Natural convection in cavity heated by below
 

Hello.
I'm studying Natural convection problem which is in enclosed cavity and heated by below. .
Until now, i have found some results that depend on resolution.
like.. 40x40, 80x80..
Anyway...
My question is in many papers , they show that the results are symmetric in 1:2, 1:3 ratio of domain. but in my results, in the steady state, the result shows big vortex instead of two symmetric vortex.
I need your comments and experiences. thanks