Three dimensional character of VAWT wakes: an experimental investigation for H-shaped and Troposkien architectures

In this paper the aerodynamics and performance of Vertical Axis Wind Turbines are discussed, on the basis of a wide set of experiments performed at Politecnico di Milano (Italy). Both an H-shaped and a Troposkien Darrieu turbines for micro-generation, characterized by the same swept area and blade section, are tested in real-scale. Performance measurements show that the Troposkien rotor outperforms the H-shaped turbine, mostly related to the larger midspan section of the Troposkien rotor (resulting by the constraint of constant swept area). These features find a further assessment in the shape of the wakes shed downstream of the two machines, which represent the focus of the present paper. The flow configuration in the wakes was accomplished by means of hot wire anemometry on several measurement traverses downstream of the turbine models. In particular, beside conventional flat measurement grids, for the Troposkien turbine a curved measurement grid was also adopted, so to keep constant the distance between the easurement surface and the curved blade. The comparison between curved and flat grid measurements allowed to investigate the mixing process downstream of the Troposkien turbine, and represented an interesting way to match the perspectives of single turbine and wind farm designers. The morphology of H-shape and Troposkien rotor wakes are shown to exhibit relevant differences, especially in the three-dimensional character and time-periodic evolution in the blade tip region. In particular, large-scale vortices dominate the tip region of the wake shed by the H-shape turbine; these vortices pulsate significantly during the period, due to the periodic fluctuation of the blade aerodynamic loading. Conversely, the highly tapered shape of the Troposkien rotor prevents the onset of tip vortices, but also induces a dramatic spanwise reduction of tip speed ratio, promoting the onset of local dynamic stall, large time-periodic unsteadiness and high turbulence in this region. The way these mechanisms affect the performance of the two classes of turbines, and the wake mixing process, are finally discussed in the framework of wind energy exploitation.