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Re: Help with cylindrical mesh (beginner)

Posted by Courtney Gras at December 10. 2012

Hello All:

I am attempting to import a .step file into Salome V6.5.0 so that I can partition this mesh for use with Elmer (following these guidelines).

I have made an attempt at meshing my geometry many times, but always receive a memory error (or the software freezes entirely). I am completely new to this procedure (and FEM in general), so I am wondering if somebody could please take a look at my .step file and recommend an approach.

I have 2 different .step files - both represent stranded cables, although one is twisted and one is straight (I originally assumed the straight cable would work since it is a simpler geometry).

A link to the straight cable is here: https://www.dropbox.com/s/51svhhpl20a4iee/Cable_Total.zip

And a link to the twisted cable is here: https://www.dropbox.com/sh/eylkf71r0ywhybv/fd3UxFaaJY

My eventual goal is to have a single mesh of the cable which can be imported into Elmer for further analysis, but partitioned such that Elmer "sees" the outer 24 wires as a separate "body" (they need to represent steel cables while the inner wires should be copper). 

Any assistance you can provide would be much appreciated. 

Best Regards,

Courtney

Re: Help with cylindrical mesh (beginner)

Posted by Saint Michael at December 11. 2012

Hello Courtney

I tried to mesh Cable_Total.stp with NETGEN 1D-2D-3D algorithm and got a failure because of an invalid 2D mesh, which appeared too coarse and self-intersecting in the middle of wire. I deduced that to use NETGEN 1D-2D-3D it's necessary to limit max size of element. Note that while computing this mesh, SALOME occupied ~4.5 Gb of RAM.

I tried to use "Extrusion 3D" algorithm. In case of Cable_Total.stp it's much faster than NETGEN, though it's a bit harder to correctly assign all needed algos and hypotheses. I wrote a script doing this automatically and got a mesh on Cable_Total.stp.

But when I tried to apply my script to 3ft wire.STEP, 3D meshing failed because 1) incorrect 2D mesh (different mesh on opposite faces of a wire, which can be fixed in principle by script correction) and 2) there are solids including several wires, which is fatal for use of "Extrusion 3D" algorithm.

3ft wire.STEP can't be meshed using NETGEN 1D-2D-3D, because faces of solids, which compose one wire, are overlapping. But you can use NETGEN 1D-2D + Tetrahedron (Netgen). I am going to try it.

 

Attached is the script I used to mesh Cable_Total.step


St.Michael

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Re: Help with cylindrical mesh (beginner)

Posted by Courtney Gras at December 11. 2012

St. Michael:

Thanks so much for your help! Please let me know the outcome of the combination NETGEN 1D-2D + Tetrahedron (Netgen) method. I will attempt the script myself, but if you would be kind enough to share the Cable_Total.stp file, that would help me a lot (especially since I am new at this, I will be able to verify everything worked correctly). If you have any further recommendations, please let me know.


Thank you again for your time!

Best Regards,

Courtney

 

 

 

Re: Help with cylindrical mesh (beginner)

Posted by Courtney Gras at December 12. 2012

A final question: I realize that I need to limit the max element size, and I am curious as to what you limited that to.

 

Thanks again,

Courtney

Re: Help with cylindrical mesh (beginner)

Posted by Saint Michael at December 12. 2012

I have not tried it yet. NETGEN 1D-2D-3D failed because there were triangles of linear size close to diameter of a wire. You are to set a max size of element to a reasonable value.

St.Michael

Re: Help with cylindrical mesh (beginner)

Posted by William Tougeron at December 13. 2012

Hi Courtney and Saint Michael,

From my point of view, it should be possible to create a structured (blocking) mesh.

I enclose screenshots of what I mean on a single "sub-wire" of the twisted one.

To do so I rebuilt the geometry in Salome (as I do very often). It is not a problem at all for the strait wire. For the stwisted wire, it can be difficult, but I created a Python function able to create the middle spline of two spline. This allows to extract the middle edge of the 'pipes' (the wires), so as to extrude newly created block faces along this spline (see the picture enclosed).

Courtney, I would like to be sure of the file to use about the twisted wire. Because from your link there are several possibilities, and the one I downloaded is not only twisted, but bended. And, it contains intersecting volumes. So it is obvious that for this case the structured mesh is not possible. Else, there is no problem.

Are you interested? And what file to use for the twisted wire? Also, what number of cells do you expect? Or what maximum cell size do you want?

Best regards,

William

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Re: Help with cylindrical mesh (beginner)

Posted by William Tougeron at December 13. 2012

Here is the function to create the middle spline.

Import this script in your study (File, Load Script...), then type in your Python console:

MakeMiddleSpline( 'Edge_1' , 'Edge_2' )

(To be adapted according to the name of your splines in the study tree, of course...)

Best regards,

William

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Re: Help with cylindrical mesh (beginner)

Posted by Courtney Gras at December 13. 2012

Hello William,

 

Thank you so much for the response - I truly appreciate your assistance! I have created a separate folder in dropbox to share the files from, so hopefully the links below will lead to the correct files.

 

 My ultimate goal is to obtain a mesh of the twisted wire, with a separate "body" (in Elmer) for the outer and inner wires as mentioned above. The straight cable was really just a backup plan in case I could not get the twisted cable to mesh properly.

 

If you are still willing to assist, the new links to these files are the following:

 

Straight wire: https://www.dropbox.com/s/6drmv687mbdz1ay/Straight_Wire.step

Twisted Wire: https://www.dropbox.com/s/29rymw4f7rok1pa/Twisted_Wire.STEP

 

As for maximum cell size, I have been exploring a number of options, but with my limited experience I would welcome you to use your discretion. The final application for this geometry is an analysis of coupled thermal and static current equations in Elmer to look for fusing current given this wire geometry and material properties (the outer cable is steel and the inner is copper).

 

Thank you again for your assistance!

 

Regards,

Courtney

 

 

 

 

 

Re: Help with cylindrical mesh (beginner)

Posted by William Tougeron at December 13. 2012

Hi, Courtney,

Unfortunatelly some solids describing wires are intersected in your twisted wire geometry (see the enclosed screenshots). So I cannot generate a structured mesh on it.

One solution is to rebuilt the geometry without intersection in Salome. On the picture you can see the intersected solids in red, and the 'stand-alone' solids in green. A solution could be to copy the stand-alone solids so as to replace the intersecting ones by other ones not intersecting (and to rebuilt the central wire which is straight by simple extrusion). Your geometry will be sligtly modified, but very similar to the original (as you can see in the picture), and compatible with structured mesh.

Is it OK for you?

Best regards,

William

PS: I have no experience with Elmer export, but I think it won't be a problem...

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Re: Help with cylindrical mesh (beginner)

Posted by Courtney Gras at December 14. 2012

Hi William, 

Thanks for the quick response. I am aware of the intersections - if this is too much of an issue (i.e. it would take a long time to repair, which is what I imagine), then I am fine with using & meshing the straight wire instead of the twisted wire for the sake of simplicity and time. (I created the straight wire from scratch in FreeCAD)

I believe that I can export the final mesh as a unv file and import to Elmer without issue. 

Thank you again for your time - please let me know your thoughts. 

Best Regards, 

Courtney

 

Re: Help with cylindrical mesh (beginner)

Posted by William Tougeron at December 17. 2012

Hello,

I finished the straigth wire meshing, so that you can begin with it.

If you can wait a bit more, I will then create the twisted mesh after removing the intersections.

Do you have any mail address to send you the files?

Best regards,

William

Re: Help with cylindrical mesh (beginner)

Posted by Courtney Gras at December 17. 2012

Hi William, 

Yes, thanks so much! My email is: cgras42@gmail.com

Best Regards, 

Courtney

Re: Help with cylindrical mesh (beginner)

Posted by William Tougeron at December 19. 2012

Hi Courtney,

Actually there are other more 'serious' intersections in your twisted wire that cannot be removed without changing the geometry. They are between the outer and the inner cables.

In the enclosed picture, I marked in red the maximum r position (in a cylindrical coordinate system) of the innner cables, and in green the minimum position of the outer cables. (The area in orange is the intersection area). To avoid intersections, the green circle should be bigger than the red one.

Is it a problem if I increase a bit the r position of the outer cables?... Else the radius of cables can be reduced.

Say me what you want to do about this.

Best regards,

William

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Re: Help with cylindrical mesh (beginner)

Posted by Courtney Gras at December 20. 2012

Hi William, 

Sorry for the delayed response. Yes, it is fine to increase the r position of the outer cables - I noticed that problem earlier in FreeCAD, but didn't know if it would be easy to modify (this .step file was given to me by another individual) . If you are able to modify it to remove the intersections, that would be great!  Thanks so much!

By the way, I was able to get the straight wire to load in Elmer (although Elmer runs sluggish unless I reduce the number of nodes). Thank you for your help and specific directions - this has helped me a lot!

 

Regards, 

Courtney

Re: Help with cylindrical mesh (beginner)

Posted by William Tougeron at December 21. 2012

Hello Courtney

The mesh is ready now (see the enclosed picture). I sent it to you by mail

Best regards ! :)

William

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