Characterisation of Low Reynolds Number Fountain Behaviour

Abstract

Experimental evidence for previously unreported fountain behaviouris presented. It has been found that the first unstable mode of a wall bounded three dimensional round fountain is a laminar flapping motion that can grow to a circling or multi-modal 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 C = FrRe2/3. The transition to turbulence occurs at Re > 120, independent of Froude number. For Fr > 20 and Re . 120, sinuous shear 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. A regime map of the fountain behaviour for 0:7 < Fr < 55 and 15 < Re < 1100 is presented and the underlying mechanisms for the observed behaviour are proposed.