Effects of atmospheric stability on the performance of a wind turbine located behind a three-dimensional hill

Understanding the effect of thermal stratification on wind turbine wakes in complex terrain is essential to optimize wind farm design. The effect of a three-dimensional hill on the performance of a downwind turbine is studied by performing large eddy simulations for different atmospheric conditions. The distance between the hill and the turbine is six times the turbine diameter, and the hill height is equal to the hub height. It is shown that the hill wake reduces the power production of the downstream turbine by 35% for the convective boundary layer case under consideration. However, surprisingly, the wind turbine power production is increased by about 24% for the stable boundary layer case considered here. This phenomenon results from the entrainment of kinetic energy from the low-level jet due to the increased mixing induced by the hill wake. This effect strongly depends on the Coriolis force-induced wind veer. The increased turbulence intensity by the hill results in a strong increase in the forces experienced by the blades, which suggest the turbine is experiencing much higher unsteady turbulence loading. It is shown that the increase in the power fluctuations may not fully reflect the increase in the force fluctuations on the blades.