Boussinesq Model is frequently used in order to model natural convection which needs buoyancy term

Perform a transient calculation. In this approach, the initial density will be computed from the initial pressure and temperature, so the initial mass is known. As the solution progresses over time, this mass will be properly conserved. If the temperature differences in your domain are large, you must follow this approach.

Perform a steady-state calculation using the Boussinesq model (described in The Boussinesq Model). In this approach, you will specify a constant density, so the mass is properly specified. This approach is valid only if the temperature differences in the domain are small. If not, you use the transient approach.

Important: For a closed domain, you can use the incompressible ideal gas law only with a fixed operating pressure. It cannot be used with a floating operating pressure. You can use the compressible ideal gas law with either floating or fixed operating pressure.

you can use UDF if the cannot use Boussinesq Model

and the thermal expansion depend on temperature (1/T)

Boussinesq Model is frequently used in order to model natural convection which needs buoyancy term

Yes you can use a UDF in ANSYS FLUENT.

Abbas J Jubear Thanks for answering. Do you have any idea about what kind of UDF should i use for temprature dependent thermal expansion coefficient?

Mohammad Vajdi I know I can use the Boussinesq Model, but i cannot use a constant thermal expansion coefficient and there is not an option in ANSYS Fluent to use a temperature dependent thermal expansion coefficient or density while using the Boussinesq Model.

Saad Najeeb Thanks for answering. Can you be more specific about the UDF that i can use for this matter?

Using a transient calculation approach, and compute the initial density from the initial pressure and temperature then, the initial mass is known.