Characteristics of Pitching and Plunging Airfoils Under Dynamic-Stall Conditions

An experimental investigation into the dynamic-stall process of a pitching and plunging airfoil at low Reynolds numbers has been carried out using direct force measurements and smoke visualization in an Eiffel-type wind tunnel. The strong influence of reduced frequency (k = pi fc/U(infinity)) on the vortical wake of both pure-plunging and pure-pitching airfoils is revealed. Here, a transition from a bluff body to a mushroom-type wake has been observed at approximately k = 0.2. Some associated lift and moment hysteresis curves for combined pitching and plunging motions are then presented with an accompanying discussion on the nature of the dynamic-stall process. For these complex motions, it is observed that both lift and moment phase lags grow with reduced frequency from k = 0.05 to 0.1. Despite substantial lift augmentation in the light- and deep-stall regimes, strong pitching-down moments are not avoided.