Development of New Free Surface Methods for a Fixed‐Grid Fully Nonlinear Potential Flow Solver Using the Immersed Boundary Method

ABSTRACT Fully nonlinear wave‐structure interaction in real sea states is complex to simulate, though important for load estimations. While Navier–Stokes equation‐based CFD solvers have proven to be capable of modelling these physics, their computational costs are often a limiting factor for their applicability. An alternative is given by FDM‐based fully nonlinear potential flow solvers, which have been under constant development in the recent years and have proven to be capable of simulating irregular nonlinear real sea states at a fraction of the computational costs needed by CFD. Most of the current fully nonlinear potential flow solvers are utilising a ‐grid domain to represent the unsteady free surface boundary, which significantly hinders the representation of structures inside the domain. In the presented work, a fixed‐grid potential flow solver utilising parallel computation on a multi‐core infrastructure is developed to allow for the implementation of structures directly in the domain. Therefore, new methods for enforcing the free surface boundary conditions inside the fixed‐grid domain are derived. Different approaches are introduced and tested to determine the most accurate one. The presented work thereby puts emphasis on building a fixed‐grid solver that can model complex wave transformations, as well as irregular real sea states in time domain with an accuracy on par with a ‐grid solver, while not limiting its flexibility with a ‐grid. The presented solver's accuracy is tested for various two‐ and three‐dimensional nonlinear wave propagation and transformation cases by comparing to nonlinear wave theory and experimental results. The results show that the solver is capable of modelling real sea states and nonlinear wave transformation and that it is of comparable accuracy to fully nonlinear potential flow solvers utilising a ‐grid.