Why the Long Phase? Disambiguating the Dynamics of a Periodically Surging Wind Turbine

Despite its relevance to floating offshore wind turbines and other non-traditional wind-energy systems, the unsteady power generation of a wind turbine translating in the streamwise direction has not been comprehensively studied in experiments. Accordingly, a horizontal-axis wind turbine was actuated in periodic surge motions in a fan-array wind tunnel at the Caltech Center for Autonomous Systems and Technologies (CAST). Experiments were conducted at a diameter-based Reynolds number of Re_D = 5.6 × 10⁵ and at tip-speed ratios between 5.7 and 9.4. Sinusoidal and trapezoidal streamwise-velocity waveforms with maximum velocities up to U/U_∞ = 0.32 were considered. Phase-averaged torque and rotation-rate measurements of the turbine in these unsteady motions showed increases in the time-averaged power in excess of 6% relative to the steady case. A first-order model was derived to capture trends in the magnitude and phase of the measured data, which allowed the aerodynamics of the turbine to be separated from other factors, such as generator characteristics and turbine inertia. These results inform the development of strategies to optimize and control the unsteady power generation of surging wind turbines.