Measurements of the instantaneous wake flow from a model wind turbine placed in a turbulent boundary layer were obtained by wall-parallel oriented particle image velocimetry (PIV) in the St. Anthony Falls Laboratory wind tunnel. PIV velocity vector fields were used to investigate mean (expansion angle, wavelength, and wake velocity) and higher order statistics (local slope, curvature, and correlation) describing meandering motions in the turbine wake. These statistics were used to compare the wakes produced by four different wind turbine operating configurations, which include a single turbine operating at two different tip-speed ratios and two turbines aligned with the mean flow. The origin of meandering motions was identified for all cases in the hub vortex signature, which evolved into a stretched or compressed low speed meander in the wall parallel plane, depending on the turbine operating conditions and on the interaction with the wake shear layer. Finally, both autocorrelation and scale-dependent statistics on the velocity minima fluctuations about the meander signature suggest that small scale vortices, found in the hub shear layer and in the wake shear layer, interact with the hub vortex and govern its spatial evolution into large scale wake meandering.

To improve power production and structural reliability of wind turbines, there is a pressing need to understand how turbines interact with the atmospheric boundary layer. However, experimental techniques capable of quantifying or even qualitatively visualizing the large-scale turbulent flow structures around full-scale turbines do not exist today. Here we use snowflakes from a winter snowstorm as flow tracers to obtain velocity fields downwind of a 2.5-MW wind turbine in a sampling area of B36 Â 36 m 2 . The spatial and temporal resolutions of the measurements are sufficiently high to quantify the evolution of bladegenerated coherent motions, such as the tip and trailing sheet vortices, identify their instability mechanisms and correlate them with turbine operation, control and performance. Our experiment provides an unprecedented in situ characterization of flow structures around utility-scale turbines, and yields significant insights into the Reynolds number similarity issues presented in wind energy applications.

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