Effects of Advance Ratio and Radial Location on the Vortex Structure on a Rotating Blade in Reverse Flow

At high advance ratio, rotor blades encounter reversed flow over substantial portions of the retreating blade side. Prior approaches have modeled these using quasi-static separation with airfoil data modified for yaw, vortex shedding, dynamic pitch oscillations, and reverse dynamic stall of an airfoil. We pursue the hypothesis that development of a sharp-edge vortex (SEV) on a highly swept edge at angle of attack, modified with rotation effects, is a more viable model. Stereo Particle Image Velocimetry (SPIV) is used under a reversing blade of a 2-bladed rotor with cyclic pitch at advance ratios from 0.7 to 1.0 in a low speed wind tunnel. A sharp-edged vortex (SEV) develops and grows, starting in the forward-yaw region between 180° and 270° azimuth, with a significant inboard-directed core flow and outboard-directed surface layer flow. As yaw decreases, the vortex stops growing, and bursts. By 270° this vortex detaches at some locations and convects. In the backward-yaw region beyond 270°, the vortex again grows outboard, with the pressure gradient due to the vortex causing an inboard-directed surface pressure gradient that overcomes centrifugal effects. The occurrence of the vortex ahead of the sharp edge at some stations negates models based on sharp-edge shedding. Dynamic lift is not observed prior to stall. There is no evidence of unsteady shedding from the sharp edge.