[Gmsh] double surfaces/lines between adjacent bodies

[Peter Raback]

Hi,

There are some tests called periodic1 and periodic2 among the source files in trunk/fem/tests. These may show the sif file syntax.

The periodic conditions basically glue the values of two boundaries together so that the are the same. I don't know that this feature would have been used for internal boundaries but I see no reason why it wouldn't work. Maybe Juha as the author sees some? A practical problem is that if you want to glue two boundaries together at the same position it may be tricky with the GUI as its difficult to pick either one. But if you know the boundary indexing by some other means you might be able to write the sif manually.

BR, Peter

________________________________________
From: Zenker, Dr. Matthias [Matthias.Zenker at erbe-med.de]
Sent: 12 March 2009 11:01
To: Peter Raback; elmerdiscussion at postit.csc.fi; gmsh at geuz.org
Subject: RE: double surfaces/lines between adjacent bodies

Hi Peter,

that solves my problem, thank you very much!!
I just processed my test case successfully with the merge option.
The nodes were at least close together, maybe even on identical
coordinates, before the merge was done.
For more complicated cases (e.g. I want to merge nodes in a region where
the node spacing is larger than elsewhere): where can I find more info
(or an example) for the periodic BC thing? From a quick look into the
ElmerSolver Manual, I must admit that I don't fully understand what this
is all about.

Thank oyu,

Matthias

> -----Original Message-----
> From: Peter Raback [mailto:Peter.Raback at csc.fi]
> Sent: Wednesday, March 11, 2009 4:14 PM
> To: Zenker, Dr. Matthias; elmerdiscussion at postit.csc.fi; gmsh at geuz.org
> Subject: RE: double surfaces/lines between adjacent bodies
>
> Hi Matthias,
>
> Do the lines share the same nodes?
>
> If yes, then you could include the command line option
> '-merge eps' in the gmsh import in ElmerGUI/ElmerGrid. This
> will join nodes that are separated by a distance smaller than eps.
>
> If no, this might be treated by ElmerSolver with the same
> functionality that is used for the periodic BCs, I think? But
> then you would have to have to have both lines existing so
> you can define a linear dependence between them.
>
> BR, Peter
>
> ________________________________________
> From: Zenker, Dr. Matthias [Matthias.Zenker at erbe-med.de]
> Sent: 11 March 2009 16:21
> To: elmerdiscussion at postit.csc.fi; gmsh at geuz.org
> Subject: [Elmerdiscussion] double surfaces/lines between
> adjacent bodies
>
> Hi Elmer and gmsh  list,
>
> I use gmsh and Elmer to simulate an electrical current
> flowing through a conductive medium between two electrodes. I
> get the geometry from my CAD colleagues in STEP format,
> import it in gmsh, mesh it, read the mesh with Elmer and do
> the FEM calculation there. Most of the time, the calculations
> are 2D. This works in principle, but there is the following problem:
>
> Electrode and medium have common surfaces (resp. lines in
> 2D). When I get the geometry in STEP format, every part
> (Electrodes and Medium) has its own outer surface (line).
> This leads to double surfaces (lines) between the adjacent
> bodies. For geometry and mesh import, this is no problem,
> neither for gmsh nor for elmer - the mesh is imported OK in
> Elmer. But when I do the simulation, I see that no current
> flows through this double surface (although only one of them
> is defined as physical in gmsh, and only that one is seen in Elmer).
>
> Removing the double surface/line in gmsh is rather tedious
> (Identify the problematic regions where there is a double
> line between adjacent surfaces. Delete at least one of the
> adjacent surfaces. Delete the corresponding line at the
> interface. Delete its points. Redefine the surface(s) which
> was (were) deleted before. It gets more complicated in case
> of only partially overlapping lines which have to be
> partially reconstructed.).
>
> Question: How do I deal with this problem? Is there a more
> elegant (and less time consuming) way than manual removal of
> the double lines? (The autocoherence function in gmsh which
> is intended to remove double duplicate entities in geometries
> is switched on, but does not seem to have any effect.)
>
> I imagine that this must be a known problem, since doing
> simulations with geometries imported from 3D CAD systems is
> not so uncommon after all.
>
> Further it is not totally clear to me where the problem
> really comes from. I suspect non-identical mesh nodes at the
> interface, which inhibit current flow across the border.
>
> Ideas and suggestions are more than welcome...
>
> Thank you,
>
> Matthias
>
> ----------------------------------------------------------------
> ERBE Elektromedizin GmbH
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> Dipl. Phys.
> Grundlagenentwicklung Technologie
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> 72072 Tuebingen
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> ----------------------------------------------------------------
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>
>
>
> ERBE Elektromedizin GmbH
> Firmensitz: 72072 Tuebingen
> Geschaeftsfuehrer: Christian O. Erbe, Reiner Thede
> Registergericht: Stuttgart HRB 380137
>
>

_____________________________________________________________________
ERBE Elektromedizin GmbH
Firmensitz: 72072 Tuebingen
Geschaeftsfuehrer: Christian O. Erbe, Reiner Thede
Registergericht: Stuttgart HRB 380137