence the accuracy of the simulations. The discretization of the delta function introduces an artificial length scale, which effectively changes the radius and the deformability of the capsule. We discuss possibilities of reducing the computing time of simulations of deformable objects immersed in a fluid while maintaining high accuracy. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.camwa.2010.03.057
  • 2011 • 63 Finite element simulations of poly-crystalline shape memory alloys based on a micromechanical model
    Junker, P. and Hackl, K.
    Computational Mechanics 47 505-517 (2011)
    We present a finite element implementation of a micromechanically motivated model for poly-crystalline shape memory alloys, based on energy minimization principles. The implementation allows simulation of anisotropic material behavior as well as the pseudo-elastic and pseudoplastic material response of whole samples. The evolving phase distribution over the entire specimen is calculated. The finite element model predicts the material properties for a relatively small number of grains. For different points of interest in the specimen the model can be consistently evaluated with a significantly higher number of grains in a post-processing step, which allows to predict the re-orientation of martensite at different loads. The influence of pre-texture on the material's properties, due to some previous treatment like rolling, is discussed. © Springer-Verlag 2010.
    view abstractdoi: 10.1007/s00466-010-0555-4
  • 2011 • 62 Finite-element simulation of the anti-buckling-effect of a shape memory alloy bar
    Richter, F. and Kastner, O. and Eggeler, G.
    Journal of Materials Engineering and Performance 20 719-730 (2011)
    Shape memory alloys (SMA) are characterized by an intricate stress-strain curve modified by temperature, posing thermomechanically coupled problems. A model able to address this feature is the Mü ller-Achenbach- Seelecke (MAS) model which had been ported into the user material interface in the finite-element (FEM) simulation software ABAQUS. The literature on this model mainly focuses on pseudo-elasticity of SMA at elevated temperature. We address a numerical investigation in the low-temperature pseudo-plastic regime. The present publication deals with the little-known anti-buckling effect which occurs in de-twinned and pre-bent martensitic bars under axial compression. It was experimentally demonstrated by Urushiyama et al. (JSME (The Japan Society of Mechanical Engineers) Int. J. Ser. A, Solid Mech. Mater. Eng., 2003, 46(1), p 60-67). This study reveals that the origin of this effect roots in an interplay of inhomogeneous stress states and mechanically induced twin-twin phase transformations. The proper explanation of the anti-buckling effect can be inferred from the explicit knowledge of the martensitic phase composition of the bar during the process.We show that the MAS model is capable to resolve this matter in detail, hence addressing the reliability of this particular model also in the pseudo-plastic regime of SMA. The study thereby implies that the MAS model is an excellent modeling tool for the analysis of complex, thermomechanically coupled processes. © ASM International.
    view abstractdoi: 10.1007/s11665-010-9797-8
  • 2011 • 61 Grout and bentonite flow around a TBM: Computational modeling and simulation-based assessment of influence on surface settlements
    Nagel, F. and Meschke, G.
    Tunnelling and Underground Space Technology 26 445-452 (2011)
    Adequate consideration of the various interactions between the Tunnel Boring Machine (TBM) and the surrounding underground is a pre-requisite for reliable prognoses in shield supported tunneling based upon numerical analysis. In addition to face support and the grouting of the annular gap the contact conditions along the shield skin between the moving TBM and the surrounding, deforming soil constitute the most relevant component of TBM-soil interactions in mechanized tunneling. This paper is concerned with the analysis of the interface conditions between the shield skin and the soil and its adequate numerical representation in the context of a process-oriented numerical simulation model for mechanized tunneling. The situation around the shield skin is influenced by the design of the Tunnel Boring Machine, the deformational behavior of the surrounding underground and by a possible inflow of process liquids into the steering gap. A novel simulation method is proposed which allows to model the viscous flow of the process liquids into the steering gap and its interactions with the face support, the tail void grouting, the deforming soil and the moving TBM. The proposed numerical model for the TBM-soil interaction is part of a recently developed three-dimensional, process-oriented finite element model for shield tunneling (Nagel et al., 2010). It allows to investigate the effects of the inflow of process liquids into the steering gap during TBM advance considering realistic machine-related and geological conditions. It is, in particular, capable to compute the pressure distribution within the developing liquid film in association with the face support and grouting conditions and to predict its influence on the surface settlements and the overall TBM-soil interaction affecting, e.g. the hydraulic jack forces or shield deformations. © 2010 Elsevier Ltd.
    view abstractdoi: 10.1016/j.tust.2010.12.001
  • 2011 • 60 In-vitro investigation of magnetron-sputtered coatings based on silicon-substituted hydroxyapatite
    Surmeneva, M.A. and Surmenev, R.A. and Pichugin, V.F. and Chernousova, S.S. and Epple, M.
    Journal of Surface Investigation 5 1202-1207 (2011)
    Silicon-containing calcium phosphate (Si-CaP) coatings on titanium and austenite steel substrates have been prepared by method of high-frequency magnetron sputtering. The powder of silicon-containing hydroxyapatite Ca 10(PO 4) 6 - x(SiO 4) x(HO) 2 - x (Si-HA), where x = 0. 5 obtained using a mechanochemical technique, was used as a target material. The obtained coatings were X-ray amorphous; the elemental composition of the coatings depended on the composition of the target to be sputtered. The coatings were heated in air for 3 hours to the temperature 700°C with the aim of changing their structure. The bioactivity of the coatings was studied using in-vitro tests. The solution of the simulated body fluid (SBF) oversaturated with respect to HA was used as a model medium. The phase elemental composition and morphology of the deposited and annealed Si-CaP coatings before and after submersion into the solution were controlled using the methods of X-ray diffraction (XRD), energy-dispersive X-ray analysis (EDAX), and scanning electron microscopy (SEM). According to the XFA and IR-spectroscopy data, heat treatment in the air yields the formation of an apatite-like phase in the coating. Thermostating of "metal + coating" specimens in the solution of simulated body fluid revealed that all obtained coatings were biologically active, and a calcium phosphate layer was formed on the coating surface during mineralization. The annealed coatings show a higher chemical stability under physiological conditions as compared to amorphous coatings. © 2011 Pleiades Publishing, Ltd.
    view abstractdoi: 10.1134/S1027451011120135
  • 2011 • 59 Inverse method for identification of initial yield locus of sheet metals utilizing inhomogeneous deformation fields
    Güner, A. and Yin, Q. and Soyarslan, C. and Brosius, A. and Tekkaya, A.E.
    International Journal of Material Forming 4 121-128 (2011)
    Accurate finite element simulation of sheet metal forming processes requires among others accurate description of plastic behaviour of materials. This is achieved by utilization of sophisticated yield criteria having several material parameters. This work proposes a procedure which makes use of the distribution of strains to identify the initial yield locus of sheet metals by the help of inverse analysis. For this purpose a flat specimen having a varying cross-section is introduced, which is capable of revealing different deformation states in one test. Numerical simulations are performed with 2 representative materials for steel and aluminium, using the material model Yld2000-2d. The results of these simulations are treated as experimentally obtained results and with the inverse methods it is tried to obtain the given yield locus. The relation between the supplied input and the outcome of the inverse algorithm is studied by examining different objective function definitions. The numerical studies show that inclusion of the strain distribution in the definition of objective function is a key issue in identification of the yield locus. The orientation of the specimen with respect to the rolling direction also determines the amount and quality of the information used for parameter identification. Consequently the circumstances, under which the inverse method can predict the initial yield locus, are defined. © 2010 Springer-Verlag France.
    view abstractdoi: 10.1007/s12289-010-1009-4
  • 2011 • 58 Large-Eddy Simulation and experiments on non-premixed highly turbulent opposed jet flows
    Pettit, M.W.A. and Coriton, B. and Gomez, A. and Kempf, A.M.
    Proceedings of the Combustion Institute 33 1391-1399 (2011)
    An experimental and computational study is presented on highly turbulent non-premixed counterflows under both isothermal and reactive conditions. Experimentally, Hot Wire Anemometry (HWA), two-dimensional Particle Image Velocimetry (PIV) and OH Planar Laser Induced Fluorescence (PLIF) were applied. Computationally, Large-Eddy Simulations (LES) with a steady flamelet model were used to simulate the flow inside the nozzles and in the opposed flow region, using three different grid resolutions between 1.0 and 0.2 mm (0.5-70 million cells). The combined experimental and computational approach enabled the cross-validation of the simulation, and provided additional insight into the flow field. Both isothermal and burning conditions were examined with turbulent Reynolds numbers reaching a value of 900, demonstrating the system capability of reaching conditions of relevance to practical systems. Importantly, the simplicity of a compact, bench-top experiment is retained. The extension of the computational domain to a region within the nozzles with no optical access reveals the mechanism by which a specially designed turbulence generating plate (TGP) and burner housing yield turbulence intensities well exceeding 20%. The simulated and measured data were found to be in good agreement for first and second velocity moments, for the axial velocity autocorrelation function and for the normalised mean OH fluorescence. Similarity of OH-based flame morphology between experiments and computations also confirms that the LES successfully captures key features of the flow. © 2010 Published by Elsevier Inc. on behalf of The Combustion Institute. All rights reserved.
    view abstractdoi: 10.1016/j.proci.2010.06.140
  • 2011 • 57 Lattice Boltzmann modeling of dendritic growth in forced and natural convection
    Sun, D.K. and Zhu, M.F. and Pan, S.Y. and Yang, C.R. and Raabe, D.
    Computers and Mathematics with Applications 61 3585-3592 (2011)
    A two-dimensional (2D) coupled model is developed for the simulation of dendritic growth during alloy solidification in the presence of forced and natural convection. Instead of conventional continuum-based NavierStokes (NS) solvers, the present model adopts a kinetic-based lattice Boltzmann method (LBM), which describes flow dynamics by the evolution of distribution functions of moving pseudo-particles, for the numerical computations of flow dynamics as well as thermal and solutal transport. The dendritic growth is modeled using a solutal equilibrium approach previously proposed by Zhu and Stefanescu (ZS), in which the evolution of the solid/liquid interface is driven by the difference between the local equilibrium composition and the local actual liquid composition. The local equilibrium composition is calculated from the local temperature and curvature. The local temperature and actual liquid composition, controlled by both diffusion and convection, are obtained by solving the LB equations using the lattice BhatnagarGrossKrook (LBGK) scheme. Detailed model validation is performed by comparing the simulations with analytical predictions, which demonstrates the quantitative capability of the proposed model. Furthermore, the convective dendritic growth features predicted by the present model are compared with those obtained from the ZhuStefanescu and NavierStokes (ZSNS) model, in which the fluid flow is calculated using an NS solver. It is found that the evolution of the solid fraction of dendritic growth calculated by both models coincides well. However, the present model has the significant advantages of numerical stability and computational efficiency for the simulation of dendritic growth with melt convection. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.camwa.2010.11.001
  • 2011 • 56 Microfluidic emulsion separation - Simultaneous separation and sensing by multilayer nanofilm structures
    Uhlmann, P. and Varnik, F. and Truman, P. and Zikos, G. and Moulin, J.-F. and Müller-Buschbaum, P. and Stamm, M.
    Journal of Physics Condensed Matter 23 (2011)
    Emulsion separation is of high relevance for filtration applications, liquid-liquid-partitioning of biomolecules like proteins and recovery of products from droplet microreactors. Selective interaction of various components of an emulsion with substrates is used to design microfluidic flow chambers for efficient separation of emulsions into their individual components. Our lab-on-a-chip device consists of an emulsion separation cell with an integrated silicon sensor chip, the latter allowing the detection of liquid motion via the field-effect signal. Thus, within our lab-on-a-chip device, emulsions can be separated while the separation process is monitored simultaneously. For emulsion separation a surface energy step gradient, namely a sharp interface between the hydrophobic and hydrophilic parts of the separation chamber, is used. The key component of the lab-on-a-chip system is a multilayer and multifunctional nanofilm structure which not only provides the surface energy step gradient for emulsion separation but also constitutes the functional parts of the field-effect transistors. The proof-of-principle was performed using a model emulsion consisting of immiscible aqueous and organic solvent components. Droplet coalescence was identified as a key aspect influencing the separation process, with quite different effects during separation on open surfaces as compared to slit geometry. For a detailed description of this observation, an analytical model was derived and lattice Boltzmann computer simulations were performed. By use of grazing incidence small angle x-ray scattering (GISAXS) interfacial nanostructures during gold nanoparticle deposition in a flow field were probed to demonstrate the potential of GISAXS for insitu investigations during flow. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/23/18/184123
  • 2011 • 55 Molecular dynamics simulation study of microstructure evolution during cyclic martensitic transformations
    Kastner, O. and Eggeler, G. and Weiss, W. and Ackland, G.J.
    Journal of the Mechanics and Physics of Solids 59 1888-1908 (2011)
    Shape memory alloys (SMA) exhibit a number of features which are not easily explained by equilibrium thermodynamics, including hysteresis in the phase transformation and reverse shape memory in the high symmetry phase. Processing can change these features: repeated cycling can train the reverse shape memory effect, while changing the amount of hysteresis and other functional properties. These effects are likely to be due to formations of localised defects and these can be studied by atomistic methods. Here we present a molecular dynamics simulation study of such behaviour employing a two-dimensional, binary LennardJones model. Our atomistic model exhibits a symmetry breaking, displacive phase transition from a high temperature, entropically stabilised, austenite-like phase to a low temperature martensite-like phase. The simulations show transformations in this model material proceed by non-diffusive nucleation and growth processes and produce distinct microstructures. We observe the generation of persistent lattice defects during forward-and-reverse transformations which serve as nucleation centres in subsequent transformation processes. These defects interfere the temporal and spatial progression of transformations and thereby affect subsequent product morphologies. During cyclic transformations we observe accumulations of lattice defects so as to establish new microstructural elements which represent a memory of the previous morphologies. These new elements are self-organised and they provide a basis of the reversible shape memory effect in the model material. © 2011 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmps.2011.05.009
  • 2011 • 54 Monte carlo simulation for aggregative mixing of nanoparticles in two-component systems
    Zhao, H. and Kruis, F.E. and Zheng, C.
    Industrial and Engineering Chemistry Research 50 10652-10664 (2011)
    Gas-to-particle synthesis under high temperature is one of the most important methods for producing multicomponent nanoparticles. The volume enlargement of particles due to aggregation accompanies the component mixing within particles in a nonreactive system. To tailor nanocomposites, it is essential to gain an insight into the dynamic evolution of compositional distributions. In this paper, the differentially weighted Monte Carlo (DWMC) method for population balance modeling is used to simulate the process of aggregative mixing. On the methodological end, a new shift action is proposed to regulate a limited number of simulation particles to be distributed as homogeneously as possible over high-dimensional and inhomogeneous joint space of multiple components, where some simulation particles in less-populated regions are split into more simulation particles in order to increase sample space for stochastic statistics and then fatigue against statistical noise, at the same time a certain number of simulation particles in densely populated regions are randomly removed from the simulation to reduce computational demands. The DWMC with the new shift action is used to simulate the aggregative mixing process of bicomponent nanoparticles with compositional-independent or -dependent Brownian coagulation kernel in the free-molecular regime. It is found that the compositional distributions satisfy self-preserving formulation as the particle size distribution in monocomponent systems; and the extent of the time evolution of the degree of mixing (the mass-normalized power density of excess component A) corresponds with that of self-preserving distributions. The compositional distributions and the degree of mixing predicted by the DWMC agree well with theoretical models, while the constant-number method (using equally weighted simulation particles) fails in the more advanced stages of aggregative mixing. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ie200780q
  • 2011 • 53 Neural network potential-energy surfaces in chemistry: A tool for large-scale simulations
    Behler, J.
    Physical Chemistry Chemical Physics 13 17930-17955 (2011)
    The accuracy of the results obtained in molecular dynamics or Monte Carlo simulations crucially depends on a reliable description of the atomic interactions. A large variety of efficient potentials has been proposed in the literature, but often the optimum functional form is difficult to find and strongly depends on the particular system. In recent years, artificial neural networks (NN) have become a promising new method to construct potentials for a wide range of systems. They offer a number of advantages: they are very general and applicable to systems as different as small molecules, semiconductors and metals; they are numerically very accurate and fast to evaluate; and they can be constructed using any electronic structure method. Significant progress has been made in recent years and a number of successful applications demonstrate the capabilities of neural network potentials. In this Perspective, the current status of NN potentials is reviewed, and their advantages and limitations are discussed. © the Owner Societies 2011.
    view abstractdoi: 10.1039/c1cp21668f
  • 2011 • 52 NMR studies of benzene mobility in metal-organic framework MOF-5
    Hertel, S. and Wehring, M. and Amirjalayer, S. and Gratz, M. and Lincke, J. and Krautscheid, H. and Schmid, R. and Stallmach, F.
    EPJ Applied Physics 55 (2011)
    The concentration and temperature dependence of the self-diffusion of benzene adsorbed in the metal-organic framework MOF-5 (IRMOF-1) is studied by pulsed field gradient (PFG) NMR spectroscopy. When increasing the loading from 10 to 20 molecules per unit cell of MOF-5, the experimental diffusion data drop by a factor of about 3 while current molecular dynamic (MD) simulations predict slightly increasing diffusion coefficients for this range of loadings. The observation is rationalized using the recently predicted clustering of adsorbate molecules in microporous systems for temperatures well below the adsorbate critical temperature. Necessary improvements of molecular simulation models for predicting diffusivities under such conditions are discussed. © EDP Sciences, 2011.
    view abstractdoi: 10.1051/epjap/2011100370
  • 2011 • 51 Numerical simulation of hemodynamics in the ascending aorta induced by different aortic cannulas
    Stühle, S. and Wendt, D. and Jakob, H. and Kowalczyk, W.
    Minimally Invasive Therapy and Allied Technologies 20 125-131 (2011)
    There is still a lack of quantitative information concerning optimal blood flow in the aorta and in the carotid arteries during extracorporeal circulation (ECC). Problems are not only based on the location of the aortic cannula, they are furthermore associated with the cannula design itself and the effects on blood cells and aortic wall shear stresses. We simulated a two-phase fluid flow induced by different cannulas in the ascending aorta during ECC. Three commercially available cannulas were examined according to their influence on red blood cells (RBC). Additionally, mass flow in the carotid vessels and wall shear stresses acting on the aortic wall were evaluated. A constant volume flow of blood (3.4 L/min) was applied. Numerical results demonstrate a strong relation between the mass flow rate in the carotid vessels and the geometry of the aortic outflow cannula. RBC distributions both in the aorta and the carotid vessels changed depending on cannula geometry. Maximum blood velocities, shear stresses on the aortic wall, and the fluid mechanical load acting on RBCs varied depending on each cannula design. This numerical approach demonstrates the significant influence of the cannula design on the distribution of RBCs in the carotid vessels during ECC. © 2011 Informa Healthcare.
    view abstractdoi: 10.3109/13645706.2011.553957
  • 2011 • 50 Partitioned model vs parallelized solver
    Mikelsons, L. and Menager, N. and Schramm, D.
    Proceedings of the ASME Design Engineering Technical Conference 4 1101-1109 (2011)
    Nowadays, engineers still search for more efficient methods in order to decrease simulation times. However, most simulation environments do not use the full power provided by modern PCs. Even though every modern computer is equipped with a multicore processor, only very few simulation environments use more than one core for simulations. There are various possibilities to parallelize simulations. One approach is to partition the model into several submodels. Using adequate solvers for each submodel can result in lower computation times, especially if there is a significant difference in the time constants of the submodels. Other approaches are based on parallelization of the ODE solver. For example, it is possible to parallelize the linear algebra methods inside the solver. Parallelization of the solver itself is another way to use the multicore architecture. From the modeling and simulation point of view, the latter approach is more interesting. Consequently, the question is whether it is beneficial to partition the model or to use a parallelized solver. In this paper this question is answered at least for an example system. However, the more efficient approach may not be the better approach for the usage inside a simulation environment. Therefore, it is discussed which approach can be automated and integrated easier into a simulation environment. Copyright © 2011 by ASME.
    view abstractdoi: 10.1115/DETC2011-47645
  • 2011 • 49 Phase-field simulation of diffusion couples in the Ni-Al system
    Zhang, L. and Steinbach, I. and Du, Y.
    International Journal of Materials Research 102 371-380 (2011)
    By linking thermodynamic and atomic mobility databases with two-dimensional phase-field simulation, the evolution of interdiffusion microstructures in a series of Ni-Al diffusion couples associated with the γ, γ', and b-phases was studied. The formation and subsequent growth of the γ'- phase layer in β/γ and γ' + β/γ diffusion couples reproduced the experimental observations well. Moreover, the effect of coherent strain on the γ - γ' microstructural evolution, as well as that of an external compressive force on the γ + γ'/γ + γ' diffusion couple, was investigated. The phase-field simulated concentration profiles of some of the Ni-Al diffusion couples were also compared with the corresponding experimental data and the results of one-dimensional DICTRA (DIffusion Controlled TRAnsformations) simulations. A discussion of the rafting direction was also made by comprehensively comparing the phase-field simulations with the predicted results from an elastic model. © Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/146.110493
  • 2011 • 48 Quality issues in combustion les
    Kempf, A.M. and Geurts, B.J. and Ma, T. and Pettit, M.W.A. and Stein, O.T.
    Journal of Scientific Computing 49 51-64 (2011)
    Combustion LES requires modelling of physics beyond the flow-field only. These additional models lead to further quality issues and an even stronger need to quantify simulation and modelling errors. We illustrate stability problems, the need for consistent modelling in premixed and non-premixed combustion, and show how RANS models that have frequently been applied in an LES context can lead to strong conceptual errors. We outline the application of the error landscape approach to a complex non-premixed flame, and investigate several error indicators that have been developed for situations where no experimental reference data is available. © 2011 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s10915-011-9481-7
  • 2011 • 47 Relevance of the light line in planar photonic crystal waveguides with weak vertical confinement
    Kaspar, P. and Kappeler, R. and Erni, D. and Jäckel, H.
    Optics Express 19 24344-24353 (2011)
    The concept of the so-called light line is a useful tool to distinguish between guided and non-guided modes in dielectric slab waveguides. Also for more complicated structures with 2D mode confinement, the light lines can often be used to divide a dispersion diagram into a region of a non-guided continuum of modes, a region of discrete guided modes and a forbidden region, where no propagating modes can exist. However, whether or not the light line is a concept of practical relevance depends on the geometry of the structure. This fact is sometimes ignored. For instance, in the literature on photonic crystal waveguides, it is often argued that substrate-type photonic crystal waveguides with a weak vertical confinement are inherently lossy, since the entire bandgap including the line defect modes is typically located above the light line of the substrate. The purpose of this article is to illustrate that this argument is inaccurate and to provide guidelines on how an improved light line concept can be constructed. © 2011 Optical Society of America.
    view abstractdoi: 10.1364/OE.19.024344
  • 2011 • 46 Reply to "comment on 'Paramagnetic and ferromagnetic resonance studies on dilute magnetic semiconductors based on GaN' " [Phys. Status Solidi A 205, 1872 (2008)]
    Kammermeier, T. and Ney, A. and Wieck, A.D.
    Physica Status Solidi (A) Applications and Materials Science 208 1957-1959 (2011)
    In a comment by Gehlhoff et al. on our work on "Paramagnetic and ferromagnetic resonance studies on dilute magnetic semiconductors based on GaN" [Phys. Status Solidi A 205, 1872 (2008)] the authors claim a contradictory explanation of angular dependent magnetic resonance measurements. Here we discuss their - solely based on computer simulations - claims. We show that an interpretation limited on magnetic impurities can hardly be used to explain the complex magnetic behavior. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201127237
  • 2011 • 45 Simulation method for LWIR radiation distribution using a visual ray-tracer
    Utz, A. and Gendrisch, L. and Weiler, D. and Kolnsberg, S. and Vogt, H.
    Proceedings of the International Conference on Numerical Simulation of Optoelectronic Devices, NUSOD 57-58 (2011)
    Infrared cameras with passive, uncooled sensor chips utilize the longwave-infrared (LWIR) range of the electromagnetic spectrum with wavelength between 8 and 14μm for image generation. The reason for this is that every object is self-luminous at room temperature at that wavelength. Therefore, every surface acts as a source of radiation in an LWIR scenario. To gain an impression and to model the effects and circumstances in an infrared scenario, a simulation method is required. In the visual domain this task is accomplished by ray-tracing software, which allows the generation of synthetic images as well as the analysis of irradiance distribution in a given scene. In this paper we demonstrate a way to apply one of such ray-tracers to a LWIR scenario. We will also demonstrate an application of the proposed simulation method. © 2011 IEEE.
    view abstractdoi: 10.1109/NUSOD.2011.6041135
  • 2011 • 44 Simulation of fracture in heterogeneous elastic materials with cohesive zone models
    Prechtel, M. and Ronda, P.L. and Janisch, R. and Hartmaier, A. and Leugering, G. and Steinmann, P. and Stingl, M.
    International Journal of Fracture 168 15-29 (2011)
    In brittle composite materials, failure mechanisms like debonding of the matrix-fiber interface or fiber breakage can result in crack deflection and hence in the improvement of the damage tolerance. More generally it is known that high values of fracture energy dissipation lead to toughening of the material. Our aim is to investigate the influence of material parameters and geometrical aspects of fibers on the fracture energy as well as the crack growth for given load scenarios. Concerning simulations of crack growth the cohesive element method in combination with the Discontinuous Galerkin method provides a framework to model the fracture considering strength, stiffness and failure energy in an integrated manner. Cohesive parameters are directly determined by DFT supercell calculations. We perform studies with prescribed crack paths as well as free crack path simulations. In both cases computational results reveal that fracture energy depends on both the material parameters but also the geometry of the fibers. In particular it is shown that the dissipated energy can be increased by appropriate choices of cohesive parameters of the interface and geometrical aspects of the fiber. In conclusion, our results can help to guide the manufacturing process of materials with a high fracture toughness. © 2010 Springer Science+Business Media B.V.
    view abstractdoi: 10.1007/s10704-010-9552-z
  • 2011 • 43 Simulation of the temperature distribution in NC-milled workpieces
    Surmann, T. and Ungemach, E. and Zabel, A. and Joliet, R. and Schröder, A.
    Advanced Materials Research 223 222-230 (2011)
    In most cases the simulation of temperature distributions in machined workpieces is carried out by moving a heat source along a predefined workpiece model within a commercial FEM-system. For performance reasons, the material removal is often neglected or performed by removing small predefined parts of the workpiece. Furthermore, the heat source often has a constant heat flux and therefore it is not dependent on the current tool engagement. In this paper we present a voxel-based finite difference method for the thermal behavior of the process-state dependent workpiece, which is integrated into the milling simulation system NCChip, developed at the ISF. This simulation is capable of modeling the cutting forces along any arbitrary NC-path. Since the tool rotation and the cutting edges in this time domain simulation are divided into discrete angle steps and cutting wedges respectively, the thermal energy that is applied to the workpiece at each time step and at each cutting wedge can be computed as a fraction of the corresponding cutting work. In this way, the correct heat is introduced to the workpiece exactly at the current contact zone of the tool. © (2011) Trans Tech Publication.
    view abstractdoi: 10.4028/
  • 2011 • 42 Space-resolved characterization of high frequency atmospheric-pressure plasma in nitrogen, applying optical emission spectroscopy and numerical simulation
    Rajasekaran, P. and Ruhrmann, C. and Bibinov, N. and Awakowicz, P.
    Journal of Physics D: Applied Physics 44 (2011)
    Averaged plasma parameters such as electron distribution function and electron density are determined by characterization of high frequency (2.4GHz) nitrogen plasma using both experimental methods, namely optical emission spectroscopy (OES) and microphotography, and numerical simulation. Both direct and step-wise electron-impact excitation of nitrogen emissions are considered. The determination of space-resolved electron distribution function, electron density, rate constant for electron-impact dissociation of nitrogen molecule and the production of nitrogen atoms, applying the same methods, is discussed. Spatial distribution of intensities of neutral nitrogen molecule and nitrogen molecular ion from the microplasma is imaged by a CCD camera. The CCD images are calibrated using the corresponding emissions measured by absolutely calibrated OES, and are then subjected to inverse Abel transformation to determine space-resolved intensities and other parameters. The space-resolved parameters are compared, respectively, with the averaged parameters, and an agreement between them is established. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/44/48/485205
  • 2011 • 41 Spatial dynamics of helium metastables in sheath or bulk dominated rf micro-plasma jets
    Niermann, B. and Hemke, T. and Babaeva, N.Y. and Böke, M. and Kushner, M.J. and Mussenbrock, T. and Winter, J.
    Journal of Physics D: Applied Physics 44 (2011)
    Space resolved concentrations of helium He (3S1) metastable atoms in an atmospheric pressure radio-frequency micro-plasma jet were measured using tunable diode laser absorption spectroscopy. The spatial profile of metastable atoms in the volume between the electrodes was deduced for various electrode gap distances. Density profiles reveal the sheath structure and reflect the plasma excitation distribution, as well as the dominance of the α-mode discharge. Gap width variations show the transition from a normal glow plasma to a pure sheath discharge. In order to analyse and verify the experimentally observed profiles of the metastable atoms, a two-dimensional simulation model was set up. Applying an appropriate He/N2/O 2 chemistry model, the correlation between the metastable profiles and the underlying excitation mechanisms was obtained. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/44/48/485204
  • 2011 • 40 Spatially resolved simulation of a radio-frequency driven micro-atmospheric pressure plasma jet and its effluent
    Hemke, T. and Wollny, A. and Gebhardt, M. and Brinkmann, R.P. and Mussenbrock, T.
    Journal of Physics D: Applied Physics 44 (2011)
    Radio-frequency driven plasma jets are frequently employed as efficient plasma sources for surface modification and other processes at atmospheric pressure. The radio-frequency driven micro-atmospheric pressure plasma jet (μAPPJ) is a particular variant of that concept whose geometry allows direct optical access. In this work, the characteristics of the μAPPJ operated with a helium-oxygen mixture and its interaction with a helium environment are studied by numerical simulation. The density and temperature of the electrons, as well as the concentration of all reactive species are studied both in the jet itself and in its effluent. It is found that the effluent is essentially free of charge carriers but contains a substantial amount of activated oxygen (O, O3 and O2(1Δ)). The simulation results are verified by comparison with experimental data. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/44/28/285206
  • 2011 • 39 Stability and dynamics of droplets on patterned substrates: Insights from experiments and lattice Boltzmann simulations
    Varnik, F. and Gross, M. and Moradi, N. and Zikos, G. and Uhlmann, P. and Müller-Buschbaum, P. and Magerl, D. and Raabe, D. and Steinbach, I. and Stamm, M.
    Journal of Physics Condensed Matter 23 (2011)
    The stability and dynamics of droplets on solid substrates are studied both theoretically and via experiments. Focusing on our recent achievements within the DFG-priority program 1164 (Nano-and Microfluidics), we first consider the case of (large) droplets on the so-called gradient substrates. Here the term gradient refers to both a change of wettability (chemical gradient) or topography (roughness gradient). While the motion of a droplet on a perfectly flat substrate upon the action of a chemical gradient appears to be a natural consequence of the considered situation, we show that the behavior of a droplet on a gradient of topography is less obvious. Nevertheless, if care is taken in the choice of the topographic patterns (in order to reduce hysteresis effects), a motion may be observed. Interestingly, in this case, simple scaling arguments adequately account for the dependence of the droplet velocity on the roughness gradient (Moradi et al 2010 Europhys. Lett. 8926006). Another issue addressed in this paper is the behavior of droplets on hydrophobic substrates with a periodic arrangement of square shaped pillars. Here, it is possible to propose an analytically solvable model for the case where the droplet size becomes comparable to the roughness scale (Gross et al 2009 Europhys. Lett. 8826002). Two important predictions of the model are highlighted here. (i)There exists a state with a finite penetration depth, distinct from the full wetting (Wenzel) and suspended (Cassie-Baxter, CB) states. (ii)Upon quasi-static evaporation, a droplet initially on the top of the pillars (CB state) undergoes a transition to this new state with a finite penetration depth but then (upon further evaporation) climbs up the pillars and goes back to the CB state again. These predictions are confirmed via independent numerical simulations. Moreover, we also address the fundamental issue of the internal droplet dynamics and the terminal center of mass velocity on a flat substrate. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/23/18/184112
  • 2011 • 38 Strategies for springback compensation regarding process robustness
    Gösling, M. and Kracker, H. and Brosius, A. and Kuhnt, S. and Tekkaya, A.E.
    Production Engineering 5 49-57 (2011)
    In this article, strategies which compensate geometrical deviations caused by springback are discussed using finite element simulations and statistical modelling techniques. First of all the ability to predict springback using a finite element simulation model is analysed. For that purpose numerical predictions and experiments are compared with each other regarding the amount of springback. In a next step, different strategies for compensating springback such as a modification of stress condition, component stiffness and tool geometry are introduced. On the basis of finite element simulations these different compensation strategies are illustrated for a stretch bending process and experimentally checked for an example. Finally springback simulations are compared regarding their robustness against noise variables such as friction and material properties. Thereby a method based on statistical prediction models is introduced which allows for an accurate approximation of the springback distribution with less numerical effort in comparison to a classical Monte-Carlo method. © 2010 German Academic Society for Production Engineering (WGP).
    view abstractdoi: 10.1007/s11740-010-0251-4
  • 2011 • 37 Structural analysis of articular cartilage using multiphoton microscopy: Input for biomechanical modeling
    Lilledahl, M.B. and Pierce, D.M. and Ricken, T. and Holzapfel, G.A. and Davies, C.D.L.
    IEEE Transactions on Medical Imaging 30 1635-1648 (2011)
    The 3-D morphology of chicken articular cartilage was quantified using multiphoton microscopy (MPM) for use in continuum-mechanical modeling. To motivate this morphological study we propose aspects of a new, 3-D finite strain constitutive model for articular cartilage focusing on the essential load-bearing morphology: an inhomogeneous, poro-(visco)elastic solid matrix reinforced by an anisotropic, (visco)elastic dispersed fiber fabric which is saturated by an incompressible fluid residing in strain-dependent pores. Samples of fresh chicken cartilage were sectioned in three orthogonal planes and imaged using MPM, specifically imaging the collagen fibers using second harmonic generation. Employing image analysis techniques based on Fourier analysis, we derived the principal directionality and dispersion of the collagen fiber fabric in the superficial layer. In the middle layer, objective thresholding techniques were used to extract the volume fraction occupied by extracellular collagen matrix. In conjunction with information available in the literature, or additional experimental testing, we show how this data can be used to derive a 3-D map of the initial solid volume fraction and Darcy permeability. © 2011 IEEE.
    view abstractdoi: 10.1109/TMI.2011.2139222
  • 2011 • 36 Structure and flow of droplets on solid surfaces
    Müller-Buschbaum, P. and Magerl, D. and Hengstler, R. and Moulin, J.-F. and Körstgens, V. and Diethert, A. and Perlich, J. and Roth, S.V. and Burghammer, M. and Riekel, C. and Gross, M. and Varnik, F. and Uhlmann, P. and Stamm, ...
    Journal of Physics Condensed Matter 23 (2011)
    The structure and flow of droplets on solid surfaces is investigated with imaging and scattering techniques and compared to simulations. To access nanostructures at the liquid-solid interface advanced scattering techniques such as grazing incidence small-angle x-ray scattering (GISAXS) with micro-and nanometer-sized beams, GISAXS and insitu imaging ellipsometry and GISAXS tomography are used. Using gold nanoparticle suspensions, structures observed in the wetting area due to deposition are probed insitu during the drying of the droplets. After drying, nanostructures in the wetting area and inside the dried droplets are monitored. In addition to drying, a macroscopic movement of droplets is caused by body forces acting on an inclined substrate. The complexity of the solid surfaces is increased from simple silicon substrates to binary polymer brushes, which undergo a switching due to the liquid in the droplet. Nanostructures introduced in the polymer brush due to the movement of droplets are observed. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/23/18/184111
  • 2011 • 35 System-theoretical analysis and modeling of pulsed thermal Time-of-Flight flow sensor
    Ecin, O. and Engelien, E. and Viga, R. and Hosticka, B.J. and Grabmaier, A.
    2011 7th Conference on Ph.D. Research in Microelectronics and Electronics, PRIME 2011 - Conference Proceedings 149-152 (2011)
    A system-theoretical model of an air flow sensor based on the principle of pulsed thermal Time-of-Flight (TTOF) is described. The region of interest (ROI) in the pipe flow is regarded as a linear time-invariant (LTI) system with an input and output signal. Based on the experimental results the simulation model is created and applied for analysis of different input signals and system's responses with respect to the flow velocity. Experimental and simulated results are compared in an air flow velocity range between 0.1 m/s and 0.7 m/s. © 2011 IEEE.
    view abstractdoi: 10.1109/PRIME.2011.5966239
  • 2011 • 34 Temperature measurements at thoriated tungsten electrodes in a model lamp and their interpretation by numerical simulation
    Bergner, A. and Westermeier, M. and Ruhrmann, C. and Awakowicz, P. and Mentel, J.
    Journal of Physics D: Applied Physics 44 (2011)
    An atmospheric pressure argon arc is operated with dc currents of different amplitudes in a model lamp between electrodes made of pure and thoriated tungsten. Temperature measurements are performed at these electrodes with a CCD camera being calibrated at =890nm in absolute units of surface radiance and an interference filter for this wavelength. Temperature distributions are deduced from the CCD camera records of the electrodes assuming that they are grey body radiators. The records show a diffuse mode of attachment at the cathode. Doping the electrode with ThO 2 causes a reduction in the cathode temperature by an amount of the order of 1000K. On the other hand the anode temperature is weakly increased by a doping with ThO 2. A reduction in the work function of the cathode from 4.55 to 3eV is found by a comparison with cathode temperatures obtained by a numerical simulation of the diffuse mode of arc attachment with a well established cathode boundary layer model. Moreover, it is noted that the reduction is independent of the amount of ThO 2 by which the electrode material is doped indicating that the work function of thoriated cathodes is the result of a self adjustment to the work function minimum at a thorium coverage of 0.5. The weak influence of ThO 2 on the anode temperature shows that the average work function of the anode does not depend on the thorium content of the electrode. The results are explained by a thorium ion current, by which evaporated thorium is repatriated to the cathode surface. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/44/50/505203
  • 2011 • 33 The collagen fibril architecture in the lamina cribrosa and peripapillary sclera predicted by a computational remodeling approach
    Grytz, R. and Meschke, G. and Jonas, J.B.
    Biomechanics and Modeling in Mechanobiology 10 371-382 (2011)
    The biomechanics of the optic nerve head is assumed to play an important role in ganglion cell loss in glaucoma. Organized collagen fibrils form complex networks that introduce strong anisotropic and nonlinear attributes into the constitutive response of the peripapillary sclera (PPS) and lamina cribrosa (LC) dominating the biomechanics of the optic nerve head. The recently presented computational remodeling approach (Grytz and Meschke in Biomech Model Mechanobiol 9:225-235, 2010) was used to predict the micro-architecture in the LC and PPS, and to investigate its impact on intraocular pressure-related deformations. The mechanical properties of the LC and PPS were derived from a microstructure-oriented constitutive model that included the stretch-dependent stiffening and the statistically distributed orientations of the collagen fibrils. Biomechanically induced adaptation of the local micro-architecture was captured by allowing collagen fibrils to be reoriented in response to the intraocular pressure-related loading conditions. In agreement with experimental observations, the remodeling algorithm predicted the existence of an annulus of fibrils around the scleral canal in the PPS, and a predominant radial orientation of fibrils in the periphery of the LC. The peripapillary annulus significantly reduced the intraocular pressure-related expansion of the scleral canal and shielded the LC from high tensile stresses. The radial oriented fibrils in the LC periphery reinforced the LC against transversal shear stresses and reduced LC bending deformations. The numerical approach presents a novel and reasonable biomechanical explanation of the spatial orientation of fibrillar collagen in the optic nerve head. © 2010 Springer-Verlag.
    view abstractdoi: 10.1007/s10237-010-0240-8
  • 2011 • 32 The influence of additions of Al and Si on the lattice stability of fcc and hcp Fe-Mn random alloys
    Gebhardt, T. and Music, D. and Ekholm, M. and Abrikosov, I.A. and Vitos, L. and Dick, A. and Hickel, T. and Neugebauer, J. and Schneider, J.M.
    Journal of Physics Condensed Matter 23 (2011)
    We have studied the influence of additions of Al and Si on the lattice stability of face-centred-cubic (fcc) versus hexagonal-closed-packed (hcp) Fe-Mn random alloys, considering the influence of magnetism below and above the fcc Néel temperature. Employing two different ab initio approaches with respect to basis sets and treatment of magnetic and chemical disorder, we are able to quantify the predictive power of the ab initio methods. We find that the addition of Al strongly stabilizes the fcc lattice independent of the regarded magnetic states. For Si a much stronger dependence on magnetism is observed. Compared to Al, almost no volume change is observed as Si is added to Fe-Mn, indicating that the electronic contributions are responsible for stabilization/destabilization of the fcc phase. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/23/24/246003
  • 2011 • 31 The multipole resonance probe: Characterization of a prototype
    Lapke, M. and Oberrath, J. and Schulz, C. and Storch, R. and Styrnoll, T. and Zietz, C. and Awakowicz, P. and Brinkmann, R.P. and Musch, T. and Mussenbrock, T. and Rolfes, I.
    Plasma Sources Science and Technology 20 (2011)
    The multipole resonance probe (MRP) was recently proposed as an economical and industry compatible plasma diagnostic device (Lapke et al 2008 Appl. Phys. Lett. 93 051502). This communication reports the experimental characterization of a first MRP prototype in an inductively coupled argon/nitrogen plasma at 10 Pa. The behavior of the device follows the predictions of both an analytical model and a numerical simulation. The obtained electron densities are in excellent agreement with the results of Langmuir probe measurements. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0963-0252/20/4/042001
  • 2011 • 30 Thermodynamics and molecular dynamics investigation of a possible new critical size for surface and inner cohesive energy of Al nanoparticles
    Chamaani, A. and Marzbanrad, E. and Rahimipour, M. R. and Yaghmaee, M. S. and Aghaei, A. and Kamachali, R. D. and Behnamian, Y.
    Journal of Nanoparticle Research 13 6059--6067 (2011)
    In this study, the authors first review the previously developed, thermodynamics-based theory for size dependency of the cohesion energy of free-standing spherically shaped Al nanoparticles. Then, this model is extrapolated to the cubic and truncated octahedron Al nanoparticle shapes. A series of computations for Al nanoparticles with these two new shapes are presented for particles in the range of 1-100 nm. The thermodynamics computational results reveal that there is a second critical size around 1.62 and 1 nm for cubes and truncated octahedrons, respectively. Below this critical size, particles behave as if they consisted only of surface-energy-state atoms. A molecular dynamics simulation is used to verify this second critical size for Al nanoparticles in the range of 1-5 nm. MD simulation for cube and truncated octahedron shapes shows the second critical point to be around 1.63 and 1.14 nm, respectively. According to the modeling and simulation results, this second critical size seems to be a material property characteristic rather than a shape-dependent feature.
    view abstractdoi: 10.1007/s11051-011-0258-6
  • 2011 • 29 Variable window adaptive Kernel Principal Component Analysis for nonlinear nonstationary process monitoring
    Khediri, I.B. and Limam, M. and Weihs, C.
    Computers and Industrial Engineering 61 437-446 (2011)
    On-line control of nonlinear nonstationary processes using multivariate statistical methods has recently prompt a lot of interest due to its industrial practical importance. Indeed basic process control methods do not allow monitoring of such processes. For this purpose this study proposes a variable window real-time monitoring system based on a fast block adaptive Kernel Principal Component Analysis scheme. While previous adaptive KPCA models allow only handling of one observation at a time, in this study we propose a way to fast update or downdate the KPCA model when a block of data is provided and not only one observation. Using a variable window size procedure to determine the model size and adaptive chart parameters, this model is applied to monitor two simulated benchmark processes. A comparison of performances of the adopted control strategy with various Principal Component Analysis (PCA) control models shows that the derived strategy is robust and yields better detection abilities of disturbances. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.cie.2011.02.014
  • 2010 • 28 A computational remodeling approach to predict the physiological architecture of the collagen fibril network in corneo-scleral shells
    Grytz, R. and Meschke, G.
    Biomechanics and Modeling in Mechanobiology 9 225-235 (2010)
    Organized collagen fibrils form complex networks that introduce strong anisotropic and highly nonlinear attributes into the constitutive response of human eye tissues. Physiological adaptation of the collagen network and the mechanical condition within biological tissues are complex and mutually dependent phenomena. In this contribution, a computational model is presented to investigate the interaction between the collagen fibril architecture and mechanical loading conditions in the corneo-scleral shell. The biomechanical properties of eye tissues are derived from the single crimped fibril at the micro-scale via the collagen network of distributed fibrils at themeso-scale to the incompressible and anisotropic soft tissue at the macro-scale. Biomechanically induced remodeling of the collagen network is captured on the meso-scale by allowing for a continuous re-orientation of preferred fibril orientations and a continuous adaptation of the fibril dispersion. The presented approach is applied to a numerical human eye model considering the cornea and sclera. The predicted fibril morphology correlates well with experimental observations from X-ray scattering data. © Springer-Verlag 2009.
    view abstractdoi: 10.1007/s10237-009-0173-2
  • 2010 • 27 A general approach to simulating workpiece vibrations during five-axis milling of turbine blades
    Biermann, D. and Kersting, P. and Surmann, T.
    CIRP Annals - Manufacturing Technology 59 125-128 (2010)
    Workpiece vibrations have a significant influence on the machining process and on the quality of the resulting workpiece surface, particularly when milling thin-walled components. In this paper a simulation system, consisting of an FE model of the workpiece coupled with a geometric milling simulation for computing regenerative workpiece vibrations during the five-axis milling process, is presented. Additionally, a modeling method for visualizing the resulting surface is described. In order to validate the simulation model, turbine blades were machined and the experimental results were compared to the simulation results. © 2010 CIRP.
    view abstractdoi: 10.1016/j.cirp.2010.03.057
  • 2010 • 26 A novel isotropic quasi-brittle damage model applied to LCF analyses of Al2024
    Kintzel, O. and Khan, S. and Mosler, J.
    International Journal of Fatigue 32 1948-1959 (2010)
    The current paper deals with the assessment and the numerical simulation of low cycle fatigue of an aluminum 2024 alloy. According to experimental observations, the material response of Al2024 is highly direction-dependent showing a material behavior between ductile and brittle. In particular, in its corresponding (small transversal) S-direction, the material behavior can be characterized as quasi-brittle. For the modeling of such a mechanical response, a novel, fully coupled isotropic ductile-brittle continuum damage mechanics model is proposed. Since the resulting model shows a large number of material parameters, an efficient, hybrid parameter identification strategy is discussed. Within this strategy, as many parameters as possible have been determined a priori by exploiting analogies to established theories (like Paris' law), while the remaining free unknowns are computed by solving an optimization problem. Comparisons between the experimentally observed and the numerically simulated lifetimes reveal the prediction capability of the proposed model. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijfatigue.2010.07.001
  • 2010 • 25 Acidity constants from DFT-based molecular dynamics simulations
    Sulpizi, M. and Sprik, M.
    Journal of Physics Condensed Matter 22 (2010)
    In this contribution we review our recently developed method for the calculation of acidity constants from density functional theory based molecular dynamics simulations. The method is based on a half reaction scheme in which protons are formally transferred from solution to the gas phase. The corresponding deprotonation free energies are computed from the vertical energy gaps for insertion or removal of protons. Combined to full proton transfer reactions, the deprotonation energies can be used to estimate relative acidity constants and also the Brønsted pKa when the deprotonation free energy of a hydronium ion is used as a reference. We verified the method by investigating a series of organic and inorganic acids and bases spanning a wide range of pKa values (20 units). The thermochemical corrections for the biasing potentials assisting and directing the insertion are discussed in some detail. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/22/28/284116
  • 2010 • 24 Acoustic driven flow and lattice Boltzmann simulations to study cell adhesion in biofunctionalized μ-fluidic channels with complex geometry
    Fallah, M.A. and Myles, V.M. and Krüger, T. and Sritharan, K. and Wixforth, A. and Varnik, F. and Schneider, S.W. and Schneider, M.F.
    Biomicrofluidics 4 (2010)
    Accurately mimicking the complexity of microvascular systems calls for a technology which can accommodate particularly small sample volumes while retaining a large degree of freedom in channel geometry and keeping the price considerably low to allow for high throughput experiments. Here, we demonstrate that the use of surface acoustic wave driven microfluidics systems successfully allows the study of the interrelation between melanoma cell adhesion, the matrix protein collagen type I, the blood clotting factor von Willebrand factor (vWF), and microfluidic channel geometry. The versatility of the tool presented enables us to examine cell adhesion under flow in straight and bifurcated microfluidic channels in the presence of different protein coatings. We show that the addition of vWF tremendously increases (up to tenfold) the adhesion of melanoma cells even under fairly low shear flow conditions. This effect is altered in the presence of bifurcated channels demonstrating the importance of an elaborate hydrodynamic analysis to differentiate between physical and biological effects. Therefore, computer simulations have been performed along with the experiments to reveal the entire flow profile in the channel. We conclude that a combination of theory and experiment will lead to a consistent explanation of cell adhesion, and will optimize the potential of microfluidic experiments to further unravel the relation between blood clotting factors, cell adhesion molecules, cancer cell spreading, and the hydrodynamic conditions in our microcirculatory system. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3396449
  • 2010 • 23 An elasto-plastic three phase model for partially saturated soil for the finite element simulation of compressed air support in tunnelling
    Nagel, F. and Meschke, G.
    International Journal for Numerical and Analytical Methods in Geomechanics 34 605-625 (2010)
    This paper presents a fully coupled finite element formulation for partially saturated soil as a triphasic porous material, which has been developed for the simulation of shield tunnelling with heading face support using compressed air. While for many numerical simulations in geotechnics use of a two-phase soil model is sufficient, the simulation of compressed air support demands the use of a three-phase model with the consideration of air as a separate phase. A multiphase model for soft soils is developed, in which the individual constituents of the soil-the soil skeleton, the fluid and the gaseous phase-and their interactions are considered. The triphasic model is formulated within the framework of the theory of porous media, based upon balance equations and constitutive relations for the soil constituents and their mixture. An elasto-plastic, cam-clay type model is extended to partially saturated soil conditions by incorporating capillary pressure according to the Barcelona basic model. The hydraulic properties of the soil are described via DARCY's law and the soil-water characteristic curve after VAN GENUCHTEN. Water is modelled as an incompressible and air as a compressible phase. The model is validated by means of selected benchmark problems. The applicability of the model to geotechnical problems is demonstrated by results from the simulation of a compressed air intervention in shield tunnelling. © 2009 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/nag.828
  • 2010 • 22 An investigation of frictional and collisional powder flows using a unified constitutive equation
    Langroudi, M.K. and Turek, S. and Ouazzi, A. and Tardos, G.I.
    Powder Technology 197 91-101 (2010)
    This is an experimental and numerical study of dry, frictional powder flows in the quasi-static and intermediate regimes using the geometry of the Couette device. We measure normal and shear stresses on the shearing surface and extract from the data, constitutive equations valid in the slow frictional, quasi-static and the intermediate (dense), collisional regimes of flow. This constitutive equation is then used in a new, specially developed FEM solver (FeatFlow-Ouazzi et al., 2005 [18]) to obtain solutions of the continuum equations of motion as well as stress and velocity distributions in the powder. While the measurements to obtain the constitutive equation are performed in a concentric Couette device, the numerical scheme is used to predict the torque and stresses in two additional geometries. These geometries are an eccentric Couette where the inner, rotating cylinder is placed off-center with different eccentricities and a more complicated geometry where a cylindrical body is introduced in the middle between the rotating and stationary cylinders and obstructs part of the shearing gap. The purpose of these calculations is to show the versatility of the numerical solution. © 2009 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.powtec.2009.09.001
  • 2010 • 21 Comparison of finite element and fast Fourier transform crystal plasticity solvers for texture prediction
    Liu, B. and Raabe, D. and Roters, F. and Eisenlohr, P. and Lebensohn, R.A.
    Modelling and Simulation in Materials Science and Engineering 18 (2010)
    We compare two full-field formulations, i.e. a crystal plasticity fast Fourier transform-based (CPFFT) model and the crystal plasticity finite element model (CPFEM) in terms of the deformation textures predicted by both approaches. Plane-strain compression of a 1024-grain ensemble is simulated with CPFFT and CPFEM to assess the models in terms of their predictions of texture evolution for engineering applications. Different combinations of final textures and strain distributionsare obtained with the CPFFT and CPFEM models for this 1024-grain polycrystal. To further understand these different predictions, the correlation between grain rotations and strain gradients is investigated through the simulation of plane-strain compression of bicrystals. Finally, a study of the influence of the initial crystal orientation and the crystallographic neighborhood on grain rotations and grain subdivisions is carried out by means of plane-strain compression simulations of a 64-grain cluster. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/18/8/085005
  • 2010 • 20 Coupled DEM-CFD simulation of pneumatically conveyed granular media
    Sturm, M. and Wirtz, S. and Scherer, V. and Denecke, J.
    Chemical Engineering and Technology 33 1184-1192 (2010)
    A DEM-CFD coupling for the simulation of gas-solid flows was successfully implemented and simulations were performed for the application to industrial-scale pneumatic conveying. Therefore, all particle collisions and phase interactions were considered and porosity determination was optimized. The aim of this work is to show the applicability of the presented simulation model to the different regimes of pneumatic conveying systems. As a first test case a dense vertical pneumatic conveying system was chosen and an individual plug was investigated in detail. Variations of the conveying air velocity were also considered. As a second test case dilute conveying in a horizontal-to-vertical pipe bend was simulated. The occurrence of roping and the reduction of particle velocity is of high interest for the design of specific pneumatic systems. It is shown that both regimes can be captured reasonably well and the results are rich in details. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/ceat.201000162
  • 2010 • 19 Coupled Discrete Element (DEM) - Continuous Fluid (CFD) method for the application of pneumatic conveyed granular media
    Sturm, M. and Wirtz, S. and Scherer, V. and Denecke, J.
    American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP 4 547-556 (2010)
    A Coupled Discrete Element - Continuous Fluid Method is developed, which allows the numerical simulation of the behaviour of pneumatic conveyed granular media. While the fluid is simulated by computational fluid dynamics, the soft-sphere discrete element approach is used for the particle system. A "four-way coupling" is implemented to account for phase interaction on both sides and for the collisions among particles and with the pipe walls. The purpose of this work is to show the applicability of the presented simulation model to dense pneumatic conveying systems. Unfortunately, only few complete experimental data are available in this field. In this work a pneumatic conveying system for dense phase regime with a vertical pipe of 2 m height and a diameter of 50 mm is investigated. Periodic conditions are applied to both phases, the fluid and the particle system. The particles, which leave the pipe at one end, are inserted back at the other end of the pipe, so that a much larger simulation domain is represented. After an initial set up and some intermediate time steps a stable mode is reached and one single plug forms, which is then investigated in detail. The pressure distribution along the plug which coincides with the porosity distribution, the length of the plug and its velocity are important parameters which are examined and compared to experimental investigations. Variations of the conveying air velocity and their influence are also considered. Copyright © 2009 by ASME.
    view abstractdoi: 10.1115/PVP2009-77240
  • 2010 • 18 Effect of Ni4Ti3 precipitation on martensitic transformation in Ti-Ni
    Zhou, N. and Shen, C. and Wagner, M.F.-X. and Eggeler, G. and Mills, M.J. and Wang, Y.
    Acta Materialia 58 6685-6694 (2010)
    Precipitation of Ni4Ti3 plays a critical role in determining the martensitic transformation path and temperature in Ni-Ti shape memory alloys. In this study, the equilibrium shape of a coherent Ni 4Ti3 precipitate and the concentration and stress fields around it are determined quantitatively using the phase field method. Most recent experimental data on lattice parameters, elastic constants, precipitate-matrix orientation relationship and thermodynamic database are used as model inputs. The effects of the concentration and stress fields on subsequent martensitic transformations are analyzed through interaction energy between a nucleating martensitic particle and the existing microstructure. Results indicate that R-phase formation prior to B19′ phase could be attributed to both direct elastic interaction and stress-induced spatial variation in concentration near Ni4Ti3 precipitates. The preferred nucleation sites for the R-phase are close to the broad side of the lenticular-shaped Ni4Ti3 precipitates, where tension normal to the habit plane is highest, and Ni concentration is lowest. © 2010 AWE and Crown Copyright. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.08.033
  • 2010 • 17 Efficient computation of the elastography inverse problem by combining variational mesh adaption and a clustering technique
    Arnold, A. and Reichling, S. and Bruhns, O.T. and Mosler, J.
    Physics in Medicine and Biology 55 2035-2056 (2010)
    This paper is concerned with an efficient implementation suitable for the elastography inverse problem. More precisely, the novel algorithm allows us to compute the unknown stiffness distribution in soft tissue by means of the measured displacement field by considerably reducing the numerical cost compared to previous approaches. This is realized by combining and further elaborating variational mesh adaption with a clustering technique similar to those known from digital image compression. Within the variational mesh adaption, the underlying finite element discretization is only locally refined if this leads to a considerable improvement of the numerical solution. Additionally, the numerical complexity is reduced by the aforementioned clustering technique, in which the parameters describing the stiffness of the respective soft tissue are sorted according to a predefined number of intervals. By doing so, the number of unknowns associated with the elastography inverse problem can be chosen explicitly. A positive side effect of this method is the reduction of artificial noise in the data (smoothing of the solution). The performance and the rate of convergence of the resulting numerical formulation are critically analyzed by numerical examples. © 2010 Institute of Physics and Engineering in Medicine.
    view abstractdoi: 10.1088/0031-9155/55/7/016
  • 2010 • 16 Efficient monolithic simulation techniques for the stationary Lattice Boltzmann equation on general meshes
    Hübner, T. and Turek, S.
    Computing and Visualization in Science 13 129-143 (2010)
    In this paper, we present special discretization and solution techniques for the numerical simulation of the Lattice Boltzmann equation (LBE). In Hübner and Turek (Computing, 81:281-296, 2007), the concept of the generalized mean intensity had been proposed for radiative transfer equations which we adapt here to the LBE, treating it as an analogous (semi-discretized) integro-differential equation with constant characteristics. Thus, we combine an efficient finite difference-like discretization based on short-characteristic upwinding techniques on unstructured, locally adapted grids with fast iterative solvers. The fully implicit treatment of the LBE leads to nonlinear systems which can be efficiently solved with the Newton method, even for a direct solution of the stationary LBE. With special exact preconditioning by the transport part due to the short-characteristic upwinding, we obtain an efficient linear solver for transport dominated configurations (macroscopic Stokes regime), while collision dominated cases (Navier-Stokes regime for larger Re numbers) are treated with a special block-diagonal preconditioning. Due to the new generalized equilibrium formulation (GEF) we can combine the advantages of both preconditioners, i.e. independence of the number of unknowns for convection-dominated cases with robustness for stiff configurations. We further improve the GEF approach by using hierarchical multigrid algorithms to obtain grid-independent convergence rates for a wide range of problem parameters, and provide representative results for various benchmark problems. Finally, we present quantitative comparisons between a highly optimized CFD-solver based on the Navier-Stokes equation (FeatFlow) and our new LBE solver (FeatLBE). © 2009 Springer-Verlag.
    view abstractdoi: 10.1007/s00791-009-0132-6
  • 2010 • 15 Electronic excitations in B12AS2 and their temperature dependence by vacuum ultraviolet ellipsometry
    Bakalova, S. and Gong, Y. and Cobet, C. and Esser, N. and Zhang, Y. and Edgar, J.H. and Zhang, Y. and Dudley, M. and Kuball, M.
    Journal of Physics Condensed Matter 22 (2010)
    The dielectric response function of epitaxial B12As2 films on 4H-SiC was determined at room temperature and at 10 K in the spectral region of 3.6-9.8 eV, i.e., in the vacuum ultraviolet (VUV) spectral region, by synchrotron ellipsometry. The experimental dielectric function was simulated with the critical point parabolic band model. The parameters of the dispersive structures were derived by numerical fitting of the experimental data to the proposed model. New high energy optical transitions are resolved at 5.95, 7.8 and 8.82 eV and their lineshape and origin are discussed. The temperature dependence of the critical point energies and transition strengths was determined, and the excitonic effect is considered. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/22/39/395801
  • 2010 • 14 Electronic structure of self-assembled InGaAs/GaAs quantum rings studied by capacitance-voltage spectroscopy
    Lei, W. and Notthoff, C. and Lorke, A. and Reuter, D. and Wieck, A.D.
    Applied Physics Letters 96 (2010)
    Self-assembled InGaAs quantum rings, embedded in a GaAs matrix, were investigated using magneto-capacitance-voltage spectroscopy. The magnetic-field dispersion of the charging energies exhibits characteristic features for both the first and second electron, which can be attributed to a ground state transition from l=0 into l=-1, and a ground state transition from l=-1 into l=-2, respectively. Furthermore, using a combination of capacitance-voltage spectroscopy and one-dimensional numerical simulations, the conduction band structure of these InGaAs quantum rings was determined. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3293445
  • 2010 • 13 Experimental and computer simulation determination of the structural changes occurring through the liquid-glass transition in Cu-Zr alloys
    Mendelev, M.I. and Kramer, M.J. and Ott, R.T. and Sordelet, D.J. and Besser, M.F. and Kreyssig, A. and Goldman, A.I. and Wessels, V. and Sahu, K.K. and Kelton, K.F. and Hyers, R.W. and Canepari, S. and Rogers, J.R.
    Philosophical Magazine 90 3795-3815 (2010)
    Molecular dynamics (MD) simulations were performed of the structural changes occurring through the liquid-glass transition in Cu-Zr alloys. The total scattering functions (TSF), and their associated primary diffuse scattering peak positions (Kp), heights (Kh) and full-widths at half maximum (KFWHM) were used as metrics to compare the simulations to high-energy X-ray scattering data. The residuals of difference between the model and experimental TSFs are ∼0.03 for the liquids and about 0.07 for the glasses. Over the compositional range studied, Zr1-xCux (0.1 ≤ x ≤ 0.9), Kp, Kh and KFWHM show a strong dependence on composition and temperature. The simulation and experimental data correlate well between each other. MD simulation revealed that the Cu-Zr bonds undergo the largest changes during cooling of the liquid, whereas the Cu-Cu bonds change the least. Changes in the partial-pair correlations are more readily seen in the second and third shells. The Voronoi polyhedra (VP) in glasses are dominated by only a few select types that are compositionally dependent. The relative concentrations of the dominant VPs rapidly change in their relative proportion in the deeply undercooled liquid. The experimentally determined region of best glass formability, xCu 65%, shows the largest temperature dependent changes for the deeply undercooled liquid in the MD simulation. This region also exhibits very strong temperature dependence for the diffusivity and the total energy of the system. These data point to a strong topological change in the best glass-forming alloys and a concurrent change in the VP chemistry in the deeply undercooled liquid. © 2010 Taylor &amp; Francis.
    view abstractdoi: 10.1080/14786435.2010.494585
  • 2010 • 12 Hydrogen vibrational modes on graphene and relaxation of the C-H stretch excitation from first-principles calculations
    Sakong, S. and Kratzer, P.
    Journal of Chemical Physics 133 (2010)
    Density functional theory (DFT) calculations are used to determine the vibrational modes of hydrogen adsorbed on graphene in the low-coverage limit. Both the calculated adsorption energy of a H atom of 0.8 eV and calculated C-H stretch vibrational frequency of 2552 cm-1 are unusually low for hydrocarbons, but in agreement with data from electron energy loss spectroscopy on hydrogenated graphite. The clustering of two adsorbed H atoms observed in scanning tunneling microscopy images shows its fingerprint also in our calculated spectra. The energetically preferred adsorption on different sublattices correlates with a blueshift of the C-H stretch vibrational modes in H adatom clusters. The C-H bending modes are calculated to be in the 1100 cm-1 range, resonant with the graphene phonons. Moreover, we use our previously developed methods to calculate the relaxation of the C-H stretch mode via vibration-phonon interaction, using the Born-Oppenheimer surface for all local modes as obtained from the DFT calculations. The total decay rate of the H stretch into other H vibrations, thereby creating or annihilating one graphene phonon, is determined from Fermi's golden rule. Our calculations using the matrix elements derived from DFT calculations show that the lifetime of the H stretch mode on graphene is only several picoseconds, much shorter than on other semiconductor surfaces such as Ge(001) and Si(001). © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3474806
  • 2010 • 11 Information acquisition for modelling and simulation of logistics networks
    Kuhnt, S. and Wenzel, S.
    Journal of Simulation 4 109-115 (2010)
    Design, organisation and management of Large Logistics Networks (LLN) usually involve model-based analyses of the networks. The usefulness of such an analysis highly depends on the quality of the input data, which of course should be best possible to capture the real circumstances. In this paper, an advanced procedure model for a structured, goal- and task-oriented information and data acquisition for the model-based analyses of LLN is proposed. This procedure model differs from other approaches by focussing on information acquisition rather than solely on data acquisition, and by employing a consequent verification and validation concept. All steps of the procedure modelGoal Setting, Information Identification, Preparation of Information and Data Collection, Information and Data Collection, Data Recording, Data Structuring, Statistical Data Analysis, Data Usability Test-are described and exemplified for a network of air-freight-flow. © 2010 Operational Research Society Ltd. All rights reserved.
    view abstractdoi: 10.1057/jos.2009.9
  • 2010 • 10 Investigation of the internal substructure of microbands in a deformed copper single crystal: Experiments and dislocation dynamics simulation
    Dmitrieva, O. and Svirina, J.V. and Demir, E. and Raabe, D.
    Modelling and Simulation in Materials Science and Engineering 18 (2010)
    We investigate the internal structure of microbands in a shear-deformed copper single crystal. The microstructure is characterized using high-resolution electron backscatter diffraction. The occurrence of microbands is due to the alternation of local orientation, which is characteristic of a deformation laminate. These microbands contain a substructure consisting of further local 1°-orientation alternations. A two-dimensional discrete dislocation dynamics model is used to describe the orientation substructure within the microbands. The boundary conditions for the simulation were estimated from the distribution of the geometrically necessary dislocation density obtained from the orientation map. The dislocation arrangement in the dynamic simulation explains the formation of the experimentally observed substructure. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/18/8/085011
  • 2010 • 9 Joining of lightweight frame structures by die-less hydroforming
    Marré, M. and Gies, S. and Maevus, F. and Tekkaya, A.E.
    International Journal of Material Forming 3 1031-1034 (2010)
    A successful approach to achieve a reduction of a car's total weight is the implementation of lightweight strategies in the design process, e. g. using lightweight materials. An interesting alternative to conventional welding and riveting processes is joining by die-less hydroforming. This work describes an analytical model which can be used to calculate the strengths of these joints, taking into account the material parameters, joint geometry and process parameters. Additionally, validation of the model by both finite element simulations and experiments will be provided. Furthermore, investigations were carried out to implement the described methodology for a multi-joint used in a space frame structure. © 2010 Springer-Verlag France.
    view abstractdoi: 10.1007/s12289-010-0946-2
  • 2010 • 8 Non-thermal atmospheric pressure HF plasma source: Generation of nitric oxide and ozone for bio-medical applications
    Kühn, S. and Bibinov, N. and Gesche, R. and Awakowicz, P.
    Plasma Sources Science and Technology 19 (2010)
    A new miniature high-frequency (HF) plasma source intended for bio-medical applications is studied using nitrogen/oxygen mixture at atmospheric pressure. This plasma source can be used as an element of a plasma source array for applications in dermatology and surgery. Nitric oxide and ozone which are produced in this plasma source are well-known agents for proliferation of the cells, inhalation therapy for newborn infants, disinfection of wounds and blood ozonation. Using optical emission spectroscopy, microphotography and numerical simulation, the gas temperature in the active plasma region and plasma parameters (electron density and electron distribution function) are determined for varied nitrogen/oxygen flows. The influence of the gas flows on the plasma conditions is studied. Ozone and nitric oxide concentrations in the effluent of the plasma source are measured using absorption spectroscopy and electro-chemical NO-detector at variable gas flows. Correlations between plasma parameters and concentrations of the particles in the effluent of the plasma source are discussed. By varying the gas flows, the HF plasma source can be optimized for nitric oxide or ozone production. Maximum concentrations of 2750 ppm and 400 ppm of NO and O3, correspondingly, are generated. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0963-0252/19/1/015013
  • 2010 • 7 Nonlinear reaction coordinate analysis in the reweighted path ensemble
    Lechner, W. and Rogal, J. and Juraszek, J. and Ensing, B. and Bolhuis, P.G.
    Journal of Chemical Physics 133 (2010)
    We present a flexible nonlinear reaction coordinate analysis method for the transition path ensemble based on the likelihood maximization approach developed by Peters and Trout [J. Chem. Phys. 125, 054108 (2006)]. By parametrizing the reaction coordinate by a string of images in a collective variable space, we can optimize the likelihood that the string correctly models the committor data obtained from a path sampling simulation. The collective variable space with the maximum likelihood is considered to contain the best description of the reaction. The use of the reweighted path ensemble [J. Rogal, J. Chem. Phys. 133, 174109 (2010)] allows a complete reaction coordinate description from the initial to the final state. We illustrate the method on a z-shaped two-dimensional potential. While developed for use with path sampling, this analysis method can also be applied to regular molecular dynamics trajectories. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3491818
  • 2010 • 6 Observation of breathing-like modes in an individual multiwalled carbon nanotube
    Spudat, C. and Müller, M. and Houben, L. and Maultzsch, J. and Goss, K. and Thomsen, C. and Schneider, C.M. and Meyer, C.
    Nano Letters 10 4470-4474 (2010)
    We study collective vibrational breathing modes in the Raman spectrum of a multiwalled carbon nanotube. In correlation with high-resolution transmission electron microscopy, we find that these modes have energies differing by more than 23% from the radial breathing modes of the corresponding single-walled nanotubes. This shift in energy is explained with intershell interactions using a model of coupled harmonic oscillators. The strength of this interaction is related to the coupling strength expected for few-layer graphene. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/nl102305a
  • 2010 • 5 On the action of magnetic gradient forces in micro-structured copper deposition
    Mutschke, G. and Tschulik, K. and Weier, T. and Uhlemann, M. and Bund, A. and Fröhlich, J.
    Electrochimica Acta 55 9060-9066 (2010)
    In order to shed more light on the role of magnetic gradient forces and Lorentz forces on the deposition pattern found recently at copper electrodes, experiments and numerical simulations have been performed in a simple geometry that consists of a single small cylindrical permanent magnet which is placed behind the cathode. The cylinder axis coincides with the magnetization direction and points normal to the electrode surface. The electrode is oriented vertically which allows a separate discussion of the influence of both forces. Experiments and numerical simulations are found to give very good qualitative agreement with respect to the deposition pattern. Our analysis clearly shows that the major influence is due to the action of the magnetic gradient force. Numerical simulations prove that the separate action of the Lorentz force does not reproduce the deposition structure. A detailed analytical discussion of the motion forced by the different magnetic forces in superposition with natural convection is given. © 2010 Elsevier Ltd All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2010.08.046
  • 2010 • 4 Prediction of tautomer ratios by embedded-cluster integral equation theory
    Kast, S.M. and Heil, J. and Güssregen, S. and Schmidt, K.F.
    Journal of Computer-Aided Molecular Design 24 343-353 (2010)
    The "embedded cluster reference interaction site model" (EC-RISM) approach combines statistical-mechanical integral equation theory and quantum-chemical calculations for predicting thermodynamic data for chemical reactions in solution. The electronic structure of the solute is determined self-consistently with the structure of the solvent that is described by 3D RISM integral equation theory. The continuous solvent-site distribution is mapped onto a set of discrete background charges ("embedded cluster") that represent an additional contribution to the molecular Hamiltonian. The EC-RISM analysis of the SAMPL2 challenge set of tautomers proceeds in three stages. Firstly, the group of compounds for which quantitative experimental free energy data was provided was taken to determine appropriate levels of quantum-chemical theory for geometry optimization and free energy prediction. Secondly, the resulting workflow was applied to the full set, allowing for chemical interpretations of the results. Thirdly, disclosure of experimental data for parts of the compounds facilitated a detailed analysis of methodical issues and suggestions for future improvements of the model. Without specifically adjusting parameters, the EC-RISM model yields the smallest value of the root mean square error for the first set (0.6 kcal mol-1) as well as for the full set of quantitative reaction data (2.0 kcal mol-1) among the SAMPL2 participants. © 2010 Springer Science+Business Media B.V.
    view abstractdoi: 10.1007/s10822-010-9340-x
  • 2010 • 3 Sea salt concentrations across the European continent
    Manders, A.M.M. and Schaap, M. and Querol, X. and Albert, M.F.M.A. and Vercauteren, J. and Kuhlbusch, T.A.J. and Hoogerbrugge, R.
    Atmospheric Environment 44 2434-2442 (2010)
    The oceans are a major source for particles that play an important role in many atmospheric processes. In Europe sea salt may contribute significantly to particulate matter concentrations. We have compiled sodium concentration data as a tracer for sea salt for 89 sites in Europe to provide more insight in the distribution of sea salt across Europe. The annual average sea salt concentrations above land were estimated to range between 0.3 and almost 13 μg m-3. Maximum concentrations are found at the Irish coast. At coastal sites along the Atlantic and North Sea coast concentrations tend to be around 5 μg m-3. More inland locations up to about 300 km away from the coast tend to show concentrations between 2 and 5 μg m-3, whereas sites further away from the coast are characterized by lower concentrations. An analysis of the representativity of the data with respect to a long term average showed that the long average is associated with a standard deviation of around 15%. The compilation of observations provides an improved overview of sea salt concentrations in Europe as well as an improved basis for model validation. Verification of the results of the LOTOS-EUROS model learned that the model represents well the spatial variability of the observed sea salt concentrations very well. However, the absolute concentrations are significantly overestimated due to large uncertainties in the emission and dry deposition parameterizations. Using the high explained variability in the gradients across Europe, the bias-corrected modelled distribution serves as a best estimate of the sea salt distribution across Europe for 2005. © 2010.
    view abstractdoi: 10.1016/j.atmosenv.2010.03.028
  • 2010 • 2 Second-order convergence of the deviatoric stress tensor in the standard Bhatnagar-Gross-Krook lattice Boltzmann method
    Krüger, T. and Varnik, F. and Raabe, D.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 82 (2010)
    It is shown numerically that the deviatoric stress tensor is second-order accurate in the bulk Bhatnagar-Gross-Krook lattice Boltzmann (LB) method. In an earlier work, we have already predicted the second-order convergence. However, numerical simulations using a duct flow were not fully in line with this prediction. In particular, the convergence rate of the stress tensor was observed to depend on the LB boundary condition. In the present paper, we examine a pure bulk system, the decaying Taylor-Green vortex flow. Our prediction on the second-order accuracy of the stress tensor is unambiguously evidenced via these studies. © 2010 The American Physical Society.
    view abstractdoi: 10.1103/PhysRevE.82.025701
  • 2010 • 1 Two-beam high-order harmonics from solids: Coupling mechanisms
    Tarasevitch, A. and Wieczorek, J. and Kohn, R. and Bovensiepen, U. and Von Der Linde, D.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 82 (2010)
    The polarization of the two beam (driver-probe) high-order harmonic generation from solids is measured. The experiments, together with computer simulations, allow us to distinguish two different coupling mechanisms of the driver and the probe, resulting in different harmonic efficiencies and spectral slopes. We find that in the nonrelativistic regime the coupling is mostly due to the nonlinear plasma density modulation. © 2010 The American Physical Society.
    view abstractdoi: 10.1103/PhysRevE.82.056410