I am trying to implement the periodic boundary condition on Gambit for fluent 5. When I export the mesh file I get errors about the periodic boundary conditions. The two faces I am specifying to have periodic boundary conditions have identical geometry and mesh topology. The error message is:

ERR[6006] periodic boundary condition on face 1 is incorrect and will not be written to the neutral file.

Do you have any suggestions??

I have had this type of problem before with geomesh and Fluent/UNS.

I would recommend that you assign a boundary condition other than periodic to the boundaries e.g. wall. Call one boundary, say, wall-periodic-1 and the other, say, wall-periodic-2. Once the file has been exported to Fluent then you can simply assign the walls as the periodic and shadow entities, respectively. Good luck.

Yep, Allan's suggestion should work. Gambit has had a problem with defining periodic boundaries - you have to define them as walls and then re-define them as periodic once you have the mesh inside Fluent (if they are rotationally periodic don't forget to set the axis of rotation on the fluid-domain first). This might have been fixed in the latest version of Gambit though, I haven't tried it.

You could try before meshing in gambit create a hardlink between the faces, that are going to be a periodic boundary. It worked out in my case.

Good Luck

As suggested by Kostya, creating a hard link between the periodic faces will eliminate this error. "hard link" is also necessary to ensure periodicity for unstructured mesh( quad/tri pave).

However, Fluent 5 may still report error for periodic bcs in some cases. This is due to the fact that Fluent 5 requires node numbers on periodic faces to be written in a particular order.Tfilter fe2ram which is used to translate the mesh from Gambit format to Fluent 5 format ensures only face pairing. So in some cases node pairings for periodics written by tfilter fe2ram may not match the Fluent 5 convention.

We will be implementing direct Fluent 5 export from Gambit(in place of using fe2ram) later this year which will fix this problem.

For now, please create periodics inside the solver using "make periodic" command as suggested by Allan and Jonas. slitting the periodic and remaking them again will also fix the problem.