Instability of Poiseuille flow in a fluid overlying a glass bead packed porous layer

The instability of Poiseuille flow in a fluid overlying a glass bead packed porous layer saturated with the same fluid is studied. A three layer configuration is adopted, where a variable effective viscosity is modelled between the outer fluid and porous homogeneous regions. Previous work on the stability of this system has assumed that all regions are homogeneous. The results demonstrate that there are two modes of instability corresponding to the fluid and porous layers, respectively.