A24D-01 Atmospheric boundary layer wind shear and veer reshape wind turbine aerodynamics and efficiency (Invited)

Abstract: Wind shear and wind veer, gradients of wind speed and direction, respectively, are ubiquitous in the atmospheric boundary layer (ABL). Field experiments have revealed statistically significant variations in power production efficiency (quantified by a power coefficient) upwards of ±15% linked to the magnitude of shear and veer flowing in to the turbines. Yet the magnitude of these variations in efficiency caused by shear and veer are not reproduced by standard wind power engineering modeling tools. We first perform concurrent-precursor large-eddy simulations (LES) of an actuator disk-modeled wind turbine across ABL conditions to demonstrate that LES yields similar behavior to the field experiments: shear and veer associated with stable ABL conditions can reduce wind power efficiency by more than 20% for a 240 m-diameter turbine. We hypothesize that shear and veer trigger new, non-linear, sub-kilometer processes through interactions with the wind turbine drag forcing. To elucidate the driving mechanisms, we perform simplified, controlled inflow LES where shear, veer, and turbine drag coefficient are controlled independently. Using these controlled simulations, we demonstrate that shear and veer effects on wind turbines can each be decomposed into: (1) geometric effects, due to changes in rotor-normal flow velocity that are straightforward to model, and (2) inductive effects, which change the velocities induced by the turbine's drag force through processes that previously were not well understood. Inductive effects of wind speed shear modulate the power coefficient through heterogeneity in the local induced velocity, which is triggered by changes to the local, nondimensional drag coefficient. Inductive effects of wind veer reduce the power coefficient by generating an adverse pressure gradient at the turbine (drag element) scale that is linked to the response of flow vorticity to the turbine and its wake. The geometric and inductive effects approximately linearly superimpose, with increasing losses as shear and veer magnitudes increase or as the turbine size increases. Future work will consider extending this analysis methodology to elucidate the potential impact of shear and veer on the drag coefficient of other drag morphologies in the atmosphere, such as buildings and vegetation.