Large-eddy simulation study of wind turbine array above swell sea

Swells are common in sea areas, but how swells affect the operation of an offshore wind farm is poorly understood. To fill in this knowledge gap, large-eddy simulations of turbine arrays above swells are performed. Specifically, downwind, upwind, and lateral swells with three different wave ages are considered. The results show that downwind and upwind swells respectively lead to increased and decreased wind speeds with weakened and enhanced turbulence. The wakes above high-wave-age downwind swells go through three distinct phases, featuring slow, fast, and moderate wake recovery rates and low, high, and moderate turbulence levels. Compared to the no swell case, downwind swells lead to higher power outputs far downstream, whereas upwind swells lead to lower or slightly higher power outputs. Lateral swells give rise to spanwise motions at sea level, resulting in a wind profile that changes direction as a function of the height. The nonzero spanwise velocity deflects the wakes and reduces sheltering among the wind turbines. The net effect is increased turbine power outputs. In addition, a peak emerges at the swells’ intrinsic frequency in the power output spectra of the first turbines above high-wave-age upwind and downwind swells and all the turbines above lateral swells.