In this paper the aerodynamics and performance of two Vertical Axis Wind Turbines are discussed, on the basis of a wide set of experiments performed at Politecnico di Milano (Italy). A H-shaped and a Troposkien Darrieus turbine 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) and to the non-aerodynamic struts of the H-shaped rotor. These features are consistent with the character of the wakes shed by the turbines, measured by means of hot wire anemometry on several surfaces downstream of the models. The morphology of H-shape and Troposkien rotor wakes 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 marked by high periodic and turbulent unsteadiness in the tip region of the wake. The way in which these mechanisms affect the wake evolution and mixing process for the two classes of turbines is investigated for different tip speed ratios, highlighting some relevant implications in the framework of wind energy exploitation.

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