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.
There are certain limitations with this type of conversion:
The following grid manipulation tools are not available on polyhedral meshes:
extrude-face-zone under the
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
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 Section
26.2.1), as well as meshes generated by the
GAMBITHex Core meshing scheme or the
TGridHexcore menu option (hexcore meshes). Note that if the mesh has undergone conformal adaption (see Section
26.2.2) or it is a hexcore mesh in which the transitional cells have been converted to polyhedra (see Section
31.5.2), then it does not have hanging nodes and can therefore be converted.