Why the background mesh is removed? (Fluent, Overset, 3D)

mesemese Member

I have a 3D Fluent model where a disc (with a partly cogged surface) is rotating into a fluid medium. The disc diameter is 80 mm and its length is 0.25 mm. The depth of the background mesh is equal to the length of the disc (the disc fits into the background mesh).

The model consists of a background mesh (figures 1) and a component mesh (figures 2). The component mesh is a ring of fluid around the disc. I have considered a cylindrical hole in the middle of the background. The hole and disc are concentric and the diameter of the hole is somewhat smaller than the disc so that a layer of mesh will exist between the surfaces of the hole and disc (figure 3). I expected that after initialization, only this layer of mesh becomes “dead” and is removed from the solution domain.

Figure 1.A. The front view for the background mesh.

Figure 1.B. A zoomed-in view for the background mesh (the part which is shown into the rectangle in Figure 1.A.) 

Figure 1.C. A side view for the background mesh showing its depth. 

Figure 2.A. The front view for the disc mesh.

Figure 2.B. A zoomed-in view for the disc mesh (the part which is shown in the rectangle in Figure 2.A.)

Figure 3. a zoomed-in view for the background and disc meshes all together.

The problem is that when I want to display contours, the whole background mesh is disappeared as if it has been removed. In particular, with Results -> Graphics -> Mesh, when the option Overset is ticked, only the mesh for the component (disc) is shown, but when that option is not ticked, the background mesh is visible as well (see figure 4).

Figure 4.


1. I use overset mesh because there is a tiny gap (a few microns) between the disc and a blade and it was too difficult to preserve the tolerances with sliding mesh.

2. The reason for considering a hole into the background is to save memory usage, because without the hole, we will need to generate a huge number of elements in the background mesh while we know that the elements which fall into a solid object will be removed by the solver after initialization.

3. The front and rear surfaces of both background and disc are introduced as symmetry boundary conditions.


  • kkourbatkkourbat Forum Coordinator

    I assume the case has been initialized, otherwise the overset mesh cannot be displayed. I assume also that there is no orphans, or the number of orphans are small. This is a 3d case, no surface has been selected in the display Mesh panel. Use boundary distance donor priority method. Gaps are small, make sure to use double precision solver.

  • mesemese Member

    Thank you. Yes, the displayed results are after initialization. Please note that in Figure 4, I have not shown the selected surfaces. In fact, for illustrations, I have selected all items in the surfaces list.

    The model is with double precision. I used boundary distance donor priority method but still the background mesh is not visible.

    Let’s have a look to the orphan cells. There are 170292 overset orphan cells in the overset interface and that is the same with both donor priority methods.

    Figure 5 (below) shows orphan cells by the command define/overset-interfaces/mark-cells then writing orphan then the command define/overset-interface/display-cells. The figure is the same with both donor priority methods. We see that a row of orphan cells is formed in the outer layer of the component mesh. Sounds interesting.

    Figure 5. A zoomed-in view showing orphan cells in red colour.

    I guess the background mesh is actually removed by the solver and I think the reason is related to the fact that a physical boundary of the background is surrounded by a wall of the component mesh (see figure 3), which as a result, a physical boundary is forced to remove. If that is the case (?) I do not know based on which rule Fluent makes this decision.

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