TGrid-FLUENT - How to transition from hexes to tets without pyramids ('integral face match')
Typically, pyramids are used to transition from hex cells to tet cells. In some cases, small clearances make the introduction of an additional pyramidal 'layer' troublesome. An alternate approach is to split each quad into 2 trianges. The hex cell 'sees' 2 tri's making up one quad. This is sometimes referred to as a type of 'integral face match'.
To use this capability, you must first create a 'tri' version of the quad zone. It is neccessary that the original quad shape is preserved so that each quad is split into 2 component tris. This is achieved in TGrid using the /boundary/create-tri-surface command. Assume the original wall is called wall-quad, and the 'triangularized' version is called wall-tri.
Next, it is neccessary to merge the nodes between the two zones. This is performed in the TGrid GUI. Select wall-quad in one column, and wall-tri in the other column. Be sure 'free nodes' in both columns is deselected, and press 'merge'. Now there are 2 face zones but a single set of shared nodes.
Create a tetrahedral subdomain using wall-tri, and generate the tet mesh. Save the final mesh.
(This mesh can also be generated in another pre-processor: it is simply required that each quad is split into two tris, the quads and tris are in separate face zones, and the nodes are shared between the two face zones. Furthermore, until these separate face zones are 'merged' in FLUENT, it is neccessary that the face zones have an external boundary type assignment).
Read the mesh into FLUENT and enter the /grid text user interface menu. Execute the make-hanging-interface command (can be abbreviated mhi), it will ask for the quad zone, then the tri zone:
> /grid> mhi
> Quad face zone id/name  wall-quad
> Tri face zone id/name  wall-tri
> Created hanging interface. Moved wall-tri to interior-14
At this point, there is a single interior zone.
The /grid/make-hanging-interface command leverages the parent-child face relationship structure used for hanging node adaption.