PIV analysis of the near wake behind a dynamically pitching horizontal axis wind turbine

The near wake regions behind wind turbines are not well understood, and their complexity has been at the center of many theoretical and experimental studies. Floating offshore wind turbines are a novel adaptation of traditional wind farms, proving difficult to experimentally model due to the unpredictability of ocean movement and the validity of turbine model scaling. Stereo particle image velocimetry (PIV) experiments were conducted in a wind tunnel to investigate the near wake characteristics of a model turbine pitching at various frequencies, similar to those that an actual turbine would experience due to ocean waves. The model turbine has a diameter of D = 0.4m and was placed in a wind tunnel at 3 streamwise locations so that we can assemble a composite wake downstream of the turbine. A crank-lever system was used to dynamically pitch the turbine at various frequencies. Experiments were conducted at a Reynolds number of Re =1x10⁵ and a tip speed ratio (TSR) of 5.54 which corresponds to the maximum power output of the turbine. Phase-averaged velocity fields were computed to study the downstream development of the wake behind the turbine, and to compare and contrast the effects of different pitching frequencies on the turbine. We observe the formation of helical tip vortices as well as vortex sheets shed in the wake along the length of the rotor blades. The tip vortices trended upwards when the turbine was pitching back, -5° < 𝛼 < 0°, and downwards when the turbine was pitching forward, 0° < 𝛼 < 5°. The vortex sheets were shed from the trailing edge of the rotor blades at the angle that the blades were positioned, and in every case deformed as they progressed downstream until they were angled ~ 45° relative to their starting position at 1.25D downstream from the turbine.