15:30
Computational Fluid Dynamics III
Chair: Pierre Ferrant
15:30
30 mins
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Unsteady Aerodynamic Simulations of Floating Offshore Wind Turbines with Coupled Periodic Surge and Pitch motions
Ping Cheng, Yong Ai, Decheng Wan
Abstract: Compared with bottom-fixed wind turbines, a floating offshore wind turbine (FOWT) has an extra level of complexity on aerodynamic performance because of the motions of the supporting platform. In this paper, the unsteady aerodynamic performance of the NREL-5MW Baseline wind turbine with coupled periodic surge and pitch motions of its supporting platform are investigated. The three-dimensional Reynolds Averaged Navier-Stokes equations are solved for the aerodynamic numerical simulation. The naoe-FOAM-os-SJTU solver, which is based on OpenFOAM and overset grid technique, is employed. From the simulation, the time series of the unsteady torque and thrust are obtained, together with the detailed information of the wake flow field to clarity the detailed flow filed information. The simulation results are compared both with those obtained from aerodynamic simulation of wind turbine with effects of platform’s periodic surge motion, and with other approaches with different numerical methods.
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16:00
30 mins
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CFD SIMULATION OF WAVE IMPACT ON A SEMI-SUBMERSIBLE: A NUMERICAL CASE STUDY
Peter van der Plas, Arthur Veldman
Abstract: In offshore applications, extreme events of wave impact on rigid and floating structures are of high interest. In the past the CFD simulation tool ComFLOW [2] has been successfully used for these purposes. For accurate prediction of wave run-up and wave loading on offshore structures high resolution is required in the areas of interest. In order to reduce the number of grid points absorbing boundary conditions are used to limit the computational domain. The computational time is further reduced by application of local grid refinement and parallelization.
This case study concerns the application of CFD to the simulation of wave impact on a semi-submersible [1,3]. Various regular waves are considered, of which the properties are only known within a given uncertainty range. Small variations in the incoming wave can cause large differences in the measured impact forces on the structure. This presents a complicating factor in interpreting the CFD results. In a similar fashion, small changes in the simulation setup can cause seemingly disproportionate differences in the solution if interpreted in a deterministic sense.
The variation observed in the experimental measurement poses several challenges. Firstly, the experiment has to be reconstructed in the numerical simulation. A representative wave is constructed by means of averaging of the experiment data [1]. Secondly, given the variation of the flow conditions around the semi-submersible in the test basin, a deterministic interpretation of the numerical results is of limited value. In the current study the deterministic comparison is complemented by a statistical approach to assess the average and variation of the impact forces. The statistical approach is also applied to investigate grid convergence behaviour.
[1] B. Iwanowski, M. Lefranc and R. Wemmenhove, “CFD Simulation of Wave Run-up on a Semi-Submersible and Comparison with Experiment”, ASME 2009 28th Int. Conf. on Ocean, Offshore and Arctic Eng. (2009).
[2] A.E.P. Veldman, R. Luppes et al., “Turbulence Modelling, Local Grid Refinement and Absorbing Boundary Conditions for Free-Surface Flow Simulations in Offshore Applications”, ASME 2014 33rd Int. Conf. on Ocean, Offshore and Arctic Eng. (2014).
[3] R. Wemmenhove, R. Luppes et al., “Numerical simulation of hydrodynamic wave loading by a compressible two-phase flow method”, Computers & Fluids, Vol. 114, pp. 218-231, (2015)
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16:30
30 mins
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NUMERICAL SIMULATIONS OF HYDRODYNAMIC TESTS FOR UNDERWATER VEHICLES
Sertaç ARSLAN, Emrah GÜLAY, Mete INEKÇI
Abstract: Hydrodynamic coefficients must be obtained to examine controllability and maneuverability characteristics of an Autonomous Underwater Vehicle (AUV). Experimental, numerical and empirical methods are different ways to predict hydrodynamic coefficients. Various captive tests, such as; straight-line towing, Rotating Arm (RA), Planar Motion Mechanism (PMM), and Coning Motion Mechanism (CMM) tests can be conducted to obtain hydrodynamic coefficients or maneuvering derivatives that are necessary for system simulation of AUVs. Although the most reliable way to determine hydrodynamic coefficients of an AUV is conducting hydrodynamic tests, thanks to developments in computer technology it is possible to solve flow problems by using Computational Fluid Dynamics (CFD) methods.
In this study, CFD modelling techniques are developed to simulate straight-line towing tests, RA tests and PMM tests. These test scenarios are modeled by using CFD methods. FLUENT commercial flow solver is used to solve these flow problems. Firstly, Defense Advanced Research Projects Agency (DARPA) and UK Natural Environment Research Council’s Autosub test-case models are used for straight-line towing test simulations. Then RA and PMM tests are simulated for only Autosub test-case model. Rotating arm tests are simulated for constant angular velocity and constant angular acceleration. Furthermore, pure heave motion is simulated in the scope of PMM studies. FLUENT user defined functions are used for the analyses that mesh motions are needed. Analyses results are compared with experimental data which is available in literature. Finally, hydrodynamic database of Autosub model, which is produced by CFD methods, used for stability and performance analyses to determine maneuverability characteristics of Autosub model.
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