## 6.4.1 Non-Conformal Grid Calculations

To compute the flux across the non-conformal boundary, 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).

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

Fluxes across the grid 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.

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 bringing information into cell IV from cells I and III.

Non-Conformal Interface Algorithms

Non-conformal interface calculations are handled using the following two approaches:

• Triangular face approach: triangulates the polygon intersection faces, and stores triangular faces. This approach involves node movement and water tight cells.

• Virtual polygon approach: stores area vector and centroid of the polygon faces. This approach does not involve node movement and cells are not necessarily water tight cells. Hence gradients are corrected to take into account the missing cell area.

Both the algorithms calculate the intersection of the interface meshes, however using different approach for calculations.

Note:   The former approach of making cells water tight sometimes results in concave cells, causing the interface creation to fail. For this reason the virtual polygon approach is more stable than the triangular face approach.

 Previous versions of FLUENT (version 6.1 and earlier) use only the triangular face approach. Current versions of FLUENT support both approaches.

Case files from FLUENT 6.1 or earlier can be read and run normally in the current version of FLUENT. By default, these cases will use the triangular face approach. You can also manually switch to the virtual polygon approach for such cases.

To use the new algorithm, use the use-virtual-polygon-approach TUI command.

define grid-interfaces use-virtual-polygon-approach

 /define/grid-interfaces> use-virtual-polygon-approach /define/grid-interfaces> 

During sliding mesh calculations, left handed faces can be generated due to bad grids. FLUENT corrects the left handedness of these faces automatically. But in extreme cases, it cannot be fully corrected. Such faces are deleted automatically, so that the solution does not diverge.

Left handed cells can also be created for the geometries that contains sharp edges and corners, which may affect the final solution. For such geometries, it is recommended to first separate the zones and then create the interfaces separately to get the better solution.

Additional input of angle/translation-vector may be required to recreate face-periodic interfaces. Also, with the new interface in parallel, there will be no need for encapsulation.

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