32.5.2 Preparing Hexcore Meshes for Partitioning

If you generated your mesh using the GAMBIT Hex Core meshing scheme or the TGrid
Mesh/Hexcore menu option, the mesh may contain features that need to be addressed prior to partitioning. Such features include hanging nodes and overlapping parent-child faces, and are located at the transition between the core of hexahedral cells and the surrounding body-fitted mesh. If the interface boundary between the partitions happens to go through these parent-child faces, the solution may fail or produce inaccurate results.

One way to address the overlapping parent-child faces in a hexcore mesh is to encapsulate the neighboring cells. If you use the ANSYS FLUENT serial solver to do the partitioning or the parallel solver with auto-partitioning, the encapsulation is done automatically while balancing the partitions. However, if there are too many encapsulated cells clustered together, the load may not be evenly distributed among the compute nodes. Furthermore, the size of the interface boundary between partitions may be increased due to these constraints. Either of these circumstances can affect the performance of the solution.

Therefore, it is recommended that you remove the hanging nodes and overlapping parent-child faces before you partition a hexcore mesh by converting the transitional hexahedral cells to polyhedra. Each transitional cell that is converted will retain the same overall dimensions, but will have more than the original 6 faces. The conversion to polyhedra should take place prior to reading the mesh into ANSYS FLUENT, and can be done using either the tpoly utility or TGrid.

When you use the tpoly utility, you must specify an input case file that contains a hexcore mesh. This file can either be in ASCII or Binary format, and the file should be unzipped. If the input file does not contain a hexcore mesh, then none of the cells are converted to polyhedra. When you use the tpoly utility, you should specify an output case file name. After the input file has been processed by the tpoly filter, an ASCII output file is generated.

The output case file resulting from a tpoly conversion only contains mesh information. None of the solver-related data of the input file is retained.

To convert a file using the tpoly filter, before starting ANSYS FLUENT, type the following:

utility tpoly input_filename output_filename

You can also use TGrid to convert the transitional cells to polyhedra. You must either read in or create the hexcore mesh in TGrid, and then save the mesh as a case file with polyhedra. To do this, use the File/Write/Case... menu option, being sure to enable the Write As Polyhedra option in the Select File dialog box.

Limitations

Converted hexcore meshes have the following limitations:

• The following mesh manipulation tools are not available on polyhedral meshes:
  • extrude-face-zone under the modify-zone option

  • 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.