Prof. Dr.-Ing. Bettar Ould el Moctar

Ship & Offshore Technology
University of Duisburg-Essen

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  • A numerical method to compute global resonant vibrations of ships at forward speed in oblique waves
    Riesner, M. and el Moctar, O.
    Applied Ocean Research 108 (2021)
    Resonant wave-induced vibrations of a ship hull girder, known as “springing”, is an important issue when addressing fatigue aging of the steel structure. To compute resonant wave-induced vibrations, fluid-structure interactions and associated structural and hydrodynamic properties need to be addressed. This paper introduces a time-domain numerical method that predicts higher-order springing taking into account forward speed. Structural dynamics were computed based on a beam element approach that considers vertical and horizontal bending as well as nonuniform torsion. Furthermore, mass and stiffness matrices accounted for strong coupling effects between hull girder bending and torsion. The hydrodynamic solver coupled the fully nonlinear stationary free surface flow with the oscillatory flow and considered geometrical nonlinearities caused by the changing wetted surface due to the incident waves, nonlinear rigid body motions and linear elastic vibrations. Numerical predictions were validated against model tests of a post-Panamax containerships at forward speed. Dry and wet natural frequencies and midship vertical bending, horizontal bending and torsional moments induced by higher order springing vibrations compared favourably to experimental measurements. © 2020
    view abstract10.1016/j.apor.2020.102520
  • Assessment of wave induced higher order resonant vibrations of ships at forward speed
    Riesner, M. and el Moctar, O.
    Journal of Fluids and Structures 103 (2021)
    An efficient nonlinear time domain method computed higher order springing induced vertical bending vibrations of ships in waves. A weakly nonlinear time domain approach obtained the hydrodynamic response of the elastic hull girder, and a linear finite element model based on Timoshenko's beam theory calculated its structural response. Coupling of the fully nonlinear stationary forward speed problem with the weakly nonlinear elastic body seakeeping problem constituted the major progress. We demonstrated that accounting for the nonlinear stationary forward speed problem significantly affected the prediction of springing-induced vibrations. Rigid body motions were computed via a nonlinear equations of motion. Elastic vibrations were computed using the modal superposition technique based on linear elastic motion equations. Radiation forces of the moving and vibrating hull structure (rigid body and elastic) were computed via convolution integrals, and Froude–Krylov and hydrostatic forces were combined and integrated over the instantaneous wetted surface. A waterline integral accounted for nonlinear effects of radiation and diffraction forces due to the changing wetted surface. A two-way coupling algorithm ensured accurate convergence. Comparisons of the numerically calculated midship vertical bending moment of a large container ship with experimental results showed good agreement. Investigations of the subject container ship advancing at forward speeds of 15 and 22kn in regular head waves focused on second, third, and fourth order springing. © 2021 Elsevier Ltd
    view abstract10.1016/j.jfluidstructs.2021.103262
  • Deformation measurement of a monopile subject to vortex- induced vibration using digital image correlation
    Tödter, S. and el Sheshtawy, H. and Neugebauer, J. and el Moctar, O. and Schellin, T.E.
    Ocean Engineering 221 (2021)
    Monopile towers used for offshore wind turbines are sensitive to Vortex-Induced Vibration (VIV). Here, their structural response to VIV was experimentally investigated with models of a partially and a fully immerged offshore monopile. The partially and fully immersed model cylinders had aspect ratios of 18.75 and 28.13, respectively. They were subject to reduced velocities of up to 9.78 and 5.45, respectively, corresponding to Reynolds numbers of up to 77120 and 63680, respectively. Normalised transverse motion amplitudes of up to 1.19 were measured. “Strouhal-like” numbers down to 0.13 were obtained, and they decreased with increasing flow velocity and corresponding motion amplitude. The 3D Digital Image Correlation (DIC) processing method was used to measure response without influencing model properties and the flow field. To assess the suitability of DIC for VIV investigations, results obtained from the conventional technique using triaxial accelerometers were compared. The influence of different processing methods, pattern designs and their assemblies, the repeatability of the model tests, and the influence of ventilation were investigated also. Additionally, forces and torques were measured directly. Results from these three measurement techniques were compared and discussed. © 2020
    view abstract10.1016/j.oceaneng.2020.108548
  • Mooring-configurations induced decay motions of a buoy
    Jiang, C. and El Moctar, O. and Schellin, T.E.
    Journal of Marine Science and Engineering 9 (2021)
    The hydrodynamic damping of a buoy stationed with three different mooring configurations was estimated using a Navier-Stokes (NS) equations solver coupled with a dynamic mooring model. The mooring configurations comprised a catenary system, a catenary system with sub floaters, and a catenary system with sub floaters and clump weights. Systematic simulations were achieved by adopting the overset grid scheme instead of the conventional morphing grid scheme, which required regenerating the entire mesh when the buoy’s initial position changed, thereby avoiding mesh distortions and numerical instabilities. Motion decay simulations in heave, pitch, and surge were conducted with and without various mooring systems. The analyzed results comprised decaying oscillating motions, natural periods, and associated amounts of damping. The mooring systems influenced not only restoring force characteristics, but also total damping of the moored buoy, which demonstrated the importance of considering mooring-induced damping when investigating moored offshore structures. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/jmse9030350
  • Comparative study of mathematical models for mooring systems coupled with CFD
    Jiang, C. and el Moctar, O. and Schellin, T.E. and Paredes, G.M.
    Ships and Offshore Structures (2020)
    A numerical method that coupled a Reynolds-averaged Navier-Stokes solver with a quasi-static and a dynamic mooring model was employed and compared. Decay tests of the freely floating and the moored buoy in calm water determined the system's linear and quadratic damping characteristics. In regular waves, predicted heave and pitch amplitudes and high-frequency surge amplitudes based on the quasi-static mooring model were similar to those based on the dynamic mooring model and similar to comparable measurements. However, the low-frequency surge and pitch motions based on the quasi-static model deviated significantly from the corresponding motions based on the dynamic model and from measurements. In irregular waves, buoy motion amplitudes, especially in surge, based on the quasi-static model were overpredicted. The accurate evaluation of the moored buoy in irregular waves, especially when the moored buoy underwent large translations, was possible only by accounting also for the dynamic loads acting on the mooring lines. © 2020, © 2020 Informa UK Limited, trading as Taylor & Francis Group.
    view abstract10.1080/17445302.2020.1790294
  • Control of unsteady partial cavitation and cloud cavitation in marine engineering and hydraulic systems
    Kadivar, E. and Timoshevskiy, M.V. and Nichik, M.Y. and El Moctar, O. and Schellin, T.E. and Pervunin, K.S.
    Physics of Fluids 32 (2020)
    Cavitation is a process of liquid evaporation, bubble or vapor sheet formation, and further collapse of vapor structures, which plays a destructive role in many industrial applications. In marine transport and hydraulic machinery, cavitation usually occurs nearby the surface of a ship propeller and rudder, impeller blades in a pump, and distributor vanes and runner blades in a hydroturbine and causes various undesirable effects such as vibrations of frameworks and/or moving parts, material erosion, and noise enhancement. Based on an extensive literature review, this research is aimed at an experimental investigation of a passive approach to control cavitation on a benchmark hydrofoil using a wedge-type vortex generator in different flow regimes with a high Reynolds number. In this study, we employed a high-speed imaging method to explore the spatial patterns and time evolutions of cavitation structures and utilized a hydroacoustic pressure transducer to record and analyze local pressure pulsations due to the collapse of the cavities in the hydrofoil wake region. The results show that the examined control technique is quite effective and capable of hindering the formation of cloud cavities and reducing the amplitude of pressure pulsations associated with unsteady cavitation dynamics. This study provides important experimental information, which can be useful for improving industrial technologies and for promoting new developments in this particular research field. © 2020 Author(s).
    view abstract10.1063/5.0006560
  • Dynamics of a cavitation bubble near a solid surface and the induced damage
    Sagar, H.J. and el Moctar, O.
    Journal of Fluids and Structures 92 (2020)
    Numerical and experimental studies of the dynamics of a cavitating bubble near a resilient metal surface were performed. To augment the experimental flow visualizations of a collapsing bubble, numerical simulations were conducted to more thoroughly identify the collapse dynamics and analyze the flow. A bubble collapse was captured using a high-speed camera and back illumination. The metal sample was made of pure aluminum placed near a collapsing cavitation bubble at various distances from the metal surface. Width, depth, and volume of the induced material deformations were measured using an optical microscope and a three-dimensional profilometer and then compared against existing experimental data from the literature. The cavitating bubble's dynamics and the related flow were simulated numerically using the open source finite volume based flow solver CavitatingFOAM. This code solved the Navier–Stokes equations for compressible two-phase flows using an Euler–Euler approach, including the barotropic equations of state. Bubble shapes, collapse times, and obtained damage parameters were compared to experimental observations. Impact velocities, pressures, shear rates, and various flow phenomena were discussed, providing broad insight into bubble dynamics and the induced damage. © 2019 Elsevier Ltd
    view abstract10.1016/j.jfluidstructs.2019.102799
  • Experimental investigation of tip vortex influence on viv of a circular cylinder at high reynolds numbers
    Youssef, M. and Tödter, S. and Neugebauer, J. and El Moctar, O. and Schellin, T.E.
    Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE 6A-2020 (2020)
    Lower aspect ratio cylinders are of interest as these are used for, e.g., wind turbine towers. We present a study predicting the vortex-induced vibration (VIV) characteristics of a cantilever circular cylinder with an aspect ratio (length-to-diameter ratio) of 10. The free end condition caused tip vortices to generate endcell- induced vibration (ECIV) occurring at wind speeds higher than the threshold wind speed for VIV. Wind tunnel experiments were conducted at Reynolds numbers ranging from 3.6 × 104to 3.26 × 105. The cantilever circular cylinder was made of a polypropylene pipe mounted on a six-degree-of-freedom load cell. Its upper end was free to oscillate in the streamline and transverse directions. An accelerometer was placed at the top of the cylinder to measure accelerations and, via twofold integration, retrieve cylinder motions at its free end. Down-wash tip vortices occurred. The response amplitude with vortex shedding was finite, even at the synchronization point. It increased with flow velocity and affected the synchronization behavior. Results suggested that nonlinearities occurring when vortices were Shedd were due to free end conditions and higher Reynolds numbers. © 2020 American Society of Mechanical Engineers (ASME). All rights reserved.
    view abstract10.1115/OMAE2020-19133
  • Hydrodynamic optimisation of a multi-purpose wind offshore supply vessel
    Feng, Y. and Moctar, O.E. and Schellin, T.E.
    Ship Technology Research 67 (2020)
    The aim of this study was to reduce the total resistance of a multi-purpose wind offshore supply vessel by optimising its hull. Resistance was computed using a potential flow boundary element method and a Reynolds-averaged Navier–Stokes equations solver. Optimised hull forms were obtained for the ship advancing at different ship speeds under calm water conditions, employing the two multi-objective optimisation algorithms, Non-dominated Sorting Genetic Algorithm II (NSGA-II) and Multi-Objective Simulated Annealing (MOSA). Using NSGA-II yielded slightly larger reductions of total resistances than MOSA. The greatest reductions were achieved at ship speeds between 11 and 14knots. At these speeds, a thinner and longer bulbous bow reduced resistance. At speeds greater than 15knots, a bloated bulbous bow was more helpful to reduce resistance. © 2019, © University of Duisburg-Essen 2019.
    view abstract10.1080/09377255.2019.1602976
  • Hydrodynamic optimization of a containership
    Feng, Y. and El Moctar, O. and Schellin, T.E.
    Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE 2B-2020 (2020)
    Using open-source software Dakota, this paper describes the process of generating an optimal parametric hull shape for a generic containership. Selected design variables defined the ship's hull, and the influence of these variables on calm water resistance was analyzed. Computations of the flow around the hull were obtained from a validated nonlinear potential flow boundary element method. Using the multi-objective optimization algorithm Surrogate Based Global Optimization (SBGO) reduced the computational effort. The hydrodynamic calm water ship resistance defined the objective function. Compared with the original hull, wave resistance of the optimal hull was significantly reduced for the ship at Froude numbers corresponding to its design speeds. Copyright © 2020 ASME.
    view abstract10.1115/OMAE2020-18616
  • Investigation of a semi-submersible floating wind turbine in surge decay using CFD
    Burmester, S. and Vaz, G. and Gueydon, S. and el Moctar, O.
    Ship Technology Research 67 (2020)
    The hydrodynamic damping estimation of the surge motion and the flow characteristics of a moored semi-submersible floating offshore wind turbine is the focus of this paper. The numerical surge decay tests were investigated using a Reynolds-Averaged Navier–Stokes solver. Solution verification on the numerical simulations was performed by estimation of the numerical errors and uncertainties. A linear stiffness matrix and a non-linear quasi-static mooring model were used in the equations of motion. Several surge decay simulations were performed to understand the effects of wave radiation, coupled motions and non-linear moorings on the hydrodynamic damping and the flow field around the floater. The numerical results were compared with experimental data. The free surface had major effects on the hydrodynamic damping and the flow field. © 2018, © University of Duisburg-Essen 2018.
    view abstract10.1080/09377255.2018.1555987
  • Numerical assessment of cavitation-induced erosion using a multi-scale Euler-Lagrange method
    Peters, A. and El Moctar, O.
    Journal of Fluid Mechanics (2020)
    A multi-scale Euler-Lagrange method was developed and applied to numerically assess cavitation-induced erosion based on the collapse dynamics of Lagrangian bubbles. This approach linked macroscopic and microscopic scales and captured large vapour volumes on an Eulerian frame, while small vapour volumes were treated as spherical Lagrangian bubbles. Interactions between vapour bubbles and the liquid phase were considered via a two-way coupling scheme. A verification and sensitivity study of the developed procedure to transform vapour volumes between Eulerian and Lagrangian frames was performed. First, the developed method was validated for bubble dynamics, using analytical and experimental data. Second, the cavitating flow through an axisymmetric nozzle was simulated using a measurement-based distribution of cavitation nuclei. Details of single bubble collapses were used to assess cavitation erosion. Based on well-recognised fundamental experiments and theoretical considerations from the literature, model assumptions were derived to account for the effects of a bubble's stand-off distance on the bubble's motion and its radiated pressure during an asymmetric near-wall bubble collapse. Computed maximum collapse radii of bubbles correlated well with diameters of measured erosion pits. Considering a nonlinear dependence of erosion on impact pressure, calculated erosion potentials compared well to measured erosion depths. © 2020 The Author(s). Published by Cambridge University Press
    view abstract10.1017/jfm.2020.273
  • Numerical study on wave-induced motions and steady wave drift forces for ships in oblique waves
    Zhang, W. and el Moctar, O. and Schellin, T.E.
    Ocean Engineering 196 (2020)
    This paper considers numerical predictions of wave-induced motions and steady wave drift forces for ships in oblique waves. The velocity potential around the ship is computed via a time domain Rankine panel method. Based on the computed velocity potential, the six degree of freedom wave-induced motions were computed and then the second order wave loads were evaluated using a near field method. An artificial soft spring system was employed to control horizontal ship motions. The steady longitudinal and transverse wave drift forces and the steady yaw moment were computed for the KVLCC2 tanker at zero forward speed and for the S-175 containership at constant forward speed. Predicted results generally compared favorably to published experimental measurements. A systematic analysis of the effects of the soft spring system on wave-induced motions and steady wave drift forces demonstrated that, although the increased stiffness of the system changed the corresponding motion response of the ships, a spring's influence was negligibly small if its stiffness was weak enough. © 2019 Elsevier Ltd
    view abstract10.1016/j.oceaneng.2019.106806
  • Towards credible CFD simulations for floating offshore wind turbines
    Burmester, S. and Vaz, G. and el Moctar, O.
    Ocean Engineering 209 (2020)
    The credibility in the results needs to be demonstrated in order to be able to use computational fluid dynamics (CFD) as an engineering tool. This may be obtained by sufficient verification and validation studies involving error and uncertainty quantification. This study investigates how to perform credible CFD simulations of floating offshore wind turbines (FOWTs). Three methods to estimate discretisation errors were compared for three different problems related to FOWTs: wave propagation in 2D, wave loads on a circular cylinder, and surge decay of a semi-submersible FOWT. The three discretisation error estimation methods are a least-squares formulation using the observed order of convergence in combination with a data quality measure for different spatial and temporal refinement, another least-squares fit method using the theoretical order of convergence for constant Courant number grid refinement studies, and the factor of safety method applying a ratio between observed and theoretical order of convergence. We compared the final results to an analytical solution of the 2D wave signal, and to experimental data for the wave loads and surge decay motions, for validation purposes. The results of this work show the advantages and disadvantages of the three error estimation methods. The uncertainty bars for the discretisation uncertainty of the numerical simulations were mostly larger than the comparison error with the model test data and analytical solution. © 2020 Elsevier Ltd
    view abstract10.1016/j.oceaneng.2020.107237
  • Validation of a dynamic mooring model coupled with a RANS solver
    Jiang, C. and el Moctar, O. and Moura Paredes, G. and Schellin, T.E.
    Marine Structures 72 (2020)
    Standard design procedures and simulation tools for marine structures are aimed primarily for use by the offshore oil and gas. Mooring system restoring forces acting on floating offshore structures are obtained from a quasi-static mooring model alone or from a coupled analysis based on potential flow solvers that do not always consider nonlinear mooring-induced restoring forces, fluid structure interactions, and associated hydrodynamic damping effects. This paper presents the validation of a dynamic mooring system analysis technique that couples the dynamic mooring model with a Reynolds-averaged Navier-Stokes (RANS) equations solver. We coupled a dynamic mooring model with a RANS equations solver, and analyzed a moored floating buoy in calm water, regular and irregular waves and validated our motion and mooring force predictions against experimental measurements. The mooring system consisted of three catenary chains. The analyzed response comprised decaying oscillating buoy motions, linear and quadratic damping characteristics, and tensile forces in mooring lines. The generally favorable comparison of predicted buoy motions and mooring forces to experimental data confirmed the reliability of our implemented coupling technique to predict system response. Additional comparative results from a potential flow solver demonstrated the benefits of the coupled dynamic mooring model with RANS equations. The successful validated tool of coupling the dynamic mooring model with the RANS solver is available as open source, and it shows the potential of the coupled methodology to be used for analyzing the moored offshore structures. © 2020 Elsevier Ltd
    view abstract10.1016/j.marstruc.2020.102783
  • A hybrid method for ship response coupled with sloshing in partially filled tanks
    Lyu, W. and Riesner, M. and Peters, A. and Moctar, O.E.
    Marine Structures 67 (2019)
    An efficient hybrid numerical method was developed to study wave-induced ship response coupled with sloshing-induced compressible fluid response in partially filled tanks of a 138,000 m3 liquefied natural gas carrier at zero speed in regular head and beam waves. A nonlinear boundary element Rankine source code, solving the incompressible incident flow field, accounted for the nonlinear stationary forward speed problem and the weakly nonlinear seakeeping problem. A Reynolds-averaged Navier-Stokes equations solver, computing the compressible sloshing flow in tanks, considered the associated discontinuous numerical domains of coupled simulations. Grid sensitivity studies were performed. Comparative available experimental model test measurements of ship response and free surface sloshing elevations in tanks validated the numerical method. Further investigations regarding the influence of wave steepness and tank fill levels on sloshing were performed. Results demonstrated that liquid sloshing in tanks had a large influence on ship surge and roll responses but hardly effected ship heave motions. © 2019 Elsevier Ltd
    view abstract10.1016/j.marstruc.2019.102643
  • Assessment of LNG pump tower loads
    Thome, M. and Neugebauer, J. and El Moctar, O.
    Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE 2 (2019)
    The assessment of design loads acting on Liquefied Natural Gas (LNG) pump tower are widely based on Morison equation. However, the Morison equation lacks consideration of transverse flow, impact loads and the interaction between fluid and structure. Studies dealing with a direct simulation of LNG pump tower loads by means of Computational Fluid Dynamics (CFD), which can cover the aforementioned effects, are currently not available. A comparative numerical study on LNG pump tower loads is presented in this paper focusing on the following two questions: Are impact loads relevant for the structural design of LNG pump towers? In which way does the fluid-structure interaction influence the loads? Numerical simulations of the multiphase problem were conducted using field methods. Firstly, Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations, extended by the Volume of Fluid (VoF) approach were used to simulate the flow inside a three-dimensional LNG tank in model scale without tower structure. The results were used to validate the numerical model against model tests. Motion periods and amplitudes were systematically varied. Velocities and accelerations along the positions of the main structural members of the pump tower were extracted and used as input data for load approximations with the Morison equation. Morison equation, URANS and Delayed Detached Eddy Simulation (DDES) computed tower loads were compared. Time histories as well as statistically processed data were used. Global loads acting on the full (with tower structure) and simplified structure (no tower structure, but using Morison equation) are in the same order of magnitude. However, their time evolution is different, especially at peaks, which is considered significant for the structural design. Copyright © 2019 ASME
    view abstract10.1115/OMAE2019-96138
  • Boundary layer instability control in the unsteady cloud cavitating flow
    Kadivar, E. and Moctar, O.E.
    IOP Conference Series: Earth and Environmental Science 240 (2019)
    In this article, we propose a passive boundary layer control method to control the vortex structure of the cavity on the suction side and wake region of the CAV2003 benchmark hydrofoil. This method may be used in different applications such as marine, turbomachinery and hydraulic machinery. First, we used a hybrid URANS model for turbulence to simulate the 3D unsteady cloud cavitating flow and validated it based on experimental data. We performed the numerical simulations using open source code OpenFOAM and an Euler-Euler cavitation model. Second, we studied the effect of passive boundary layer control method on vortex structure on the suction side of the hydrofoil and in wake region. We showed that this control method may influence the boundary layer structure on the hydrofoil surface and also near the trailing edge. Using this technique the pressure distribution and the fluctuating part of the velocity field on the hydrofoil surface were modified over the chord length. This method induced a stabilization of the boundary layer and delay its separation. Therefore a significant reduction in cavitation-induced vibration may be expected. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1755-1315/240/6/062061
  • Comparison and validation of numerical methods to assess hydrodynamic loads on mechanical coupling of multiple bodies
    Ghesmi, M. and von Graefe, A. and Shigunov, V. and Friedhoff, B. and el Moctar, O.
    Ship Technology Research 66 (2019)
    In the design of articulated systems, accurate prediction of loads on mechanical couplings is important for the dimensioning of the coupling, as well as an assessment of local and global structural loads. The paper studies a twofold pushing convoy in shallow water. Several numerical approaches for the prediction of articulation loads are compared with each other and with model tests. Kinematic constraints are applied to couple the bodies at articulation points to model hinge and rigid mechanical couplings. For comparison, contact elements are also used to simulate hinges. Depending on the articulation model numerical computations are conducted in frequency or time domain. Comparison between different numerical methods and with model experiments shows that the proposed methods can predict articulation loads with sufficient accuracy for design purposes. The frequency domain approach with kinematic constraints seems more suitable to compute the hinge coupling forces than time-domain simulations with contact elements. © 2018, © University of Duisburg-Essen 2018.
    view abstract10.1080/09377255.2018.1482100
  • Corrigendum to “Investigation of the effect of cavitation passive control on the dynamics of unsteady cloud cavitation” Journal of Applied Mathematical Modelling 64 (2018) 333–356 (Applied Mathematical Modelling (2018) 64 (333–356), (S0307904X18303305), (10.1016/j.apm.2018.07.015))
    Kadivar, E. and el Moctar, O. and Javadi, K.
    Applied Mathematical Modelling 72 (2019)
    The authors apologize for omitting to acknowledge the prior work of Javadi et al. wherein they used an appendage as a passive controller to stabilize the cavitation on a 2-D hydrofoil [1]. This paper is an extension of the ideas presented in [1]. Herewith, we would like to acknowledge the omission of the mentioned paper through citing this article in the original paper, as listed below. © 2019 Elsevier Inc.
    view abstract10.1016/j.apm.2019.04.001
  • Experimental investigation of free-running ship manoeuvers under extreme shallow water conditions
    Mucha, P. and Dettmann, T. and Ferrari, V. and el Moctar, O.
    Applied Ocean Research 83 (2019)
    We present an experimental investigation of a free-running manoeuvring inland waterway ship at extreme shallow water conditions. Physical tests of zig-zag manoeuvres at two different water depths were performed in model scale and investigated with regards to the effects of limited under-keel clearance. Experimental data comprise results from repeatability studies and may serve for validation of manoeuvring simulations. © 2018 Elsevier Ltd
    view abstract10.1016/j.apor.2018.09.008
  • Numerical prediction of wave added resistance using a Rankine Panel method
    Zhang, W. and Moctar, O.E.
    Ocean Engineering (2019)
    Based on a time domain Rankine panel method, a computer code was developed to predict wave added resistance. The unknown variables of the discretized flow field boundaries were represented using the quadratic B-spline basis function; the temporal derivative of the free surface conditions were approximated using an Euler scheme. The added resistance was computed via a near-field method. Three numerical approaches were introduced, namely, choosing the boundary end condition, neglecting the second order derivative of the disturbed potential, and computing the water line integral. Numerical computations were conducted in short and long waves for a Wigley hull, the S-175 container ship, and the KVLCC2 tanker. The double-body linearization and the Neumann–Kelvin linearization were tested, and numerical results were compared to published experimental measurements and numerical data. Fairly good agreement was found between the results of double-body linearization and the published data. Effects of wave radiation and diffraction on added resistance were analyzed, demonstrating that the interaction between radiation and diffraction should not be omitted in long waves. © 2019 Elsevier Ltd
    view abstract10.1016/j.oceaneng.2019.02.055
  • Prediction of hydrodynamic damping of moored offshore structures using CFD
    Jiang, C. and El Moctar, O. and Schellin, T.E.
    Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE 2 (2019)
    Usually, mooring system restoring forces acting on floating offshore structures are obtained from a quasi-static mooring model alone or from a coupled analysis based on potential flow solvers that do not always consider nonlinear mooring-induced phenomena or fluid-structure interactions and the associated viscous damping effects. By assuming that only the mooring system influences the restoring force characteristics, the contribution of mooring-induced damping to total system damping is neglected. This paper presents a technique to predict hydrodynamic damping of moored structures based on coupling the dynamic mooring model with a Reynolds-averaged Navier-Stokes (RANS) equations solver. We obtained hydrodynamic damping coefficients using a least-square algorithm to fit the time trace of decay tests. We analyzed a moored offshore buoy and validated our predictions against experimental measurements. The mooring system consisted of three catenary chains. The analyzed response comprised the decaying oscillating buoy motions, the natural periods, and the associated linear and quadratic damping characteristics. Predicted motions, natural periods, and hydrodynamic damping generally well agreed to comparable experimental data. Copyright © 2019 ASME
    view abstract10.1115/OMAE2019-95935
  • Rankine source time domain method for nonlinear ship motions in steep oblique waves
    Riesner, M. and Chillcce, G. and el Moctar, O.
    Ships and Offshore Structures 14 (2019)
    This article presents a weakly nonlinear time domain Rankine source boundary element method to calculate wave-induced ship motions. Radiation forces were calculated using convolution integrals, whereas Froude–Krylov and hydrostatic forces were obtained from ship positions relative to the instantaneously wetted surface. The nonlinear rigid body motion equations were coupled with the flow equations to enable reliable predictions also in finite amplitude (steep) waves. A system of soft springs prevented excessive low-frequency motions of the ship, but allowed it to move in six degrees of freedom. Numerical results of a 14,000TEU container ship at two different forward speeds in oblique waves of different steepness were validated against comparable ship motions obtained from available experimental towing tank measurements and a frequency domain method. The influence of wave steepness and wave heading on ship motions was of particular interest. © 2018, © 2018 Informa UK Limited, trading as Taylor & Francis Group.
    view abstract10.1080/17445302.2018.1498568
  • Research towards goal based standards for container shipping
    Shigunov, V. and Rathje, H. and El Moctar, O.
    Fluid Mechanics and its Applications 119 (2019)
    Analysis and verification of rule-related technical aspects of safe and efficient container shipping are an important part of R&D activities of classification societies. Casualty statistics show that container loss in heavy weather is an important issue for innovative container ship designs. The paper demonstrates two examples of research activities aiming at the reduction of cargo losses. One example is ship-specific operational guidance, assisting the ship master to avoid excessive motions and accelerations in heavy weather. The design accelerations underlying the operational guidance are part of classification rules, requiring understanding of the physics of dynamic loads on containers and lashing. The status of the ongoing research in this area is shown, in particular, the study of the effects of container flexibility and dynamic load amplification, not addressed explicitly in the present classification rules. © Springer Nature Switzerland AG 2019.
    view abstract10.1007/978-3-030-00516-0_39
  • Stabilization of cloud cavitation instabilities using Cylindrical Cavitating-bubble Generators (CCGs)
    Kadivar, E. and Moctar, O.E. and Javadi, K.
    International Journal of Multiphase Flow (2019)
    Unsteady cloud cavitation phenomenon is an important subject due to its undesirable effects in various applications such as ship propeller, rudder and hydraulic machinery systems. We present an efficient passive control method to control the cavitation instabilities which may be caused by the shedding of cavity structures in the vicinity of the solid surface of an immersible body. We proposed a passive control method so called Cylindrical Cavitating-bubble Generators (CCGs) on the surface of a benchmark hydrofoil and analyzed the effects of this passive controller on the dynamics of the unsteady cloud cavitation. First we modeled the unsteady cavitating flow around the hydrofoil without CCGs using a hybrid URANS model which was implemented in an open source code. Next, we studied the effect of CCGs on the mechanism of the unsteady cloud cavitation. The results show that using this method, the unsteady cavity structure was changed to a quasi-stable cavity structure compared with the cloud cavity shedding in the case without CCGs. We observed that the instability behavior of the unsteady cloud cavitation was mitigated and only small-scale cavity may be shedded from the hydrofoil in the free stream flow away from the hydrofoil surface. A notable reduction in cavitation-induced vibration and high wall-pressure peaks on the solid surface was observed. © 2019 Elsevier Ltd
    view abstract10.1016/j.ijmultiphaseflow.2019.03.019
  • Wind and current loads on barges and ships
    El Moctar, O. and Schellin, T.E. and Neugebauer, J.
    Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE 2 (2019)
    Increased waterborne trade has led to the construction of ever larger ships and barges as oversized modules are transported by sea. The provision of tugs for towing such vessels has become a serious issue, especially in restricted areas often characterized by coastal regions of limited water depth. Wind loads are most relevant for towing operations because large forces act on the sail area and submerged hull of the towed vessels, such as construction barges when carrying oversized modules or fully laden containerships. Systematic steady and unsteady numerical simulations were performed using a RANS-based field method to predict wind and current forces and moments acting on representative construction barges, containerships, tankers, and passenger ships. Aero- and hydrodynamic computations were carried out separately. Aerodynamic computations considered various deck load configurations to represent realistic loading conditions; hydrodynamic computations accounted for finite water depth. Agreement with available wind tunnel experimental data was generally favorable. Our purpose was to provide a reference for wind and current loads on different barge and ship types. The objective was not to investigate flow details needed for, e.g., smoke propagation, helicopter landing, etc. This has been covered by other researchers. Copyright © 2019 ASME
    view abstract10.1115/OMAE2019-95716
  • Numerical and experimental investigation of added resistance of different ship types in short and long waves
    Sigmund, S. and el Moctar, O.
    Ocean Engineering 147 (2018)
    Added resistance in short and long regular head waves for four different ship types was systematically and extensively investigated using Reynold-averaged Navier Stokes solvers. Computations, which included ship motions and added resistance, were validated against scale model test measurements. Effects of ship speed, skin friction, wave steepness, ship type, as well as wave radiation and wave diffraction on added resistance were analyzed. © 2017 Elsevier Ltd
    view abstract10.1016/j.oceaneng.2017.10.010
  • Inland waterway ship test case for resistance and propulsion prediction in shallow water
    Mucha, P. and el Moctar, O. and Dettmann, T. and Tenzer, M.
    Ship Technology Research 64 (2017)
    A novel test case representative of modern inland waterway ships is established for provision of reference data for benchmarking of numerical methods. Hull geometry, main particulars and appendages for propulsion and manoeuvring are introduced. Results of resistance and propulsion model tests in shallow water condition are described and discussed. Design peculiarities, challenges for model experiments and computations are addressed. © 2017, © University of Duisburg-Essen and Federal Waterways Engineering and Research Institute 2017.
    view abstract10.1080/09377255.2017.1349723
  • Nonlinear computational methods for hydroelastic effects of ships in extreme seas
    el Moctar, O. and Ley, J. and Oberhagemann, J. and Schellin, T.
    Ocean Engineering 130 (2017)
    We present computational methods to assess slamming-induced hull whipping on sectional loads of ships in regular and irregular waves. The numerical methods solved the Reynolds-averaged Navier-Stokes (RANS) equations coupled with the nonlinear rigid body motion equations of the elastic ship hull. We numerically investigated three containerships in regular waves, in random irregular long crested waves, and in deterministic wave sequences. Comparisons to experimental measurements agreed favorably. We relied on different wave models, including second order Stokes waves and nonlinear wave fields obtained from the solution of nonlinear Schrodinger equations (NLS). Simulations in random irregular waves provided short-term ship response probability distributions under sea state conditions relevant for design loads. © 2016 Elsevier Ltd
    view abstract10.1016/j.oceaneng.2016.11.037
  • Numerical and experimental analysis of added resistance of ships in waves
    el Moctar, O. and Sigmund, S. and Ley, J. and Schellin, T.E.
    Journal of Offshore Mechanics and Arctic Engineering 139 (2017)
    Two Reynolds-Averaged Navier-Stokes (RANS) based field methods numerically predicted added resistance in regular head waves for a 14,000 TEU containership and a medium size cruise ship. Long and short waves of different frequencies were considered. Added resistance was decomposed into diffraction and radiation force components, whereby diffraction forces were obtained by restraining the ship in waves and radiation forces by prescribing the motions of the ship in calm water. In short waves, the diffraction part of total resistance was dominant as almost no ship motions were induced. In long waves, the sum of diffraction and radiation forces exceeded total resistance, i.e., the interaction of these two force components, which caused the reduction of total resistance, needed to be accounted for. Predictions were compared with model test measurements. Particular emphasis was placed on the following aspects: discretization errors, frictional resistance as part of total added resistance in waves, and diffraction and radiation components of added resistance in waves. Investigations comprised two steps, namely, a preliminary simulation to determine calm water resistance and a second simulation to compute total resistance in waves, always using the same grids. Added resistance was obtained by subtracting calm water resistance from total averaged wave resistance. When frictional resistance dominated over calm water resistance, which holds for nearly all conventional ships at moderate Froude numbers, high grid densities were required in the neighborhood surrounding the hull as well as prism cells on top of the model's surface. © 2017 by ASME.
    view abstract10.1115/1.4034205
  • Numerical and experimental investigation of propulsion in waves
    Sigmund, S. and el Moctar, O.
    Ocean Engineering 144 (2017)
    The effects of head waves on propulsion characteristics of a single and a twin screw ship were investigated based on a Reynolds-averaged Navier-Stokes (RANS) solver and physical tests. Initially, propeller open water characteristics were determined in a homogeneous inflow. Next, computations of the towed models without propeller(s) in calm water and in waves were performed to obtain calm water resistances and waves added resistances. Finally, forces acting on the self-propelled models in calm water and in regular head waves were computed. Using obtained results propulsion characteristics in calm water and in waves were determined. Computations were performed using a RANS based flow-solver coupled with the nonlinear six-degrees-of-freedom equations of motion. When needed, the sliding interface method was used, enabling rotation of the geometrically modeled propeller. All computations were performed on the same numerical grid to keep errors originating from different spatial and temporal discretizations as small as possible. Grid studies were conducted to evaluate discretization errors. Computational results were compared to experimental results obtained from physical model tests. It was shown that the RANS solver is capable of investigating the propulsion behavior of a ship in regular head waves. Fair agreement between numerical and experimental results was obtained. © 2017 Elsevier Ltd
    view abstract10.1016/j.oceaneng.2017.08.016
  • Numerical and experimental investigations of wave-induced second order hydrodynamic loads
    Lyu, W. and el Moctar, O.
    Ocean Engineering 131 (2017)
    We present computational methods to reliably predict second order forces and moments acting on ships in waves. Second order forces and moments are useful to assess a number of operational aspects, such as minimum power requirements, maneuvering capabilities, and towing forces. Our methods comprise a boundary element Rankine source method and an extended Reynolds-averaged Navier-Stokes (RANS) equations solver. The Rankine source method accounts for the ship's forward speed by considering the nonlinear stationary flow including squat; the RANS solver is coupled with the nonlinear rigid body equations of motion. Investigations dealt with the influence of ship speed, hull shape, and encounter wave angle on second order forces and moments. Furthermore, we calculated diffraction induced added resistances. Results of experimental model test measurements of a tanker, a large containership, and a cruise ship are presented and used to validate our numerical methods. © 2017 Elsevier Ltd
    view abstract10.1016/j.oceaneng.2016.11.047
  • Numerical and experimental investigations of ship maneuvers in waves
    El Moctar, O. and Sprenger, F. and Schellin, T.E. and Papanikolaou, A.
    Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE 2 (2016)
    Assuring a ship's maneuverability under diverse conditions is a fundamental requirement for safe and economic ship operations. Considering the introduction of the Energy Efficiency Design Index (EEDI) for new ships and the related decreasing installed power on ships, the necessity arose to more accurately predict the maneuverability of ships in severe seas, strong winds, and confined waters. To address these issues, extensive experimental and numerical investigations were performed within the European funded Project SHOPERA. Here, second order forces and moments for a containership and a tanker were measured in model tests and computed by solving the Reynolds-Averaged Navier-Stokes (RANS) equations. Generally, these measured and computed second order loads (drift forces and yaw moments, added resistance) compared favorably. Furthermore, the effects of waves on zig-zag and turning circle maneuvers were investigated. Copyright © 2016 by ASME.
    view abstract10.1115/OMAE2016-54847
  • Numerical prediction of the propulsion characteristics of ships in waves
    Sigmund, S. and El Moctar, O.
    Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE 7 (2016)
    The influence of regular head waves on the propulsion characteristics of a twin screw cruise ship is investigated using RANS based flow-solver. Propeller open water characteristics are determined at first by computing the propeller forces in homogeneous inflow. Then, computations of the towed model without propeller and of the self-propelled model in calm water are performed to obtain the propulsion characteristics in calm water. Afterwards, the total resistance as well as the forces of the self-propelled model in regular head waves are computed. All computations are performed using a RANS based flow-solver coupled with the six-degrees-of-freedom equations of motion. The sliding interface method is used to allow the rotation of the geometrically modelled propeller, when needed. Computations are performed using the same numerical grids to keep errors originating on different spatial and temporal discretization as small as possible. Grid studies are performed to evaluate discretization errors of each mesh region, namely the hull region and the rotating propeller region, separately. The numerical results are compared with experimental results obtained from physical tests. It is shown, that RANS is capable of investigating the propulsion behavior of a ship in regular head waves, but to the cost of high computational effort. Fair agreement between numerical and experimental results is obtained. All results show that the propulsion characteristic change in waves. It is also found, that this is mainly caused by the change of the propeller efficiency due to a different propulsion point as a consequence of the added resistance in waves. Copyright © 2016 by ASME.
    view abstract10.1115/OMAE2016-54793
  • Prediction of non-linear ship responses in waves considering forward speed effects
    Riesner, M. and von Graefe, A. and Shigunov, V. and el Moctar, O.
    Ship Technology Research 63 (2016)
    This article describes the development of a non-linear time-domain boundary element method to determine non-linear ship responses (motions and loads) in waves. The general approach by Cummins was used to express the equation of motion in the time domain. Hydrodynamic forces were split into inertia, radiation, diffraction, Froude–Krylov and restoring components. Radiation forces were determined in time domain by convolution of the impulse response of the ship with the motion velocity. The Froude–Krylov and restoring forces were computed over the instantaneous wetted surface, taking into account ship motions, the undisturbed wave and stationary wave system. Diffraction forces were computed from the complex force amplitude resulting from the linear problem of the incident wave diffraction. The impulse response of the ship can be determined using the linear hydrodynamic damping coefficients or added masses. In this work, these two approaches were compared. Ship motions calculated with the developed method were compared with model test and RANSE-based simulations as well as with linear frequency domain results for three different ship types at various forward speeds. It was shown that the convolution integral represents the radiation forces satisfactorily. The convolution integral based on damping coefficients showed distinct advantages at zero speed. With increasing forward speed, the added mass-based convolution integral leaded in some cases to better results. © 2016, © University of Duisburg-Essen 2016.
    view abstract10.1080/09377255.2016.1221607
  • A mathematical model for roll damping prediction
    Piehl, H. and El Moctar, O.
    Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE 2 (2015)
    A ship in seaway is always prone to roll motion. For the safety of personnel, ship and cargo it is essential to optimize the roll damping properties of the hull shape in order to prevent exceeding roll angles. Therefore, a tool for the prediction of roll damping is an important requirement during the design phase of ship hulls. The objective of this study is to use regression analysis and numerical simulation of roll motion to develop an analytic expression for the determination of roll damping. The development procedure starts with a variation of several hull shape parameter that influence the roll damping. For each of the parameter variants, a numerical roll simulation is conducted and the according roll damping coefficients are determined by time series analysis. Finally, regression analysis is applied to the computed results in order to derive a mathematical model that allows to determine the roll damping coefficient depending on the hull shape parameter. © 2015 by ASME.
    view abstract10.1115/OMAE2015-41642
  • Calculation of horizontal sectional loads and torsional moment
    Shigunov, V. and von Graefe, A. and el Moctar, O.
    Journal of Offshore Mechanics and Arctic Engineering 137 (2015)
    Horizontal sectional loads (horizontal shear force and horizontal bending moment) and torsional moment are more difficult to predict with potential flow methods than vertical loads, especially in stern-quartering waves. Accurate computation of torsional moment is especially important for large modern container ships. The three-dimensional (3D) seakeeping code GL Rankine has been applied previously to the computation of vertical loads in head, following and oblique waves; this paper addresses horizontal loads and torsional moment in oblique waves at various forward speeds for a modern container ship. The results obtained with the Rankine source-patch method are compared with the computations using zero-speed free-surface Green functions and with model experiments. © 2015 by ASME.
    view abstract10.1115/1.4029483
  • Experimental investigation of impact loads during water entry
    Tenzer, M. and El Moctar, O. and Schellin, T.E.
    Ship Technology Research 62 (2015)
    The objective of the present investigation was to provide reliable experimental data suitable to validate numerical tools aimed at predicting impact loads on and elastic deformations of wedge-shaped structures. To investigate impact-induced hydroelastic effects on slamming pressure peaks, four test bodies were examined. Two bodies were fitted with stiffened, rigid bottom plating and two bodies with thin elastic bottom plating, each case with 5° and 10° deadrise angles. Results comprised impact-induced pressures, accelerations, forces, and structural strains. Measurement repeatability, sampling rate effects, and hydroelastic effects were emphasised. Measured pressures and forces were compared with published experimental data. Additionally, this paper documents body geometries, test rig set-ups including instrumentation, and experimental procedures. © University of Dulsburg-Essen 2015.
    view abstract10.1179/0937725515Z.0000000003
  • Numerical and experimental analysis of added resistance of ships in waves
    El Moctar, O. and Sigmund, S. and Schellin, T.E.
    Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE 11 (2015)
    A RANS-based field method numerically predicted added resistance in regular head waves for a 14000 TEU containership (Duisburg Test Case) and a medium-size cruise ship. We concentrated our investigations on short waves. For different frequencies, we decomposed added resistance into diffraction and radiation force components, whereby diffraction forces were obtained by restraining the ship in waves and radiation forces, by prescribing the motions of the ship in calm water. In short waves, the diffraction part of total resistance was dominant as almost no ship motions were induced. In long waves, the sum of diffraction and radiation forces exceeded total resistance, i.e., the interaction of these two force components, which caused the reduction of total resistance, had to be accounted for. Predictions were compared with model test measurements. Particular emphasis was placed on the following aspects: discretization errors, frictional resistance as part of total added resistance in waves, diffraction and radiation components of added resistance in waves, and the influence of surge motion on added resistance. Investigations comprised two steps, namely, a preliminary simulation to determine calmwater resistance and a second simulation to compute total resistance in waves, always using the same grids. Added resistance was obtained by subtracting calm-water resistance from total averaged wave resistance. When frictional resistance dominated calm-water resistance, which holds for nearly all conventional ships at moderate Froude numbers, high grid densities were required in the neighborhood surrounding the hull. © 2015 by ASME.
    view abstract10.1115/OMAE201542403
  • Numerical modelling and prediction of cavitation erosion
    Peters, A. and Sagar, H. and Lantermann, U. and el Moctar, O.
    Wear 338-339 (2015)
    This paper addresses the prediction of cavitation erosion using a numerical flow solver together with a new erosion model. Numerical flow simulations were conducted with an implicit, pressure-based Euler-Euler multiphase flow solver in combination with the developed erosion model. The erosion model refers to the microjet hypothesis and uses information from the flow solution to assess the occurrence of microjets in specific areas. The ability of the numerical code to simulate cavitating flows was shown by comparison with experimental tests of sheet cavitation over a NACA 0009 hydrofoil. The numerical prediction of cavitation erosion was compared to measured erosion in experimental tests of an axisymmetric nozzle and shows good agreement regarding the erosive areas in general and the areas of highest erosion. Aim of this work is the assessment of erosion sensitive areas, as well as the erosion potential of cavitational flow during the incubation period. © 2015.
    view abstract10.1016/j.wear.2015.06.009
  • Numerical Modelling and Prediction of Erosion Induced by Hydrodynamic Cavitation
    Peters, A. and Lantermann, U. and El Moctar, O.
    Journal of Physics: Conference Series 656 (2015)
    The present work aims to predict cavitation erosion using a numerical flow solver together with a new developed erosion model. The erosion model is based on the hypothesis that collapses of single cavitation bubbles near solid boundaries form high velocity microjets, which cause sonic impacts with high pressure amplitudes damaging the surface. The erosion model uses information from a numerical Euler-Euler flow simulation to predict erosion sensitive areas and assess the erosion aggressiveness of the flow. The obtained numerical results were compared to experimental results from tests of an axisymmetric nozzle.
    view abstract10.1088/1742-6596/656/1/012054
  • Rankine source method for ship-ship interaction problems
    Graefe, A. and Shigunov, V. and el Moctar, O.
    Journal of Offshore Mechanics and Arctic Engineering 137 (2015)
    A Rankine source method is extended and applied to ship-ship interaction problems. The method covers the nonlinear steady flow and linear seakeeping in the frequency domain. The nonlinear steady flow solution accounts for the nonlinear free-surface conditions, ship wave, and dynamic trim and sinkage. Periodic flow due to waves is linearized with respect to the wave amplitude, taking into account interactions with the nonlinear steady flow following Hachmann approach, which considers the steady perturbation potential as constant in the body-fixed reference frame. This is advantageous for the prediction of ship motions at moderate to high Froude numbers. In this context, a new formulation of the boundary condition for the multibody case is derived. Two examples are considered, overtaking in calm water and replenishment at sea. For a feeder vessel overtaken by a container ship, horizontal forces and yaw moment are computed and compared with reference data. As an example of replenishment operation, interaction between a frigate and a supply vessel is studied. Ship motions are computed for two relative positions and three forward speeds and compared with model test data for the largest forward speed. The Rankine source method proves as more accurate compared with a zero-speed freesurface Green function method. © 2015 by ASME.
    view abstract10.1115/1.4029316
  • Revisiting mathematical models for manoeuvring prediction based on modified Taylor-series expansions
    Mucha, P. and El Moctar, O.
    Ship Technology Research 62 (2015)
    This paper deals with the investigation into mathematical modelling of ship manoeuvring motions based on modified Taylor-series expansions. In light of a renewed interest in ship manoeuvring prediction methods, we revisited fundamentals and developments in mathematical modelling of hydro-dynamic forces in the manoeuvring equations of motion. In our summary, we embark from the primal formulation of non-linear multivariate polynomial models, followed by a review of critical reception and consequent modifications in the hydrodynamic community, up to application in modern manoeuvring prediction methods using computational fluid dynamics (CFD) Our actual analysis relied on a published mathematical model. Our goal was to assess the significance of single terms in predicting standard rudder manoeuvres, by way of resembling and extending systematic sensitivity studies. © University of Duisburg-Essen 2015.
    view abstract10.1179/0937725515Z.00000000015
  • A rankine panel method for added resistance of ships in waves
    Söding, H. and Shigunov, V. and Schellin, T.E. and El Moctar, O.
    Journal of Offshore Mechanics and Arctic Engineering 136 (2014)
    A new Rankine panel method and an extended Reynolds-Averaged Navier-Stokes (RANS) solver were employed to predict added resistance in head waves at different Froude numbers of a Wigley hull, a large tanker, and a modern containership. The frequency domain panel method, using Rankine sources as basic flow potentials, accounts for the interaction of the linear periodic wave-induced flow with the nonlinear steady flow caused by the ship's forward speed in calm water, including nonlinear free surface conditions and dynamic squat. Added resistance in waves is obtained by the pressure integration method. The time domain RANS solver, based on a finite volume method, is extended to solve the nonlinear equations of the rigid body six-degrees-of-freedom ship motions. The favorable comparison of the panel and RANS predictions demonstrated that the Rankine method is suitable to efficiently obtain reliable predictions of added resistance of ships in waves. Comparable model test predictions correlated less favorably, although the overall agreement was felt to be acceptable, considering the difficulties associated with the procedures to obtain accurate measurements. © 2014 by ASME.
    view abstract10.1115/1.4026847
  • An enhanced 1-way coupling method to predict elastic global hull girder loads
    Ley, J. and El Moctar, O.
    Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE 4B (2014)
    This paper introduces a numerical method to predict global hull girder loads of sea-going vessels, taking into account the structural elasticity. A field method based on a Finite Volume discretisation is applied to simulate the nonlinear rigid ship motions and provides the external loads at the hull surface. The structural response is computed in a full transient 3D-Finite-Element Analysis. The lowest global structural mode shapes and eigenfrequen-cies are covered by the 3D-FE model. The mapping between the Finite Volume mesh and Finite Element grid, is performed by the Mesh-Based Code Coupling Interface (MpCCI). As long as only global vertical bending modes are considered, simplified beam models may sufficiently cover the structural response. However, the use of the 3D-FE model is motivated by the prediction of the global torsional and local loads that are influenced by hydroe-lastic effects. A 1-way coupling method is applied. To account for hydromass effects, the Finite-Element model is enhanced by acoustic elements. Acoustic wave equations are solved to simulate the sound wave propagation in water and to obtain realistic eigenfrequencies of the wetted hull. Structural and hydrody-namic damping is controlled by the Rayleigh-Damping method. Simulations are performed for an ultra large container vessel sailing in regular head waves. The computed time histories of the vertical bending moment are compared with experimental data and with numerical simulations using a strong 2-way coupling simulation that employs a Finite-Element Timoshenko-Beam. Copyright © 2014 by ASME.
    view abstract10.1115/OMAE2014-24199
  • Bilge keel forces and vortex shedding - A numerical analysis with OpenFOAM
    Piehl, H. and El Moctar, O.
    Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE 2 (2014)
    The damping effectiveness of a bilge keel depends on the velocity of the flow and the roll period of the ship. The Morison equation defines the relation between these motion parameter and the forces acting on the bilge keel. Sarpkaya and O'Keefe published in 1996 an article with their results of several flow experiments conducted over a wide range of Keulegan-Carpenter period parameter. They provide the KC dependent drag and inertia coefficients obtained from the measured forces on a bilge keel and discuss the vortex pattern generated by the oscillating bilge keel. In the present study the CFD code OpenFOAM was used to compute the flow around a bilge keel and the related forces. The numerical and experimental results are compared. Based on the test conditions a finite volume mesh was generated and the flow was computed with a transient, turbulent flow solver. In order to investigate the boundary layer on an oscillating wall and the dissipation of the vortex structures different turbulence models (kωSST, low Re and LES) were used. The experiments were conducted at zero forward speed. However, the computations were performed for different ship speeds. Copyright © 2014 by ASME.
    view abstract10.1115/OMAE2014-24079
  • Calculation of horizontal sectional loads and torsional moment
    Shigunov, V. and Von Graefe, A. and El Moctar, O.
    Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE 7 (2014)
    Horizontal sectional loads (horizontal shear force force and horizontal bending moment) and torsional moment are more difficult to predict with potential flow methods than vertical loads, especially in stern-quartering waves. Accurate computation of torsional moment is especially important for large modern container ships. The 3D seakeeping code GL Rankine has been applied previously to the computation of vertical loads in head, following and oblique waves; this paper addresses horizontal loads and torsional moment in oblique waves at various forward speeds for a modern container ship. The results obtained with the Rankine source-patch method are compared with the computations using zero-speed wave Green functions and model experiments. Copyright © 2014 by ASME.
    view abstract10.1115/OMAE2014-23487
  • 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 abstract10.1115/1.4026885
  • Numerical prediction of the added resistance of ships in waves
    Ley, J. and Sigmund, S. and El Moctar, O.
    Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE 2 (2014)
    The added resistance in waves is computed for different ship types using two different Reynolds-Averaged Navier-Stokes Equations (RANSE) solvers, namely Comet and inter-Foam (OpenFOAM). Hence, the RANS equations are implicitly coupled with the non-linear equations of motions for six degrees of freedom and the solvers are extended by algorithms for mesh morphing to account for ship motions. The computational effort for these simulations is high compared to potential flow based simulations, especially for short waves. However, to understand the physics related to added resistance of ships and to investigate influencing parameters, field methods based on RANS equations may be suitable. The prediction of the added resistance in waves consists of two steps; the computations of the calm water resistance and the total resistance in waves. The discretisation errors as well as the influence of the surge motions on the added resistance are investigated. Further, the added resistance is decomposed in diffraction and radiation problems as it is commonly done in potential theory. Copyright © 2014 by ASME.
    view abstract10.1115/OMAE2014-24216
  • First order reliability analogies of nonlinear bending moments in ships
    Oberhagemann, J. and Shigunov, V. and El Moctar, O.
    Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE 9 (2013)
    Hydroelasticity codes based on the solution of the Navier-Stokes equations are beneficial for extreme value predictions of hull girder loads. Since direct long-term analyses using these numerical methods are prohibitive due to excessively large required computation times, strategies are sought to reduce the computational effort. An extrapolation approach allows reducing the required simulation duration significantly. Application to hogging bending moments of flexible containerships agrees with Monte-Carlo simulations in random sea state realisations. Copyright © 2013 by ASME.
    view abstract10.1115/OMAE2013-10547
  • Rankine source method for ship-ship interaction problems
    Von Graefe, A. and Shigunov, V. and El Moctar, O.
    Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE 9 (2013)
    A Rankine source method is extended and applied to ship-ship interaction problems. The method covers the nonlinear steady flow and linear seakeeping in the frequency domain. The nonlinear steady flow solution accounts for the nonlinear free-surface conditions, ship wave and dynamic trim and sinkage. Periodic flow due to waves is linearized with respect to the wave amplitude, taking into account interactions with the nonlinear steady flow following Hachmann approach, which considers the steady perturbation potential as constant in the body-fixed reference frame. This is advantageous for the prediction of ship motions at moderate to high Froude numbers. In this context, a new formulation of the boundary condition for the multi-body case is derived. Two examples are considered, overtaking in calm water and replenishment at sea. For a feeder vessel overtaken by a container ship, computed forces and yaw moment are compared with model test data. As an example of replenishment operation, interaction between a frigate and a supply vessel is studied. Ship motions are computed for two relative positions and three forward speeds and compared with model test data for the largest forward speed. The Rankine source method proves as more accurate compared with a zero-speed free-surface Green function method. Copyright © 2013 by ASME.
    view abstract10.1115/OMAE2013-10810
  • Roll decay model test of a post panamax container ship - Experimental and numerical analysis
    Piehl, H. and El Moctar, O.
    Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE 7 (2013)
    The slender shape of a modern ship hull, exhibits a roll motion that is - in contrast to heave and pitch motion - submitted to large amplitudes and weak damping. This effect explains the importance of determining the roll damping of a hull and denotes the difficulty to measure such a small dynamic property. For the joint research project Best-Roll a series of roll decay tests were conducted with the model of a post panamax container ship. To capture the influence of the draft and the ship velocity, the roll motion was measured for a systematic variation of these parameter. In addition the tests were performed with an initial roll angle of 20 degrees, in order to investigate the nonlinear damping behavior at large roll angles. The objective of the subsequent study was to test the capability of OpenFOAM to compute the overall roll damping of a ship hull and appendages. For the numerical comparison, the roll decay test was simulated with OpenFOAM, using a transient multiphase solver with a RANS turbulence model, dynamicmesh motion and a rigid body motion model of the ship. The results were compared by calculating the roll damping coefficient for both experiment and numerical simulation by means of time series analysis of the roll angle. Copyright © 2013 by ASME.
    view abstract10.1115/OMAE2013-11065
  • Ship-bank interaction of a large tanker and related control problems
    Mucha, P. and El Moctar, O.
    Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE 5 (2013)
    The objective of this work is to establish a synthesis between modern methodology in the field of ship maneuvering and control theory using the example of hydrodynamic ship-bank interactions for a large tanker. Evolving technologies have paved the way for developing increasingly sophisticated modeling techniques to study ship flows. These tecnologies have made it possible to resemble Planar Motion Mechanism (PMM) tests in numerical simulations using Reynolds-averaged Navier-Stokes (RANS) equations. These advances give way for the numerical determination of hydrodynamic derivatives as present in the maneuvering equations. This methodology is adopted in the present investigation to obtain these coefficients for various separation distances to a vertical wall. Likewise, control theory has experienced vital progress enabling engineers to apply elaborate control policies in their systems. Special attention has been payed to the distinct discipline of optimal control theory and the family of Linear Quadratic (LQ) regulators. Among the popular class of conventional Proportional-Integral-Derivative (PID) controllers rather heuristic design procedures are applied; appealing to the practitioners but might not be suitable for special applications. The work presented investigates the suitablity of deriving hydrodnamic properties by means of Virtual Planar Motion Mechanism (VPMM) tests for the KVLCC2 tanker travelling at various distances to a vertical wall of infinite depth. In subsequent maneuvering simulations the performance of the introduced controllers is discussed. Copyright © 2013 by ASME.
    view abstract10.1115/OMAE2013-11099
  • A Rankine panel method for added resistance of ships in waves
    Söding, H. and Shigunov, V. and Schellin, T.E. and El Moctar, O.
    Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE 4 (2012)
    A new Rankine panel method and an extended RANS solver were employed to predict added resistance in head waves at different Froude numbers of a Wigley hull, a large tanker, and a modern containership. The frequency domain panel method, using Rankine sources as basic flow potentials, accounts for the interaction of the linear periodic wave-induced flow with the nonlinear steady flow caused by the ship's forward speed in calm water, including nonlinear free surface conditions and dynamic squat. Added resistance in waves is obtained by pressure integration method. The time domain RANS solver, based on a finite volume method, is extended to solve the nonlinear equations of the rigid body six-degrees-of-freedom ship motions. The favorable comparison of panel and RANS predictions demonstrated that the Rankine method is suitable to efficiently obtain reliable predictions of added resistance of ships in waves. Comparable model test predictions correlated less favorably although overall agreement was felt to be acceptable, considering the difficulties associated with procedures to obtain accurate measurements. Copyright © 2012 by ASME.
    view abstract10.1115/OMAE2012-83457
  • Prediction of ship response statistics in severe sea conditions using RANS
    Oberhagemann, J. and Ley, J. and El Moctar, B.O.
    Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE 2 (2012)
    The International Association of Classification Societies (IACS) promotes the paradigm shift in structural design rules for ships towards risk based approaches. This requires improvements in the assessment of structural design loads and new methods for estimation of wave loads and responses, amongst others with respect to extreme value distributions. In this paper we present a numerical method based on the solution of RANS equations to deal with large wave-induced ship motions and corresponding loads for different ship types. Nonlinearities of wave excitation and ship response are included. Short-term ship response distributions from time domain simulations are compared with model test data. Significant deviations from Rayleigh distribution of amplitudes are observed, especially for hull girder loads including effects of structural elasticity. Copyright © 2012 by ASME.
    view abstract10.1115/OMAE2012-83995
  • 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 (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 abstract10.1115/OMAE2012-83450
  • 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 (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 abstract10.1115/OMAE2012-84013
  • Assessing the dynamic stability of an offshore supply vessel
    Shigunov, V. and El Moctar, O. and Schellin, T.E. and Kaufmann, J. and Stute, R.
    Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE 1 (2011)
    The dynamic stability was investigated of a typical offshore service vessel operating under stability critical operating conditions. Excessive roll motions and relative motions at the stern were studied for two loading conditions for ship speeds ranging from zero to the design speed. A linear frequencydomain seakeeping analysis was followed by nonlinear timedomain simulations of ship motions in waves. Based on results from these methods, critical scenarios were selected and simulated using finite-volume solvers of the Reynolds-averaged Navier-Stokes equations to understand the phenomena related to dynamically unstable ship motions as well as to confirm the results of the simpler analysis methods. Results revealed the possibility of excessive roll motions and water run-up on deck; counter measures such as a ship-specific operational guidance are discussed. Copyright © 2011 by ASME.
    view abstract10.1115/OMAE2011-49043
  • Wave-in-deck load analysis for a jack-up platform
    Schellin, T.E. and Perić, M. and el Moctar, O.
    Journal of Offshore Mechanics and Arctic Engineering 133 (2011)
    This paper describes the prediction of environmental loads on a typical three-leg jack-up platform under freak wave conditions. Considered were cases where the air gap is small and the hull is subject to impact-related wave-in-deck loads. The technique to predict wave loads was based on the use of a validated CFD code that solves the Reynolds-averaged Navier-Stokes equations. This code relies on the interface-capturing technique of the volume-of-fluid type to account for highly nonlinear wave effects. It computes the two-phase flow of water and air to describe the physics associated with complex free-surface shapes with breaking waves and air trapping, hydrodynamic phenomena that had to be considered to yield reliable predictions. The Stokes fifth-order wave theory initialized volume fractions of water, velocity distributions in the solution domain, and time-dependent boundary conditions at inlet and outlet boundaries. This paper demonstrates that this technique can be a valuable numerical tool for preliminary designs as well as subsequent safety assessments. In particular, it shows that effects of different operating and design parameters on wave-in-deck loads, such as wave direction, wave height, wave period, and wind speed, can be evaluated with an affordable computing effort. © 2011 American Society of Mechanical Engineers.
    view abstract10.1115/1.4002047
  • finite volume method

  • fluid dynamics

  • fluid-structure interaction

  • hydrodynamics

  • modelling and simulation

  • multi-phase fluids

  • numerical methods

  • offshore technology

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