An experimental and numerical study of the vortex structure in the wake of a wind turbine

An experimental investigation into the properties of the vortex wake behind a wind turbine rotor has been carried out at model scale, using Particle Image Velocimetry (PIV). The two-blade model was operated at tip speed ratios in the range λ=3–8, and chord Reynolds numbers Re=6400–16 000. The blades were untwisted, with flat-plate aerofoil profile. Measurements of wake velocity and vorticity were obtained for a two-dimensional flow field representing an axial cross-section of the wake, extending 2.9 rotor diameters downstream of the rotor. The vorticity maps were compared with calculations made using the Rotor Vortex Lattice Method (ROVLM), an inviscid free-wake code recently developed at the University of Stuttgart. The PIV and ROVLM data show qualitative agreement in terms of the shape of the wake boundary, including downstream wake contraction, and quantitative agreement in terms of the tip vortex pitch. It appears that the fundamental behaviour of the helical vortex wake may be relatively insensitive to blade chord Reynolds number, so long as similarity of tip speed ratio is observed.