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The linear instability of the stratified plane Couette flow

Published online by Cambridge University Press:  23 August 2018

Giulio Facchini ,
Benjamin Favier Open the ORCID record for Benjamin Favier [Opens in a new window] ,
Patrice Le Gal ,
Meng Wang Open the ORCID record for Meng Wang [Opens in a new window]  and
Michael Le Bars
Giulio Facchini*
Affiliation:
Aix Marseille Univ., CNRS, Centrale Marseille, IRPHE UMR 7342, 49 rue F. Joliot Curie, Marseille, 13013, France
Benjamin Favier
Affiliation:
Aix Marseille Univ., CNRS, Centrale Marseille, IRPHE UMR 7342, 49 rue F. Joliot Curie, Marseille, 13013, France
Patrice Le Gal
Affiliation:
Aix Marseille Univ., CNRS, Centrale Marseille, IRPHE UMR 7342, 49 rue F. Joliot Curie, Marseille, 13013, France
Meng Wang
Affiliation:
Department of Mechanical Engineering, University of California Berkeley, 6121 Etcheverry Hall, Berkeley, CA 94720-1740, USA
Michael Le Bars
Affiliation:
Aix Marseille Univ., CNRS, Centrale Marseille, IRPHE UMR 7342, 49 rue F. Joliot Curie, Marseille, 13013, France
*
†Email address for correspondence: giuliofacchini@gmail.com
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Abstract

We present the stability analysis of a plane Couette flow which is stably stratified in the vertical direction orthogonal to the horizontal shear. Interest in such a flow comes from geophysical and astrophysical applications where background shear and vertical stable stratification commonly coexist. We perform the linear stability analysis of the flow in a domain which is periodic in the streamwise and vertical directions and confined in the cross-stream direction. The stability diagram is constructed as a function of the Reynolds number $Re$ and the Froude number $Fr$, which compares the importance of shear and stratification. We find that the flow becomes unstable when shear and stratification are of the same order (i.e. $Fr\sim 1$) and above a moderate value of the Reynolds number $Re\gtrsim 700$. The instability results from a wave resonance mechanism already known in the context of channel flows – for instance, unstratified plane Couette flow in the shallow-water approximation. The result is confirmed by fully nonlinear direct numerical simulations and, to the best of our knowledge, constitutes the first evidence of linear instability in a vertically stratified plane Couette flow. We also report the study of a laboratory flow generated by a transparent belt entrained by two vertical cylinders and immersed in a tank filled with salty water, linearly stratified in density. We observe the emergence of a robust spatio-temporal pattern close to the threshold values of $Fr$ and $Re$ indicated by linear analysis, and explore the accessible part of the stability diagram. With the support of numerical simulations we conclude that the observed pattern is a signature of the same instability predicted by the linear theory, although slightly modified due to streamwise confinement.

JFM classification

Waves/Free-surface Flows: Channel flow Instability: Instability Geophysical and Geological Flows: Internal waves
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 853 , 25 October 2018 , pp. 205 - 234
Copyright
© 2018 Cambridge University Press 

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