You should be aware of the following geometry setup and grid construction requirements at the beginning of your problem setup:
The conformal periodic boundaries can be created in GAMBIT or TGrid when you are generating the volume mesh. See the GAMBIT Modeling Guide or the TGrid User's Guide for more information about creating periodic boundaries in GAMBIT or TGrid.) Alternatively, you can create the conformal periodic boundaries in FLUENT using the make-periodic text command (see Section 6.8.4 for details).
Although GAMBIT and TGrid can produce true periodic boundaries, most CAD packages do not. If your mesh was created in such a package, create the periodic boundaries using the non-conformal periodic option in FLUENT (see Section 6.8.4 for details). However, this option is recommended only for periodic zones that are planar.