Swell Impacts on an Offshore Wind Farm in Stable Boundary Layer: Wake Flow and Energy Budget Analysis

Abstract. A parameterization of wave-induced stress is employed in an open-source large-eddy simulation code to investigate the swell impacts on the wake flow and the power output of a real offshore wind farm under a stable atmospheric boundary layer for the first time. The output module of the code is extended to include all source and sink terms of the kinetic energy equation. Two typical scenarios in the North Sea area with modest wind speeds and wind-following/opposing fast waves are considered. Results show that swells significantly affect the profiles of wind speed and turbulence intensity across the entire operational height of the wind turbines. Such influences are prominently observed in the inflow and progressively diminish in the wake flow downstream. Through kinetic energy budget analysis, we discover that the wave effects are primarily exerted through indirect modification of the advection of energy in streamwise and vertical dimensions instead of the direct wave-induced energy input/output. The wind shift and yawing adjustment caused by waves play a crucial role in the energy harvesting rate, depending on the specific inflow direction and wind farm layout. The absolute wave-induced changes in wind speed and turbulence intensity progressively decrease downstream, and the relative changes in total power production reach up to 20.0 %/−27.3 % for the wind-following/opposing wave scenarios respectively.