A hybrid 2D-3D CFD model is developed for studying water wave loads on a slender pile in a pile group. In the hybrid model approach, a one-way link is established between a model for the far-field and another for studying the fluid-structure interaction in the near field. In the far-field a 2D incompressible Navier-Stokes multiphase solver is considered for the proper reproduction of phase-focused (freak) waves produced in physical experiments. The near-field model is a multiphase 3D CFD model that utilizes compressible Navier-Stokes equations to enhance the simulation of entrapped air compressibility effects during breaking wave impact on structures. Both models use the Volume-Of-Fluid (VOF) method to capture the air-water interface and alternatively a RANS or LES turbulence model. An overlap zone is introduced to both models, in which fluid kinematics and surface elevation are sampled from the far-field model and introduced via a relaxation function to the overlap zone in the near-field model. In the 3D model, the use of a relaxation approach provides absorption for reflected waves from the structure. Further, a procedure is outlined to achieve/enhance the 3D model convergence. This is necessary in case of the development of artificial high velocities at water-air interface at the end of a short overlap (relaxation) zone for wave inlet (or near the boundary if only a wave inlet boundary condition is considered). The model system is developed using the OpenFOAM® framework. The overlap zone is implemented as an extension to the waves2Foam [1] toolbox.

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