Turbulence Measurements in Blade Tip Vortices Using Dual-Plane Particle Image Velocimetry

The mean and turbulent flow characteristics of the blade tip vortices generated by a hovering rotor were studied using dual-plane stereoscopic digital particle image velocimetry (DPS-DPIV). The DPS-DPIV technique permitted the non-invasive measurement of the three components of the unsteady velocity field, and the nine components of the velocity gradient tensor. DPS-DPIV is based on coincident flow measurements made over two differentially-spaced laser sheet planes, thus allowing for velocity gradient calculations to be made also in a direction orthogonal to the measurement planes. A polarization-based technique employing beam-splitting optical cubes and filters was used to give the two laser sheets orthogonal polarizations, and to ensure that the cameras imaged Mie scattered light from only one or other of the laser sheets. The digital processing of the images used a deformation grid correlation algorithm, optimized for the high velocity gradients and small-scale turbulent flows found inside vortices. Detailed turbulence measurements were used to derive the fluctuating terms that are involved in the Reynolds-averaged stress transport equations. The results have shown that the turbulence distribution is anisotropic, and that the stress in the flow cannot be represented as a linear function of its strain. The measurement of all nine velocity gradients also allowed for precise measurements of the vorticity vector, as well as the inclination between the vortex axis and the measurement plane. This inclination was found to be almost orthogonal at all early wake ages. ∗ NDSEG Fellow. bjo212@umd.edu † Assistant Research Scientist. mani@umd.edu ‡ Minta Martin Professor. leishman@umd.edu Presented at the 34th European Rotorcraft Forum, Arena and Convention Center, Liverpool, England, September 16–19, 2008. c © 2008 by Johnson, Ramasamy & Leishman. All rights reserved. Published by the Royal Aeronautical Society of Great Britain with permission. Nomenclature

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