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The structure of the near wake of a sphere moving horizontally in a stratified fluid

Published online by Cambridge University Press:  26 April 2006

J. M. Chomaz ,
P. Bonneton  and
E. J. Hopfinger
J. M. Chomaz
Affiliation:
Meteo-France CNRM Toulouse, 42 avenue Coriolis, 31057 Toulouse, France LADHYX, Ecole Polytechnique, 91128 Palaiseau-Cedex, France
P. Bonneton
Affiliation:
Meteo-France CNRM Toulouse, 42 avenue Coriolis, 31057 Toulouse, France
E. J. Hopfinger
Affiliation:
LEGI-IMG, BP 53, 38041 Grenoble-Cedex, France
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Abstract

We present experimental results for the wake structure of spheres moving in homogeneous and stratified fluid. In homogeneous fluid, the results of Kim & Durbin (1988) are confirmed and it is shown that the two characteristic frequencies of the wake correspond to two instability modes, the Kelvin–Helmholtz instability and a spiral instability. For the stratified wake four general regimes have been identified, depending principally on the Froude number F. For F > 4.5 the near wake is similar to the homogeneous case, and for F < 0.8 it corresponds to a triple-layer flow with two lee waves, of amplitude linear in F, surrounding a layer dominated by quasi-two-dimensional motion. Froude numbers close to one (F∈]0.8, 1.5[) give rise to a saturated lee wave of amplitude equal to half the sphere radius, which suppresses the separation region or splits it into two. Between F = 1.5 and 4.5 a more complex regime exists where the wake recovers progressively its behaviour in homogeneous fluid: the axisymmetry of the recirculating zone, the Kelvin–Helmholtz instability and, finally, the spiral instability.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 254 , September 1993 , pp. 1 - 21
Copyright
© 1993 Cambridge University Press

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