A series of numerical experiments concerning the phenomenon of natural convection in a two-layer composite system heated from below is reported. The composite system consists of a fluid layer bounded from below by a porous bed saturated by the same fluid. The numerical solutions are based on the full equations for time-dependent flow, and document the main features of the flow at Rayleigh numbers several orders of magnitude above critical value. The ranges covered by the numerical solutions are 102-106 for overall Rayleigh number, 10-7-10-4 for Darcy number, and 0.2-1 for the height to length ratio of the composite system. © 1986.