A Wake in the Middle of the Night: 3D-PTV Measurements around Full-Scale Vertical-Axis Wind Turbines

Studies of the wake dynamics of wind turbines are critical for the design and optimization of wind farms in order to minimize wake losses and maximize power density. To observe the inherently three-dimensional flow structures in the wakes of full-scale vertical-axis wind turbines (VAWTs), a volumetric particle-tracking velocimetry method was developed for field experiments at the Field Laboratory for Optimized Wind Energy (FLOWE) in Lancaster, CA, using six cameras and artificial snow as tracer particles. Velocity and vorticity fields extending up to three turbine diameters into the wake were measured around isolated 2-kW VAWTs: one with five straight blades, and another with three helical blades. Two tip-speed ratios were examined for each turbine. The 3D flow measurements allowed the dynamics of vortical structures in the streamwise, transverse, and wall-normal directions to be analyzed. Additionally, significant differences in wake geometry between the straight- and helical-bladed turbines were observed, which can be explained using a simple vortex-line model. These results help clarify mechanisms responsible for wake recovery in VAWTs, and thus have implications for wind-farm design.