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Ultra-fast escape of a deformable jet-propelled body

Published online by Cambridge University Press:  13 March 2013

G. D. Weymouth  and
M. S. Triantafyllou
G. D. Weymouth*
Affiliation:
Southampton Marine and Maritime Institute, University of Southampton, Southampton, SO17 1BJ, UK
M. S. Triantafyllou
Affiliation:
Center for Ocean Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
*
†Email addresses for correspondence: weymouth@mit.edu, G.D.Weymouth@soton.ac.uk
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Abstract

In this work a cephalopod-like deformable body that fills an internal cavity with fluid and expels it to propel an escape manoeuvre, while undergoing a drastic external shape change through shrinking, is shown to employ viscous as well as mainly inviscid hydrodynamic mechanisms to power an impressively fast start. First, we show that recovery of added-mass energy enables a shrinking rocket in a dense inviscid flow to achieve greater escape speed than an identical rocket in a vacuum. Next, we extend the shrinking body results of Weymouth & Triantafyllou (J. Fluid Mech., vol. 702, 2012, pp. 470–487) to three-dimensional bodies and show that three hydrodynamic mechanisms must be combined to achieve rapid escape performance in a viscous fluid: added-mass energy recovery; flow separation elimination; and an optimized energy storage and recovery. In particular, we show that the mechanism of separation elimination achieved through rapid body shrinking, coordinated with the mechanism of recovering the initially imparted added-mass energy, is critical to achieving a high escape speed. Hence a flexible, collapsing body can be vastly superior to a rigid-shell jet-propelled body.

JFM classification

Biological Fluid Dynamics: Biological Fluid Dynamics Boundary Layers: Boundary layer control Flow Control: Drag reduction
Type
Papers
Information
Journal of Fluid Mechanics , Volume 721 , 25 April 2013 , pp. 367 - 385
Copyright
©2013 Cambridge University Press

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