Diffraction-radiation codes enable to model the behaviour of Wave Energy Converters (WEC) and seakeeping of ships on many sea-states with very little computational time. However, the viscous effects are neglected and therefore the simulations lead to relatively inaccurate values. The inaccuracy mainly occurs at the resonance frequency, especially in roll motions for which viscous effects are of major importance. Classically, the viscous effects are represented by adding viscous damping coefficients obtained either from experimental data or analytical approaches based on numerous approximations. In order to improve the accuracy of the diffraction-radiation solvers, the damping coefficients can also be calculated from Computational Fluid Dynamics (CFD) simulations. The first part of this paper presents the three CFD solvers and turbulence models used in this validation study: ICARE and ISIS-CFD are developed by Ecole Centrale de Nantes and Star-CCM+ is a general purpose solver developed by CD-adapco. For each case, a preferred solver is chosen and a second solver is used for verification in most cases. The second part briefly presents the theory that obtains drag coefficients in oscillatory flows, which are closely related to damping coefficients in waves. Each of the three following parts introduces the experimental test cases to which numerical results are compared to. The numerical parameter convergence study leads to a choice of around 200 timesteps per period with an adapted mesh enabling to obtain drag coefficients with errors lower than 5%. A mesh convergence study in the wake area leads to a mesh refinement of around 2 to 2.5 % of the body characteristic length. In order to reduce the computational time, the total number of cells can be decreased by mainly refining locations where specific flow detachment occurs, such as body corners or sharp edges. Turbulence models are also varied. Validation results are finally presented in terms of single or coupled damping coefficients and added mass coefficients. They are presented for various non-dimensional numbers such as Keulegan-Carpenters and Reynolds number.
This paper compares numerical and experimental results in the study of the resonance phenomenon which appears between two side-by-side fixed barges for different sea-states. Simulations were performed using SWENSE (Spectral Wave Explicit Navier-Stokes Equations) approach and results are compared with experimental data on two fixed barges with different headings and bilges. Numerical results, obtained using the SWENSE approach, are able to predict both the frequency and the magnitude of the RAO functions.
Computational Fluid Dynamics (CFD) tools have progressed greatly in the past decades in such a manner that they have become a recurring tool during the workflow of an engineering project. In the specific case of Marine and Offshore Engineering, these tools are more and more used to predict forces on structures with a very detailed level of precision, to optimize hull forms to conceive more efficient and environmentally friendly designs and also to reassure structural engineers with respect to the assumptions made during complex engineering problems. In this article several examples of how these tools and methodologies have been employed in the design process of Marine and Offshore structures will be presented. Validations compared to tank tests and wind tunnels will be presented for damping coefficients, green water impact, aerodynamic coefficients and non-linear wave loading studies. Throughout these examples it will be shown how the need of dedicated tools/methodologies is necessary in order to efficiently use the CFD tools during the Naval and Offshore design process. Very often, distinct phenomena need to be modeled in order to correctly apprehend the complexity of the flows. To do so, coupling distinct approaches is sometimes necessary: CFD solver to a potential one for instance. In the paragraphs below four examples are illustrated: nonlinear wave loading computations, damping coefficients computation, aerodynamics winf loads estimation on complex topside structures, and prediction of green water impact on a FPSO.
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