Subfilter Wave Drag Modeling for LES of Offshore Wind Farms

Achieving an accurate understanding and representation of the momentum transfer between wind and ocean waves without having to resolve the ocean surface is a key component of constructing computationally efficient simulation tools for full-scale offshore wind farms. Large Eddy Simulations are conducted with a Dynamic Wave Spectrum Drag Model (Dyn-WaSp) that represents the momentum transfer from a full spectrum of ocean waves using a phase-aware wall model, leading to spatially and temporally varying drag (more accurate than static roughness model) without requiring the wave surface to be resolved. In this study, the subfilter wave drag model of the Dyn-WaSp model is investigated; variation in the slip velocity between the wind and ocean waves causes the dynamic coefficient to become dependent on filter size (scale variant). A new formulation of the dynamic subfilter roughness model is developed that accounts for slip at the ocean surface by introducing a normalized velocity difference between the surface wind and a characteristic subfilter wave speed. The performance of this model is compared to a no slip roughness model in a variety of wind and wave conditions.