Flowfield Characteristics on a Retreating Rotor Blade

Abstract

Abstract : The aerodynamics of a rotor become uncertain when the rotor is in edgewise flight at advance ratios approaching unity. The retreating blade encounters reverse flow over significant portions of the span. Traditional approaches have attempted to make yaw corrections to 2-dimensional airfoil aerodynamics. This project used stereo particle image velocimetry on a 2-bladed rotor at advance ratios of 0.7, 0.85 and1.0, focusing on rotor azimuths 240, 270 and 300 degrees. These complemented load measurements and tuft visualization on the same blade used as a fixed wing in forward and backward sweep in forward and reversed flow. SPIV was also used on the blade held fixed at discrete yaw attitudes. The results vindicated a vortex flow model for reverse flow aerodynamics. Soon after 180 degrees azimuth, a strong vortex forms under the sharp edge of the blade, similar to that on the leeside of a delta wing or strake at angle of attack. By 240 degrees the vortex is strong, similar to that on a forward-swept delta wing, with the highest dynamic pressure near the root. As the yaw decreases, the vortex bursts. By 270 degrees, the vortex is burst and detached. It convects with the blade, and is seen under the blade at 300 degrees. The pressure distribution under the blade is explained using vortex interactions. Three-dimensional separation over the blunt edge does not generate periodic shedding in these experiments. Extraction of static pressure from the velocity field is explored, and gives useful results.