A Virtual Free Surface (VFS) model for efficient wave–current CFD simulation of fully submerged structures

• A reduced model for efficient wave and wave–current CFD simulation is introduced.
• The method is verified with analytical and free surface reference results.
• A modeling strategy to incorporate wave–current interaction is described.
• The model is applied to simulate a tidal turbine and its support structure.
• A significant speedup at good agreement compared to free surface models is achieved.

In ocean engineering, complex design problems are frequently analyzed by applying computational fluid dynamics (CFD). When these problems involve waves and currents, a CFD simulation of a multiphase flow is one possible approach to gain valuable information. However, in terms of the computational costs, free surface flow simulations are quite restrictive. Neglecting viscous effects, significantly simpler and faster models are available, but may not provide sufficient information regarding the true nature of the flow. As an alternative, a single phase Virtual Free Surface (VFS) model is introduced in this paper. The methodology allows for efficient CFD simulations of fully submerged structures subjected to pure waves and combined wave–current scenarios, under consideration of wave–current interaction. The velocities and pressures computed with the model are analyzed and compared to theoretical reference solutions. Then, a correction model is introduced that reduces deviations observed in the pressure solution. The model is applied to the investigation of a tidal turbine and its support structure. All results computed with the VFS model are compared to reference free surface computations. Overall the VFS results agree well with the reference solutions, at significantly lower computational cost. Therefore, the method is ideal for optimization problems involving a large number of structural variations subjected to specific offshore conditions.