Published online by Cambridge University Press: 22 November 2013
Convection in a porous medium at high Rayleigh number   $\mathit{Ra}$  exhibits a striking quasisteady columnar structure with a well-defined and
 $\mathit{Ra}$  exhibits a striking quasisteady columnar structure with a well-defined and   $\mathit{Ra}$ -dependent horizontal scale. The mechanism that controls this scale is not currently understood. Motivated by this problem, the stability of a density-driven ‘heat-exchanger’ flow in a porous medium is investigated. The dimensionless flow comprises interleaving columns of horizontal wavenumber
 $\mathit{Ra}$ -dependent horizontal scale. The mechanism that controls this scale is not currently understood. Motivated by this problem, the stability of a density-driven ‘heat-exchanger’ flow in a porous medium is investigated. The dimensionless flow comprises interleaving columns of horizontal wavenumber   $k$  and amplitude
 $k$  and amplitude   $\widehat{A}$  that are driven by a steady balance between vertical advection of a background linear density stratification and horizontal diffusion between the columns. Stability is governed by the parameter
 $\widehat{A}$  that are driven by a steady balance between vertical advection of a background linear density stratification and horizontal diffusion between the columns. Stability is governed by the parameter   $A= \widehat{A}\mathit{Ra}/ k$ . A Floquet analysis of the linear-stability problem in an unbounded two-dimensional domain shows that the flow is always unstable, and that the marginal-stability curve is independent of
 $A= \widehat{A}\mathit{Ra}/ k$ . A Floquet analysis of the linear-stability problem in an unbounded two-dimensional domain shows that the flow is always unstable, and that the marginal-stability curve is independent of   $A$ . The growth rate of the most unstable mode scales with
 $A$ . The growth rate of the most unstable mode scales with   ${A}^{4/ 9} $  for
 ${A}^{4/ 9} $  for   $A\gg 1$ , and the corresponding perturbation takes the form of vertically propagating pulses on the background columns. The physical mechanism behind the instability is investigated by an asymptotic analysis of the linear-stability problem. Direct numerical simulations show that nonlinear evolution of the instability ultimately results in a reduction of the horizontal wavenumber of the background flow. The results of the stability analysis are applied to the columnar flow in a porous Rayleigh–Bénard (Rayleigh–Darcy) cell at high
 $A\gg 1$ , and the corresponding perturbation takes the form of vertically propagating pulses on the background columns. The physical mechanism behind the instability is investigated by an asymptotic analysis of the linear-stability problem. Direct numerical simulations show that nonlinear evolution of the instability ultimately results in a reduction of the horizontal wavenumber of the background flow. The results of the stability analysis are applied to the columnar flow in a porous Rayleigh–Bénard (Rayleigh–Darcy) cell at high   $\mathit{Ra}$ , and a balance of the time scales for growth and propagation suggests that the flow is unstable for horizontal wavenumbers
 $\mathit{Ra}$ , and a balance of the time scales for growth and propagation suggests that the flow is unstable for horizontal wavenumbers   $k$  greater than
 $k$  greater than   $k\sim {\mathit{Ra}}^{5/ 14} $  as
 $k\sim {\mathit{Ra}}^{5/ 14} $  as   $\mathit{Ra}\rightarrow \infty $ . This stability criterion is consistent with hitherto unexplained numerical measurements of
 $\mathit{Ra}\rightarrow \infty $ . This stability criterion is consistent with hitherto unexplained numerical measurements of   $k$  in a Rayleigh–Darcy cell.
 $k$  in a Rayleigh–Darcy cell.
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               . Science
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               . Science
               325, 1656–1658.CrossRefGoogle Scholar ${\mathrm{CO} }_{2} $
                     
                   storage in saline aquifers. Adv. Water Resour.
               33, 443–455.CrossRefGoogle Scholar
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                   storage in saline aquifers. Adv. Water Resour.
               33, 443–455.CrossRefGoogle Scholar