PIV analysis of the near wake behind a statically pitched 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 to characterize the near wake of a model turbine in various static pitched positions. The model turbine has a diameter of D = 0.4m and was placed in a wind tunnel at 3 streamwise locations to capture a composite image of the near wake. The wind turbine was angled at 𝛼 = 5°, 0°, and -5° and PIV data was synchronized with the blade position. Experiments were conducted at a Reynolds number of Re =1x10⁵ and a tip speed ratio (TSR) of 5.54 - corresponding to the maximum power output for this particular turbine. Phase-averaged velocity fields were computed to characterize the downstream development of the wake behind the turbine, and to compare and contrast the effects of the pitch angle of 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 vortex trajectory trended upwards when the turbine was pitched back, 𝛼 = -5°, and downwards when the turbine was pitched forward, 𝛼 = 5°. The vortex sheets shed from the trailing edge of the blades deformed as they progressed downstream until they were angled ~ 45° relative to their starting position at 1.25D downstream from the turbine.