Wake Dynamics of a Wind Turbine with an Oscillating Rotation Rate at High Reynolds Numbers.

Understanding the wake behavior of wind turbines under unsteady conditions is critical for optimizing wind farm power generation. Dynamic induction control is a method which imposes unsteady thrust conditions on turbines to allow for earlier wake recovery. This improves the power density of wind farms, as downstream turbines no longer sit in the low-energy wake of upstream turbines. In order to study the effects of dynamic induction control at utility-scale Reynolds numbers, a horizontal-axis wind turbine is operated in high-pressure wind tunnel with a Reynolds number of 4*10⁶. The tip-speed ratio is periodically oscillated at varying Strouhal numbers. Force measurements are taken to calculate the thrust and power coefficients of the turbine. A hot-wire anemometer is used to characterize the velocity measurements in both streamwise and spanwise sweeps of the wake. The results show that a travelling wave of velocity fluctuations propagates downstream through the wake. The travelling wave behaves differently at different mean tip-speed ratios. Two mechanisms for the generation of the travelling wave are considered: the oscillation of the thrust coefficient and the change in location of helical tip vortex breakdown. The interplay of these two mechanisms causes differing behavior in the travelling wave at different mean tip-speed ratios. These results help to clarify the behavior of utility-scale wind turbine wakes in unsteady flows.