The effects of wind shear on rotor aerodynamics*

Wind speed and direction variations can affect rotor power, thrust force, and structural loading. The effects of wind shear increase as turbines become larger and extend farther into the atmospheric boundary layer, where wind conditions can be more complex than those in the surface layer. Conventional turbine power models, like those prescribed by the IEC for wind resource assessment, assume incident wind conditions do not affect airfoil efficiency or induced rotor velocity, which determines the coefficient of power. These models are therefore limited in their ability to account for the aerodynamic effects of shear that modify turbine power production. We find that the common rotor-equivalent wind speed (REWS) model predicts power with similar accuracy to estimating power based on hub height wind speed alone. In this study, we investigate an actuator disk in large eddy simulations to resolve the aerodynamic interactions between the disk and inflow wind profiles. These simulations demonstrate that induction increases monotonically as the magnitude of directional shear over the rotor increases, which lowers the disk velocity and power production of the turbine. For wind conditions near uniform, 1D momentum theory overpredicts induction on the rotor by 2%, and underpredicts by as much as 3% as average directional shear over the rotor increases. These results point to coupling between the rotor and sheared inflow wind that causes average induction to deviate nonlinearly from what is predicted by 1D momentum theory.