How to modify high aspect ratio's of imported stl

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Tim
Mesher
Beiträge: 34
Registriert: Fr 11. Mai 2012, 19:29

How to modify high aspect ratio's of imported stl

Beitrag von Tim »

Does anyone know a free solution to altering high aspect ratio of facets typically produced in cylindrical 3D models.
As i show, in Tetgen meshing the solution is very complex / irregular and can cause computation problems. However, it doesn't alter the facet length, so the curve has the original stl imported accuracy which produces
good hole stress plotting.
In NetGen meshing, the stl is converted into coarse facets which leads to higher stress results compared to Tetgen - its easy to see why.

Please see the images. ( this is my first try at image loading - so there might be a delay.)

Netgen corner stress: 1 of 2
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Netgen corner stress: 2
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Tetgen corner stress: 1 of 2
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Tetgen corner stress: 2
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Some of the images have been cut of by the photo hosting provider - i hope all is understood.

looking forward to any helpful answers..

Tim
Mesher
Beiträge: 34
Registriert: Fr 11. Mai 2012, 19:29

Re: How to modify high aspect ratio's of imported stl

Beitrag von Tim »

Here is another example using Netgen at the finest mesh size possible. In this case size 1.5 - this is NOT the correct procedure as its a waste of computational time and produces massive data which is slow to work with.
The only advantage is finer facets but these are still not fine enough and are inaccurate compared to Tetgen results.

However, as the computed stress values are higher, it is a SAFE result. So there is not a problem in working with Z88 at all.

I want to find a process to alter the aspect ratio of triangles. I can produce step / stl data. Can anyone help / point me in a good direction. I don't mind paying a little if need be.

Z88 manuals state parameters of stl import ( which are helpful) and the necessity of providing a good stl. - i wish there was some examples showing Good / Bad etc.

Meshing and refinement - works very well. :thumbup:
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Model faceting / Computing stresses perfectly to mesh :thumbup:
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Appreciate any contributions.

Tim.

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MNeidnicht
Alumni
Beiträge: 47
Registriert: Do 2. Sep 2010, 16:01

Re: How to modify high aspect ratio's of imported stl

Beitrag von MNeidnicht »

Hi Tim,

we know well the problems that occur with netgen and tetgen. Tetgen accepts the geometry from STL exactly, but provides poor quality element. Netgen creates very "beautiful" elements, but the volume of the part changes. ... Unfortunately mostly at the details that are of interest for the calculation. But the two free-meshers are not developed by us and we have actually no influence on the results. Our local refinement decomposes only the initial elements - so bad angle are inherited.

Conclusion: As long as the STL contains bad angle, we can not anything do for now. :(

So please look into the program, that exported the STL. Often there is an opportunity to enter a minimum aspect ratio or minimum angle or something else. What you need is a increase of the facet-number.

Allow me one more tip: If you compare the stresses on the Gauss points in two different meshes, you compare the stresses at different locations, too. Note that the Gauss points are situated inside of the element. In small elements, they are thus very close to the surface. Gauss points of elements, which are just a bit bigger are situated quite deeper in the part. Maybe corner node stresses are more interesting? Sometime, after a certain fineness of the mesh must occur convergence.

I added a picture to show you, what I mean.
circ_necking.png
(239.07 KiB) 2071-mal heruntergeladen

selopez
Preprocessor
Beiträge: 69
Registriert: Sa 24. Mär 2012, 03:10

Re: How to modify high aspect ratio's of imported stl

Beitrag von selopez »

tim.jpeg
(134.65 KiB) 2050-mal heruntergeladen
Tim, as you can see at the attached image I tried to reproduce the case that you exposed. Then I applied a particular procedure for prepare the STL file and then meshed it at Aurora's Tetgen for quadratic tetrahedrons. If that what you see at the image is what's you're looking for, I'll be glad to email you the description of the procedure.

Best regards

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auroraIco
Site Admin
Beiträge: 153
Registriert: Di 24. Aug 2010, 12:49

Re: How to modify high aspect ratio's of imported stl

Beitrag von auroraIco »

This looks very handsome!

Tim
Mesher
Beiträge: 34
Registriert: Fr 11. Mai 2012, 19:29

Re: How to modify high aspect ratio's of imported stl

Beitrag von Tim »

Yes - very neat. Selopez, i need your recipe!

Incase there's any other interested parties out there,
I have been experimenting with splitting surfaces by 'imprinting lines'

1. This gives a control of triangle ratios in the stl conversion process which my CAD
software didn't provide.

2. Provides / maintains positions from CAD surface positions to Tetgen free meshs.
I believe this is a standard procedure. These examples are just to shown what is possible.

Generally the split surface area should be smaller than the mesh size, however, small areas
can be combined to make larger areas than the mesh size thus keeping position on the surface
to apply BC at exact places.

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Maintaining surface positions in Tetgen 'Free Meshing':
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Another example:
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I hope this information helps some users in improved meshing and applying
loads / BC at exact positions on surfaces.

Comments + / - or neutral welcome. Are there any other ways to do the above?

Tim.

Tim
Mesher
Beiträge: 34
Registriert: Fr 11. Mai 2012, 19:29

Re: How to modify high aspect ratio's of imported stl

Beitrag von Tim »

Confession time.

I have spent too much time on .stl file translations and the problems with the traingles and meshing.

If you have not been using the .stp input, i would recommend experimentating with it.
The .stp file translation automatically overides CAD face / part facet setting and converts all facets according to a certain parameter within the .stp translation process - it looks good/better than the .stl
process to me. :D -although, there are always exceptions!
When using the Tetgen meshing process, the .stp translation allows a more regular, low ratio triangle mesh on cylindrical / toroidal surfaces without the local high intensities of illregular triangles.

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Hope this helps.

Regards to all.

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