## 8.3.1 Defining Density for Various Flow Regimes

The selection of density in FLUENT is very important. Set the density relationship based on your flow regime.

• 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.

Mixing Density Relationships in Multiple-Zone Models

If your model has multiple fluid zones that use different materials, you should be aware of the following:

• For calculations with the pressure-based solver that do not use one of the general multiphase models, the compressible ideal gas law cannot be mixed with any other density methods. This means that if the compressible ideal gas law is used for one material, it must be used for all materials.

This restriction does not apply to the density-based solvers.

• There is only one specified operating pressure and one specified operating temperature. This means that if you are using the ideal gas law for more than one material, they will share the same operating pressure. If you are using the Boussinesq model for more than one material, they will share the same operating temperature.

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