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Stable and unstable monopolar vortices in a stratified fluid

Published online by Cambridge University Press:  26 April 2006

J. B. Flór  and
G. J. F. Van Heijst
J. B. Flór
Affiliation:
Fluid Dynamics Laboratory, Department of Technical Physics, Eindhoven University of Technology, PO Box 513, 5600MB Eindhoven, The Netherlands Present address: University of Camrbidge, Department of Applied Mathematics and Theoretical Physics, Silver Street, Cambridge CB3 9EW, UK.
G. J. F. Van Heijst
Affiliation:
Fluid Dynamics Laboratory, Department of Technical Physics, Eindhoven University of Technology, PO Box 513, 5600MB Eindhoven, The Netherlands
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Abstract

This paper presents experiments on planar monopolar vortex structures generated in a non-rotating, stratified fluid. In order to study the dynamics of such planar vortices in the laboratory, angular momentum was generated in a specific horizontal layer of the stratified fluid, by using three different generation mechanisms. The lens-shaped monopolar vortices thus created were in some cases stable and conserved their circular symmetry, while in other cases they appeared to be unstable, leading to the formation of a multipoled vortex with a different topology. Characteristics such as cross-sectional profiles (angular velocity and vorticity) and vorticity-stream function scatter plots have been measured experimentally by using digital image processing techniques. The characteristics of the monopolar vortices are compared with analytical vortex models known from literature. Simple models, based on vertical diffusion of vorticity, are proposed to describe the monopolar vortex decay; they show reasonable agreement with the experimental results.

From the multipolar structures, the tripolar vortex and a specific case of a triangular vortex, neither having been observed before in a stratified fluid, are studied in detail. A comparison with point-vortex models yields good agreement. Although these multipolar vortices appear to persist for a long while, they are found eventually to be unstable and to transform into a monopolar vortex.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 311 , 25 March 1996 , pp. 257 - 287
Copyright
© 1996 Cambridge University Press

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