numerical symmetry

tj

Hi all, I'm simulating the buoyant flow from a naturally ventilated building in still atmospheric conditions. I have noticed that the plume emitted from the building (roof apex) looses its symmetry during the solution procedure. I have read that this is a result of "unbalanced lateral forces". If I force numerical symmetry by incorporating a symettrical plane then the solution converges fine. Could anyone explain to me these unbalanced lateral forces that occur during the solution process? Do i always need to force numerical symmetry in these type of simulations? Thanks for any help