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The classical Gill's stability problem for the stationary and parallelbuoyant flow in a vertical porous slab with impermeable and isothermalboundaries kept at different temperatures is reconsidered in a differentperspective. A three-layer slab is studied instead of a homogeneous slab as inGill's problem. The three layers have a symmetric configuration where the twoexternal layers have a high thermal conductivity, while the core layer has amuch lower conductivity. A simplified model is set up where the thermalconductivity ratio between the external layers and the internal core is assumedas infinite. It is shown that a flow instability in the sandwiched porous slabmay arise with a sufficiently large Rayleigh number. It is also demonstratedthat this instability coincides with that predicted in a previous analysis fora homogeneous porous layer with permeable boundaries, by considering thelimiting case where the permeability of the external layers is much larger thanthat of the core layer.

Gill's problem in a sandwiched porous slab