Effects of combined horizontal and vertical heterogeneity on the onset of transient convection in a porous medium

The effects of both horizontal and vertical hydrodynamic and thermal heterogeneity on the onset of convection in a horizontal layer of a saturated porous medium, uniformly heated from below but with a nonuniform basic temperature gradient resulting from transient heating or otherwise, are studied analytically using linear stability theory for the case of weak heterogeneity. It is found that the effect of such heterogeneity on the critical value of the Rayleigh number Ra based on mean properties is of second order if the properties vary in a piecewise constant or linear fashion. The effects of horizontal heterogeneity and vertical heterogeneity are then comparable once the aspect ratio is taken into account and, to a first approximation, are independent.