### 7.6.1. Fluid Buoyancy Model

#### 7.6.1.1. Density Difference

This is the default option for all multiphase flows. The additional buoyancy force is modeled by considering the difference in density between phases. If you are using a constant density fluid, you should consider the Boussinesq model.

| (7–1)

#### 7.6.1.2. Boussinesq

This option may be selected for constant density fluids for multiphase flows with heat transfer, but it is not the default option. You should use this model to model natural convection effects within such a fluid (that is, if density fluctuations due to temperature changes are important). Note that the default option, `Density Difference`, neglects such effects if the fluid density is constant.

| (7–2)

Note further that the `Boussinesq` option may be more difficult to converge than the `Density Difference` option, because of coupling between heat transfer and momentum. It may be advantageous to obtain a converged solution first using the `Density Difference` option. This may then be used as an initial guess for a solution using the `Boussinesq` option.

Multiphase simulations use the full buoyancy model because there is usually a significant difference in density between the phases. However, you can select to use the Boussinesq buoyancy model within each phase to account for buoyancy effects due to temperature differences within the phase.

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