Traveling waves in the wakes of dynamically controlled wind turbines

The wake dynamics of wind turbines in unsteady flow conditions may have a substantial impact on the wake losses in and overall power density of wind farms. These effects are particularly relevant for wind turbines under dynamic induction control and for floating offshore turbines undergoing platform oscillations in the streamwise direction. In this study, we investigate the effects of periodic oscillations in the rotation rate and streamwise position of wind turbines on their wake dynamics and recovery. We derive an analytical model for the near wake that describes the formation and propagation of traveling waves in the streamwise velocity and wake radius. The predictions of the model show good qualitative agreement with phase-averaged flow-field measurements from a periodically surging turbine in an optically accessible towing tank. These measurements also demonstrate that the identified unsteady flow mechanisms can lead to reductions in the streamwise extent of the wake by over 45%. We also conduct experiments in a high-pressure wind tunnel using a turbine with a periodically varying rotation rate, at Reynolds numbers closer to those of utility-scale turbines. Similar traveling-wave dynamics are evident in the wake, though no significant differences in wake recovery are observed. These results highlight unsteady flow mechanisms in wind-turbine wakes that could be leveraged to enhance the power generation of fixed-bottom and floating offshore wind farms.