Experimental study of transonic flow over a wind turbine airfoil

Abstract. For the largest wind turbines currently being designed, operation at cutout conditions can lead to the tip airfoil experiencing transonic flow conditions. To date, this phenomenon has primarily been explored through numerical simulations, but modelling uncertainties limits the reliability of these predictions. In response to this challenge, our study marks the first experimental investigation of a wind turbine airfoil under transonic conditions, for which we consider the FFA-W3-211 airfoil. Measurements are conducted in the high-subsonic range (Mach 0.5 and 0.6), utilizing Schlieren visualization and Particle Image Velocimetry (PIV) to characterise the airfoil across a range of angles of attack expected at cutout conditions. Unsteady shock wave formation is observed for the higher Mach number, with the shock oscillation range increasing with steeper angles of attack. Also, it is found that the presence of a local supersonic flow region does not consistently result in a shock wave. Our findings reveal that while calculations based on isentropic flow theory are reasonably effective in predicting the onset of transonic flow, they fail to predict the intensity of transonic flow effects, in particular, the formation and unsteady nature of shock waves. This underscores the need for higher-fidelity tools and experiments to capture the dynamic transonic flow effects on wind turbine airfoils.