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11.4.2 Setting Up the Problem

The steps for setting up a sliding mesh problem are listed below. (Note that this procedure includes only those steps necessary for the sliding mesh model itself; you will need to set up other models, boundary conditions, etc. as usual.)

1.   Enable the appropriate option for modeling unsteady flow in the Solver panel. (See Section  25.17 for details about the unsteady modeling capabilities in FLUENT.)

Define $\rightarrow$ Models $\rightarrow$ Solver...

2.   Set boundary conditions for the sliding action:

Define $\rightarrow$ Boundary Conditions...

(a)   Change the zone type of the interface zones of adjacent cell zones to interface in the Boundary Conditions panel.

(b)   In the Fluid panel or Solid panel for each moving fluid or solid zone, select Moving Mesh in the Motion Type drop-down list and set the translational and/or rotational velocity. (Note that a solid zone cannot move at a different speed than an adjacent fluid zone.)


Note that simultaneous translation and rotation can be modeled only if the rotation axis and the translation direction are the same (i.e., the origin is fixed).

By default, the velocity of a wall is set to zero relative to the motion of the adjacent mesh. For walls bounding a moving mesh this results in a "no-slip'' condition in the reference frame of the mesh. Therefore, you need not modify the wall velocity boundary conditions unless the wall is stationary in the absolute frame, and therefore moving in the relative frame. See Section  7.13.1 for details about wall motion.

See Chapter  7 for details about input of boundary conditions.

3.   Define the grid interfaces in the Grid Interfaces panel (Figure  11.4.1).

Define $\rightarrow$ Grid Interfaces...

Figure 11.4.1: The Grid Interfaces Panel

(a)   Enter a name for the interface in the Grid Interface field.

(b)   Specify the two interface zones that comprise the grid interface by selecting one or more zones in the Interface Zone 1 list and one or more zones in the Interface Zone 2 list. (The order does not matter.)

(c)   Set the Interface Type, if appropriate. There are two options:

  • Enable Periodic for periodic problems.


    Periodic is not a valid option when more than one zone is selected in each Interface Zone.

  • Enable Coupled if the interface lies between a solid zone and a fluid zone.

(d)   Click on Create to create a new grid interface.

For all types of interfaces, FLUENT will create boundary zones for the interface (e.g., wall-9, wall-10), which will appear under Boundary Zone 1 and Boundary Zone 2. You can use the Boundary Conditions panel to change them to another zone type (e.g., pressure far-field, symmetry, pressure outlet).

If you have enabled the Coupled option, FLUENT will also create wall interface zones (e.g., wall-4, wall-4-shadow), which will appear under Interface Wall Zone 1 and Interface Wall Zone 2.

If you create an incorrect grid interface, you can select it in the Grid Interface list and click on the Delete button to delete it. (Any boundary zones that were created when the interface was created will also be deleted.)


When you have completed the problem setup, you should save an initial case file so that you can easily return to the original grid position (i.e., the positions before any sliding occurs). The grid position is stored in the case file, so case files that you save at different times during the unsteady calculation will contain grids at different positions.


For cases with strong impeller-baffle interactions, it is recommended that you switch from an MRF model setup to a sliding mesh setup using the following text command:

grid $\rightarrow$ modify-zones $\rightarrow$ mrf-to-sliding-mesh

To successfully switch from an MRF to a sliding mesh, you must provide the ID of the fluid zone. FLUENT identifies all the zones belonging to this fluid zone as well as fluid zones shared in the domain. FLUENT then splits these zones into walls, afterwhich the walls are slit converted to interfaces. FLUENT then changes the boundary condition of the fluid zone to Moving Mesh in the Fluid panel. The sliding mesh solution tends to be more robust than the MRF solution.

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