Mechanism to Improve Propulsive Performance of an Oscillating Airfoil Employing Hybrid Heave Motions

A number of studies of unsteady airfoil dynamics have been inspired by animal locomotion. In the present research, we employ a sports-mimetic approach, where we study unsteady airfoil motions inspired by sail dynamics. Olympic sailors use various unsteady aerodynamic techniques to increase propulsion. One such technique is “sail flicking”, whereby sailors use their bodyweight to roll the boat about its longitudinal axis, flicking the sail periodically. This leads to a distinct high-lift mode and a low-lift mode, depending on the angle of the sail oscillations relative to the incoming apparent wind. By replacing the sail by a NACA 0012 airfoil, we show that this “hybrid-heave” motion, wherein the airfoil oscillates at non-normal angles to the incoming flow, exhibits strong lift amplification, up to 7-8 times the lift of a non-oscillating airfoil. Vortex dynamics and force measurements are made with reference to the airfoil trajectory through the fluid, indicating that the high-lift mode is associated with an in-phase variation of velocity and time-varying angle of attack of the airfoil.