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Buoyancy-driven instabilities of miscible two-layer stratifications in porous media and Hele-Shaw cells

Published online by Cambridge University Press:  31 January 2011

P. M. J. TREVELYAN ,
C. ALMARCHA  and
A. DE WIT
P. M. J. TREVELYAN
Affiliation:
Nonlinear Physical Chemistry Unit, Center for Nonlinear Phenomena and Complex Systems, Faculté des Sciences, Université Libre de Bruxelles (ULB), CP 231, 1050 Brussels, Belgium
C. ALMARCHA
Affiliation:
Nonlinear Physical Chemistry Unit, Center for Nonlinear Phenomena and Complex Systems, Faculté des Sciences, Université Libre de Bruxelles (ULB), CP 231, 1050 Brussels, Belgium
A. DE WIT*
Affiliation:
Nonlinear Physical Chemistry Unit, Center for Nonlinear Phenomena and Complex Systems, Faculté des Sciences, Université Libre de Bruxelles (ULB), CP 231, 1050 Brussels, Belgium
*
†Email address for correspondence: adewit@ulb.ac.be
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Abstract

Buoyancy-driven instabilities of a horizontal interface between two miscible solutions in the gravity field are theoretically studied in porous media and Hele-Shaw cells (two glass plates separated by a thin gap). Beyond the classical Rayleigh–Taylor (RT) and double diffusive (DD) instabilities that can affect such two-layer stratifications right at the initial time of contact, diffusive-layer convection (DLC) as well as delayed-double diffusive (DDD) instabilities can set in at a later time when differential diffusion effects act upon the evolving density profile starting from an initial step-function profile between the two miscible solutions. The conditions for these instabilities to occur can therefore be obtained only by considering time evolving base-state profiles. To do so, we perform a linear stability analysis based on a quasi-steady-state approximation (QSSA) as well as nonlinear simulations of a diffusion–convection model to classify and analyse all possible buoyancy-driven instabilities of a stratification of a solution of a given solute A on top of another miscible solution of a species B. Our theoretical model couples Darcy's law to evolution equations for the concentration of species A and B ruling the density of the miscible solutions. The parameters of the problem are a buoyancy ratio R quantifying the ratio of the relative contribution of B and A to the density as well as δ, the ratio of diffusion coefficients of these two species. We classify the region of RT, DD, DDD and DLC instabilities in the (R, δ) plane as a function of the elapsed time and show that, asymptotically, the unstable domain is much larger than the one captured on the basis of linear base-state profiles which can only obtain stability thresholds for the RT and DD instabilities. In addition the QSSA allows one to determine the critical time at which an initially stable stratification of A above B can become unstable with regard to a DDD or DLC mechanism when starting from initial step function profiles. Nonlinear dynamics are also analysed by a numerical integration of the full nonlinear model in order to understand the influence of R and δ on the dynamics.

JFM classification

Convection: Buoyancy-driven instability Convection: Convection in porous media Convection: Double diffusive convection
Type
Papers
Information
Journal of Fluid Mechanics , Volume 670 , 10 March 2011 , pp. 38 - 65
Copyright
Copyright © Cambridge University Press 2011

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