24.2.5 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. ANSYS 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, this equation in the separate Theory Guide , with an additional term involving corrections to the body force. This results in extra body force correction terms in this equation in the separate Theory Guide , and allows the flow to achieve a realistic pressure field very early in the iterative process.

To include this body force, enable Gravity in the Operating Conditions dialog box and specify the Gravitational Acceleration.

Cell Zone Conditions Operating Conditions...

For VOF calculations, you should also enable the Specified Operating Density option in the Operating Conditions dialog box, 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 enable the Implicit Body Force treatment for the Body Force Formulation in the Multiphase Model dialog box. This treatment improves solution convergence by accounting for the partial equilibrium of the pressure gradient and body forces in the momentum equations. See Section  24.2.5 for details.