Phase-Averaged Dynamics of a Periodically Surging Wind Turbine

TL;DR

This study experimentally investigates the unsteady power dynamics of a wind turbine subjected to periodic surge motions, developing a simple model and observing power fluctuations that inform optimization strategies for non-traditional wind energy systems.

Contribution

It introduces a linear first-order differential equation model for surging turbines that accurately predicts their dynamic response without unsteady calibration, supported by experimental data.

Findings

Power increases up to 6.4% at high tip-speed ratios and surge velocities.

Power decreases at low tip-speed ratios due to blade stall.

The model accurately predicts amplitude and phase responses.

Abstract

The unsteady power generation of a wind turbine translating in the streamwise direction is relevant to floating offshore wind turbines, kite-mounted airborne wind turbines, and other non-traditional wind-energy systems. To study this problem experimentally, measurements of torque, rotor speed, and power were acquired for a horizontal-axis wind turbine 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 $Re_D=6.1\times10^5$ and at tip-speed ratios between 5.2 and 8.8. Sinusoidal and trapezoidal surge-velocity waveforms with maximum surge velocities up to 23% of the free-stream velocity were tested. A model in the form of a linear ordinary differential equation (first-order in time) was derived to capture the time-resolved dynamics of the surging turbine. Its coefficients were obtained using torque measurements from a stationary turbine, without the need for unsteady calibrations. Its predictions compared favorably with the measured amplitude- and phase-response data. Furthermore, increases in the period-averaged power of up to 6.4% above the steady reference case were observed in the experiments at high tip-speed ratios and surge velocities, potentially due to unsteady or nonlinear aerodynamic effects. Conversely, decreases in mean power with increased surge velocity at low tip-speed ratios were likely a result of the onset of stall on the turbine blades. These results inform the development of strategies to optimize and control the unsteady power generation of periodically surging wind turbines, and motivate further investigations into the unsteady aerodynamics of wind-energy systems.