Phase Averaged Flow Statistics of a Wind Turbine's Wake at High Reynolds Numbers

Wind turbine wakes are dominated by a wide range of turbulent structures, with the helical tip vortex being one of the most recognizable. To better understand the tip vortex's contributions on the wake's evolution, a wind tunnel study was conducted at a Reynolds number of 4 million with a 20cm diameter model turbine. High Reynolds number conditions were achieved using a pressurized wind tunnel facility, the High Reynolds number Test Facility (HRTF), at Princeton University. The tip vortex has been shown to be affected by a turbine's rotation, such as a more energetic tip vortex for smaller tip speed ratios. Therefore, experiments were conducted at tip speed ratios of 4 and 7. Velocity measurements were obtained via a nanoscale hot-wire anemometer, to reduce spatial filtering of the velocity signal. The tip vortex is expected to primarily dominate the flow behavior in the near wake region, so measurements were restricted to 0.75 to 2 diameters downstream of the turbine. Time-averaged statistics are insufficient to completely capture the turbulence contributions of the tip vortex, so phase averaged statistics, such as the variance, are presented. The phase averaged results support the inverse relationship between tip speed ratio and tip vortex strength and capture tip vortex breakup, an essential component for understanding bulk wake recovery.