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

  • Oliver G. W. Cassells (a1), Tony Vo (a1), Alban Pothérat (a2) and Gregory J. Sheard (a1)

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.

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Corresponding author

Email address for correspondence: greg.sheard@monash.edu

References

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

  • Oliver G. W. Cassells (a1), Tony Vo (a1), Alban Pothérat (a2) and Gregory J. Sheard (a1)

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