On the effect of atmospheric stability on the efficacy of wind turbine blade pitch control strategies*

Upscaling of wind turbines has been the general tendency to increase their power production. However, increasing the dimensions of turbines poses new engineering challenges to maintain their structural integrity.  Cyclic loads due to turbulence, gravity, and wind shear are detrimental for the blades. Atmospheric flow stratification aggravates the magnitude of these loads as it causes an increase in wind shear, often accompanied by veering in wind direction with height. In this work, we delve into using the individual pitch control (IPC) of the blades for load reduction due to flow heterogeneity at the rotor. Large-eddy simulations of a turbine in stable and neutral atmospheric flow have been performed. The turbine blades and nacelle have been modeled with the actuator surface model. LES results using the baseline controller and IPC were compared to investigate their effects on the blades' bending moment fluctuations. Our study shows a larger magnitude of cyclic loads due to high flow heterogeneity impinging the rotor for stable atmospheric conditions than in neutral. In addition, the IPC reduces the amplitude of these cyclic loads.