6.7.2 Converting Skewed Cells to Polyhedra

Another method of cell agglomeration is the skewness-based cluster approach. This type of conversion is designed to convert only part of the domain. The objective is to convert only skewed tetrahedral cells above a specified cell equivolume skewness threshold into polyhedra. By converting the highly skewed tetrahedral cells, the quality of the mesh can be improved significantly.

A different algorithm is used for local conversion. This algorithm evaluates each highly skewed tetrahedral cell and all of the surrounding cells, to select an edge on the highly skewed cell that best matches criteria for cell agglomeration. Then all of the cells which share this edge are combined into a polyhedral cell. During the process, the data is interpolated from the original cells to the resultant polyhedra.

Limitations

There are certain limitations with this type of conversion:

• The following mesh manipulation tools are not available on polyhedral meshes:
  • the mesh/modify-zones/extrude-face-zone-delta text command

  • the mesh/modify-zones/extrude-face-zone-para text command

  • fuse

  • skewness smoothing

  • swapping will not affect polyhedral cells

• The polyhedral cells that result from the conversion are not eligible for adaption. For more information about adaption, see Chapter  27.

• Only tetrahedral cells are converted, as all other cells are skipped.

• Meshes with hanging nodes will not be converted. This includes meshes that have undergone hanging node adaption (see this section in the separate Theory Guide), as well as meshes generated by the GAMBIT Hex Core meshing scheme or the TGrid Hexcore menu option (hexcore meshes). Note that if the mesh is a hexcore mesh in which the transitional cells have been converted to polyhedra (see Section  32.5.2), then it does not have hanging nodes and can therefore be converted.

Using the Convert Skewed Cells Dialog Box

To convert skewed cells in your domain to polyhedral cells, go to the Convert Skewed Cells dialog box.

Mesh $\rightarrow$ Polyhedra $\rightarrow$ Convert Skewed Cells...

Figure 6.7.7: The Convert Skewed Cells Dialog Box

1.   Select the zone(s) you want to consider for local polyhedra conversion from the Cell Zones selection list. After the zone selection is made, the Current value of the Maximum Cell Skewness and the percentage of Cells Above Target are displayed.

2.   Specify the maximum allowable cell skewness in the Target text-entry box, and press <Enter> to update the Cells Above Target.

The Cells Above Target (%) should be only a couple of percentage points, otherwise the conversion will be ineffective due to the high face count.

3.   Click the Convert button.

The number of created polyhedra and the resulting maximum cell skewness will be printed in the console.