Dear Mr. Mahdi

         The Boussinesq equation had some assumptions for forced or natural convection with laminar or turbulent fluid flow. The pressure and velocity schemes Body Force weight and the Semi Implicit method for pressure linked equation (SIMPLE) is the proper scheme in thermal processes is so fine to have the converge for laminar forced convection.

Hi Ansari,

Can you be more specific about your problem/ model?

Dear Mehdi,

As I remember, we have used "Body Force Weighted" or "Second Order" for pressure and solved a transient problem when modelling the natural convection. And, of course, the natural-convective flow is almost always turbulent in real life. That's all with most general tricks :)

The chapter "Natural Convection and Buoyancy-Driven Flows" of the Fluent User's guide seems to be quite appropriate for future guidance. 

Hope, that helps.

Dear

The pressure scheme shold be "Body force weighted" or "PRESTO". And the velocity scheme is "First order" or "Second order". The SIMPLE scheme is sufficient for natural covection. As for Boussinesq density evaluation, well, it is depends on your problem. If your problem is a closed system, the density should be the fluid density refers to the average temperature. 

Hi Mahdi

The pressure and velocity schemes Body Force and Second order respectively shoud be used. Semi Implicit method for pressure linked equation (SIMPLE) is proper scheme in thermal processes.

For compressible flows, the ideal gas law is the appropriate density relationship. 

For incompressible flows, you may choose one of the following methods: 

Constant density , if you do not want density to be a function of temperature. 

The incompressible ideal gas law , when pressure variations are small enough that the flow is fully incompressible but you wish to use the ideal gas law to express the relationship between density and temperature (e.g., for a natural convection problem). 

Density as a polynomial, piecewise-linear, or piecewise-polynomial function of temperature , when the density is a function of temperature only, as in a natural convection problem. 

The Boussinesq model , for natural convection problems involving small changes in temperature.