Trailed circulation of hovering rotors with leading-edge protuberances

Abstract

The effect of leading-edge protuberances on the wake of a rotor hovering out of ground effect was examined using performance and phase-resolved particle image velocimetry measurements. Protuberance amplitude and wavelength were investigated using a set of baseline rectangular blades and four blade sets with various sinusoidal leading-edges. For blade loading coefficients below 0.06, protuberances had minimal effect on rotor performance, but as the thrust increased the modified blades began to incur an additional power requirement. The rotor wake was significantly affected by protuberance amplitude. Specifically, increasing protuberance amplitude generated a more uniform inflow distribution, as well as increased size, magnitude, and strength of the vorticity trailed into the wake sheet. Furthermore, an examination of the spanwise distribution of circulation trailed into the wake sheets showed that the modified blades produced a sinusoidal variation in both the positive and negative circulation distribution. As compared to the baseline blade, the highest amplitude blades also increased the magnitude of the mean circulation (both positive and negative) about which the sinusoidal oscillation occurred.