Page 128 - ansys

8.2 Explicit Schemes

Explicit schemes take on different forms:

1. Time-dependent with the explicit interpolation scheme: Since the donor-acceptor scheme is
available only for quadrilateral and hexahedral meshes, it cannot be used for a hybrid mesh
containing twisted hexahedral cells. For such cases, you should use the time-dependent explicit
scheme. This formulation can also be used for other cases in which the geometric reconstruction
scheme does not give satisfactory results, or the flow calculation becomes unstable. Note that the
CICSAM scheme or the modified HRIC scheme can be computationally inexpensive when
compared to the geometric reconstruction scheme and improves the robustness and stability of
the calculations. The Volume Fraction discretizations, Modified HRIC and CICSAM, are available in
the Solution Controls task page when the explicit scheme is selected.
2. Time-dependent with the geometric reconstruction interpolation scheme: This for-mulation
should be used whenever you are interested in the time-accurate transient behavior of the VOF
solution.

To use this formulation, make sure Explicit is selected as the VOF Scheme in the Solution Methods task
page, then select Geo-Reconstruct as the Volume Fraction Discretization scheme in the Solution
Methods task page.

3. Time-dependent with the donor-acceptor interpolation scheme: This formulation should be used
instead of the time-dependent formulation with the geometric re-construction scheme if your mesh
contains highly twisted hexahedral cells. For such cases, the donor-acceptor scheme may
provide more accurate results.

The Donor-Acceptor spatial discretization scheme is used when Explicit is selected as the Scheme in
the Multiphase Models dialog box. Initially, Donor-Acceptor is not available in the Solution Methods
task page GUI. To make it available, use the following text command:
Solve –› set –› expert

You will be asked a series of questions, one of which is If your response is yes, then many more
discretization schemes will be available for your selection. You can now use this formulation by
selecting Donor-Acceptor as the Volume Fraction Spatial Discretization in the Solution Methods task
page.

4. The CICSAM scheme gives interface sharpness of the same level as the geometric reconstruction
scheme and is particularly suitable for flows with high viscosity ratios between the phases.
To use this formulation, select Explicit as the volume fraction Scheme in the Mul-tiphase Model dialog
box, then select CICSAM as the Volume Fraction Spatial Dis-cretization in the Solution Methods task
page.

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