Development of the Turbulent Vortex Sheet in the Wake of a Hovering Rotor

Flow visualization and particle image velocimetry experiments were conducted in the wake of a hovering two-bladed rotor to better understand the development and evolution of the trailed vortex wake sheet. Two sets of blades with different twist distributions were tested: 1. A baseline untwisted blade, and 2. A linearly twisted blade with 16 degrees of nose-down twist over its span. Each rotor was also tested at two blade loading coefficients of 0.053 and 0.08. Flow visualization and PIV measurements were made for multiple regions of interest in the rotor wake to understand the development and evolution of the vortex sheets as a function of wake age, and its possible interactions with the tip vortex. The vortex sheet was shown to be comprised primarily of small counterrotating vortex pairs, often called Taylor-Görtler vortices, which also were the source of significant amounts of turbulence in the rotor wake. However, the sign of the phase-averaged vorticity contained in the vortex sheet followed the expected distribution of bound circulation along the blade. The results also showed that the development of the vortex sheet and its subsequent dynamics depends on blade twist and rotor thrust. The use of twist was found to generate a more uniform axial velocity profile, which caused the vortex sheet to convect more uniformly in the wake. Whereas, the wake sheet produced by the rotor with untwisted blades convect non-uniformly because of the more linear axial velocity distribution in the wake. At early wake ages, a sinusoidal wave was seen to form in the vortex sheets, but its development was suppressed by axial stretching as it convected in the wake. The sinusoidal wave in the vortex sheet appeared to initiate from the influence of the developing tip vortices as well as self-induced effects of the vorticity contained within the sheet itself.