6.7.1 Converting the Domain to a Polyhedra

Conversion of a mesh to polyhedra only applies to 3D meshes that contain tetrahedral and/or wedge/prism cells. To begin the conversion process, ANSYS FLUENT automatically decomposes each non-hexahedral cell into multiple sub-volumes called "duals" (the shaded regions seen in the 2D example in Figure  6.7.1). Each dual is associated with one of the original nodes of the cell. These duals are then agglomerated into polyhedral cells around the original nodes. Thus, the collection of duals from all cells sharing a particular node makes up each polyhedral cell (see Figure  6.7.2). The node that is now within the polyhedral cell is no longer needed and is removed.

To better understand how duals are formed, you can consider the straightforward case of a tetrahedral mesh. Each of the cells are decomposed in the following manner: first, new edges are created on each face between the face centroid and the centroids of the edges of that face. Then, new faces are created within the cell by connecting the cell centroid to the new edges on each face. These interior faces establish the boundaries between the duals of a cell, and divide the cell into 4 sub-volumes. These dividing faces may be adjusted and merged with neighboring faces during the agglomeration process, in order to minimize the number of faces on the resultant polyhedral cell.

Figure 6.7.1: Connection of Edge Centroids with Face Centroids

Figure 6.7.2: A Polyhedral Cell

Hexahedral cells are not converted to polyhedra when the domain is converted, except when they border non-hexahedral cells. When the neighboring cell is reconfigured as polyhedra, the shared face of the hexahedral cell is decomposed into multiple faces as well, resulting in a polyhedral cell. In such a case the shape of the original hexahedral cell is preserved (i.e. the overall dimensions of the cell stay the same), but the converted cell has more than the original 6 faces (see Figure  6.7.3).

Figure 6.7.3: A Converted Polyhedral Cell with Preserved Hexahedral Cell Shape

Conversion proceeds in a slightly different manner in boundary layers that are modeled using thin wedge/prism cells. These cells are decomposed in the plane of the boundary surface, but not in the direction normal to the surface. The resulting polyhedra will therefore preserve the thickness of the original wedge/prism cells (Figure  6.7.4). In most cases, the cell count in the new polyhedral boundary layer will be lower than the original boundary layer.

Figure 6.7.4: Treatment of Wedge Boundary Layers

To convert the entire domain of your mesh, use the Mesh/Polyhedra/Convert Domain menu.

Mesh $\rightarrow$ Polyhedra $\rightarrow$ Convert Domain

The following is an example of the resulting message printed in the console:

Setup conversion to polyhedra.
Converting domain to polyhedra...

        Creating polyhedra zones.
        Processing face zones...........
        Processing cell zones...
        Building polyhedra mesh...................
        Optimizing polyhedra mesh.....
>> Reordering domain using Reverse Cuthill-McKee method:
      zones, cells, faces, done.
   Bandwidth reduction = 1796/247 = 7.27
   Done.

Figure  6.7.5, the original tetrahedral mesh of a section of a manifold, is compared to Figure  6.7.6 which is the resulting mesh after the entire domain is converted to a polyhedra.

Figure 6.7.5: The Original Tetrahedral Mesh

Figure 6.7.6: The Converted Polyhedral Mesh

Limitations

Some limitations you will find with polyhedral meshes that you generally do not experience with other cell types include:

• Meshes that already contain polyhedral cells cannot be converted.

• 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.

• The following mesh manipulation tools are not available for 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

• Meshes in which the domain has been converted to polyhedral cells are not eligible for adaption. For more information about adaption, see Chapter  27.

• The dynamic mesh model cannot be used on polyhedral meshes. The dynamic mesh model will automatically be disabled if a case with the dynamic mesh option enabled is converted.