Williamson, N., Srinarayana, N., Armfield, S.W., McBain, G.D., and Lin, W. (2008) Low-Reynolds-number fountain behaviour. Journal of Fluid Mechanics, 608. pp. 297-317.

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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 0.7Fr 100 and 15Re 1900 is presented and the underlying mechanisms for the observed behaviour are proposed. Movies are available with the online version of the paper.

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