6.4.1 Non-Conformal Mesh Calculations

To compute the flux across the non-conformal boundary, ANSYS FLUENT must first compute the intersection between the interface zones that comprise the boundary. The resulting intersection produces an interior zone where the two interface zones overlap (see Figure  6.4.1).

Figure 6.4.1: Completely Overlapping Mesh Interface Intersection

If one of the interface zones extends beyond the other (Figure  6.4.2), by default ANSYS FLUENT will create additional wall zones for the portion(s) of the boundary where the two interface zones do not overlap.

Figure 6.4.2: Partially Overlapping Mesh Interface Intersection

Fluxes across the mesh interface are computed using the faces resulting from the intersection of the two interface zones, not from the interface zone faces.

In the example shown in Figure  6.4.3, the interface zones are composed of faces A-B and B-C, and faces D-E and E-F.

Figure 6.4.3: Two-Dimensional Non-Conformal Mesh Interface

The intersection of these zones produces the faces a-d, d-b, b-e, and e-c. Faces produced in the region where the two cell zones overlap (d-b, b-e, and e-c) are grouped to form an interior zone, while the remaining face (a-d) forms a wall zone.

To compute the flux across the interface into cell IV, face D-E is ignored and instead faces d-b and b-e are used to bring information into cell IV from cells I and III.

While the previous discussion described the default treatment of a non-conformal interface, there are several options you can enable to revise the treatment of the fluxes at the interface:

• periodic boundary condition

• periodic repeats

• coupled wall

These non-conformal interface options are described in the sections that follow.

The Periodic Boundary Condition Option

Non-conformal interfaces can be used to implement a periodic boundary condition like that described for conformal periodic boundaries (see Section  7.3.16). The advantage of using a mesh interface is that, unlike the standard periodic boundary condition, the nodes of the two zones do not have to match one-for-one.

The interface zones that utilize the periodic boundary condition option (Figures  6.4.4 and 6.4.5) are coupled in the manner described in the previous section, except that the zones do not overlap (i.e., the zones are not spatially coincident at any point). In order to generate the new faces that will be used to compute the fluxes across the interface, the nodes of the first zone are either translated or rotated (about a given axis) onto the other zone. The distance / angle that the nodes are translated / rotated is called the "periodic offset''. The new faces will be defined between all of the combined nodes, and then applied to each of the original zones.

Figure 6.4.4: Non-Conformal Periodic Boundary Condition (Translational)

Figure 6.4.5: Non-Conformal Periodic Boundary Condition (Rotational)

The Periodic Repeats Option

The periodic repeats option is appropriate when each of the interface zones is adjacent to a pair of conformal periodic zones (see Figures  6.4.6 and 6.4.7). The periodic repeats option takes into account the repeating nature of the flow solutions in the two cell zones in the following manner. Wherever the interface zones overlap (i.e., wherever interface zone 1 and 2 are spatially coincident), the fluxes on either side of the interface are coupled in the usual way. The portion of interface zone 1 that does not overlap is coupled to the non-overlapping portion of interface zone 2, by translating or rotating the fluxes by the periodic offset. This is similar to the treatment of non-conformal periodic boundary conditions. The periodic repeats option is typically used in conjunction with the sliding mesh model when simulating the interface between a rotor and stator.

Figure 6.4.6: Translational Non-Conformal Interface with the Periodic Repeats Option

Figure 6.4.7: Rotational Non-Conformal Interface with the Periodic Repeats Option

The Coupled Wall Option

As described previously, the typical function of non-conformal interfaces is to couple fluid zones, so as to permit fluid flow to pass from one mesh interface to the other. Another available option is to create a coupled wall boundary at the interface. In such a case, fluid flow would not pass across the interface, as the interface is acting as a wall zone. Coupled wall heat transfer, on the other hand, would be permitted. Such an interface is required if one or both of the cell zones is a solid. It is also allowable if both of the cell zones are fluids; for example, you can model a thin wall or baffle separating the two fluid zones. Figure  6.4.8 illustrates coupled walls with both solid and fluid zones.

Note that coupled walls can also make use of the periodic repeats option. That is, both options can be invoked simultaneously (see Section  6.4.4 for more details).

Figure 6.4.8: Non-Conformal Coupled Wall Interfaces