Validation of CFD for the Determination of Damping Coefficients for the Use of Wave Energy Converters Modelling

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.Copyright © 2013 by ASME