6.4.3 Requirements and Limitations of Non-Conformal Meshes

This section describes the requirements and limitations of non-conformal meshes:

• The mesh interface can be of any shape (including a non-planar surface, in 3D), provided that the two interface boundaries are based on the same geometry. If there are sharp features (e.g., 90-degree angles) or curvature in the mesh, it is especially important that both sides of the interface closely follow that feature.

  For example, consider the case of two concentric circles that define two fluid zones with a circular, non-conformal interface between them, as shown in Figure  6.4.9. Because the node spacing on the interface edge of the outer fluid zone is coarse compared to the radius of curvature, the interface does not closely follow the feature (in this case, the circular edge.)

  The maximum tolerance between two interfaces should not be larger than their adjacent cell size at that location That is no cell should be completely enclosed between two interfaces.

  Figure 6.4.9: A Circular Non-Conformal Interface

  

• If you create a single mesh with multiple cell zones separated by a non-conformal boundary, you must be sure that each cell zone has a distinct face zone on the non-conformal boundary.

  The face zones for two adjacent cell zones will have the same position and shape, but one will correspond to one cell zone and one to the other. It is also possible to create a separate mesh file for each of the cell zones, and then merge them as described in Section  6.3.15.

• All periodic zones must be correctly oriented (either rotational or translational) before you create the non-conformal interface.

• You must not enable the periodic boundary condition option if the interface is adjacent to another non-conformal interface.

• In order for the periodic boundary condition option or periodic repeats option to be valid, the edges of the second interface zone must be offset from the corresponding edges of the first interface zone by a uniform amount (either a uniform translational displacement or a uniform rotation angle). This is not true for non-conformal interfaces in general.

  The periodic boundary condition option has the additional requirement that the angle associated with a rotational periodic must be able to divide 360 without remainder.

• The periodic repeats option requires that some portion of the two interface zones must overlap (i.e., be spatially coincident).

• The periodic repeats option requires that the non-overlapping portions of the interface zones must have identical shape and dimensions. If the interface is part of a sliding mesh, you must define the mesh motion such that this criterion is met at all times.

• The periodic repeats option requires one pair of conformal periodic zones adjacent to each of the interface zones. For example, when you calculate just one channel and blade of a fan, turbine, etc., you must have conformal periodics on either side of the interface threads. This will not work with non-conformal periodics.

  Note that for 3D cases, you cannot have more than one pair of conformal periodic zones adjacent to each of the interface zones.

• You must not have a single non-conformal interface where part of the interface is made up of a coupled two-sided wall, while another part is not coupled (i.e., the normal interface treatment). In such cases, you must break the interface up into two interfaces: one that is a coupled interface, and the other that is a standard fluid-fluid interface. See Section  6.4.4 for information about creating coupled interfaces.