how to create hanging grid interface?

Hanging grid interface can be used to enforce connectivity between two different type of meshes, such as hex with tet mesh. As compared with non-confromal grid-interfaces, hanging grid-interface is more accurate.The requirement for this is to have all the nodes (boundary and interior nodes) on these two faces be merged in TGrid.
Lets say we have a small box (box-1) inside a big box (box-2). We would like to have hex mesh inside the small box and tet mesh outside the small box. Lets say all the surfaces of the big box (surf-2) is meshed with triangles (tris); since we have hex mesh, all the surfaces of the small box (surf-1) must be quad. surf-1 is the interface between the two boxes.

1. TGrid: Split the Quads into Tris:
type the following in Text User Interface (TUI):
/boundary/create-tri-surface/surf-1 no

It will copy surf-1 and split the quad in the copied one. The copied one will be free (standalone), which needs to be connected.

The copied one will get a default name. Lets call it surf-1-c.

2. TGrid: Merge Nodes

-Using the Graphical User Interface (GUI), go to Boundary->Nodes
-select only surf-1 under 'Boundary Face Zones' on the right under 'Compare ...'
-de-select 'Only Free Nodes' on the left side under 'Compare ...'

-select only surf-1-c under 'Boundary Face zones' on the right under 'With...'
-de-select 'Only Free Nodes' on the left side under 'Compare ...'

-click on Merge

This should merge ALL the nodes on one zone into another. You may need to display 'multi' on either surf-1 or surf-1-c to make sure they are all connected.

3. TGrid: Create Tet Mesh

Using GUI, go to: Mesh -> Domains
-select only surf-2, and surf-1-c under 'Boundary Zones'
-click on New

This will create a sub-domain, which is needed to create the tet mesh.

Using GUI: Mesh -> Auto Mesh -> Init & Refine

Save mesh.

4. Fluent: Create Hanging Interface

In TUI, type:

/grid/make-hanging-interface surf-1 surf-1-c

It will create hanging interface. It will move surf-1-c into an interior, having the same zone ID.

NOTES: We can NOT create hanging interface if the interface is wall. For wall type, we have three options:
I. either grow pyramids to make the mesh conformal.

II. create coupled non-conformal interface between hex and tet. Perform only steps 1 and 3 above. When in Fluent:

-Define -> Boundary Conditions
Change the type for surf-1 and surf-1-c to Interface type.

-Define -> Grid Interface
-enter any name under 'Grid Interface'
-select surf-1 under 'Interface Zone 1'
-select surf-1-c under 'Interface Zone 2'
-select Coupled under 'Interface Type'
-click on create.

The coupled option will make non-conformal interface if the interface is wall type. It will create wall and a shadow, but are not accessible by user.

III. If heat transfer is not important through this interface, step 1, and 3 need only be performed. In Fluent, by default, surf-1 and surf-1-c will be boundary walls and adiabatic.

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