Leading Edge Vortex Development on a Waving Wing at Reynolds Numbers Between 10,000 and 60,000

Abstract

The waving wing experiment is a fully three-dimensional simplification of the flapping wing motion observed in nature in which the spanwise velocity gradient and wing starting and stopping acceleration that exist on an insect-like flapping wing are generated by rotational motion of a finite span wing. The flow development around a waving wing at Reynolds numbers 10,000, 30,000, and 60,000 was studied using high speed PIV to capture the unsteady velocity field. Vorticity fields were computed and a vortex detection algorithm implemented in order to identify individual vortices in the flow. Vortex development was quantified by computing the circulation of the leading edge vortex as a function of time. The leading edge vortex was found to develop more quickly (in a non-dimensional time) at lower Reynolds numbers. Lift and drag forces were measured using a two-component force balance. The lift curve shape was similar at all of the Reynolds numbers tested. A transient high lift peak approximately 1.5 times the quasi-steady value occurred in the first chord-length of travel, caused by the formation of a strong attached leading edge vortex. Although the shape of the curve was similar, the timing of the transient lift peak varied with Reynolds number. The maximum lift at a Reynolds number of 30,000 occurred at a stroke angle of approximately θ = 3.6 deg, but at a Reynolds number of 60,000 the lift peak occurred at θ = 7.4 deg. Although Reynolds number was not found to affect the fundamental structure of the flow development, but it can affect the time-scale of the development of flow structures and thus the lift force produced.