THROUGHFLOW EFFECTS ON CONVECTIVE INSTABILITY IN SUPERPOSED FLUID AND POROUS LAYERS

We implement a linear stability analysis of the convective instability in superposed horizontal fluid and porous layers with throughflow in the vertical direction. It is found that in such a physical configuration both stabilizing and destabilizing factors due to vertical throughflow can be enhanced so that a more precise control of the buoyantly driven instability in either a fluid or a porous layer is possible. For zeta = 0.1 (zeta, the depth ratio, defined as the ratio of the fluid-layer depth to the porous-layer depth), the onset of convection occurs in both fluid and porous layers, the relation between the critical Rayleigh number R(m)c and the throughflow strength gamma-m is linear and the Prandtl-number (Pr(m)) effect is insignificant. For zeta greater-than-or-equal-to 0.2, the onset of convection is largely confined to the fluid layer, and the relation becomes R(m)c approximately gamma-m2 for most of the cases considered except for Pr(m) = 0.1 with large positive gamma-m where the relation R(m)c approximately gamma-m3 holds. The destabilizing mechanisms proposed by Nield (1987 a, b) due to throughflow are confirmed by the numerical results if considered from the viewpoint of the whole system. Nevertheless, from the viewpoint of each single layer, a different explanation can be obtained.