Journal of Fluid Mechanics


Ultra-fast escape of a deformable jet-propelled body

G. D. Weymoutha1 c1 and M. S. Triantafylloua2

a1 Southampton Marine and Maritime Institute, University of Southampton, Southampton, SO17 1BJ, UK

a2 Center for Ocean Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA


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.

(Received December 05 2012)

(Revised January 18 2013)

(Accepted January 23 2013)

(Online publication March 13 2013)

Key words

  • biological fluid dynamics;
  • boundary layer control;
  • drag reduction


c1 Email addresses for correspondence:,

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