Closed surface within closed surfaces (for cfd)

Rhino
1

Hi

I’m a CAD modeller for CFD. I often get into trouble when I have to make an air part.
ex: I have a room with a duct. I only need the air in the room, which means I have to subtract the duct from the air in the room. But that gives the Objects do not intersect.
Does anybody know a work around for this issue?

ex:

image
image775×504 8.62 KB

2

Hej @Astrid_Kjærbølling
Are you, by chance, meshing your geometry before exporting to whatever CFD program you are using? If yes, it’s as simple as meshing the parts beforehand using the Mesh command, flipping the normals of the inner mesh (in this case the duct) using the Flip command and then joining the two meshes (making sure that JoinDisjointMeshes=Yes in the command prompt. Start the Join command with no objects pre-selected to make the options available).
HTH, Jakob

Edit: And if you ever need to split seperate meshes that you have previously joined, the command is SplitDisjointMesh.

3

Good to know.
Normally we would like to have nurb geometry for the cfd program. So the person doing the CFD make the tessellation.

4

Hi again @Astrid_Kjærbølling
It’s still do-able - it’s just a bit more convoluted.

  1. “Join” the parts using NonManifoldMerge
  2. Use CreateRegions to create all possible regions (geometries) from the non-manifold geometry.
  3. Select and delete all the regions you don’t need. It can sometimes be an advantage to create a clipping plane, so that it’s easier to see what you are deleting, as all the regions perfectly overlap.
  4. Export as STP (or whatever format you use - but STP supports nonmanifold geometries).
    HTH, Jakob
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5

Thanks that seems to work.
Can wait to test if its okay in the cfd program aswell!

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