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Reynolds-number scaling of vortex pinch-off on low-aspect-ratio propulsors

Published online by Cambridge University Press:  23 June 2016

John N. Fernando  and
David E. Rival
John N. Fernando*
Affiliation:
Department of Mechanical and Materials Engineering, Queen’s University, Kingston, ON K7L 3N6, Canada
David E. Rival
Affiliation:
Department of Mechanical and Materials Engineering, Queen’s University, Kingston, ON K7L 3N6, Canada
*
†Email address for correspondence: john.fernando@queensu.ca
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Abstract

Impulsively started, low-aspect-ratio elliptical flat plates have been investigated experimentally to understand the vortex pinch-off dynamics at transitional and fully turbulent Reynolds numbers. The range of Reynolds numbers investigated is representative of those observed in animals that employ rowing and paddling modes of drag-based propulsion and manoeuvring. Elliptical flat plates with five aspect ratios ranging from one to two have been considered, as abstractions of propulsor planforms found in nature. It has been shown that Reynolds-number scaling is primarily determined by plate aspect ratio in terms of both drag forces and vortex pinch-off. Due to vortex-ring growth time scales that are longer than those associated with the development of flow instabilities, the scaling of drag is Reynolds-number-dependent for the aspect-ratio-one flat plate. With increasing aspect ratio, the Reynolds-number dependency decreases as a result of the shorter growth time scales associated with high-aspect-ratio elliptical vortex rings. Large drag peaks are observed during early-stage vortex growth for the higher-aspect-ratio flat plates. The collapse of these peaks with Reynolds number provides insight into the evolutionary convergence process of propulsor planforms used in drag-based swimming modes over diverse scales towards aspect ratios greater than one.

JFM classification

Biological Fluid Dynamics: Propulsion Biological Fluid Dynamics: Swimming/flying Vortex Flows: Vortex dynamics
Type
Rapids
Information
Journal of Fluid Mechanics , Volume 799 , 25 July 2016 , R3
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Copyright
© 2016 Cambridge University Press 

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