Raylegh-Benard Convection in a porous media

Hi everyone, I'm studying Raylegh-Benard convection in a porous media in a very simple domain: an impermeable box heated from below. I experienced some problems trying to simulate steady convection, I noticed (using transient simulation) that only under certain condition of depth, permeability and porosity I reach a quite steady result.

Someone knows if there is a theoretical rule to sum up these condition? a sort of Raylegh number I suppose.

Thanks