Effect of Mean Angle of Attack Modulation on Dynamic Stall

Wind tunnel experiments at $M=0.2$ were conducted on a cambered airfoil instrumented with surface pressure transducers that was oscillated with two independent frequencies. The primary input, $f_1$, corresponds to a range of reduced frequencies, while the slower, secondary input, $f_2$, drives the modulation of the mean angle of attack, thus varying the stall-penetration angle, $\alpha_{pen}$. Various combinations transitioned different regimes of dynamic stall from ``light" to ``deep". Results suggest that when $\alpha_{pen}$ is falling between consecutive cycles, the aerodynamic loads do not fully recover to the values seen when $\alpha_{pen}$ is rising, even though the airfoil recedes to $\alpha_{pen}<0$ during each oscillation. The experimental data is presented in terms of load coefficients, aerodynamic damping, and their phase relationships to pitch angle.