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5 Replies
Last post: September 13, 2010 10:11am UTC
September 8, 2010 11:40am UTC
Strange behaviour when using pairs
Hi,
I have observed the following strange behaviour in Comsol 3.5a:
- I draw two blocks, one of top of the other.
- I draw a few points on one of the suface of the lower block, to force the meshing to go through these points.
- If I mesh directly, the mesh indeed goes through these points.
- If I first create a pair (without imprint) between the two blocks, the mesh does not go through these points.
The tow cases are attached to this post.
Any explanations? How can I force the mesh to go through the points even when I define a pair?
Thanks for any help.
Alois
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September 8, 2010 9:10pm UTC in response to Alois Pfenniger
Re: Strange behaviour when using pairs
Hi
if you are plying with pairs, you have probably turned on the "assembly mode". Then all items are "assembled, but single lines and points are considered as orphans in assembly mode. You must first group (union) the items (lines and points and at least one face/volume. If you have several parts/geometrical items, but only one common border, yu should group everything to end up with only two entities with the common border. Then you will see your points and lines appear again. Items that are grouped by an union, before exiting the "draw mode" (or passing the Finish line in V4) are considered as a single part (even if subcomposed of several items) for the assembly process.
hope I'm clear
--
Good luck
Ivar
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September 9, 2010 7:26am UTC in response to Ivar Kjelberg
Re: Strange behaviour when using pairs
Hi,
1000 thanks for your great hint. Indeed Comsol switches automatically to Assembly mode when creating a pair.
I was not able to make a union between points and solids: when I go to the "Create Composite Object" function, I can either select "Solids", "Faces", "Lines" or "Points", but not several at the same time. Furthermore, the function "Union" is only available for Solids.
But I found a workaround by using the "Coerce To" -> "Solid" function of the draw mode. Now it works perfectly, I have my identity pair and the mesh goes through the points I defined.
Regards,
Alois
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September 10, 2010 8:51am UTC in response to Alois Pfenniger
Re: Strange behaviour when using pairs
Hi
Well you must proceed systematicaly points => lines => surfaces/faces =>volumes, with your raw 0D->3D geometry,
this then becomes "points, edges, boundaries, (sub)domains" with no overlappings nor orphants.
Except in "assembly mode", where boundaries or edges might be dedoubled, hence the need to define "pairs"
--
Good luck
Ivar
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September 13, 2010 8:17am UTC in response to Ivar Kjelberg
Re: Strange behaviour when using pairs
Hi,
In the "Create Composite Object" dialog box, you cannot combine objects from different types. If you select for example "Object type" -> "Solids", and if you type the formula manually by typing the name of a solid and the name of a point (e.g. "BLK1+PT1"), the following error message appears:
Error: 9161
Unknown variable name.
- Name: PT1
I think that it is not possible to create a composite object (union) of a solid and a point. For that, one has to use the "Coerce To" function. See also following paragraph from the Comsol Multiphysics User's Guide, p.33
Forming Composite Geometry Objects from Nonsolid Geometry Objects
To create composite objects formed from a combination of solids, faces, curves, and points, you can also use the Coerce to Solid, Coerce to Face, or Coerce to Curve buttons on the Draw toolbar. For more information on coercing geometry objects, see “Coercing Geometry Objects” on page 34.
Either I am missing your point, or it is indeed not possible to directly unit objects of different types.
Have a nice day,
Alois
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September 13, 2010 10:11am UTC in response to Alois Pfenniger
Re: Strange behaviour when using pairs
Hi you are right
In V3.5
You must first split items to the same level, then coerce them back to lines/solids. This means that individual points do not appear (are considered as orphans) so you can only get them by splitting a surface by a line (and split the line to have a common "point" where you want the fixed point to appear.
now you points are still there (select point view) even if not part of a solid. Points outside of a domain are greyed (ignored), points inside a domain are accesible and the mesher will consider them as "hord points" to mesh at
This limitation is still logical when thinking on the role of the "analysed" geometrica entities (items where to define BCs and physics and or probes). But sometimes rather restrictive: i.e. structural to have a point (outside any domain) where to add a lumped mass linked by a Weak contribution to a domain/boundary
--
Good luck
Ivar
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