Search for: All records

Abstract The aerodynamics of vertical axis wind turbines (VAWTs) are inherently unsteady because the blades experience large angle of attack variations throughout a full turbine revolution. At low tip speed ratios, this can lead to a phenomenon known as dynamic stall. To better characterise the unsteady aerodynamics and represent them in models and simulations, data from studies of individual static or pitching airfoils are often applied to VAWT blades. However, these studies often involve sinusoidally pitching airfoils, whereas the pitching motions experienced by VAWTs are more complex. Here, the pressures and forces on an airfoil undergoing VAWT-shaped pitch motions corresponding to various tip speed ratios are compared to those of a sinusoidally pitching airfoil in order to assess whether a sinusoidal motion represents a reasonable approximation of the motions of a VAWT blade. While the lift development induced by the sinusoidal motion yields good agreement with that induced by the VAWT-shaped motion at the higher tip speed ratios, notable discrepancies exist at the lower tip speed ratios, where the VAWT motion itself deviates more from the sinusoid. Comparison with sinusoidal motions at reduced frequencies corresponding to the upstroke or downstroke of the VAWT-shaped motion yield better agreement in terms of the angle of stall onset. 

Abstract Unsteady airfoil experiments were conducted in a high-pressure wind tunnel at chord Reynolds numbers of Re c = 3.0 × 10 6 . A moderately thick NACA0021 airfoil was pitched from rest beyond the static stall angle in six individual ramp tests with increasing and decreasing angles of attack. The variant types of motion of the pitching maneuvers were characterized by constant angular velocity, angular acceleration and angular jerk, respectively. The ramp-up experiments revealed a substantial and time-dependent excess of the aerodynamic forces from static values in all three test cases and exhibited a distinct time delay as a consequence of the variant motion types. Similarly, the ramp-down experiments were largely impacted by the progression of the pitching motion, resulting in pronounced differences in the temporal development of lift and drag. Results are shown as time series of integrated forces and surface pressure distributions.