Effects of Body Shapes on Rotor In-Ground-Effect Aerodynamics

Abstract

Videographic flow visualization and two-component particle image velocimetry (PIV) measurements were performed to examine the developing vortical wake produced by a two-bladed hovering rotor at a height of one rotor radius above a horizontal ground plane. The experiments were performed with an isolated rotor and with three different bodies placed in the wake below the rotor. The bodies examined had circular, elliptical, and rectangular cross-sections, respectively. Flow measurements were taken in planes that covered the nose and tail region of each body. The objective of the study was to gain a better understanding of the nature of the flow at the ground plane and to assess the overall effects of a body in the rotor wake, and in particular to document the nature of the unsteady, turbulent boundary layer flow over the ground. The flow visualization and PIV were performed using a Nd:YAG laser that illuminated a radial plane of the flow, with imaging performed with a CCD camera. Measurements of the spatial locations of the tip vortices as a function of wake age were obtained to quantify the wake distortion produced by each body shape. The outward flow over the ground plane was shown to have similar characteristics to a classical turbulent wall-jet; these similarities were especially apparent further downstream on the ground plane away from the rotor. The results showed that the flow over the nose of each of the bodies was similar to that of the isolated rotor, but with some minor differences in the flow at the ground. The slipstream boundary was shown to be severely disrupted by the tail of each body, and showed larger variations from that produced by the isolated rotor. Wake impingement on the body was shown to cause catastrophic bursting of the blade tip vortices. The body with a rectangular cross-section was generally found to produce the greatest differences in the overall flow characteristics near the ground plane compared to that of the isolated rotor. The work has relevance to the better understanding the problem of rotorcraft brownout, where the near-wall flow drives the mobilization and uplift of dust.