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Ultra-fast escape of a deformable jet-propelled body

Ultra-fast escape of a deformable jet-propelled body
Ultra-fast escape of a deformable jet-propelled body
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.
biological fluid dynamics, boundary layer control, drag reduction
0022-1120
367-385
Weymouth, G.D.
b0c85fda-dfed-44da-8cc4-9e0cc88e2ca0
Triantafyllou, M.S.
8b2b42be-39f5-41ab-b9c8-5ba019b04b6d
Weymouth, G.D.
b0c85fda-dfed-44da-8cc4-9e0cc88e2ca0
Triantafyllou, M.S.
8b2b42be-39f5-41ab-b9c8-5ba019b04b6d

Weymouth, G.D. and Triantafyllou, M.S. (2013) Ultra-fast escape of a deformable jet-propelled body. Journal of Fluid Mechanics, 721, 367-385. (doi:10.1017/jfm.2013.65).

Record type: Article

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.

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e-pub ahead of print date: 13 March 2013
Published date: 25 April 2013
Keywords: biological fluid dynamics, boundary layer control, drag reduction
Organisations: Fluid Structure Interactions Group

Identifiers

Local EPrints ID: 349868
URI: http://eprints.soton.ac.uk/id/eprint/349868
ISSN: 0022-1120
PURE UUID: 1028a62f-b8ad-4faa-a5ff-43fb899d236b
ORCID for G.D. Weymouth: ORCID iD orcid.org/0000-0001-5080-5016

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Date deposited: 13 Mar 2013 10:57
Last modified: 09 Jan 2022 03:44

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Contributors

Author: G.D. Weymouth ORCID iD
Author: M.S. Triantafyllou

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