On the convection in a porous medium with inclined temperature gradient and vertical throughflow. Part II. absolute and convective instabilities, and spatially amplifying waves.

In this second part of our analysis of the destabilization of transverse modes in an extended horizontal layer of a saturated porous medium with inclined temperature gradient and vertical throughflow, we apply the mathematical formalism of absolute and convective instabilities to studying the nature of the transition to instability of such modes by assuming on physical grounds that the transition is triggered by growing localized wavepackets. It is revealed that in most of the parameter cases treated in the first part of the analysis (Brevdo and Ruderman 2009), at the transition point the evolving instability is convective. Only in the cases of zero horizontal thermal gradient, and in the cases of zero vertical throughflow and the horizontal Rayleigh number R h < 49, the instability is absolute implying that, as the vertical Rayleigh number, R v, increases passing through its critical value, R vc, the destabilization tends to affect the base state throughout and eventually destroys it at every point in space. For the parameter values considered, for which the destabilization has the nature of convective instability, we found that, as R v, increases beyond the critical value, while the horizontal Rayleigh number, R h, and the Péclet number, Q v, are kept fixed, the flow experiences a transition from convective to absolute instability. The values of the vertical Rayleigh number, R v, at the transition from convective to absolute instability are computed. For convectively unstable, but absolutely stable cases, the spatially amplifying responses to localized oscillatory perturbations, i.e., signaling, are treated and it is found that the amplification is always in the direction of the applied horizontal thermal gradient.