See if this paper can be of any assistance:

https://www.researchgate.net/publication/281411068_A_NUMERICAL_STUDY_OF_TURBULENT_BOUNDARY_LAYER_FLOWS_GENERATED_BY_NATURAL_CONVECTION

Conference Paper A NUMERICAL STUDY OF TURBULENT BOUNDARY LAYER FLOWS GENERATE...

Hey Raf. It helped a lot, thank you so much.

I'm happy to know it helped you! Best Regards!!

Dear Mohammad-Reza

By my experience using constant temperature is a suitable way for boundary condition. But  operating conditions and material have an important role in simulating natural convection.

1. You should use Boussinesq  approximation for your density, this method is so useful for low temperature differences,

2. You should activate gravity in your operating condition.

3. You should use PRESTO! model for your pressure discretization.

Good Luck

Dear Nima, 

I found your instructions very useful.

I really appreciate your sense of responsibility to spread knowledge.

Dear Mohammad-Reza

Your Welcome. I hope you found that useful :)

Best Regards