Unsteady effects on a pitching airfoil at conditions relevant for large vertical axis wind turbines

Interest in offshore vertical axis wind turbines (VAWTs) has created a need to study VAWTs at much higher Reynolds numbers than they have previously been studied at. VAWTs are characterised by unsteady aerodynamics, and high Reynolds numbers have the potential to alter the blade aerodynamics significantly. Here, results are reported on an airfoil that is pitched sinusoidally around zero angle of attack at Re_c = 1 × 10⁶, at a reduced frequency k = 0.15 and amplitudes of 5° ≤ α ≤20°. Since the static stall angle is not exceeded, no stall effects occur. Nevertheless, lift, drag and moment coefficients show noticeable hysteresis. Despite the high amplitudes of oscillation, lift and moment coefficients show reasonable agreement with unsteady aerodynamic theories by Theodorsen and Motta et al. with regard to the width of the hysteresis loops, but have noticeably steeper slopes.