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Shear flow instabilities in shallow-water magnetohydrodynamics

Published online by Cambridge University Press:  14 January 2016

J. Mak ,
S. D. Griffiths  and
D. W. Hughes
J. Mak
Affiliation:
Department of Applied Mathematics, University of Leeds, Leeds LS2 9JT, UK
S. D. Griffiths
Affiliation:
Department of Applied Mathematics, University of Leeds, Leeds LS2 9JT, UK
D. W. Hughes
Affiliation:
Department of Applied Mathematics, University of Leeds, Leeds LS2 9JT, UK
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Abstract

Within the framework of shallow-water magnetohydrodynamics, we investigate the linear instability of horizontal shear flows, influenced by an aligned magnetic field and stratification. Various classical instability results, such as Høiland’s growth-rate bound and Howard’s semi-circle theorem, are extended to this shallow-water system for quite general flow and field profiles. In the limit of long-wavelength disturbances, a generalisation of the asymptotic analysis of Drazin & Howard (J. Fluid Mech., vol. 14, 1962, pp. 257–283) is performed, establishing that flows can be distinguished as either shear layers or jets. These possess contrasting instabilities, which are shown to be analogous to those of certain piecewise-constant velocity profiles (the vortex sheet and the rectangular jet). In both cases it is found that the magnetic field and stratification (as measured by the Froude number) are generally each stabilising, but weak instabilities can be found at arbitrarily large Froude number. With this distinction between shear layers and jets in mind, the results are extended numerically to finite wavenumber for two particular flows: the hyperbolic-tangent shear layer and the Bickley jet. For the shear layer, the instability mechanism is interpreted in terms of counter-propagating Rossby waves, thereby allowing an explication of the stabilising effects of the magnetic field and stratification. For the jet, the competition between even and odd modes is discussed, together with the existence at large Froude number of multiple modes of instability.

JFM classification

Instability: Instability Geophysical and Geological Flows: Shallow water flows
Type
Papers
Information
Journal of Fluid Mechanics , Volume 788 , 10 February 2016 , pp. 767 - 796
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© 2016 Cambridge University Press 

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