Fluid Dynamics of Interacting Blade Tip Vortices With a Ground Plane

Experiments were conducted using flow visualization and phase-resolved digital particle image velocimetry (DPIV) to examine the fluid dynamics of a rotor wake as it interacted with a horizontal ground plane. The experiments were conducted using a rotor with a ground plane that could be moved to as close as 25% of the rotor radius to the rotor. Visualization of the rotor wake and the wake/ground interactions was performed by illuminating a smoke seeded flow using a Nd:YAG laser sheet, phase-locked to the rotational frequency of the rotor. Two-component DPIV measurements were obtained at four rotor heights off the ground. Measurements at several wake ages were obtained to examine the dynamic features of the wake/surface interaction process. The results showed that the overall downwash induced by the rotor initially becomes an unsteady radial wall jet defined between the ground and the rotor wake boundary. The viscous processes of diffusion, vortex straining, and turbulence generation are shown to be significant factors in understanding fluid dynamics of the resulting flow. At the higher rotor heights, the vortices diffused before they reached the ground. At the lower rotor heights, vortex stretching countered the effects of diffusion but turbulence in the developing wall jet sheared the vortices, accelerating their diffusion. The tip vortices still created substantial unsteady shear stresses on the ground, in some cases exceeding twice the time-averaged values. An intermediate rotor height was found to produce the highest flow velocities at the ground. Corresponding measurements of performance showed the expected reduction in rotor power and an increase in thrust for in-ground-effect operations.