Removing Invalid Cells

Poor quality cells can cause the solution to diverge or compromise the accuracy of the results. Simcenter STAR-CCM+ provides a tool for removing poor quality mesh cells based on specified mesh criteria.

In Simcenter STAR-CCM+, the Removing Invalid Cells tool allows you to remove mesh cells from the volume mesh region definition based on one or more of the following mesh criteria:

Simcenter STAR-CCM+ moves all the removed cells into a separate region that is not used in the analysis. Invalid cell boundaries are automatically added to the neighboring cell faces of cells that have been removed, minimizing the impact of these removed areas on the solution.

You can also remove invalid cells on regions that are in contact with shell regions. In this case, if any of the cells that are removed from the region are in contact with cells of the shell region, the connected shell region cells are also removed.

This option is not available for simulations containing finite element physics models.

To remove invalid cells:
  1. Open a mesh scene in the Graphics window.
  2. Right-click the region or regions of interest and select Remove Invalid Cells...

    The Remove Invalid Cells dialog appears.

  3. In the Regions box, click Select... and use the Select Objects dialog to specify the relevant regions.
  4. Enable the mesh criteria that you want to use to identify invalid cells.
  5. For the enabled criteria, specify the desired threshold values.
    The following threshold values are suggested guidelines:
    • Face Validity: 0.95 to 0.51 (default 0.51)
    • Cell Quality: 1e-05 to 1e-12 (default 1e-08)

      As the Cell Quality metric can sometimes flag valid trimmed cells (especially if they are anisotropic), you are recommended to use a low value.

    • Volume Change: 1e-05 to 1e-12 (default 1e-10)
    • Contiguous Cells: 1 to 1000000 (default 1)
    • The Connected Face Area is dependent on the case dimensions and can be estimated based on the average mesh cell size.

    If you change the metric values then they are automatically preserved as part of the Simcenter STAR-CCM+ workspace and will be used instead of the default values in future sessions. To restore the default factory values, click Reset.

  6. To preview the number of cells that each criterion would flag for removal, click Identify.

    The cells that meet the specified criteria are displayed in the active mesh scene.

  7. When you are satisfied with the criteria, click Remove to remove the cells. If you do not want to remove cells, just close the dialog without selecting Remove.
    The Output window displays the result:

Cells that meet the provided criteria are moved into a new region called Cells deleted from [Region].

New invalid cell boundaries with the names internal-0 and internal-1 are created in each region accordingly. The new region has no associated physics continuum, so it is not necessary to delete it in order to run the analysis.

Contiguous Cells

The contiguous cells metric can be used to identify discontiguous regions of cells (as determined by the connected face area metric). These cell regions can either pre-exist in the volume mesh or be created when bad quality cells are removed based on the other three invalid cell criteria. Discontiguous fluid cells would cause the solver to fail under most circumstances.

A value in the range 1 - 1000000 can be entered, where the value indicates the number of cells in the discontiguous region. On utilizing the feature, first the other criteria are applied and any invalid cells are flagged. Then any discontiguous regions of unflagged cells are checked and compared to the supplied metric value, as well as the supplied connected face area value. If the number of cells is less than the amount specified, and/or the area criterion is not met, then the region is also marked for removal.

Connected Face Area

The connected face area is the smallest area that is allowed between two connected cells, below which the cells would be considered discontiguous and subject to the contiguous cells check criterion.

It is sometimes possible during meshing and cell removal that is based on the other criteria, that two cells are left connected but with an extremely small face area between the two. Such scenarios can lead to the flux calculation becoming zero and the solution becoming unstable. Although topologically connected, if the face area between the two cells is below the specified value, then the cells are no longer considered to be connected. In this case, remove the smaller group of cells using the above contiguous cell check.

Volume

Use the volume metric to remove all the cells with volume below a specified value. You can specify any value above zero.

Function

The function metric allows you to remove the cells for which a selected field function has a value below () or above () a specified limit. To specify the field function, click Select... in the Function group box and use the Select Objects dialog.