Turbulence measurements were performed inside the vortical wake of a small hovering rotor, using stereoscopic, particle image velocimetry (PIV). An improved method of measurement correction based on the concept of helicity in the flow was used to separate the biasing effects of the inherent aperiodicity in the wake from the actual turbulence properties within the tip vortices. An analysis of the minimum number of PIV image pairs required to achieve statistical convergence for the turbulence quantities suggested at least 250 samples were needed for the first-order characteristics, and about 750 samples for the second-order characteristics. Significant turbulence activity was found up to two core radii from the tip vortex axis. The tip vortices were also shown to have asymmetric flow characteristics, with a pronounced anisotropic distribution of eddy viscosity, a typical characteristic of flows with high streamline curvature. The measurements should help to form a basis for developing improved turbulence models for vortex flows, as well as for validating existing computational fluid dynamics predictions.
Procedures for measuring the turbulence characteristics of rotor blade tip vortices
Verfahren zur Messung der Turbulenzcharakteristiken von Wirbeln an Rotorspitzen
Journal of the American Helicopter Society ; 54 , 2 ; 022006/1-022006/17
2009
17 Seiten, 18 Bilder, 45 Quellen
Aufsatz (Zeitschrift)
Englisch
Procedures for Measuring the Turbulence Characteristics of Rotor Blade Tip Vortices
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