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Plume formation and resonant bifurcations in porous-media convection

Published online by Cambridge University Press:  26 April 2006

Michael D. Graham
Affiliation:
Department of Chemical Engineering, University of Wisconsin, Madison, WI53706-1691, USA
Paul H. Steen
Affiliation:
School of Chemical Engineering, and Center for Applied Mathematics, Cornell University, Ithaca, NY14853-5204, USA

Abstract

The classical boundary-layer scaling laws proposed by Howard for Rayleigh–Bénard convection at high Rayleigh number extend to the analogous case of convection in saturated porous media. We computationally study two-dimensional porous-media convection near the onset of this scaling behaviour. The main result of the paper is the observation and study of instabilities that lead to deviations from the scaling relations.

At Rayleigh numbers below the scaling regime, boundary-layer fluctuations born at a Hopf bifurcation strengthen and eventually develop into thermal plumes. The appearance of plumes corresponds to the onset of the boundary-layer scaling behaviour of the oscillation frequency and mean Nusselt number, in agreement with the classical theory. As the Rayleigh number increases further, the flow undergoes instabilities that lead to ‘bubbles’ in parameter space of quasi-periodic flow, and eventually to weakly chaotic flow. The instabilities disturb the plume formation process, effectively leading to a phase modulation of the process and to deviations from the scaling laws. We argue that these instabilities correspond to parametric resonances between the timescale for plume formation and the characteristic convection timescale of the flow.

Type
Research Article
Copyright
Copyright © Cambridge University Press 1994

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