Unsteady flows can help turbines surge ahead in power production

To augment the performance of energy-harvesting turbines in real flow conditions, the effects of temporal variations in the streamwise inflow must be considered. These unsteady dynamics can lead to enhancements or losses in the time-averaged power extraction of a turbine, depending on the characteristics of the flow perturbation and turbine aerodynamics, and alter the flow conditions downstream of the turbine. To quantify and predict these phenomena, an analytical framework for the time-varying and time-averaged power extraction of a turbine in a periodically varying streamwise inflow is derived and validated in wind-tunnel experiments. This framework can also be used to predict the time-varying flow properties upstream of the turbine. Extensions of this modeling paradigm to the wake region downstream of the turbine are then explored and compared with flow-field measurements of a periodically surging turbine in an optical towing tank using two-dimensional particle-image velocimetry. The phase-averaged wake measurements help parameterize the effects of unsteady inflow dynamics on the time-averaged wake profile and on the contributions of vortical structures to the evolution of the wake. These results inform control schemes for enhancing the power extraction of individual turbines in unsteady flow conditions, as well as minimizing unsteady wake losses on downstream turbines in an array.