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[Getfem-users] Antwort: Re: Maximum allowed residuum

 From: Moritz Jena Subject: [Getfem-users] Antwort: Re: Maximum allowed residuum Date: Thu, 9 May 2019 08:44:24 +0200

Hello Yves,

Adapting the residuum at each iteration step is very interesting and I will try to implement this in my code.

However, my problem is still, to determine/calculate the maximum allowed residuum depending on my model.

At the moment I follow the theory, that the maximum allowed residuum max_res is calculated with respect to the external forces:

If external_forces > 1e-5 * 1000
max_res = external_forces / 1000
else
max_res = 1e-5
end

I base my theory on observations in ansys. Am I right with my theory? Or is there even another (maybe easier) way to get a plausible max_res out of GetFEM?

Best regards,

Moritz Jena

Datum:        08.05.2019 17:11
Betreff:        Re: [Getfem-users] Maximum allowed residuum

Dear Jena,

Unfortunately, from the interface you cannot adjust the maximum residual and change it from a Newton sub-iteration to another. The only mean I see from the interface is to call for only one Newton iteration each time, then change the maximum residual from a call to another and loop until the convergence criterium is not reached.

Best Regards,

Yves

----- Mail original -----
Envoyé: Mardi 7 Mai 2019 16:23:00
Objet: [Getfem-users] Maximum allowed residuum

Dear GetFEM-Users,

I have written a FEM program, with which you can calculate technical
springs numerically.
Therefore I'm using the GetFEM-MATLAB-Interface 4.3

These calculations are highly non-linear due to self-contact issues and
large deformations.
That is the reason why technical springs have to be solved iteratively in
several substeps.

The calculation by itself works very well. But I think I have a problem
with the maximum allowed residuum that is passed to the solve command of
getfem.
My theory is that the maximum allowed residuum has to be variable during
the different substeps.
By investigating these question online I found a paper of an university in
Germany that contains the following for a equation system of
A*x=b : ||delta_x||/||x|| <= Cond(A) * ||delta_b||/||b||
Unfortunatley these _expression_ is not leading to plausible results.

I'm also familiar using ansys. By using this commercial software I know
that the maximum allowed residuum is changing by each iteration and by
each substep.

All of the delivered getfem examples are using a constant maximum allowed
residuum, no matter of high deformation or not.

So my question is, how am I able to find the maximum allowed residuum and
by what _expression_? And do I have to define a variable maximum allowed
residuum during the substeps to get reliable results out of the solve
command?

Best regards,

Moritz Jena