The effect of swell on marine atmospheric boundary layer and the operation of an offshore wind turbine

Swell is a common type of sea wave, however, significant knowledge gaps exist regarding how the swells influence the marine atmospheric boundary layer and the operation of wind turbines. In this study, the effects of downwind and upwind swells with three different wave ages (vertical bar c/u*vertical bar approximate to 30, 60, and 90) are investigated using large-eddy simulation. We find that downwind swells lead to a shear layer with accelerated wind velocity and decreased turbulence intensity. In comparing with that without swells, the hub-height wind velocity above swells with c/u* approximate to 30, 60, and 90 increases by 3.7%, 16.1%, and 23.3%, respectively, while the turbulence kinetic energy decreases over 40% and varies non-monotonously with the wave age. In contrast, upwind swells induce a substantially decreased wind profile with enhanced turbulence. For turbines operating above downwind or upwind swells with vertical bar c/u*vertical bar approximate to 90, the power output is nearly enhanced by 100% or reduced by 50%, and a characteristic frequency of the turbines' power fluctuations exists identical to the swell's intrinsic frequency. The turbine wake is affected by both the swell-changed wind velocity and turbulence intensity. Above the downwind (upwind) swell, the wake shows a slower (higher) recovery and weaker (stronger) meandering. (C) 2022 Elsevier Ltd. All rights reserved.