High-Amplitude Pitch of a Flat Plate: An Abstraction of Perching and Flapping

Abstract

We compare water tunnel experiment and 2D vortex-particle computation for a generalization of the classical problem of flat-plate constant-rate pitch and related motions, at frequencies and Reynolds numbers relevant to Micro Air Vehicle applications. The motivation is problems of maneuvering, perching and gust response. All of the examined flows evince a strong leading edge vortex. Increasing pitch rate tends to tighten the leading edge vortex and to produce a trailing-edge vortex system dominated by a counter-rotating pair. Pitch pivot point location is crucial to the leading edge vortex size and formation history, and to its subsequent behavior in convecting over the airfoil suction-side. Despite the respective limitations of the experiment and computations, agreement in vorticity fields between the two at an overlapping case at Re = 10,000 is good, whence it is possible to use the computation to obtain integrated force data unavailable in the experiment. These were studied for Re= 100 and 1000. Lift prediction from the computation shows a direct proportionality of lift to the pitch rate on the pitch upstroke. Finally, we compare pitch vs. plunge, and find that quasi-steady prediction is reasonably successful in predicting a combined pitch-plunge that effectively cancels the leading edge vortex, but not in canceling the trailing vortex system.