Large eddy simulations of curled wakes from tilted wind turbines

Abstract

One control strategy to increase power production in wind farms is angling wind turbine rotors, in order to steer wakes away from downwind turbines. Although rotor yaw is the most common approach to wake steering, tilting the rotor vertically to steer the wake downward can also increase total farm power. In this study, large eddy simulations of a 15 MW turbine are performed for rotor tilt angles of 0°, 15°, and 30° with below-rated turbulent inflow. Wake characteristics are analyzed, including using circulation to quantify the curled wake's counter-rotating vortex pair and quantifying wake shapes by fitting Legendre polynomials to wake edge polar coordinates. Tilting the rotor causes downward wake steering, wider and vertically compressed wake cross-sections, and stronger counter-rotating vortices. Although the wake velocity deficit recovers similarly for tilted and non-tilted wakes, the power available to a downwind rotor recovers faster because the tilted wake is steered away from the downwind rotor area and is replaced by high-speed air from above. This also causes higher effective wind shear across the downwind rotor. Additional simulations double the gap between the ground surface and the rotor bottom, which affects the wake geometry as well as the downwind power recovery and wind shear.