Experimental analysis of the kinetic energy transport and turbulence production in the wake of a model wind turbine

The mixing properties of the self-induced flow in a wind turbine wake are studied. The wake of a model horizontal axis wind turbine is analysed with the Particle Image Velocimetry technique and a triple decomposition of the flow. The process of wake re-energising is studied and its dependency on the wake flow structures and stability is shown. The streamwise development of the wake velocity is presented, as well as its clear dependency on the onset of the pairwise instability of the tip-vortices. The mean flow kinetic energy transport and turbulence production is calculated for different regions of the wake. The main conclusion is that the stability of the tip-vortex helix has a strong influence on the mixing of the wake with the outer flow and its re-energising. A thorough estimation of the energy transport at wake scale and the modelling of its dependency on the turbine characteristics would be a first step towards a rotor design process which does not only take into account the aerodynamic and power optimisation of the rotor itself, but also the re-energising properties of the wake, namely the “design of the wake”.