### 13.1.4. Domain Interface: Mesh Connection Tab

The Mesh Connection tab contains the Mesh Connection Method settings.

#### 13.1.4.1. Mesh Connection Method

You must specify a mesh connection method for all interface models.

##### 13.1.4.1.1. Mesh Connection: Option

The following options may be available, depending on other settings:

- Automatic

- 1:1 Direct (One-to-One)

- GGI (General Grid Interface)

For details on these options, see Mesh Connection Options in the CFX-Solver Modeling Guide.

##### 13.1.4.1.2. Intersection Control

You can use the options described in this section to control the intersection of non-matching meshes for a particular interface.

Note: You can also use Solver Controls to apply default controls for the intersection of all interfaces (settings that are overwritten by Intersection Control settings that you apply individually to domain interfaces using the settings below). See Intersection Control to learn how to apply default Intersection Control settings to all interfaces.

Note:

- If Direct (one-to-one) mesh connectivity is available, the solver will ignore the Intersection Control option and will instead use a 'topological intersection', that is, use the one-to-one information to generate the intersection data.

- If you are restarting a run, the intersection step is skipped and the intersection data is read from the results file. This behavior can be overridden by setting the expert parameter `force intersection` to `True`.

###### 13.1.4.1.2.1. Intersection Control: Option

The Intersection Control options for when the Mesh Connection Option is set to GGI or Automatic are as described below. The following options can be used to control the intersection of non-matching meshes. CFX provides the GGI (General Grid Interface) capability which determines the connectivity between the meshes on either side of the interface using an intersection algorithm. In general, two intersection methods are provided:

- Bitmap Intersection:

  Two faces on either side of the interface which have to be intersected are both drawn into an equidistant 2D pixel map. The area fractions are determined by counting the number of pixels that reside inside both intersected faces (that is, within the union of the two faces). The area fraction for a face is then calculated by dividing the number of overlapping pixels by the total number of pixels in the face. This method is very robust.

- Direct Intersection (Default):

  Two faces on either side of the interface are intersected using the Sutherland-Hodgeman clipping algorithm. This method computes the exact area fractions using polygon intersection, and is much faster and more accurate than the bitmap method.

The Bitmap Resolution controls the number of pixels used to fill the 2D pixel map (see description of the bitmap intersection method above). The higher this number, the more accurate the final calculation of the area fractions. In general, the default resolution of 100 should be sufficient but large differences in the mesh resolution on both sides of the interface as well as other mesh anomalies may require the bitmap resolution to be increased. Larger numbers will cause longer intersection times, for example, doubling the bitmap resolution will approximately quadruple the GGI intersection time.

The Angle tolerance option (in degrees) is used for `Direct` intersections only. In order for two faces to be able to intersect, the angular difference between their normals must be within the specified tolerance of 180 degrees.

Both Intersection Control options enable you to set the following:

Permit No Intersection

  When the Permit No Intersection option is set, the solver will run when there is no overlap between the two sides of an interface. This parameter is mainly useful for transient cases where interface geometry is closing and opening during the run. For example, transient rotor-stator cases with rotating valves, or moving mesh cases where the GGI interface changes from overlap to non-overlap during the simulation both can exhibit this type of behavior. This parameter is not switched on by default.

Discernible Fraction

  Controls the minimum area fraction below which partially intersected faces are discarded. The following default values used by the solver depend on the intersection method:

  - Bitmap Intersection: 1/(Bitmap Resolution)^1.5

  - Direct Intersection: 1.0E-06

Edge Scale Factor

  Controls the value of the GGI edge scale factor. Control volume sector faces on GGI interfaces are detected as degenerate if two opposite edges are smaller than the GGI edge scale factor times the cube root of the corresponding sector volume. Those faces are not intersected.

Periodic Axial Radial Tolerance

  Used when determining if the surface represented by the interface is a constant axial or radial surface. For a rotational periodic GGI interface, the solver ensures that the ratio of the radial and axial extent compared to the overall extent of each interface side is bigger than the specified value and therefore, the interface vertices do not have the same radial or axial positions.

Circumferential Normalized Coordinates Option

  The Circumferential Normalized Coordinates Option is used to set the type of normalization applied to the axial or radial position coordinates (η). Mesh coordinate positions on GGI interfaces using pitch change are transformed into a circumferential (θ) and axial or radial position (η). The η coordinates span from hub to shroud and are normalized to values between 0 and 1. In cases where the hub and/or shroud curves do not match on side 1 and side 2, different approaches are available to calculate the normalized η coordinates based on side local or global minimum and maximum η values:

  - Mixed (Default for Fluid Fluid interfaces): Normalization of η is based on local minimum and maximum η values as well as the η range of side 1. This method forces the hub curves on side 1 and 2 to align. Non-overlap regions adjacent to the shroud may be produced if the shroud curves are not the same.

  - Global (Default for Fluid Solid Interfaces): Normalization of η is based on global minimum and maximum eta values. This method intersects side 1 and 2 unchanged from their relative positions in physical coordinates. If the hub and shroud curves do not match then non-overlap regions will be produced.

  - Local: Normalization of η is done locally for each side of the interface. This method will always produce an intersection of side 1 and 2, but may cause undesirable scaling of the geometry in some cases.

Face Search Tolerance Factor

  A scaling factor applied to the element sized based separation distance, which is used to find candidates for intersection. For a given face on side 1 of the interface, candidate faces for intersection are identified on side 2 using an octree search algorithm. The octree search uses this tolerance to increase the sizes of the bounding boxes used to identify candidates. Making this parameter larger will increase the size of the bounding boxes, resulting in possible identification of more candidates.

Face Intersection Depth Factor

  A scaling factor applied to the element sized based separation distance used when performing the direct or bitmap intersection. The final intersection of faces is only applied to those faces that are closer to each other than a specified distance. This distance is calculated as the sum of the average depth of the elements on side 1 and side 2 of the interface. This factor is applied as a scaling on the default distance. It might be necessary to adjust this factor if the normal element depth on the two interfaces sides varies a lot, or side 1 and 2 of the interface are separated by thin regions (for example, thin fin type geometries).

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