While applying periodic boundary conditions on set of boundaries [with identical nodes displaced in y-direction] in fluent, the defined "PERIODIC" options are interchanging to "WALL".

(1). I have not used the code for a while now, because I am busy checking out another code right now. (2). As I remembered, each time I create a model (2-D or 3-D), I can not create both periodic boundaries. I can only create one of the periodic boundary along with the inlet, exit, and maybe a body inside the domain. (3). At this point, the geometry of the computational domain always has the second periodic boundary missing. So, the geometry is not defined complete. There is an open edge ( for 2-D) or an open surface (for 3-D). (4). This missing periodic edge or surface is then created in a separate menu item by selecting and copying the first defined periodic boundary to a new location to fill the missing portion of the geometry. This must be done by translation or rotation. Naturally, all of the created geometry and mesh must be merged. This process is done to ensure that the second periodic boundary is identical to the first periodic boundary in geometry and mesh. ( so, the surface mesh has to be constructed first on the ono-complete geometry first.) (5). That's all I can remember right now. Hope this will help.