For a periodic boundary without any pressure drop, there is only one input you need to consider: whether the geometry is rotationally or translationally periodic. (Additional inputs are required for a periodic flow with a periodic pressure drop. See Section 9.4.)
Rotationally periodic boundaries are boundaries that form an included angle about the centerline of a rotationally symmetric geometry. Figure 7.15.1 illustrates rotational periodicity. Translationally periodic boundaries are boundaries that form periodic planes in a rectilinear geometry. Figure 7.15.2 illustrates translationally periodic boundaries.
You will specify translational or rotational periodicity for a periodic boundary in the Periodic panel (Figure 7.15.4), which is opened from the Boundary Conditions panel (as described in Section 7.1.4).
Note that there will be an additional item in the Periodic panel for the density-based solvers, which allows you to specify the periodic pressure jump. See Section 9.4 for details.
If the domain is rotationally periodic, select Rotational as the Periodic Type; if it is translationally periodic, select Translational. For rotationally periodic domains, the solver will automatically compute the angle through which the periodic zone is rotated. The axis used for this rotation is the axis of rotation specified for the adjacent cell zone.
Note that there is no need for the adjacent cell zone to be moving for you to use a rotationally periodic boundary. You could, for example, model pipe flow in 3D using a nonrotating reference frame with a pie-slice of the pipe; the sides of the slice would require rotational periodicity.
You can use the Grid/Check menu item (see Section 6.5) to compute and display the minimum, maximum, and average rotational angles of all faces on periodic boundaries. If the difference between the minimum, maximum, and average values is not negligible, then there is a problem with the grid: the grid geometry is not periodic about the specified axis.