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Bistability and hysteresis induced by form drag in nonlinear subcritical and supercritical double-diffusive Lapwood convection in shallow porous enclosures

Published online by Cambridge University Press:  06 January 2017

Redha Rebhi
Affiliation:
Laboratoire de Mécanique Physique et Modélisation Mathématique (LMP2M), Université Dr. Yahia Farès de Médéa, Quartier Ain D’Heb, Médéa 26000, Algérie
Mahmoud Mamou*
Affiliation:
Aerodynamics Laboratory, NRC Aerospace, National Research Council Canada, Ottawa, Ontario K1A OR6, Canada
Patrick Vasseur
Affiliation:
Ecole Polytechnique, Université de Montréal, C.P. 6079, Succ. Centre-ville, Montréal, Québec, H3C 3A7, Canada
*
Email address for correspondence: Mahmoud.Mamou@nrc-cnrc.gc.ca

Abstract

This paper considers natural Lapwood convection in a shallow porous cavity filled with a binary fluid. The investigation is mainly focused on the nonlinear behaviour of subcritical convection and the bistability phenomenon caused by the combined effects of porous medium form drag and double-diffusive convection. The Dupuit–Darcy model, which includes the effect of the form drag at high Reynolds flow, is used to describe the convective flow in the porous matrix. The enclosure is subject to vertical temperature and concentration gradients. The governing parameters of the problem under study are the Rayleigh number, $R_{T}$, the buoyancy ratio, $\unicode[STIX]{x1D711}$, the Lewis number, $Le$, the form drag coefficient, ($1/P_{r}^{\ast }$), where $P_{r}^{\ast }$ is a modified Prandtl number, and the aspect ratio of the cavity, $A$. An analytical solution, valid for shallow enclosures ($A\gg 1$), is derived on the basis of the parallel flow approximation. Among other things, this work focuses on the effects of the form drag parameter on the convective flows that occur when the thermal and solutal buoyancy forces are opposing each other. For this situation, in the absence of the form drag effect, the onset of motion is known to occur at a subcritical Rayleigh number, $R_{TC}^{sub}$, which depends upon $\unicode[STIX]{x1D711}$ and $Le$ only. The effects of $P_{r}^{\ast }$ on $R_{TC}^{sub}$ and on the subsequent convective heat and mass transfer rates are found to be significant. A new bistability phenomenon arises when the onset of subcritical convection is shifted close to or beyond the threshold of supercritical convection, whether heating or cooling isothermally or upon applying constant heat and solute fluxes, regardless of the enclosure aspect ratio value. It is demonstrated, on the basis of linear stability theory, that the form drag parameter has a stabilizing effect and considerably affects the threshold for Hopf bifurcation, $R_{TC}^{Hopf}$, which characterizes the transition from steady to unsteady convection. In the range of governing parameters considered in this study, the heat, solute and flow characteristics predicted by the analytical model are found to agree well with the numerical study of the full governing equations.

Type
Papers
Copyright
© 2017 Cambridge University Press 

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