Experiments on Free-to-Pivot Hover Motions of Flat Plates

Abstract

Using force measurements and flow visualization in a water tunnel, we consider motions of a flat plate with square edges, free to pivot about its leading edge, between incidence angle limits of ±45°. The plate’s leading edge undergoes a prescribed sinusoidal motion, either of rectilinear translation, or of pivoting or waving about a fixed point 0.5 chords away from one of the plate’s wingtips. During most of the translation semi-stroke in either direction, the plate rests against its incidence limiter to produce a positive angle of attack; this reverses on the opposite semi-stroke, producing a motion akin to normal-hover with delayed rotation. Two geometries are considered: a nominally 2D or wall-to-wall plate, and a plate of aspect ratio 3.4. Reynolds number effects in the range of 5000-20,000 were not found to be significant, but the ratio of stroke amplitude to plate chord determines Depending on stroke to chord amplitude, lag between plate rotation and translation will differ, and the resulting vortex production history and aerodynamic load production history will differ. Large stroke to chord ratios produces higher thrust coefficients and simpler vortex wakes. Thrust coefficient histories are very similar between the translating 2D and AR=3.4 plates, and resistive force coefficients are very similar amongst all three cases – suggesting that whatever distinctions between the three cases may be present in the flowfield due to putative spanwise pressure gradient, these effects do not systematically alter the integrated aerodynamic forces.