ffc team mailing list archive

["Martin Sandve Alnæs" <martinal@xxxxxxxxx>]

2008/4/16, Anders Logg <logg@xxxxxxxxx>:
> On Wed, Apr 16, 2008 at 09:24:02PM +0200, Johan Hoffman wrote:
>  > > Hi all, I'm trying to do a survey of all the different ways of enforcing
>  > > incompressibility in the Navier-Stokes equations, and FFC/FIAT of course
>  > > has
>  > > the most advanced element capabilities for doing this...
>  > >
>  > > Unfortunately, I have hit a snag (either in my ignorance or through a bug)
>  > > in my grand scheme, starting with the Oseen equations (fix the convective
>  > > velocity for fixed points, don't do the full Newton linearization).
>  > >
>  > > It seems that if you put a vector element into a mixed element and then
>  > > take
>  > > it back out, you can no longer index it.
>  > >
>  > > Putting the following in a .form (everything that doesn't involve pressure
>  > > spaces) works beatifully
>  > > #start .form file here
>  > > vel_element = VectorElement("Lagrange","triangle",2)
>  > > u = TrialFunction( vel_element )
>  > > v = TestFunction( vel_element )
>  > > u0 = Function( vel_element )
>  > >
>  > > a = D(v[i],j)*D(u[i],j)*dx + v[i]*u0[j]*D(u[i],j)*dx
>  > > #end .form file
>  > >
>  > > On the other hand, if I do:
>  > > # start new .form file here
>  > > vel_element = VectorElement("Lagrange","triangle",2)
>  > > pre_element = FiniteElement("Lagrange","triangle",1)
>  > > element = vel_element + pre_element
>  > > (u,p) = TrialFunctions( element )
>  > > (v,q) = TestFunctions( element )
>  > > u0 = Function( vel_element )
>  > >
>  > > a = D(v[i],j)*D(u[i],j)*dx + v[i]*u0[j]*D(u[i],j)*dx
>  > > # end new .form file
>  > >
>  > > I get an error:
>  > >
>  > > Preprocessing form file: Oseen.form --> Oseen.py
>  > >
>  > > *** object cannot be interpreted as an index
>  > > *** To get more information about this error, rerun ffc with the option
>  > > -d1.
>  > >
>  > > I've attached the output using -d1.
>  > >
>  > > Any corrections either of my stupidity or of FFC would be appreciated :)
>  >
>  > Hi Rob!
>  >
>  > I don't think you have to question your sanity, we have experienced
>  > similar problems when dealing with the compressible Euler equations: with
>  > scalar density and energy together with a momentum vector. We have been
>  > forced to use built in operators instead, which is non optimal since the
>  > definitions of the action of some operators is a bit unclear.
>  >
>  > I do not know what the underlying problem is, but we have brought this up
>  > before and it seems to be non-trivial? Or maybe (hopefully) I am wrong?
>  >
>  > This is also something I want to stress for the coming UFL; that to me in
>  > terms of operators, the most important is to have a fully reliable system
>  > for indexing. Then to add a nice collection of well defined operators on
>  > top is great, but secondary (to me).
>
>
> Indexing is one of many priorities for UFL.
>
>
>  > I wasn't sure I was supposed to edit the UFL page directly with this
>  > wishes? What do you say Anders?
>
>
> Yes, just edit the page with suggestions. That's what it's for.

All suggestions are welcome!

Perhaps the best way to add suggestions is to write an example form
with wanted syntax and explain with verbose comments what you mean by
it. Then I can use the code as a unit test as well.

Note that the page may be slightly out of sync with what's
implemented, I'll get around to cleaning this up in a few weeks (other
pressing matters right now), and can merge your additions in then.

--
Martin