Aerospace Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2737
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Item Contributions Towards the Detailed Understanding of Rotor Flow Fields in Ground Effect Operations(2014) Milluzzo, Joseph; Leishman, John G; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)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.Item EFFECTS OF BLADE TIP SHAPE ON ROTOR IN-GROUND-EFFECT AERODYNAMICS(2011) Milluzzo, Joseph; Leishman, John G; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)High-speed videographic flow visualization and detailed two-component particle image velocimetry (PIV) measurements were conducted to examine the wake produced by a hovering single-bladed rotor as it interacted with a horizontal ground plane. The mo- tivation of the work was to better understand the nature of the flow field at the ground and the possible aerodynamic mechanisms that create brownout dust clouds when rotorcraft take off and land over surfaces covered with loose sediment. Rotors with four different blade tips were tested: 1. A baseline rectangular tip, 2. A simple 20◦ swept tip, 3. A BERP-like tip, and 4. A slotted tip. Flow visualization was performed using a high- repetition rate Nd:YLF laser that illuminated appropriately seeded flows in radial planes, with imaging performed using a high-speed CMOS camera. PIV measurements were performed in regions near the blades and at the ground plane by using a Nd:YAG laser with a CCD camera. Measurements as functions of wake age were obtained to examine the morphology of the vortical rotor wake during its interaction with the ground. The results showed that the wake was subjected to powerful curvature and straining effects as it interacted with the ground plane and was deflected into a radially outward direction along the plane. Reintensification of the tip vortices during the interaction caused them to remain very distinct features in the flow near the ground to as old as six or more ro- tor revolutions. The unsteady outward flow over the ground plane was shown to have similarities to a classical turbulent wall jet, especially further away from the rotor. Flow measurements were obtained deep into the boundary layer region at the ground, and in some cases into the laminar sublayer. The results showed certain common flow features between the four blade tips, but also differences in the flows that may ultimately affect the problem of brownout. The slotted-tip was shown to be particularly effective in diffusing the tip vortices and reducing the overall intensity of the fluctuating aspects of the flow at the ground.