Large-eddy simulation of offshore wind farm

A hybrid numerical capability is developed for the simulation of offshore wind farms, in which large-eddy simulation is performed for the wind turbulence, and a potential flow based method is used for the simulation of the ocean wave field. The wind and wave simulations are dynamically coupled. The effect of wind turbines on the wind field is represented by an actuator disk model. This study focuses on the effect of wind-seas, and the turbine motion is treated as negligibly small. A variety of fully-developed and fetch-limited wind-sea conditions and turbine spacings are considered in the study. Statistical analyses are performed for the simulation results, with a focus on the mean wind profile, kinetic energy budget in the wind field, and the wind turbine power extraction rate. The results indicate that the waves have appreciable effect on the wind farm performance. The wind turbines obtain a higher wind power extraction rate under the fully developed wind-sea condition compared with that under the fetch-limited condition. This higher extraction rate is caused by the faster propagating waves and the lower sea-surface resistance on the wind when the wind-seas are fully developed. The wave-induced difference can be as high as 8% with the commonly used turbine spacing in commercial land-based wind farms, s(x) = 7 (with s(x) being the ratio of streamwise turbine spacing to the turbine diameter). Such level of difference is noteworthy considering the previous understanding that direct wave-induced disturbance to the wind field decays exponentially away from wave surface. (C) 2014 AIP Publishing LLC.