Experimental quantification of the entrainment of kinetic energy and production of turbulence in the wake of a wind turbine with Particle Image Velocimetry

The role of the tip-vortex pairwise instability (“leapfrogging”) is investigated in relation to the process of kinetic energy transport and turbulence production within the shear layer of a horizontal-axis wind-turbine wake. Experiments are conducted in an open-jet wind-tunnel on a wind turbine model. Stereoscopic particle image velocimetry (SPIV) is employed to obtain the velocity field in a meridian plane encompassing a large portion of the wake past the rotor model. Measurements with both phase-locked and unconditioned sampling techniques allow for a triple decomposition of the flow fields. The levels of turbulence intensity show a dominant role of random fluctuations after the location of the pairwise instability. Computation of the kinetic energy fluxes across the wake shear layer shows the presence of three main zones. Prior to the onset of the instability, vortices shed from the blade inhibit the turbulent mixing of the wake during its expansion. The region affected by leapfrogging exhibits a sudden increase of the net entrainment of kinetic energy. Downstream, the energy exchange is characterized by a pronounced turbulent mixing, only due to random turbulent motions. Leapfrogging determines the end of the wake expansion and with the onset of a more pronounced turbulent mixing, coincident with the beginning of the wake re-energising process.