Problem with mesh export in ELMER format with second order elements

1 week 6 days ago #3965 by Jeremie
Hello,

I created a volume mesh based on a simple cylinder in Netgen GUI 6.2.2004 with the option "Second order elements".

I exported the mesh in ELMER format but it seems there is an error in the mesh files. The information between mesh files are different:

mesh.boundary -> 306 type (quadratic triangle elements) ok
mesh.elements -> 510 type (quadratic tetrahedrons elements) ok

mesh.header -> indicates linear types: 303 and 504 (linear triangle and tetrahedron elements) (why?)

By using this mesh as this, the calculation in ELMER seems working. However there are pretty much triangles missing in the post-processing view of the .vtu files in Paraview.

I attached the geometry, the ELMER mesh and the .VTU output file of ELMER.

Thanks for helping.

Jeremie

File Attachment:

File Name: Geometry.stp
File Size:8 KB

File Attachment:

File Name: ElmerMesh.zip
File Size:28 KB

File Attachment:

File Name: VTU_file.zip
File Size:90 KB
Attachments:

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1 week 4 days ago #3970 by Jeremie
After discussion in the ELMER forum, it seems that the problem is coming from the ordering of nodes in the mesh.elements.

The mesh.elements contains the node indexes of each element.
For quadratic tetrahedrons, there are 10 nodes.

An illustration of the problem. For example here the node #6 is not at the correct location:

Correct node ordering: 1 2 3 4 5 6 7 8 9 10
WRONG node ordering: 1 2 3 4 5 7 8 6 9 10 (ordering coming from Netgen export to Elmer format)

Moreover as I said, the mesh.header must also be corrected:

mesh.header indicates linear types: 303 and 504 instead of 306 and 510 (linear triangles and linear tetrahedrons instead of quadratic triangles and quadratic tetrahedrons).

Please, is it possible to include this correction in the next release of Netgen ?

Any comments would be appreciated.

Thanks

Jeremie

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1 week 3 days ago #3972 by joachim
Hi Jeremie,
thank you for pointing this out, and also providing the solution.
We'll have the fix very soon in the nightly releases,
Joachim

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1 week 1 day ago #3975 by richb2k
Hello,

Two small questions about this topic.
1. Can Netgen/NGSolve generate 3D brick elements while meshing? I see that in 2D, quads can be generated, but what about bricks in 3D?
2. Where are element nodal orderings defined, such as what file(s) contains the actual internal ordering of nodes for elements generated by Netgen/NGSolve?

Thanks, Rich.

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1 week 1 day ago #3976 by matthiash
Hi Rich,

I'm currently implementing the fixes for the ELMER format export.
During this process I coincidentally did a documentaion of the node numbering in Netgen :)

The file is attached, the source code for it will be merged into master later today.

Best,
Matthias
Attachments:

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1 week 1 day ago #3977 by richb2k
Hello Mathias,

Excellent! the documentation of the node numbering looks great.

Thanks for taking care of this issue!

Rich.

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