High-resolution three-dimensional velocity field measurements and flow visualization results were acquired in the flow field near the tip of a rotor blade operating in hover. Using three-component laser Doppler velocimetry (LDV), the measurements documented the trailing vortex formation, the initial core structure, and the viscous evolution of the vortex properties. The test conditions covered a range of wake-ages from as young as one-degree, up to about one rotor revolution. For each wake age, vortex core properties were estimated from the velocity field measurements. The high spatial resolution obtained with LDV has shown that the tip vortex core radius can be less than 3% chord at early wake-ages, but grows asymptotically as it ages. A significant axial velocity deficit (on the order of the peak swirl velocity) existed in the vortex core at early wake-ages, but the deficit rapidly diminished as the vortex aged. Using a Richardson parameter combined with flow visualization, the results suggest that the inner core of the vortex is mostly laminar at the vortex Reynolds numbers tested in this experiment. It was found that bands of turbulent eddies, originating from an adjacent vortex sheet, underwent a re-laminarization process as they were entrained into the vortex core. The evidence suggests that the entire tip vortex structure is neither fully laminar nor fully turbulent, but is instead in a continuous state of dynamic evolution with a region of relatively slow laminar diffusion and a region of accelerated turbulent diffusion. It is suggested that the variation of peak swirl velocity is the result of the competing influences of an inviscid roll-up process, relaminarization inside the vortex core, and viscous diffusion around the core boundary.
High resolution trailing vortex measurements in the wake of a hovering rotor
Hochauflösende Messung der Wirbelströmung unter einem Hubschrauberrotor im Schwebflug
Journal of the American Helicopter Society ; 48 , 1 ; 39-52
2003
14 Seiten, 119 Bilder, 64 Quellen
Article (Journal)
English
High Resolution Trailing Vortex Measurements in the Wake of a Hovering Rotor
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