Learning Forum

[Nathaniel]

Hi, I am trying to perform an external flow analysis of a rocket. To find the damping, I need to run transient pitching and rolling simulations. I am using a sliding mesh approach. However, I cannot get Ansys fluent meshing to detect the face zones properly and create a non-conformal mesh, specifically when the moving domain is a sphere. Everything works correctly when the moving domain is a cylinder.

My method for generating the geometry consists of the following:

1. Use create enclosure on rocket body to create spherical or cylindrical moving domain.
2. Move the solid out of the enclosure component and rename to movingDomain and delete the empty component.
3. Use create enclosure on 'movingDomain' solid to create staticDomain.
4. Move the solid out of the enclosure component and rename to 'staticDomain' and delete the empty component.
5. Delete/hide and suppress the original rocket body
6. Create named selections:
   1. Outside faces: 'far-field'
   2. 'staticDomain' inner face(s): 'interface-static'
   3. 'movingDomain' outer face(s): 'interface-moving'
   4. 'movingDomain' object faces: 'wall-xxx'

Below are the results of this process when using a sphere as the internal sliding mesh interface or a cylinder as the interface.

1. Next I use the workbench->fluent 2024 R2->watertight workflow button to move to fluent meshing.
2. The difference between the two cases appears when running the 'generate the surface mesh' task. 
   1. For the sphere case, a new face zone is auto-created called far-field:xxxx, and no 'generate non-conformal mesh' option is available under describe geometry task
   2. This far-field:xxxx face zone remains throughout the rest of the process and then causes issues when switching to solution mode as far-field:xxx face zone is the same as the 'interface-static' zone. Basically, somehow, an extra face or face selection is created on the sliding mesh interface. Which results in an extra boundary.
   3. This also happens when selecting the share topology option in fluent meshing OR selecting share topology in SpaceClaim or Discovery before importing. (I tried this even though I need a non-conformal mesh for the sliding mesh analysis)
   4. 
3. When clicking generate surface mesh task in the cylinder case:
   1. This works as intended: No new 'fair-field:xxx' face zone is created.
   2. Non-conformal mesh between objects is available in describe geometry
   3. Finishing meshing and switching to solution mode shows that all the boundaries are generated properly and that a sliding mesh analysis can be run.
   4. 

If anyone has an idea why there might be a difference between these two cases, please let me know. It seems like a bug or error to me. The only difference I can think of is that in the case of the cylinder interface, there are a number of edges present, while in the case of the sphere, there are no edges. While waiting for a reply, I will try imprinting an edge on the sphere to see if that changes the results. 

Using a sphere as an interface is rather important for me as I need to be able to rotate the inner sphere in arbitrary directions. Generating a new mesh with a cylinder perfectly matching each angle would be basically impossible. 

Please note that I have also tried several different ways of generating the same input geometry. For example, I created the whole enclosure and sphere and then used the combine tool, or I manually created every solid.  Either way, the results are the same.

[Nathaniel]

I've continued messing around with this problem and sometimes I get the following error when generating the surface mesh. There are some warning that happen before hand. This error occurs when I generate the objects using the cylinder tool, sphere tool and then starting from the outside and working in use the combine tool to remove the sphere from the farfield domain and then again to remove the rocket from the sphere. I'd love to hear any ideas on what to try.

Error: Cannot merge zones of different types or belonging to different objects.
Error Object: #f

Error: state/object/execute-command:Exception encountered in S_ExecuteTask: Cannot merge zones of different types or belonging to different objects.
Error Object: ()

[Nathaniel]

Okay, so I've boiled it down to a minimal example, and I am very confident I ran into a bizarre and specific glitch.

1. Create a cylinder starting at (0,0,0), and put the other end at (-600mm,0,0) with a 60mm diameter (-600mm,30mm,0).
2. Create a sphere with origin at (-330, 0, 0) and put its end point at (600,0,0)
3. Combine the cylinder with the sphere and delete the cylinder and boolean cylinder to leave the sphere with the void cylinder.
4. Create a named selection for the sphere surface and the internal cylinder surface. 
5. Import into fluent and mesh with default settings
6. It will fail to mesh the sphere surface and lose the named selection. 

Now, the fun part. If you change the endpoint of the sphere to 601 or 599, it works fine. It seems like an oddly specific glitch to run into me.