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## Re: [Getfem-users] Transient analysis-dirichlet conditions

 From: Sébastien Janas Subject: Re: [Getfem-users] Transient analysis-dirichlet conditions Date: Wed, 4 Jun 2008 22:31:04 +0200

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Le 04-juin-08 à 16:01, Yves Renard a écrit :

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```On Wednesday 04 June 2008 10:30, Sébastien Janas wrote:
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```Hi all,

I want to solve the transient linear equation of diffusion.

It seems that there is no function pre-build in getfem for the
transient problems. So I assemble the mass matrix (MM) and stiffness
matrix (SM) "manually", and then construct the linear system A*X=B
using the backward euler

A = (MM + delta t * SM)
B = MM * X(t-1)

```
Then I have to impose dirichlet conditions on the surface. I import my mesh from gmsh, where the surface are tagged with a number (in my case
```200). How can I modify my linear system to take into accompt the
dirichlet conditions? All the functions I found in the documentation
are in the case where you use model bricks or standard assembly
procedures...

If I can have the list of node numbers tagged by 200 I would use the
penalty method. But I am not able to obtain these list.

I am using the python interface of getfem.

Best regards,

Janas Sébastien
```
```

This is true that there is no general strategy proposed for transient
```
problems, especially with the matlab or python interfaces. This is however not easy to build such a general strategy since the situation can vary a lot
```for differente time integration schemes.

However, You can have the list of nodes on a boundary with the command

gf_mesh_fem_get(MF, 'dof on region', [bnum]);

but you have first to tag the boundary with the command

gf_mesh_set(M, 'region', int bnum, imat CVFLST);

(see the Matlab examples).

Yves.
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```

```
Is there any systematic procedure to impose the dirichlet conditions on my system A*X=B ?
```
```
I have tried with the penalty method, but the sparse matrix format included in getfem set it very difficult (we can't make B[i] += A[i,i] * BIG * val because A[i,i] give an 1x1 array and not a scalar...)
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```

--

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Yves Renard (address@hidden) tel : (33) 04.72.43.87.08 Pole de Mathematiques, INSA de Lyon fax : (33) 04.72.43.85.29
``` 20, rue Albert Einstein
69621 Villeurbanne Cedex, FRANCE
http://math.univ-lyon1.fr/~renard

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