Flow visualization and phase‐resolved particle image velocimetry (PIV) experiments were conducted on a hovering rotor to help understand the fluid dynamics of its vortical wake as it interacted with a horizontal ground plane. Visualization was performed by illuminating submicron
tracer particles using a strobed laser sheet that was phase locked to the rotational frequency of the rotor. Two‐component PIV measurements of the flow in radial planes were obtained at four rotor heights off the ground. Measurements at several wake ages were obtained to examine the
morphology of the wake—surface interaction process. The results showed that the rotor wake and its downwash flow are initially subjected to powerful curvature and straining effects as it is deflected into a radially outward direction at the ground plane. An unsteady outward flow comprising
a wall jet then developed in the region between the ground plane and the vortical remnants of the rotor wake. The viscous processes of diffusion, vorticity intensification by straining, as well as turbulence generation were all shown to be factors affecting the fluid dynamics of the resulting
flow field. In general, the tip vortices were found to induce significantly unsteady flow velocities if they reached the ground plane without undergoing substantial diffusion, especially when they remained coherent enough for adjacent vortices to pair and possibly merge. Further away from
the rotor, the turbulence and velocity gradients in the developing wall jet were noted to shear the tip vortices and accelerate the diffusion of vorticity. An intermediate rotor height off the ground was found where the diffusion of vorticity was countered by the straining effects in the flow,
which in this case resulted in the highest flow velocities and shear stresses on the ground plane.
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Document Type: Research Article
Department of Aerospace Engineering, Glenn L. Martin Institute of Technology, University of Maryland, College Park, MD
Publication date: 2010-04-01
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