PIV in a model wind turbine rotor wake

Stereoscopic particle image velocimetry (PIV) measurements of the flow in the wake of scale model of a horizontal axis wind turbine is presented Near the rotor, measurements are made in vertical planes intersecting the rotor axis These planes capture flow effect from the tip and root vortices The stability of the tip vortices as a function of different tip speed ratios are demonstrated An instability seems to occur after a time corresponding to four blade passages The result is vortex pairing and in some cases grouping of three vortices Further downstream in the wake, measurements in planes perpendicular to the rotor axis is used to investigate the dynamics in the far wake Here, a precessing core is found and data indicate that the Strouhal number of the precessing is independent of the rotor spee