visocisty effect in divergence of BuoyantBuossinessqSimpleFoam
 

Hi I am modelling water flow inside an engine. It includes the heat transfer part as well. I am using BuoyantBuossinessqSimpleFoam for my simulation.
To start my simulation, first I put the air properties and every thing was fine for this case. When I reduce the viscosity to water I do not know why but every thing blows up after some iterations. I have used all the comments suggested on the forum (upwind for the convections, linear limited for laplacian.
The other thing that I should mentions is that every thing is fine when I run the simpleFoam (even for water). but when I turn on the energy it diverges.
I appreciate if I see your help.