7.3.16 Periodic Boundary Conditions

Periodic boundary conditions are used when the physical geometry of interest and the expected pattern of the flow/thermal solution have a periodically repeating nature. Two types of periodic conditions are available in ANSYS FLUENT. The first type does not allow a pressure drop across the periodic planes. (Note to FLUENT 4 users: This type of periodic boundary is referred to as a "cyclic'' boundary in FLUENT 4.) The second type allows a pressure drop to occur across translationally periodic boundaries, enabling you to model "fully-developed'' periodic flow. (In FLUENT 4 this is a "periodic'' boundary.)

This section discusses the no-pressure-drop periodic boundary condition. A complete description of the fully-developed periodic flow modeling capability is provided in Section  9.2.

Examples of Periodic Boundaries

Periodic boundary conditions are used when the flows across two opposite planes in your computational model are identical. Figure  7.3.26 illustrates a typical application of periodic boundary conditions. In this example the flow entering the computational model through one periodic plane is identical to the flow exiting the domain through the opposite periodic plane. Periodic planes are always used in pairs as illustrated in this example.

Figure 7.3.26: Use of Periodic Boundaries to Define Swirling Flow in a Cylindrical Vessel

Inputs for Periodic Boundaries

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  9.2.)

Rotationally periodic boundaries are boundaries that form an included angle about the centerline of a rotationally symmetric geometry. Figure  7.3.26 illustrates rotational periodicity. Translationally periodic boundaries are boundaries that form periodic planes in a rectilinear geometry. Figure  7.3.27 illustrates translationally periodic boundaries.

Figure 7.3.27: Example of Translational Periodicity - Physical Domain

Figure 7.3.28: Example of Translational Periodicity - Modeled Domain

You will specify translational or rotational periodicity for a periodic boundary in the Periodic dialog box (Figure  7.3.29), which is opened from the Boundary Conditions task page (as described in Section  7.1.4).

Figure 7.3.29: The Periodic Dialog Box

Note that there will be an additional item in the Periodic dialog box for the density-based solver, which allows you to specify the periodic pressure jump. See Section  9.2 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 Mesh/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 mesh: the mesh geometry is not periodic about the specified axis.

Default Settings at Periodic Boundaries

By default, all periodic boundaries are translational.

Calculation Procedure at Periodic Boundaries

ANSYS FLUENT treats the flow at a periodic boundary as though the opposing periodic plane is a direct neighbor to the cells adjacent to the first periodic boundary. Thus, when calculating the flow through the periodic boundary adjacent to a fluid cell, the flow conditions at the fluid cell adjacent to the opposite periodic plane are used.