This note discusses a common error encountered by users when trying chimera meshes in CFD-FASTRAN.

1. Hole-Cutting is Encroaching a Wall/Blocked Region
2. Entire zone was blanked out while cutting a chimera hole

Such errors are easy to avoid and hopefully this note will assist you.

Let’s start with a brief overview of the chimera methodology using a 2D case of flow over a cylinder. We have a background mesh encompassing the outer boundaries of the computational domain and a chimera mesh to include the cylinder as shown in Figure 1. In order to accurately represent the physics of flow over a cylinder, the region corresponding to the cylinder has to be removed from the background mesh. This, in essence is the hole-cutting operation and the resulting hole in the background mesh is the chimera hole. The errors mentioned above appear during this operation.

Figure 1.  Overview of Chimera methodology 

These errors usually occur under the following conditions:

1. Two wall boundaries are intersecting. The wall boundaries may be from a background mesh or any of the chimera meshes. An example shown in Figure 2 where the wall boundaries from two chimera meshes intersect. This condition violates the current limitations on our chimera methodology. Usually one might avoid this in the initial setup. However, when the chimera grid is moving either through prescribed motions or 6-DOF models, one might encounter this. For prescribed motions, the chimera test mode under SC → Adv → Skip Flow-field Solution can be used to test if there is a possibility for wall boundaries to intersect.
2. The mesh in which a chimera hole is being cut has a wall boundary and during the hole-cutting operation, this wall is exposed (i.e., all the cells up until the wall were removed).In the example shown in Figure 3, the background mesh has only one cell available for hole cutting and hence the wall is exposed resulting in the encroachment error. This mesh would work fine if there were no walls. The solution is to increase the mesh resolution in the background mesh such that hole cutting doesn’t remove all the cells up to the wall. About 5 cell thickness is recommended. Another variation of this error is that an entire zone is blanked out. A similar solution should be able to fix that as well.
3. A trivial but sometimes overlooked reason for the second error (entire zone blanking) is that a user forgets to specify the overset boundaries.

Figure 2.  Chimera errors due to intersecting walls

Figure 3.  Chimera errors due to insufficient mesh resolution. 

Regards,
Abraham Meganathan
ESI CFD Support Team