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From three-dimensional to quasi-two-dimensional: transient growth in magnetohydrodynamic duct flows

Published online by Cambridge University Press:  19 December 2018

Oliver G. W. Cassells
Affiliation:
Department of Mechanical and Aerospace Engineering, Monash University, Victoria 3800, Australia
Tony Vo
Affiliation:
Department of Mechanical and Aerospace Engineering, Monash University, Victoria 3800, Australia
Alban Pothérat
Affiliation:
Fluid and Complex Systems Research Centre, Coventry University, Coventry CV15FB, UK
Gregory J. Sheard*
Affiliation:
Department of Mechanical and Aerospace Engineering, Monash University, Victoria 3800, Australia
*
Email address for correspondence: greg.sheard@monash.edu

Abstract

This study seeks to elucidate the linear transient growth mechanisms in a uniform duct with square cross-section applicable to flows of electrically conducting fluids under the influence of an external magnetic field. A particular focus is given to the question of whether at high magnetic fields purely two-dimensional mechanisms exist, and whether these can be described by a computationally inexpensive quasi-two-dimensional model. Two Reynolds numbers of $5000$ and $15\,000$ and an extensive range of Hartmann numbers $0\leqslant \mathit{Ha}\leqslant 800$ were investigated. Three broad regimes are identified in which optimal mode topology and non-modal growth mechanisms are distinct. These regimes, corresponding to low, moderate and high magnetic field strengths, are found to be governed by the independent parameters; Hartmann number, Reynolds number based on the Hartmann layer thickness $R_{H}$ and Reynolds number built upon the Shercliff layer thickness $R_{S}$, respectively. Transition between regimes respectively occurs at $\mathit{Ha}\approx 2$ and no lower than $R_{H}\approx 33.\dot{3}$. Notably for the high Hartmann number regime, quasi-two-dimensional magnetohydrodynamic models are shown to be excellent predictors of not only transient growth magnitudes, but also the fundamental growth mechanisms of linear disturbances. This paves the way for a precise analysis of transition to quasi-two-dimensional turbulence at much higher Hartmann numbers than is currently achievable.

Type
JFM Papers
Copyright
© 2018 Cambridge University Press 

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