2D PIV Visualization of Dynamic Stall on a Vertical Axis Wind Turbine

Abstract

After a decrease of interest in the 1990's, the research on Vertical Axis Wind Turbines (VAWT) a has reappeared in the last years as a result of the its increasing application in the built environment, where VAWTs present several advantages over Horizontal Axis Wind Turbines (HAWT). The VAWT has an unsteady aerodynamic behavior due to the variation with the angle of rotationof the blade's sections' angle of attack, perceived velocity and consequential Reynolds number. The phenomenon of dynamic stall is then an inherent effect of the operation of a Vertical Axis Wind Turbine (VAWT) at low tip speed ratios (�), impacting both loads and power. The limited amount of experimental knowledge regarding this phenomena has lead to the development of the present work. The experimental method uses Particle Image Ve- locimetry to visualize the development of the flow over the suction side of the airfoil for two different reference Reynolds numbers (Re = 52000 and Re = 70000) and three tip speed ratios (� = 2, 3, 4), thus in the operational regime of a small urban wind turbine. The field-of-view of the experiment covers the entire rotation of the blade and almost the entire rotor area. The results describe the evolution of the flow around the airfoil and in the rotor area, with special focus on the leading edge separation vortex and trailing edge shed vorticity development. The results also allow the quantification of the flow, both the velocity field and the vorticity/circulation b , in terms of phase locked average and random component. These experimental results allow a new insight into the development of dynamic stall in a VAWT, supplying a important database for the validation of numerical models.