6.8.2 Separating Zones

Upon reading a mesh file, ANSYS FLUENT automatically performs zone separations in two conditions. If a face zone is attached to multiple cells zones in the preprocessor, the face zone will be separated so that each one is attached to only one cell zone. Furthermore, if you have defined an internal face as a wall type, an additional shadow wall zone will be generated (e.g., for a wall named baffle, a shadow wall zone named baffle-shadow will be generated).

There are several methods available in ANSYS FLUENT that allow you to manually separate a single face or cell zone into multiple zones of the same type. If your mesh contains a zone that you want to break up into smaller portions, you can make use of these features. For example, if you created a single wall zone when generating the mesh for a duct, but you want to specify different temperatures on specific portions of the wall, you will need to break that wall zone into two or more wall zones. If you plan to solve a problem using the sliding mesh model or multiple reference frames, but you forgot to create different fluid zones for the regions moving at different speeds, you will need to separate the fluid zone into two or more fluid zones.

After performing any of these separations, you should save a new case file. If data exists, it is automatically assigned to the proper zones when separation occurs, so you should also write a new data file. The old data cannot be read on top of the case file in which the zones have changed.

The maximum number of zones into which you can separate any one face zone or cell zone is 32.

There are four ways to separate face zones and two ways to separate cell zones. The face separation methods will be described first, followed by the cell separation tools. Slitting (decoupling) of periodic zones is discussed in Section  6.8.5.

Note that all of the separation methods allow you to report the result of the separation before you commit to performing it.

Separating Face Zones

Methods for Separating Face Zones

For geometries with sharp corners, it is often easy to separate face zones based on significant angle. Faces with normal vectors that differ by an angle greater than or equal to the specified significant angle will be placed in different zones. For example, if your mesh consists of a cube, and all 6 sides of the cube are in a single wall zone, you would specify a significant angle of 89 $^\circ$. Since the normal vector for each cube side differs by 90 $^\circ$ from the normals of its adjacent sides, each of the 6 sides will be placed in a different wall zone.

If you have a small face zone and would like to put each face in the zone into its own zone, you can do so by separating the faces based on face. Each individual face (triangle, quad, or polygon) will be separated into different zones.

You can also separate face zones based on the marks stored in adaption registers. For example, you can mark cells for adaption based on their location in the domain (region adaption), their boundary closeness (boundary adaption), isovalues of some variable, or any of the other adaption methods discussed in Chapter  27. When you specify which register is to be used for the separation of the face zone, all faces of cells that are marked will be placed into a new face zone. (Use the Manage Adaption Registers dialog box to determine the ID of the register you wish to use.)

Finally, you can separate face zones based on contiguous regions. For example, when you use coupled wall boundary conditions you need the faces on the zone to have a consistent orientation. Consistent orientation can only be guaranteed on contiguous regions, so you may need to separate face zones to allow proper boundary condition specification.

Inputs for Separating Face Zones

To break up a face zone based on angle, face, adaption mark, or region, use the Separate Face Zones dialog box (Figure  6.8.2).

Mesh $\rightarrow$ Separate $\rightarrow$ Faces...

Figure 6.8.2: The Separate Face Zones Dialog Box

If you are planning to separate face zones, you should do so before performing any adaptions using the (default) hanging node adaption method. Face zones that contain hanging nodes cannot be separated.

The steps for separating faces are as follows:

1.   Select the separation method ( Angle, Face, Mark, or Region) under Options.

2.   Specify the face zone to be separated in the Zones list.

3.   If you are separating by face or region, skip to the next step. Otherwise, do one of the following:

• If you are separating faces by angle, specify the significant angle in the Angle field.

• If you are separating faces by mark, select the adaption register to be used in the Registers list.

4.   (optional) To check what the result of the separation will be before you actually separate the face zone, click the Report button. The report will look like the following example:

 45 faces in contiguous region 0
 30 faces in contiguous region 1
 11 faces in contiguous region 2
 14 faces in contiguous region 3
 Separates zone 4 into 4 zone(s).

5.   To separate the face zone, click the Separate button. A report will be printed in the console like the following example:

 45 faces in contiguous region 0
 30 faces in contiguous region 1
 11 faces in contiguous region 2
 14 faces in contiguous region 3
 Separates zone 4 into 4 zone(s).
 Updating new zone information ...
   created new zone wall-4:001 from wall-4
   created new zone wall-4:002 from wall-4
   created new zone wall-4:010 from wall-4
 done.

When you separate the face zone by adaption mark, you may sometimes find that a face of a corner cell will be placed in the wrong face zone. You can usually correct this problem by performing an additional separation, based on angle, to move the offending face to a new zone. You can then merge this new zone with the zone in which you want the face to be placed, as described in Section  6.8.1.

Separating Cell Zones

Methods for Separating Cell Zones

If you have two or more enclosed cell regions sharing internal boundaries (as shown in Figure  6.8.3), but all of the cells are contained in a single cell zone, you can separate the cells into distinct zones using the separation-by-region method. Note that if the shared internal boundary is of type interior, you must change it to another double-sided face zone type ( fan, radiator, etc.) prior to performing the separation.

Figure 6.8.3: Cell Zone Separation Based on Region

You can also separate cell zones based on the marks stored in adaption registers. You can mark cells for adaption using any of the adaption methods discussed in Chapter  27 (e.g., you can mark cells with a certain isovalue range or cells inside or outside a specified region). When you specify which register is to be used for the separation of the cell zone, cells that are marked will be placed into a new cell zone. (Use the Manage Adaption Registers dialog box to determine the ID of the register you wish to use.)

Inputs for Separating Cell Zones

To break up a cell zone based on region or adaption mark, use the Separate Cell Zones dialog box (Figure  6.8.4).

Mesh $\rightarrow$ Separate $\rightarrow$ Cells...

Figure 6.8.4: The Separate Cell Zones Dialog Box

If you are planning to separate cell zones, you should do so before performing any adaptions using the (default) hanging node adaption method. Cell zones that contain hanging nodes cannot be separated.

The steps for separating cells are as follows:

1.   Select the separation method ( Mark or Region) under Options.

2.   Specify the cell zone to be separated in the Zones list.

3.   If you are separating cells by mark, select the adaption register to be used in the Registers list.

4.   (optional) To check what the result of the separation will be before you actually separate the cell zone, click the Report button. The report will look like this:

 Separates zone 14 into two zones, with 1275 and 32 cells.

5.   To separate the cell zone, click the Separate button. ANSYS FLUENT will print the following information:

 Separates zone 14 into two zones, with 1275 and 32 cells.
 No faces marked on thread, 2
 No faces marked on thread, 3
 No faces marked on thread, 1
 No faces marked on thread, 5
 No faces marked on thread, 7
 No faces marked on thread, 8
 No faces marked on thread, 9
 No faces marked on thread, 61
 Separates zone 62 into two zones, with 1763 and 58 faces.
 All faces marked on thread, 4
 No faces marked on thread, 66
 Moved 32 cells from cell zone 14 to zone 10
 Updating new zone information ...
   created new zone interior-4:010 from interior-4
   created new zone interior-6:009 from interior-6
   created new zone fluid-14:008 from fluid-14
 done.

As shown in the example above, separation of a cell zone will often result in the separation of face zones as well. When you separate by mark, faces of cells that are moved to a new zone will be placed in a new face zone. When you separate by region, faces of cells that are moved to a new zone will not necessarily be placed in a new face zone.

If you find that any faces are placed incorrectly, see the comment above, at the end of the inputs for face zone separation.