A Quantitative Analysis of Starting Jet Vortex Ring Entrainment at Low Reynolds Number
Abstract
The future propulsion of very small unmanned air and underwater vehicles operating at Reynolds numbers in the hundreds or less may be accomplished by pulsing a jet of fluid rather than more conventional methods of propulsion such as propellers and turbo jets as used by large-scale vehicles. Pulse jet propulsion produces trailing vortex rings and recent research concerning the formation and evolution of vortex rings has suggested that these structures may be exploited to augment the thrust and increase the efficiency of pulsed jet propulsion. Limited studies have indicated correlations between the length and shape of the generating pulse and the formation, fluid entrainment, and thrust augmentation provided by the resultant vortex ring at ring Reynolds number as low as approximately 13,000. The onset of vortex ring pinch-off has previously been shown to be a limit to this thrust augmentation. The design, construction, and implementation of a novel, piston-cylinder-type, vortex ring generator to confirm and further improve measurements of fluid entrainment and other formation characteristics has been accomplished. This apparatus was used in dye injection studies and stereoscopic particle image velocimetry (SPIV) studies to qualitatively examine the evolution of the vortex ring, and evaluate the effects of pulse length and shape on the formation of the vortex ring over a range of Reynolds numbers between 250 and 13,000. Results from this investigation at a Reynolds number of 13,000 are in close agreement with those of previous research, indicating that pulse shaping can delay the phenomena of vortex ring pinchoff by as much as 20% of the stroke length to diameter ratio. Results from lower Reynolds numbers show a trending toward even greater delay of pinch-off, as Reynolds number decreases. Tests conducted at Re=250 indicate a delay in pinch-off in excess of 400% over that seen at Re=13,000.
Document Details
- Document Type
- Technical Report
- Publication Date
- May 07, 2007
- Accession Number
- ADA473331
Entities
People
- Alex G. Dulude
Organizations
- United States Naval Academy