BIRCH, J.M.*; DICKINSON, M.H.: Dynamic stall of a flapping appendage in the absence of a spiral vortex

Recent studies of insect flight describe how dynamic stall and the formation of a leading edge vortex generate large lift forces for insects. This leading edge vortex, along with forces generated through rotation and wake capture, accounts for all lift necessary for insects to fly. This vortex is present throughout the translation portion of the wingbeat cycle and has been shown to grow in size from the base toward the wing tip. It has been hypothesized that fluid spirals outward through the center of this vortex, draining energy from the core, allowing the vortex to remain stable throughout the stroke. Using a robotic flapping model of a Drosophila wing, we visualized the flow of fluid around the wing and reconstructed the flow structure using digital particle image velicometry. At mid-downstroke, the leading edge vortex consists of a large sheet of vorticity that stretches back from the leading to trailing edges of the wing. However, unlike the flow pattern seen in the hawkmoth, Manduca sexta, flow within the leading edge vortex was < 5% of wing tip velocity. We tested the idea that this axial flow was necessary to maintain the stability of the leading-edge vortex. Using both a wall that circumscribed the wing sweep as well as fences on the wing, we inhibited axial flow through the vortex core. When axial flow was not permitted off the wingtip (the wall case), the tip vortex detached from the wing more distally and forces generated by the wing increased by at least 14%. Fences had little effect on force generation. From these experiments we hypothesize that in Drosophila, axial flow has little effect on the stability of the leading edge vortex, and thus its size is not the limiting factor when determining the performance of a flapping appendage.