Convective instability in porous media with throughflow

In their study of the linear stability limits for free convection in layers of porous media or packed beds with throughflow, others found that throughflow was generally stabilizing, but there was an exception. In the case of flow toward a porous boundary, they found that the numerically calculated Rayleigh-Darcy number initially decreased and went through a minimum as the appropriate Peclet number increased from zero. It was stated that the origin of the minimum was not completely understood, but others had eliminated numerical inaccuracy as a possible cause. Thus a small amount of throughflow can have a destabilizing effect in at least one situation. The primary purpose of this note is to explain this effect. The opportunity is taken to provide some supplementary material