I googled everything, I read every document I could find. The problem lies somewhere around boussinesq approximation for water. When I use air, it works, when I use constant density it also works but then there is no buoyancy and there are no layers of fluid at different temperature (hotter is above colder). Also, when I do transient calculation it works fine, but my task is to do a steady state simulation. Net heat flux is never zero, it is always around 10-30% wrong (it's always like in a picture I posted). Is it possible that it's impossible to find a steady solution, that I need to do it transient? I tried everything with boussinesq parameters and nothing helps.n