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ERROR Surface meshing

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ERROR Surface meshing

Posted by alberto at April 20. 2016

hello 

im looking for advices about to how mesh mi geometry that i goint to export.

i have a 2 solids  that are not concentric, actually one of them is sitting in the other and i cut them like the picture

and i have been trying mesh with out succes. 

 

i made one by one but the top trought an error 

and i made a partition of both whit error too.

 

Warning. NgException at Surface meshing: Problem in Surface mesh generation

Intersecting triangles

 

https://www.dropbox.com/s/ihr1qylbh4o6u8j/Captura%20de%20pantalla%20de%202016-04-20%2012-50-18.png?dl=0

 

https://www.dropbox.com/s/943db323vy7z8cb/Captura%20de%20pantalla%20de%202016-04-20%2012-50-53.png?dl=0

 

https://www.dropbox.com/s/opklg9jqucum7kb/Captura%20de%20pantalla%20de%202016-04-20%2012-51-32.png?dl=0

 

Re: ERROR Surface meshing

Posted by Saint Michael at April 21. 2016

On such a geometry, this error is very natural, since triangles on one surface can touch triangles on another very close surface. To avoid this error, the mesh must be extremely fine.

St.Michael

Re: ERROR Surface meshing

Posted by alberto at April 21. 2016

how fine do you think ? the diameter it is 12.5 mm 

Re: ERROR Surface meshing

Posted by William Tougeron at April 21. 2016

Hello Alberto,

The problem is more in the geometry topology than in the cell size.

Actually, Salome meshes sub-faces separately. That is: the boundaries of each sub-face are then present in the mesh.

The problem of having infinitely thin faces like in the picture above is that you impose in the mesh infinitely thin face elements. You have two options:

1) If you want to keep your geometry as it is, you have no other choice than set an infinitely small element size like Saint Michael suggests to help the mesher work.

2) Else, you can modify your geometry to remove thin faces or sharp angles.

Hope this helps,

Best regards,

William

Re: ERROR Surface meshing

Posted by alberto at April 22. 2016
THANKS TO BOTH OF YOU.
 
I cut the thin surface, and that allow to me made the mesh
 
I WANT TO KNOW IF I CAN MAKE SEPARATE MESHES, OF SEPARATE BODIES AND LATTER BUILD A COMPOUND MESH AND MERGE THE NODES WITH OUT PARTITION.
 
i divided mi pipe in segments to mesh different ways and the covered of mi solid and the solid like in the picture, 
 
actually i have 7 solids where 5 are fluid and 2 solids .
 

Re: ERROR Surface meshing

Posted by Saint Michael at April 22. 2016

To finally get a conform compound mesh you need to assure same positions of node of intermediate surfaces. This can be assured using "Import 1D-2D element from another mesh" algorithm. But this naturally imposes some restrictions on meshes of solid domains.

St.Michael

Re: ERROR Surface meshing

Posted by Timo Siltala at December 22. 2016

Hello,

I have a related problem. The model consists of a shaft and hub.Both hub hole face and shaft outer face have local refinement to get the 2nd degree element mid nodes following the surfaces. Everything goes fine except both faces edge mid nodes, they are located on straight line between the corner nodes. This causes the meshes overlap and terminates meshing. I assume this might even be a bug because understandable default would be all nodes belonging to the surfaces being on their faces. Is there a workaround for this? Adjusting fineness has no effect on this.

Best regards,

Timo

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Re: ERROR Surface meshing

Posted by Saint Michael at December 22. 2016

Hi Timo

If second order causes meshing failure, then you can first generate a linear mesh and apply Modification > Convert to quadratic afterwards.

St.Michael

Re: ERROR Surface meshing

Posted by Timo Siltala at December 22. 2016

Hi St. Michael

I tried it but it makes the situation even more difficult because then ALL outer cylindrical face elements are making a shortcut and any inner face node penetrates them if the meshes do not have equal node locations (which they don't because they are meshed separately due to tiny gap between the faces). The main issue is how to force nodes belonging to a face to stand exactly on it, e.g. have constant radius as the solid's surface defining the face has. Without it any contact analysis is hopeless and requires insensible fine mesh. I believe Salome has been used for cases like this but what's the trick?

Timo

Re: ERROR Surface meshing

Posted by Saint Michael at December 22. 2016

Maybe "straight-edges" are there because you mesh them using Wire Discretization w/o Quadratic Mesh hypothesis? While faces are meshed by Netgen with Second order option ON.

St.Michael

Re: ERROR Surface meshing

Posted by Timo Siltala at December 26. 2016

Merry Christmas to you St. Michael

I have tried to mesh a simple circular cylinder so that the outer surface has nice mapped quadrangle mesh (quadratic) and the volume is filled with tetras (quadratic), except the outermost layer which should be pyramids (as well quadratic) to be in contact to the quadrangle face elements. All combinations I could figure out fail.

Using Netgen 1D-2D-3D I get smooth linear mesh. Using quadratic mesh the end edges have this shortcut phenomena and also the midnodes are totally incorrectly placed. Netgen 1D-2D-3D does not take 1D hypothesis so I created a submesh to the edges. There is no option to define 1D submesh to be quadratic. So that didn't help at all. Nor did defining quadrangle quadratic face submesh to the outer surface. Combining the submeshes makes no difference.

Well, how should this be solved with Salome? I know the extrusion and radial quadrangle hypothesis but a cylinder is just my simplified test case, reality has details not allowing to use those two hypothesis. Therefore I try to find a general procedure  to mesh such parts. Any method to mesh the "skin" in wanted (contact) areas with quadrangle quadratic elements and filling inside with pyramids and tetras will do, provided that the mid nodes of pyramid bottoms can be joined to midnodes of faces.

Best reagrds,

Timo

 

Re: ERROR Surface meshing

Posted by Saint Michael at December 26. 2016

Hi Timo

Using Netgen 1D-2D-3D I get smooth linear mesh.
It seems that it remains to make it quadratic using Modification > Convert to quadratic

.

I attach a script demonstrating this idea.

St.Michael

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Activate by alberto on Apr 20, 2016 09:45 PM
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