The Impact of Atmospheric Stability and Wake Turbulence on the Wind Turbine Blade Aerodynamics

Significant challenges remain in understanding wind turbine performance within the turbulent atmospheric boundary layer (ABL) and the wakes of upstream turbines. To help address these points, large-eddy simulations of the convective ABL under a range of stability states and inflow velocities are computed with two IEA-15 megawatt turbines separated by seven rotor diameters in a 5 km by 5 km by 1 km domain using the AMR-Wind solver from NREL. A comparison is made between the length- and time-scales encountered by the turbine in the global reference frame and the relative or local scales encountered by the rotating blade as a function of the ABL state and the rotor span. Since the ABL turbulence contains length scales much larger than the blade chord length; it has often been treated as a quasi-steady change in the local blade angle of attack and relative velocity. However, recent research from Vita et al. (2020) suggests that turbulence effects are non-negligible on blade aerodynamic performance even at integral length scales up to 3 times the local chord length. This study aims to elucidate these differences as a function of the ABL state, to better isolate the quasi-steady, unsteady, and turbulent flow conditions imposed on a wind turbine blade.