Physics of Reverse Flow on Rotors at High Advance Ratios

When a rotor blade encounters reverse flow in edgewise flight at high advance ratio, a strong vortical structure forms on the leeside of the sharp blade edge. This has unexpected effects on blade aerodynamics. A differential onset velocity gradient due to rotor rotation creates an envelope of reverse flow extending from the hub. A Sharp Edge Vortex (SEV) is observed at the geometric trailing edge, forming soon after the blade enters the retreating-blade side. The SEV grows radially inwards in size and strength as the blade retreats. The SEV convects and evolves along with the rotor blade until dissipation. In this paper, a coherent vortex is observed at 240 degrees azimuth. Vortex size and strength increase as the blade retreats, ceasing to grow before 270 degrees. The reverse flow envelope increases at higher advance ratios with increased vortical strength. The solid body rotation core of the vortex stretches at lower advance ratios and shows the signs of a burst vortex in the dissipation phase. The proximity of the SEV to the blade causes large excursions in surface static pressures which in turn generates significant negative lift on the retreating blade. The attached, coherent sharp edge vortex shows similar morphological features as the leading edge vortex on a delta wing.