EVOLUTION OF INSTABILITIES IN AN INFINITE POROUS LAYER HEATED FROM BELOW: THE EFFECT OF NEAR-RESONANT THERMAL FORCING.

The onset of Rayleigh-Benard convection in a saturated porous layer is considered when the boundary temperatures vary periodically in space. Attention is focused on the case of small amplitude, in-phase thermal modulations with a wavenumber close to the critical value for the unmodulated case. Weakly nonlinear theory and a spectral method are used to determine the stability of certain roll solutions and to calculate the evolution of the cross-roll, zigzag and sideband instabilities. It is shown that steady rolls with spatially deformed axes or spatially varying wavenumbers can result.