Effect of hybrid-heave motions on the propulsive performance of an oscillating airfoil

Abstract

Inspiration for a number of unsteady aerodynamics studies has been gathered from biology, mimicking locomotion of birds, insects, and fish. We instead employ a sports-mimetic approach to 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. Because sailors do not sail directly into the wind, the boat travels at an angle relative to the wind it experiences, which is generally referred to as the “apparent wind angle.” Due to this angle, the sail oscillates at non-perpendicular angles to the incoming flow, in a motion we call “hybrid-heave”We study these hybrid-heave oscillations with a NACA 0012 airfoil, focusing on its effects on the lift force as well as the driving force, which is aligned in the direction the boat is pointing. The benefits of hybrid-heave are seen across all cases studied here, with the maximum mean lift of up to 6 times the lift of a static airfoil and 2 times the lift of classical heave (heaving perpendicular to the free-stream). We find the existence of a “high-lift mode” close to apparent wind angles (AWA) ≈ +45°, and a “low-lift mode” centered around AWA ≈ -45°. Similar gains are seen in the driving forces with more than three-fold increases compared to the static driving force at optimal conditions. The vortex dynamics of the high-lift case reveal a departure from classical pure heave whereby only one side of the airfoil generates a leading-edge vortex in every cycle, leaving only 3 vortices shed per cycle. High lift is generated for in-phase variation of the airfoil’s instantaneous angle of attack and speed with respect to time. Conversely, out-of-phase variation of these parameters leads to a reduction in lift, compared to the static airfoil case.