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Jets and plumes with negative or reversing buoyancy

Published online by Cambridge University Press:  28 March 2006

J. S. Turner
J. S. Turner
Affiliation:
Woods Hole Oceanographic Institution, Woods Hole, Massachusetts Present address: Department of Applied Mathematics and Theoretical Physics, University of Cambridge.
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Abstract

The motion of turbulent jets of heavy salt solution injected upwards into a tank of fresh water has been compared with that of plumes which are initially buoyant but become heavy as they mix with the environment. The reversal of buoyancy in the latter case is produced by using fluids having a non-linear density change on mixing, a laboratory analogue of the density changes occurring at the top of a cumulus cloud due to evaporation. The behaviour in the two cases is quite different; salt jets reach a steady height about which only small fluctuations occur, while the plumes with reversing buoyancy exhibit violent regular oscillations. This phenomenon, which is clearly a property of the ‘evaporation’ and not just of the geometry, is suggested as a likely explanation of the observed oscillation of the tops of cumulus towers. Dimensional arguments have been used to relate the experimental results to the volume, momentum and buoyancy fluxes at the source. An application of one of the deduced relations to the atmosphere gives realistic periods for the cloud-top oscillations.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 26 , Issue 4 , December 1966 , pp. 779 - 792
Copyright
© 1966 Cambridge University Press

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