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.

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.

The optimization of a centrifugal compressor impeller is a challenge for established strategies and algorithms, as the interactions between the geometric design parameters and the aerodynamic and structural performance of the system are highly complex. Furthermore many geometrically valid designs are unusable in terms of structure mechanics or flow physics. Due to the complex parameter correlations, a simple limitation of the parametric space is no option, as possibly beneficial parameter combinations could be ruled out. To obtain a meaningful optimization result, the complete operation range of the compressor has to be taken into account which adds further complexities in terms of the optimization process and the computational expense. The combination of these issues leads to a complicated optimization scenario. The aim of the presented work is the reduction of the computational expense required to generate a high quality metamodel for optimization. This goal shall be achieved by the development of a multi-fidelity sampling method. The basic idea is to use preliminary of low-fidelity information from empirical data or fast analytical methods to identify promising regions of the parameter space. Then the samples of the DOE are concentrated in these areas while still maintaining a good coverage of the whole applicable design space. This ensures that no beneficial designs are ruled out which were not recommended by preliminary information. The points of the resulting DOE are computed by CFD and FEA computations and used to generate the metamodel which is used for the optimization. The method is tested by generating a metamodel used for compressor optimization. The results are compared to an optimization using a metamodel based on a conventional Latin hypercube sampling. © Copyright 2018 ASME.

In Germany in 2016 17,085 patients received TAVI operations and 9,579 had conventional aortic valve surgery. The 'Heart Team' uses established scoring systems (EuroSCORE, STS, German AV Score) to evaluate operation risks and which technique to use. However, such risk grading fails to consider patient morphology and possible long-term behavior of the replacement valve chosen. Therefore, pre-operative simulation of the dynamic loading on the valve leaflets after TAVR provides information vital for the selection of the appropriate aortic valve therapy - interventional versus conventional. Individual aorta used in this study was captured by MRI. Segmentation and data processing were done with Mimic In-novation Suite. The available biological aortic valves prostheses were reverse engineered to create a 3D CAD model. Simulations combined bi-directional fluid structure interaction (FSI) with a first order Ogden model of the hyperelastic behavior of aortic leaflets from bovine pericardium. Movements induced by flow and the resultant tension on the biological leaflets were computed with developed simulation model. Stress analyses of the leaflets showed behavior attributable to their particular structure. Both valves showed two stress peaks within the initial 0.3 s. Maximum stress occurred, however, at other time points. Furthermore, the initial increase in stress showed a delayed onset. The patterns of movement were also significantly different. So, at opening of the valve, the freely perfused area of the valve, the freedom of leaflet movement and symmetry at closure were different in the two valves. Simulated movement of valve leaflets corresponds well with reality. The estimated stresses clearly lie below thresholds published in the literature for bovine pericardium. It is planned to further develop the current workflow to increase stability and optimize processing time, with the intention of providing the 'Heart Team' with a tool for incorporating individual anatomy when selecting the aortic valves. © 2018 Markus Bongert et al.

An in silico investigation of modelled Extracorporeal Life Support (ECLS) via a femoral arterial cannula revealed the existence of both a defined separation zone between the opposing flows (ECLS, native flow) and different ranges dependent on flow distribution. The interaction between pulsating native circulation and constant ECLS flow is dynamic. A transient simulation model was developed to investigate the dynamic influence on this fluid mechanical interaction. The in silico model is based on a CT-generated 3D model derived from a life-sized silicon aorta. A geometric standard cannula (16Fr) is inserted femoral. Inlet boundary conditions such as the temporal flow profile of a subject from the left ventricle (native circulation) and the flow from the femoral cannula are varied such that during transient simulations the summed flow (total perfusion) is 5.5 l/min. The outlet pressure boundary conditions at the branching arteries are selected such as to model the downstream vascular system. Transient simulations revealed the dynamic effects of different flow fractions (Heart - ECLS) on the flow. Stationary simulations show a separation zone between the two flows, the position of which respectively the ECLS-range, oscillates dependent of the native circulation. Furthermore, it was noted that a raised pulse was impedimental to ECLS. This can be partly compensated by increasing the length of cannula inserted. At the same time the ECLS supply for the brain can improve at the cost of performance post-bifurcation. Increasing the ECLS fraction to above 50% flow led to retrograde flow combined with blood suction from the femoral artery. The EMPAC project model has been further developed to include investigation of the dynamic effects of blood flow. This has made it possible for the first time to analyse in detail and evaluate the temporal effects of both opposing flows streams. A subsequent investigation explains whether aortic elasticity plays a significant role. © 2018 Markus Bongert et al.

The continuously rising global demand for energy together with simultaneously decreasing resources has made the topic of energy efficiency - and therefore optimization - one of the fundamental questions of our time. Turbomachinery is one of the most important parts of the process chain in nearly every case of energy conversion. This makes the turbomachine a promising approach point for optimizations. The special relevance of this topic in regard to the global challenge of climate change can be illustrated by a simple calculation: If the efficiency of a turbo compressor with a power consumption of 15MW is improved by one percent, approximately 2t CO2 per day or over 760t CO2 per year can be saved.1 This work describes the optimization of the operation characteristic of a highly stressed centrifugal compressor impeller with regard to the size of the operation range and the efficiency in the operation point. The base impeller used for this optimization has already been pre-optimized by classical engineering methods utilizing analytical and empirical models. Due to the high mechanical stress in these kind of turbo impellers, each design has to be checked for compliance with the structural constraints in addition to the fluid dynamic computations. This context results in a highly complex, multicriterial, high dimensional optimization problem. The main subjects of the presented work are a robust geometry generation and grid generation, a highly automated workflow for the computation of the operation characteristic and the mechanical results and the representation of the operation characteristic by scalar parameters. Utilizing these tools a DOE is performed and based on its results a metamodel is created. The optimization is carried out on the metamodel using a Particle Swarm algorithm The workflow presented in this work utilizes in-house preprocessing tools as well as the tools of the ANSYS Workbench. The operation characteristics are computed using an in-house tool to control the ANSYS CFX-Solver. The statistical and stochastic pre- and post-processing as well as the metamodeling are carried out in optiSLang. Copyright © 2017 ASME.

Journal bearings are an essential component in mechanical engineering. While the fundamental functional principle is well known, the internal processes in a bearing and the complex interactions between lubrication film and bearing structure have hitherto not been fully researched. Traditionally journal bearing analysis is carried out by special simulation codes based on lubrication theory or use of the Reynolds equation. To take phenomena such as turbulence, cavitation or heat transfer into account, empirical and numerical models are integrated into the calculations. These codes have proven to be efficient and sufficiently accurate for fundamental bearing analysis but are also subject to certain limitations in that they do not allow visualisation of physical phenomena such as cavitation, recirculation or elastohydrodynamic effects in complex geometries. This paper presents an approach based on state of the art numerical fluid structure interaction (FSI) methods. Application of three dimensional computational fluid dynamics (CFD) and finite element methods (FEM) allows the analysis of arbitrary bearing geometries. Furthermore, this approach also permits a detailed analysis of flow phenomena inside the bearing. Copyright © 2014 Inderscience Enterprises Ltd.