Large-eddy simulation study on wave-wake interactions past an offshore wind turbine

Abstract

Large-eddy simulations (LES) of the flow past an offshore wind turbine under different underlying wave fields have been performed. A one-way coupling between the air and water flows has been realized through a hybrid immersed-boundary/level-set method. The velocity in the water domain is forced with the potential flow solution, while the response of the atmospheric boundary layer to the changes in the sea-surface height (tracked by the level-set equation) is simulated with a LES approach coupled to a rotating actuator disk model to mimic the effect of the wind turbine. A parametric study has been performed varying the wave period and wavelength while keeping the amplitude constant, resulting in different wave age parameters ranging from young developing waves to old swell waves. The wave field has a significant effect on the lower region of the atmospheric boundary layer, slowing down the wind field in proximity of the air-water interface and considerably increasing the local turbulence kinetic energy (TKE). The interaction between the wave evolution and the TKE results in a non-monotonic trend of the wake recovery rate with the wave age, when compared to the baseline value in the wake of a turbine over a flat wall. Both developed waves results in a lower recovery rate, whereas intermediate-age waves present a larger value than the baseline. The increased TKE in the lower layers of the rotor revolution induces an increased fluctuating component in the power production and blade loads. Nevertheless, for the parameters considered in this study, the spectra of the blade loads do not show a clear signature at the wave frequency, but the increased fluctuating component occurs over a broad range of frequencies.