A moving surface drag model for LES of wind over waves with application to offshore wind turbines

Efficiently and accurately representing the complex interactions between wind and waves in large eddy simulations (LES) of the marine atmospheric boundary layer (MABL) is crucial for developing improved climate model parameterizations and for efficient design and operation of offshore wind farms. In this study, we propose a new model, the MOving Surface Drag (MOSD) model for LES, that aims to accurately capture the phase-resolved effects of waves on turbulent airflow, while maintaining computational efficiency. This approach builds upon the surface gradient-based wall model, augmenting it by computing the pressure drag resulting from ideal potential flow interacting with the forward-facing parts of the moving surface, which is approximated as piece-wise ramp flow. Horizontally unresolved waves are modeled with the standard equilibrium wall model. We demonstrate the utility of the model in studying wind-wave interactions by first demonstrating its success in capturing the impact of monochromatic waves on the turbulence statistics for flows over a range of wave conditions through comparisons with experiments, Direct Numerical Simulations (DNS) and wall-resolved LES. Results from application and comparisons with data from a laboratory wind-wave tunnel experiment with a single wind turbine are presented. The results suggest that the use of an LES with the MOSD wall-model provides a cost-effective means to simulate wind-wave interactions in the MABL and in off-shore wind energy applications.