Flow Visualization and Measurements in the Wake of a Rotor with a Subwing Tip

A rotor blade with a subwing tip was investigated to explore the influence of multiple trailed tip vortices on the evolution of a hovering rotor wake. The wake geometry was examined using strobed laser sheet flow visualization. The velocity field was measured using a three component laser Doppler velocimetry system. The subwing tip shape was found to generate two trailing vortices, one at the tip of the main blade and the other at the tip of the subwing. These two separate vortices tend to be weaker than the single tip vortex trailed from a standard blade tip. As the vortex pair convected below the rotor, the two vortices interacted with each other and eventually merged into a single vortex. The merging process of the two vortices, enhanced the viscous diffusion of the tip vortex, thereby reducing the induced velocities. However, it was found that the subwing tip modification did not significantly alter the rotor induced velocity field or the net circulation strength of the tip vortex.