Re: [Libmesh-users] Saving calculated quantity

[Benjamin S. K. <be...@cf...>]

I will be taking a few days vacation after Christmas, and I will start=20
coding up the improved system support that I've been hyping up for a=20
while.  That should make this a lot simpler.

What you can do in the meantime is add another *system* with some=20
variables that will be used for plotting purposes.  If you don't provide=20
an assemble function for this system there should be no linear solve.

Also, sometimes it is useful to have a linear solve to compute your=20
post-processing variables.  For instance, if you have a C0 solution then=20
the stresses aren't uniquely defined at the nodes, but you can solve an=20
L2 projection system to get a smooth stress field.  Something like this:

es.add_system("Post-Processing");
es("Post-Processing").add_variable("stress");

...

Then assemble the L2 projection system like this:

compute du/dx

phi(i)*phi(j)  =3D phi(i)*du/dx

and solve it in a standard way.

This type of system will be trivially easy to solve since it is SPD=20
(just a mass matrix).

Let me know if you have any more questions.

-Ben





Martin L=FCthi wrote:

>John
>
>John Peterson writes:
> > Martin L=3DFCthi writes:
> >  > Is there an easy way to output calculated quantities to the GMV ou=
tp=3D
> > ut
> >  > files? I want to calculate e.g. stresses (or some other quantitiy
> >  > which is not a system variable), and hav it appear in the output f=
il=3D
> > e.
> > You might add it as a "dummy" system variable which is just
> > computed via a post-processing step.  Then it could be written
> > out as normal to gmv.
>
>Thanks for the answer. There is a problem with this. If I add the
>variable the beginning, the system gives a null solution (because the
>variable is not used in the element loop). Adding it after just before
>the write command results in some runtime error messages.
>
>I was thinking about using an additional vector with
>system.add_vector.
>
>Best, Martin
>
>
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