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23.9.4 Including Body Forces

When large body forces (e.g., gravity or surface tension forces) exist in multiphase flows, the body force and pressure gradient terms in the momentum equation are almost in equilibrium, with the contributions of convective and viscous terms small in comparison. Segregated algorithms converge poorly unless partial equilibrium of pressure gradient and body forces is taken into account. FLUENT provides an optional "implicit body force'' treatment that can account for this effect, making the solution more robust.

The basic procedure involves augmenting the correction equation for the face flow rate, Equation  25.4-13, with an additional term involving corrections to the body force. This results in extra body force correction terms in Equation  25.4-11, and allows the flow to achieve a realistic pressure field very early in the iterative process.

To include this body force, turn on Gravity in the Operating Conditions panel and specify the Gravitational Acceleration.

Define $\rightarrow$ Operating Conditions...

For VOF calculations, you should also turn on the Specified Operating Density option in the Operating Conditions panel, and set the Operating Density to be the density of the lightest phase. (This excludes the buildup of hydrostatic pressure within the lightest phase, improving the round-off accuracy for the momentum balance.) If any of the phases is compressible, set the Operating Density to zero.


For VOF and mixture calculations involving body forces, it is recommended that you also turn on the Implicit Body Force treatment for the Body Force Formulation in the Multiphase Model panel. This treatment improves solution convergence by accounting for the partial equilibrium of the pressure gradient and body forces in the momentum equations. See Section  23.9.4 for details.

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