Unsteady Lift Generation on Rotating Wings at Low Reynolds Numbers

Abstract

The rotating wing experiment is a fully three-dimensional simplification of the flapping-wing motion observed in nature. The spanwise velocity gradient and the wing starting and stopping acceleration that exist on an insectlike flapping wing are generated by the rotational motion of a finite-span wing. The flow development around a rotating wing at Re = 60, 000 has been studied using high-speed particle image velocimetry to capture the unsteady velocity field. Lift and drag forces have been measured for several different sets of wing kinematics and angles of attack. The lift curve shape was similar in all cases. A transient high lift peak, approximately 1.5 times the quasi-steady value, occurred in the first chord length of travel, and it was caused by the formation of a strong attached leading-edge vortex. This vortex then separated from the leading edge, resulting in a sharp drop in lift. As weaker leading-edge vortices continued to form and shed, lift values recovered to an intermediate value. The circulation of the leading-edge vortex has been measured and agrees well with the force data. Wing kinematics had only a small effect on the aerodynamic forces produced by the waving wing. In the early stages of the wing stroke, the velocity profiles with low accelerations affected the timing and the magnitude of the lift peak, but at higher accelerations, the velocity profile was insignificant.