STEP import of trimmed surfaces

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pamakela
Posts: 49
Joined: 06 Mar 2010, 18:37

STEP import of trimmed surfaces

Post by pamakela » 04 Jun 2010, 12:41

Hey all,


I tried to import and mesh a trimmed NURB surface in Elmer (in .stp format), but the resulting mesh is really distorted and clearly not of any use. Is there a solution for this? I already tried using Gmsh but it produces meshes that won't work in Elmer (even though they seem to import just fine).

STEP imported, trimmed Nurb patch: http://lipas.uwasa.fi/~n77624/brokenMesh.jpg


What I want to do is simply run a 2D case with a single trimmed surface.





Thanks

pamakela
Posts: 49
Joined: 06 Mar 2010, 18:37

Re: STEP import of trimmed surfaces

Post by pamakela » 05 Jun 2010, 11:52

I managed to get down into solving this case by extruding my profile and work on a thin 3D mesh. The mesh turned out very good, I'm wondering if it is somehow possible to extract one side of the 3D mesh and use it in a 2D case?


- pm

pamakela
Posts: 49
Joined: 06 Mar 2010, 18:37

Re: STEP import of trimmed surfaces

Post by pamakela » 07 Jun 2010, 18:51

The solution for using my trimmed NURB surface in a 2D case was to use GMSH and define the surface as a physical group.

- pm

raback
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Re: STEP import of trimmed surfaces

Post by raback » 07 Jun 2010, 23:07

Hi pm

For direct meshing of 2D surfaces you might try directly defining your case in in2d format of netgen. Gmsh uses netgen propably anyway. The format is not the prettiest one but might for some uses eliminate some steps in the workflow. For a few examples see files in directory $ELMER_HOME/samples/ElmerGUI/in2d

-Peter

pamakela
Posts: 49
Joined: 06 Mar 2010, 18:37

Re: STEP import of trimmed surfaces

Post by pamakela » 08 Jun 2010, 00:59

Hi - thanks for the reply.

I noticed it is possible to import and use a 2d STEP file in Elmer, if the profile is continuous. I was able to run a simple flow case without problems. But my initial setup (see the link in 1st post) with two elliptical holes inside the mesh cause problems - even when processed through Gmsh and setting up the physical group there. Seems like closed contours immersed in meshes require some additional steps?

raback
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Re: STEP import of trimmed surfaces

Post by raback » 08 Jun 2010, 09:28

Can't comment on the STEP import but the Gmsh import should not have the described limitations. I usually choose "Save As" -> "Save all (ignore physical groups)" in Gmsh and let Elmer do the cleaning (by default). If problems persist perhaps you could attach the relevant geo and msh files.

-Peter

pamakela
Posts: 49
Joined: 06 Mar 2010, 18:37

Re: STEP import of trimmed surfaces

Post by pamakela » 09 Jun 2010, 10:37

After a little bit of tinkering I found the following:

- Make sure the units from source software match with those of Elmer! My units were * 1000 if I used Gmsh in the middle. My cad package has Iges export which allows to set a scale factor of 0.001, which remedied the unit problem.

- If your 2D case mesh lies already on the XY plane, then no physical grouping is needed. If mesh is not on XY plane, you get numerous errors about 'degenerous elements'.

- Any arbitrary 2D plane can be used in Gmsh and Elmer but in this case physical group must be defined. This way, any transformations applied to the plane do not cause mesh related errors (ie. no degenerous elements error).


Maybe someone will find this information useful.


-pm

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