solutal convection in large cavities
 

Hello,
I'm new in this forum and I'm looking for a robust approximation for solutal convection in large cavities.
I use COMSOL 4.2a as FEM-simulation software to simulate flow and mass transfer in large cavities (height: 20m, diameter: 160m). At the wall there is salt dissolution, what affects brine density.

The simulation model of transient flow runs with a simple turbulence model based on the Prandtl Mixing Length theory. But I have problems to increase time steps, what is necessary for long time simulations with heat transfer (... 365 days and more f.i.).

Is there anybody who can give me some hints for better approximation?

Thank you very much
Michael R.