Vortical Sheet Behavior in the Wake of a Rotor in Ground Effect

Particle image velocimetry measurements with high spatial resolution were conducted on a hovering two-bladed rotor system operating in ground effect above a planar surface. The objective was to gain a better understanding of the fluid dynamics of the wake sheets trailed behind rotating blades, with specific focus on their contribution to the development of the near-wall flowfield at the ground plane. Two blade sets were tested: 1) a baseline untwisted blade and 2) a twisted blade with 17 deg of nose-down linear twist. Planar particle image velocimetry measurements were made for multiple regions of interest in the rotor wake to understand the temporal development of the wake sheets as they convected from their origin at the trailing edges of the blades until they interacted with the ground. The wake sheets were shown to contain relatively large amounts of concentrated vorticity, which persisted much longer in the wake than for the corresponding out of ground effect case. The wake sheet substantially influenced the nature of the developing flow over the ground, resulting in relatively large and higher transient excursions in the near-wall flow velocities further inboard and closer to the rotor. Performance measurements were also taken for both blade sets at various heights above the ground, with the untwisted blades showing the most significant increase in performance.