Experiments on Pitching Plates: Force and Flowfield Measurements at Low Reynolds Numbers

Abstract

We discuss implementation of prescribed-motion rigs, dye injection and direct force measurement for high frequency, high-amplitude smoothed linear pitch ramps of flat-plate models in a water tunnel. We extend recent results on 2D (wall to wall) flat plates pitching from 0° to 90°, to rectangular and elliptical (Zimmerman planform) plates of aspect ratio 2. Surveying a range of pivot point locations and pitch rates, we compare the development of leading edge vortices, tip vortices and trailing edge vortices, with trends in lift coefficient and drag coefficient history. The lift peak is seen to correlate with the maximum size of the leading edge vortex, prior to its shedding and downstream convection. Tip vortices evolve more slowly than leading edge vortices, in the sense that a tip vortex remains coherent and nearly attached to the wingtip until well past leading edge vortex saturation and shedding. Peak lift is delayed to progressively higher angles of attack as the reduced frequency is increased. And the non-circulatory or apparent-mass forces are evidently additive to the circulatory aerodynamic forces, implying applicability of linear superposition.