The influence of axial gusts on the output of low-inertia rotors

Wind and hydrokinetic turbines are designed for steady operation, and hence their efficiency suffers in unsteady environments. In this study, the response of a low-inertia rotor model to axial gusts is investigated experimentally. Gust profiles are modelled as ramp functions with differing slopes, and are simulated by accelerating the rotor model in a towing-tank facility. Rotor speed and torque are recorded during the system's response to the variation in freestream velocity. Furthermore, time-resolved particle image velocimetry (PIV)is carried out in the rotor's wake to gain insights on the change of bound circulation. The power produced is observed to be higher during the gust than during steady operation. For instance, the power output of the rotor during the fastest gust is, at maximum, 27% higher than during steady operation. Using the PIV data, the circulation of the tip and trailing vortices is estimated as a proxy for the bound circulation, and is found to have higher absolute magnitudes during the gust. As a result, the increase of the circulatory forces is concluded to be the source of the increased power performance. The current findings are relevant to rotor systems with directly coupled generators and no speed control.