Journal of Fluid Mechanics

Papers

Low-Reynolds-number fountain behaviour

N. WILLIAMSONa1, N. SRINARAYANAa1, S. W. ARMFIELDa1, G. D. McBAINa1 and W. LINa2

a1 School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, NSW, 2006 Australia

a2 School of Engineering, James Cook University, Townsville, Queensland, 4811 Australia

Abstract

Experimental evidence for previously unreported fountain behaviour is presented. It has been found that the first unstable mode of a three-dimensional round fountain is a laminar flapping motion that can grow to a circling or multimodal flapping motion. With increasing Froude and Reynolds numbers, fountain behaviour becomes more disorderly, exhibiting a laminar bobbing motion. The transition between steady behaviour, the initial flapping modes and the laminar bobbing flow can be approximately described by a function FrRe2/3=C. The transition to turbulence occurs at Re > 120, independent of Froude number, and the flow appears to be fully turbulent at Re≈2000. For Fr > 10 and Re≲120, sinuous shear-driven instabilities have been observed in the rising fluid column. For Re≳120 these instabilities cause the fountain to intermittently breakdown into turbulent jet-like flow. For Fr≲10 buoyancy forces begin to dominate the flow and pulsing behaviour is observed. A regime map of the fountain behaviour for 0.7≲Fr≲100 and 15≲Re≲1900 is presented and the underlying mechanisms for the observed behaviour are proposed. Movies are available with the online version of the paper.

(Received February 11 2008)

(Revised April 20 2008)

Key words

  • Jets;
  • Instability;
  • Transition to turbulence
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