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12.7.8. Diagnosing an Overset Case

Once an overset interface is created, it is important to check and diagnose it. In this section you will learn about diagnosing an overset interface and about the different cells participating in the solution.

In overset meshing all meshes are categorized into five cell types:

•Solve – (yellow in figures Figure 12.3: Cell Marking on component (p. 438) & Figure 12.4: Cell Marking on fluid-background (p. 438)) cells that take part in the solution.

•Donor – (marked red in figures Figure 12.3: Cell Marking on component (p. 438) & Figure 12.4: Cell Marking on fluid-background (p. 438)) provide information to corresponding cell zones.

•Receptor – (marked blue in figures Figure 12.3: Cell Marking on component (p. 438) & Figure 12.4: Cell Marking on fluid-background (p. 438)) receive information from donor cells in the corresponding cell zones.

•Dead – (marked in red in figures Figure 12.5: Dead Cells in the Component (p. 440)) deactivated cells located in regions of overlap between the background and component meshes, where multiple cells are present in the same exact location. Only one set of cells in a region is allowed to take part in the solution. Additionally, cells outside of the flow regime are marked as dead.

•Orphan – receptor cells that cannot find a corresponding donor cell. Although Fluent has intelligence to deal with orphan cells, their presence should be avoided to reduce the risk of solution inaccuracies and divergence.

You can mark orphan cells using the TUI command:

define/overset-interfaces/mark-cells orphan no

This will show all of the orphan cells present in the case, unless you specify a particular zone. Marking orphan cells creates a register that you can display via the define/overset-interfaces/display- cells text command. The marked cells are in the overset-orphan-cells-r0 register.

In this case there are not any orphan cells, so nothing is displayed in the graphics window. If you mark solve cells or other cells types and display those registers, then it will appear in the graphics window.

If large number of orphan cells are generated in a case, then it is advisable to modify the mesh accordingly.

Different type of cells can also be displayed creating contours of Cell Info and displaying the Overset Cell Type for given surfaces.

Enable the following TUI command to include receptor cells in the contours display:

/define/overset-interfaces/options/render-receptor-cells? yes

The overset interface for this tutorial is created properly, but issues could arise during overset interface creation for geometries with complex topology. ANSYS Fluent provides options to diagnose issues and fix them. You can use the ‘debug hole cut’ option to understand more about flood filling of seed cells or leakage between overlapping boundaries. For detailed information on overset mesh diagnosis, refer to "Diagnosing Overset Interface Issues" in the Fluent User's Guide.

1.Start a new Fluent session, and read overset-pod-steady-state.cas.gz and overset-pod- steady-state.dat.gz.

File → Read → Case & Data...

2.Create zone surfaces for component and fluid-background.

Results → Surface → Create → Zone...

a.Select component in the Zone list.

b.Retain component for New Surface Name and click Create.

c.Similarly, create a zone surface for fluid-background.

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d.Close the Zone Surface dialog box.

3.Enter the following TUI command in the console:/define/overset-interfaces/op- tions/render-receptor-cells? Yes

4.Display contours of the overset cell type on the surface you just created.

Results → Graphics → Contours → Edit...

a.Enable Filled and disable Autorange (which enables Clip to Range) in the Options group box.

b.Disable Node Values and Global Range in the Options group box.

c.Select Cell Info... and Overset Cell Type from the Contours of drop-down lists.

d.Enter -0.5 for Min and 2.5 for Max.

Note

Dead cells cannot be displayed in contours.

e.Enter 3 for Levels.

f.Select component from the list of Surfaces.

g.Click Display.

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Figure 12.3: Cell Marking on component

h.Repeat the process for fluid-background.

Figure 12.4: Cell Marking on fluid-background

5.Marking and displaying dead cells.

a. Expand Cell Zones in the tree to check the id number.

Setup → Cell Zone Conditions

b.Mark dead cells in component with the following TUI command: define/overset-inter- faces/mark-cells dead yes 29

c.Mark dead cells in fluid-background with the following TUI command: /define/overset-in- terfaces/mark-cells dead yes 7

d.Display the dead cells by entering the define/overset-interfaces/display-cells text command in the console.

i.Enter 0 for the text command prompt to display the overset-dead-cells-component-r0 register (you can enter 0 instead of typing the full name of the register).

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Figure 12.5: Dead Cells in the Component

ii.Enter 1 for the text command prompt to display only the overset-dead-cells-fluid-background- r1 register.