Polygon geometry problem

Builder
Builder

Hi, 

 

If i understand correctly, Polygon geometry is the approximation version of original CAD. Recent, i noticed that some dimensions are offset in Polygon geometry compared to the CAD. A simple case is the arc. In CAD, the arc edge is centered at origin, but there are some offset in polygon geometry. For my case, i want the mesh nodes are exactly on the CAD geometry edge or face. I tried the command Adjust Node Proximity to CAD but did not work. So i am wondering if there are any way to avoid any approximation or abstraction in polygon geometry in fem. 

 

Thanks,

 

Kan

4 REPLIES 4

Re: Polygon geometry problem

Phenom
Phenom

Kan,

 

as nobody is answering you  I will give you advices which I know (using NX10):

I think you want to improve the quality of your mesh by means of improving the accuracy of polygon bodies itself.

 

It's correct that polygon bodies are approximation bodies. I'm really sure that there is no way to get the analytical geometry into FEM or other NURBS or spline geometry directly.

I guess the reason is managing to manipulate the geometry in FEM directly with stitching an so on.

 

So manipulating the FEM by means of STITCHING (also in an automatically way) will downgrade the accuracy of FEM.

Auto-stitching can also happen in meshing.

 

- Model Cleanup options =>

     Small feature tolerance

     Merge Edges

     Match Edges

- Mesh Quality options => Midside method

 

But you want to improve accuracy of polygon body before using it for meshing I guess.

So I've found in Help

HELP - Start => CAE => Advanced Simulation => Geometry abstraction and polygon geometry => Polygon g...

2019-03-20_094324.png

 

With default values for  "Distance Tolerance" and "Angle Tolerance" I did get the following, as you already mentioned :

 

2019-03-20_102312.png

 

2019-03-20_092814.png

 

I could improve accuracy by refining the tolerance values BEFORE creating the first CAD body in first PRT (not iPRT or FEM), Modification with factor 1000 (not visible).

(I guess switching from 25 mm to 50 mm has no effect, it was a little mistake in creating)

 

2019-03-20_100648.png

 

2019-03-20_100518.png

 

I do not know if it's suitable for you and also I don't know if it works in your case but maybe it's a starting point for further investigations.

 

Hint: The first body with 25 mm radius was a "Design feature => Cylinder" and the second one was a body made by "Revolving a rectangle 360 °". After creating I did split both twice in PRT and made an iPRT with one wave link of one quarter. That I did select for polygon body in FEM in both cases.

 

Best wishes, Michael

| Production: NX10; Development: VB, TCL/TK, FORTRAN; Testing: NX12 | engelke engineering art GmbH, Germany | 2019-07-08
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Re: Polygon geometry problem

Siemens Phenom Siemens Phenom
Siemens Phenom

Polygon bodies are graphical tessellations of the CAD data. Arcs become polylines. In previous versions we added user control of the polygon body resolution as standard, medium, and high. Then we added a custom option. These are described in the following online help article.

 

https://docs.plm.automation.siemens.com/tdoc/nx/1847/nx_help#uid:id1397479

 

Standard, medium, and fine use hard coded methods to compute the resolution based upon the CAD body's bounding box size. If a body is large, it will have a coarse resolution. If you divide the body into 10 equal divisions, then each of the 10 bodies will have 1/10th the resolution of the original body. The difference between standard, medium, and high is with respect to the level of refinement due to edge and face curvature. Medium refines due to edge curvature. High refines due to face curvature as well. Typically face curvatures match edge curvatures, but not always.

 

Finally, meshing does try to recognize arcs and place nodes on theoretical arcs rather than on the polygon edge itself. If you create a circular face and mesh it, you should see nodes off of the edge if the mesh size is smaller than the body resolution. That's because the edge nodes are using the theoretical edge of the arc rather than the polygon edge itself.

 

Regards,

Mark  

Re: Polygon geometry problem

Phenom
Phenom


Hello Mark, thank you for that information.

From which NX Simcenter Version does it come? - I've searched for it in my actual version of NX10 and there are no dialog entries like described in your article.

 

>>>

2019-04-18_095137.png

<<<

See here

>>>

2019-04-18_095204.png

<<<

 

Best wishes, Michael

| Production: NX10; Development: VB, TCL/TK, FORTRAN; Testing: NX12 | engelke engineering art GmbH, Germany | 2019-07-08
| Kudos for good posts! And if my post answers your question, please mark it as an "Accepted Solution". Kick Off (2015-09-14)On the Map (2016-10-21)1st Gear 2018 (2018-02-19)5th Birthday! (2018-02-25)Popular (100, ‎2018-10-30)Prolific (100, 2019-02-27)Philosopher (500, ‎2019-02-28)Problem Solver (50, 2019-06-25)

Re: Polygon geometry problem

Siemens Phenom Siemens Phenom
Siemens Phenom

Michael,

 

I extracted my reply from the NX version 1847 online help. It turns out that the custom polygon geometry resolution was added in 1847 (Simcenter 3D 2019.1) too. Here's a view of the new features in Simcenter 3D 2019.1:

 

https://docs.plm.automation.siemens.com/tdoc/nx/1847/nx_help#uid:index_whatsnew:xid1537725:xid153772...

 

Note the article regarding custom polygon resolution under the geometry abstraction topic.

 

https://docs.plm.automation.siemens.com/tdoc/nx/1847/nx_help#uid:index_whatsnew:xid1537725:xid153772...

 

Regards,

Mark