Wake measurements of small-scale vertical axis wind turbines at Politecnico di Milano: A critical review

In the last ten years, four measurement campaigns were performed at Politecnico di Milano on two Darrieus Vertical Axis Wind Turbines (VAWT) for micro-generation of different architecture (H-shaped vs troposkien), but sharing the blade number (3), the blade profile (NACA 0021), and the swept area (1.5 m2). The experiments, carried out in the large-scale wind tunnel of Politecnico di Milano, included detailed wake measurements. This paper presents a review of the research activities related to velocity and turbulence measurements in the wake, proposing an analysis of both the technical aspects and the scientific outcomes of the investigation. In particular, the wakes of these turbines were measured on several surfaces downstream of the rotors for different tip speed ratios and different Reynolds numbers, searching for corresponding conditions between the two rotors. The paper first presents the technical issues involved in measuring the flow velocity in the wake of VAWT rotors with intrusive techniques such as hot wire anemometers and pressure probes. The second part of the paper proposes a comprehensive analysis of the wakes shed by the tested models. The wakes appear asymmetric and roughly follow the shape of the rotor, their width and velocity deficit being strongly dependent on the tip speed ratio. Flow angle measurements show the onset of large-scale tip vortices, for both the H-shape and the troposkien rotors, even though resulting from different aerodynamic mechanisms in the two architectures. A discussion on the impact of the wake features on the implementation of VAWTs in the urban environment concludes the paper.