The problem of thermal instability of a Walters' (model B H ) viscoelastic¯uid in a porous medium is considered in the presence of a variable gravity ®eld and rotation. It is found that the principle of exchange of stabilities is valid under certain conditions. For stationary convection, the Walters' (model B H ) elastico-viscous¯uid behaves like a Newtonian¯uid. It is found that rotation has stabilizing effect as gravity increases upward and a destabilizing effect as gravity decreases upward, the medium permeability has stabilizing/destabilizing effects depending on the rotation parameter, gravity is considered to be increasing upward from its value g (i.e. ! b 0). The effects of rotation and the medium permeability on thermal instability have also been shown graphically. The suf®cient conditions for the non-existence of overstability are also obtained.
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