Discussion on the paper: Vortex instability of mixed convection boundary layer flow adjacent to a nonisothermal horizontal surface in a porousmedium with variable permeability

In this Discussion we shall reflect on various aspects of the recently published paper by Elaiw and Ibrahim (2008). Their paper considers the onset of convective instabilities of a mixed convection boundary layer induced by the combined presence of a hot horizontal surface in an unbounded porous medium and an externally imposed uniform horizontal forced convective flow. These authors also adopt a power law variation in the surface temperature and allow for the effects of non-uniform porosity caused by inefficient packing of the porous medium near the horizontal surface. The primary aim of the present Discussion is to discuss the theoretical foundations of the analysis of Elaiw and Ibrahim (2008) in order to clarify them unambiguously.