Strange issue in node numbering during meshing

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vpapanik
Posts: 46
Joined: Sat Oct 08, 2016 1:05 pm

Strange issue in node numbering during meshing

Postby vpapanik » Thu Mar 12, 2020 11:11 am

I have a simple 2D panel

b1wYagK.png
b1wYagK.png (19.34 KiB) Viewed 3174 times

which I am meshing with quadrilateral elements.

3wuxTeh.png
3wuxTeh.png (31 KiB) Viewed 3174 times

When I solve it for dead load I am getting strange results (some elements show sxx instead of syy)
because of element orientation.

tsThyLp.png
tsThyLp.png (55.08 KiB) Viewed 3174 times

That means that node numbering for some elements start from the lower left corner and some of them from the
lower right corner. When I am showing the node numbering, there are some strange gaps in number sequence.

yIBej3C.png
yIBej3C.png (85.28 KiB) Viewed 3174 times

I wonder if there's a way to read the correct orientation of elements in order to have correct results. I know
about normal but is there a way to check the starting node, or their local axes ?

Regards
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vpapanik
Posts: 46
Joined: Sat Oct 08, 2016 1:05 pm

Re: Strange issue in node numbering during meshing

Postby vpapanik » Thu Mar 12, 2020 8:42 pm

It seems that it has to do with the line sequence that created the surface. Is it possible to change (rotate) this sequence without having to delete the surface and create it again ?

I also don't know what SAME1ST/DIFF1ST means.
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escolano
Posts: 1628
Joined: Thu Sep 18, 2014 3:31 pm

Re: Strange issue in node numbering during meshing

Postby escolano » Thu Apr 09, 2020 7:26 pm

I have attached your images to this forum, to be visible.
(instead to be links to some external, and probably temporal, location)

About your issues, probably your problem is not related with node numbering (the node label), but to the element's connectivities.

Node numeration, depends on current RenumberMethod GiD variable, default value=0 provide a numeration ordering the nodes consecutivelly and by its distance to an arbitrary coordinate.

About elements connectivity. In general elements are in 3D and not aligned to global coordinates axis.
GiD provide the element connectivities ordered in a way that preserve the 'normal' of its source surface (right-hand rule), but the 'first' node of the list for a cuadrilateral element is arbitrary,
and depends as you noticed on the way of the surface is parametrized, and this depends also on how is created (by boundary, by extrusion, etc.)

Probably in 2D do you want that the elements have its normal pointing ot +Z axis, must check the surface normals to see if they have this orientation, and swap if doen't match your requirements
View->Normals->Surfaces...
Utilities->Swap normals->Surfaces...

By the images is not possible to know how are the element connectivities, attach a zip with the model
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vpapanik
Posts: 46
Joined: Sat Oct 08, 2016 1:05 pm

Re: Strange issue in node numbering during meshing

Postby vpapanik » Fri Apr 10, 2020 8:20 am

Thanks a lot for the information, you are right. I had to cycle the connectivity for problematic elements by calculating their direction vectors, so that their first line is horizontal. That fixed the problem, since there is no other way to read/write their local axes.

It is the same thing as trying to display a result vector ON the quadrilateral element, it's not possible without transformations first.

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