Contributions Towards the Detailed Understanding of Rotor Flow Fields in Ground Effect Operations

Abstract

High-speed flow visualization and high-resolution particle image velocimetry experiments were conducted on a two-bladed rotor that was operated in a hovering state, both out of ground effect (well away from the ground) and in ground effect at several rotor heights. Recent advances in flow diagnostic instrumentation allowed measurements of the rotor wake to be performed with unprecedented levels of resolution. In particular, the goal of the present work was to gain a better understanding of the fluid dynamics of the wake sheets (and the blade tip vortices) that were trailed from the blades. The present work examined the effects produced by two blade sets: 1. A baseline untwisted blade, 2. A twisted blade with $-17^{\circ}$ of linear twist, and has revealed fluid dynamic details of the wake sheet that were hitherto unknown. For the measurements made with the rotor operating out of ground effect, the blade sets were tested at two blade loading coefficients of 0.053 and 0.08, although only the higher loading condition was tested with the rotors operating in ground effect. For the rotor operating out of ground effect, a helicoidal tip vortex was shown to form at the blade tip and the associated wake sheets were initially laid down as small-scale counterrotating vortical pairs. However, this initial vorticity quickly diffused and the sheet was then convected as a concentrated bands of turbulence, including several dominant eddies. Several types of sheet dynamics were documented in the rotor wake, including sheet interactions with the tip vortices, the detailed behavior of this interaction depending on both the blade twist and the rotor thrust. At earlier wake ages, a sinusoidal wave-like perturbation was seen to be formed on the wake sheets, although the growth in wave amplitude was limited as the sheets were convected and stretched in the velocity gradients in the downstream wake. When the rotor was operated in ground effect, the vorticity in the wake sheets persisted to much older wake ages. Wave-like perturbations did not form on the wake sheets when the rotors were operating in ground effect because the outward radial stretching of the rotor wake in the presence of the ground suppressed their development. The wake sheets were found to convect to the ground and introduce significant vorticity into the near-wall flow field closer to the rotor, contributing to fluctuations in the local flow velocities. The flow field near the ground was also observed to be significantly affected by the use of twist on the blade, with the wake impinging on the ground further inboard and closer to the rotor, which also resulted in higher flow velocities being produced further downstream.