A study concerning the flow of a Newtonian fluid through a porous medium for the particular case natural convection is produced by hot and cold spots placed in the solid phase is presented. Results involving the interaction of forced convection with thermal spots are reported to visualize the mechanisms associated with the generation of complex flow patterns in the porous medium. For this purpose the computation of a two-field model is carried out. Two systems are studied: one is a rectangular porous cavity (RPC) of square cross section and the other is an annular porous cavity (APC) comprised by two concentric vertical cylindrical walls. It is shown, in general, that the flow patterns associated with each configuration and intensities of the thermal spots may be qualitatively inferred by following rules that are established through a basic study of mixed convection in the RPC. (c) 2005 Elsevier Ltd. All rights reserved.