[lordvon]

Hello all,
Can someone please explain what criteria a cell has to meet before being categorized in the group 'twoInternalFacesCells'?

[GerhardHolzinger]

If you have a 2D mesh, then in the corners, the cells have only two internal faces. You can visualize face or cell sets in paraview, so you can check where these cells are located.

I hope this ascii sketch of a 2D mesh works out. The internal faces are marked red. The cells in the corner have two wall faces and two internal faces. The remaining two faces of the hex are of type emtpy. So, we end up with four cells with only two internal faces.

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In a 3D mesh the presence of cells with only two internal faces might indicate a problem with the mesh. But in a 2D case at least four of those cells are always present.