Page 131 - ansys

Each item in the Phases list in this task page is one of two types: a Primary-Phase indicates that the
selected item is the primary phase, and Secondary-Phase indicates that the selected item is a
secondary phase. To specify any interaction between the phases, click the Interaction... button.

(a) 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.
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 recom-mended that you also enable the
Implicit Body Force treatment for the Body Force Formulation in the Multiphase Model dialog box.
This treatment im-proves solution convergence by accounting for the partial equilibrium of the
pressure gradient and body forces in the momentum equations.
(b) Including Mass Transfer Effects
As discussed in Section mass transfer effects in the framework of ANSYS FLUENT's general multiphase
models (i.e., Eulerian multiphase, mixture multiphase, or VOF multiphase) can be modeled in one of
three ways:

w Unidirectional constant rate mass transfer (not available for VOF calculations)
w UDF-prescribed mass transfer
w mass transfer through cavitation

To define mass transfer in a multiphase simulation, as unidirectional constant, using a UDF, through
population balance, cavitation, or evaporation and condensation, you will need to use the Phase
Interaction dialog box.
Phases –› Interaction

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