## 6.4.2 Requirements and Limitations of Non-Conformal Grids

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

• The grid 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  6.4.4. 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.

• A face zone cannot share a non-conformal interface with more than one other face zone.

This is illustrated by an example, shown in Figure  6.4.5. Each volume in the figure is meshed separately and does not match node-to-node at the interface. In this example, a non-conformal interface is not allowed to be created between the three surfaces shown: one side of the box (rectangle 1) and an end cap from each pipe (circle 1 and circle 2). In order to create a non-conformal interface, rectangle 1 is split into two surfaces, rectangle 1a and rectangle 1b. Then two non-conformal interfaces are created between rectangle 1a and circle 2 and rectangle 1b and circle 1, respectively, as shown in Figure  6.4.6.

• If you create a single grid 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 grid 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.

Periodic non-conformal interfaces must overlap exactly. That is they need to have the same rotational or translational extent and also have the same axial extent. This is not true for interfaces in general, where a wall zone is created for non-overlapping regions.

• For 3D cases, if the interface is periodic, only one pair of periodic boundaries can neighbor the interface.

• Periodic interfaces require conformal periodics adjacent to it. For example, when you calculate just one channel and blade of a fan, or turbine, etc., you must have conformal periodics on either side of the interface threads. This will not work with nonconformal periodics.