Effect of wind–wave angle on the power production and wake characteristics of an offshore wind farm at various wave ages

The turbine wake and performance of an offshore wind farm are strongly influenced by wind direction and sea state, where significant knowledge gaps remain. To address the research need, the present study focuses on the effects of wind–wave angle and wave age on the wind farm performance and turbine wake characteristics using large-eddy simulation and an actuator line model. We investigate the influences of swell at various wave ages (c/u∗≈30, 60 and 90, where c is the wave phase speed and u∗ is the airflow friction velocity) and different wind–wave angles (α0=0∘–180°). Simulation results indicate that the swell following wind substantially enhances the wind farm power output and wake velocity at c/u∗≈90 compared with those at c/u∗≈30 and 60. For the effect of wind–wave angle, wind farm power output increases by approximately 50.8%, 55.3%, and 75.7% at α0=45∘, 90°, and 135°, respectively, whereas it decreases by 4.3% at α0=180∘ compared to the no-swell case. This enhancement of wind farm power output is attributed to the wake deflection. On the turbine wake characteristics, it is found that the wake is deflected considerably from the wind direction due to the deflection by lateral and oblique swells, resulting in an additional lateral shift in the wake trajectory. Low- and high-speed streaky structures are observed on the sides of the wake deflection line, which are associated with high turbulent kinetic energy there.