On 23/07/10 18:44, Felipe Montefuscolo wrote:
Yes, I can. But I think that would be annoying for a user user.
But I really liked the first option !! it is very convenient for many
cases,as you do
not need memory to store the labels of the nodes, etc.
a big thanks, your tip was very helpful!
In all our codes we just tag the elements, exactly for the reasons you
outlined below. Indeed, mathematically, what you want to do is impose
the boundary condition on the *face* of the cube. The easiest is thus to
put the tag on the triangles on the face, then iterate over the nodes of
these triangles (if your unknowns are associated with nodes---which is a
discretization detail).
Felipe,
Brazil
---------- Forwarded message ----------
From: *Geordie McBain* <[email protected]
<mailto:[email protected]>>
Date: 2010/7/23
Subject: Re: [Gmsh] about .mesh file format
To: Felipe Montefuscolo <[email protected]
<mailto:[email protected]>>
Cc: [email protected] <mailto:[email protected]>
2010/7/23 Felipe Montefuscolo <[email protected]
<mailto:[email protected]>>:
> hi again, thanks for the answer.
> Let-me explain my problem with a example. If I make a cube, I can
> label the faces (with Physical groups) that are on the boundary so
that all
> nodes that are in
> these faces will be labeled too. Perfect, that's what I want, because
> with this I can impose the Dirichlet boundary conditions in my fem code.
> The problem is: and nodes that are in the corners of the cube ? these
nodes
> belong to three faces that may have different labels, and how to decide
> which
> face does he live ? that is why it would be desirable to add a label
to them
> in medit file.
> This detail can deteriorate a solution of a lid-driven cavity
problem, for
> example.
Thanks for the clarification. I see the issue.
A lot of finite element programs (e.g. libMesh, FreeFem++) actually
impose Dirichlet conditions as Robin conditions with very high
`conductances' between the solution at the boundary and what it should
be. This makes all the boundary conditions `natural' and none
`essential'. It means you don't need to worry about which nodes are
on Dirichlet boundaries, because none are. This technique is
described in e.g.
Becker, E. B., G. F. Carey, & J. T. Oden (1981). Finite Elements, An
Introduction, Volume 1 of The Texas Finite Element Series. Englewood
Cliffs, New Jersey: Prentice-Hall, pp. 121--122
Ern, A., & J.-L. Guermond (2004). Theory and Practice of Finite
Elements, Volume 159 of Applied Mathematical Sciences. New York:
Springer, section 8.4.4, pp. 379--380
but of course you might not want to do this, I know.
I can't really think of a way to impose true Dirichlet conditions on
corners a mesh defined by a MEDIT .mesh mesh file, because of this
restriction of one tag per node... Actually here's a hack: for nodes
on just one patch it's easy enough, as you've worked out. For nodes
on an intersection of two (or more) patches, can you define that
intersection as another patch and give it a special tag, meaning that
any node with that tag is actually on each of the patches?
--
Felipe Montefuscolo
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Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science
http://www.montefiore.ulg.ac.be/~geuzaine
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