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While the explicit time-dependent formulation is less computationally expensive than the geometric
reconstruction scheme, the interface between phases will not be as sharp as that predicted with the
geometric reconstruction scheme. To reduce this diffusivity, it is recommended that you use the
second-order discretization scheme for the volume fraction equations. In addition, you may want to
consider turning the geometric reconstruction scheme back on after calculating a solution with the
implicit scheme, in order to obtain a sharper interface.
In general, Geo-Reconstruct, Modified HRIC, and CICSAM are applied to cases with sharp interfaces,
while First Order Upwind and QUICK are applied to diffused interfaces. Note that Geo-Reconstruct and
CICSAM schemes are available with the Eulerian multiphase model, only when the Immiscible Fluid
Model is enabled. First Order Upwind is available only for the Eulerian multiphase explicit scheme. It is
not available when the Immisible Fluid Model is enabled. However, it can be made available in the GUI
for the VOF multiphase explicit scheme when the solve/set/expert text command is invoked

When the Eulerian model is used with the Explicit scheme and the Immiscible Fluid Model is disabled,
you can apply First Order Upwind, QUICK, and Modified HRIC. If the Immiscible Fluid Model is
enabled, you can apply Geo-Reconstruct, CICSAM, QUICK, and Modified HRIC.

8.3 Implicit Schemes

Implicit schemes take on the following forms:

1. Time-dependent with the implicit interpolation scheme: This formulation can be used if you are
looking for a steady-state solution and you are not interested in the intermediate transient flow
behavior, but the final steady-state solution is dependent on the initial flow conditions and/or you
do not have a distinct inflow boundary for each phase.

To use this formulation, select Implicit as the volume fraction Scheme in the Multiphase Model
dialog box, and enable a Transient calculation in the General task page.
2. Steady-state with the implicit interpolation scheme: This formulation can be used if you are
looking for a steady-state solution, you are not interested in the intermediate transient flow
behavior, and the final steady-state solution is not affected by the initial flow conditions and there
is a distinct inflow boundary for each phase. Note that the implicit modified HRIC scheme can be
used as a robust alternative to the explicit geometric reconstruction scheme.

To specify the formulation when using the VOF multiphase model, select Implicit as the volume
fraction Scheme in the Multiphase Model dialog box, then select First Order Upwind, Second Order
Upwind, Modified HRIC, or QUICK as the Volume Fraction Spatial Discretization in the Solution
Methods task page. When using the Eulerian multiphase model, select First Order Upwind, QUICK, or
Modified HRIC as the Volume Fraction Spatial Discretization in the Solution Methods task page.

Examples
To help you determine the best formulation to use for your problem, some examples that use different
formulations are listed below:

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