Simulation of wave-structure interaction by a two-way coupling between a fully nonlinear potential flow model and a Navier-Stokes solver

This thesis deals with the development and application of a two-way coupling procedure between a fully nonlinear potential flow model and a Navier-Stokes solver to study wave-structure interaction applied to offshore wind turbines. The coupling strategy relies on a domain decomposition method, in which the wave field close to the structure of interest is simulated with the Navier-Stokes solver Code Saturne, an open-source Finite Volume code capturing the free surface with a Volume of Fluid method. Away from the structure, where viscous and turbulent effects may be neglected, the potential code, solving the Laplace equation for the velocity potential with a boundary integral formulation, is applied to model the large scale wave field. Generation and absorption of waves in this three-dimensional hybrid numerical wave tank take place in the outer potential domain. The potential and Navier-Stokes codes exchange data in the region around their common boundaries. The coupling may thus be referred to as 'two-way', enabling one to propagate waves in and out of the viscous subdomain, and making the hybrid algorithm suitable to study wave diffraction on fixed offshore wind turbines, while keeping the viscous subdomain as small as possible. Each code uses its own mesh and time step. Subdomains are overlapping, therefore a velocity continuity condition and a free surface continuity condition have to be verified on two distinct coupling surfaces at any time. Parallel implementation of the coupling strategy with communications between the models relying on the Message Passing Interface (MPI) library allows calculations to be run on large spatial and temporal scales on distributed memory clusters. The coupling algorithm is tested for various incident wave conditions, including solitary waves and regular nonlinear waves. It is then applied to the simulation of wave loads exerted on a vertical monopile and numerical results are compared with experimental measurements performed in a wave flume considering various values of the period and steepness of incident waves. Attention is paid to the analysis of high-order components of the nonlinear horizontal force. (author)