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# Mesh/Clipping problem

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### Mesh/Clipping problem

Posted by erichu at March 28. 2013

Hello everyone,

I have just started to use Salome and I find it really good. Right now I want to create a structural mesh on a cylinder or any similar shapes, i.e. the shape below (try to imagine yourself a nozzle). The center axis is the bottom line.

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However, some things regarding meshing makes me slightly confused, in particular the following problem.

I have created a simple geometry using the 2d sketcher, then build a face followed by revolution to create a 3d solid. I am then entering the mesh module to setup this mesh;

1) Create the outer mesh (around axial direction) / quadrangle and wire discretization

2) Create the end meshes / radial1d2d - 5 layers and the other layer is a projection

3) Set the 3d mesh to 3d extrusion

So far everything OK. The result is a mesh of hex and prism.  See attached file no clipping

Next step is to create a clip plane using clipping tool. However, something weird happens and I do not know if it is just an graphical artifact or a real mesh problem. Please see picture clipping below.

Now to the question: Is this just an artifact or can someone explain this behaviour?

Furthermore, does anyone have tips related to meshing of cylinders or similar shapes?

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### Re: Mesh/Clipping problem

Posted by erichu at March 28. 2013

here is the clipped picture

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### Re: Mesh/Clipping problem

Posted by erichu at March 29. 2013

Update:

The images I posted yesterday are not quite the same. The top one shows faces and the bottom one only volumes.

As I changed distro today, things appeared a bit different. When I use a overlay faces/volumes, the problem becomes much clearer. The white part of the image represents faces, I think, and the blue is the volume.

As almost all parts of the mesh look good, is the distortion a bug in Salome 6.6.0?

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### Re: Mesh/Clipping problem

Posted by SMESH expert at April 01. 2013

Hello erichu

Apparently this case reveals a bug of 3D meshing. Without clipping everything looks OK because all volumes are logically hidden by faces and are not visualized. With the clipping the distorted volumes becomes visible.

It would be easier to fix this issue if you provided your study (with not computed mesh) and a Python dump of your study.

As a workaround of this problem, consider creating a mesh by rotation. I mean you can create a 2D mesh of a axial section of your nozzle (you drew it in your first post). Then you use rotation to get a 3D mesh from the 2D one.

About creation of groups: if you create groups of segments before the rotation, then the rotation creates corresponding groups of faces.

Edward

### Re: Mesh/Clipping problem

Posted by erichu at April 18. 2013

Previously SMESH/MED expert wrote:

Hello erichu

Apparently this case reveals a bug of 3D meshing. Without clipping everything looks OK because all volumes are logically hidden by faces and are not visualized. With the clipping the distorted volumes becomes visible.

It would be easier to fix this issue if you provided your study (with not computed mesh) and a Python dump of your study.

As a workaround of this problem, consider creating a mesh by rotation. I mean you can create a 2D mesh of a axial section of your nozzle (you drew it in your first post). Then you use rotation to get a 3D mesh from the 2D one.

About creation of groups: if you create groups of segments before the rotation, then the rotation creates corresponding groups of faces.

Edward

Hello Edward,

Thanks for you answer. I've tried it out and find it very easy, however, I got some problems opening up the exported UNV file in paraView. It appears that some elements are missing (cells at some locations in the structure). Is it necessary to do any operations afterwards to close the 360 deg revolution?

Furthermore, does anyone know how to create such a nozzle structure using only hex directly in a 3D structure (without revolving the mesh - rather revolve the goemetry). Please see the attached picture nozzle

Thanks

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### Re: Mesh/Clipping problem

Posted by Saint Michael at April 19. 2013

I've tried it out and find it very easy, however, I got some problems opening up the exported UNV file in paraView. It appears that some elements are missing (cells at some locations in the structure). Is it necessary to do any operations afterwards to close the 360 deg revolution?

Try to merge nodes and elements after revolution.

St.Michael

### Re: Mesh/Clipping problem

Posted by erichu at April 21. 2013

Thanks Michael,

I have tried to merge nodes and elements with the result that some elements and nodes were merged together. However, when I open the mesh in paraView (via OpenFOAM) I get some weird faces. The mesh viewed in Salome can be seen in the attached file. In the next reply I have attached the paraView screenshot.

How can I remove those internal surfaces?

Previously Saint Michael wrote:

I've tried it out and find it very easy, however, I got some problems opening up the exported UNV file in paraView. It appears that some elements are missing (cells at some locations in the structure). Is it necessary to do any operations afterwards to close the 360 deg revolution?

Try to merge nodes and elements after revolution.

St.Michael

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### Re: Mesh/Clipping problem

Posted by erichu at April 21. 2013

Previously erichu wrote:

Thanks Michael,

I have tried to merge nodes and elements with the result that some elements and nodes were merged together. However, when I open the mesh in paraView (via OpenFOAM) I get some weird faces. The mesh viewed in Salome can be seen in the attached file. In the next reply I have attached the paraView screenshot.

How can I remove those internal surfaces?

Previously Saint Michael wrote:

I've tried it out and find it very easy, however, I got some problems opening up the exported UNV file in paraView. It appears that some elements are missing (cells at some locations in the structure). Is it necessary to do any operations afterwards to close the 360 deg revolution?

Try to merge nodes and elements after revolution.

St.Michael

Attachments

### Re: Mesh/Clipping problem

Posted by Saint Michael at April 23. 2013

How can I remove those internal surfaces?

If you speak about removal of the faces that were rotated to create a 3D mesh, you can (after merging nodes and elements) create a Group On Filter including these faces and remove this group with its contents. When creating the group, use "Belong to geom" criterion with threshold = the geom face you used to create the faces that you want to remove.

St.Michael

### Re: Mesh/Clipping problem

Posted by Patti Sheaffer at April 19. 2014

This has been a very helpful thread.  I didn't realize you could mesh a 2D surface, then rotate the mesh to create a "2.5D mesh," or even a 3D mesh.  The 2014 SALOME is very good.