ffc team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Fri, Jan 29, 2010 at 10:31:05AM +0000, Garth N. Wells wrote:
>
>
> Anders Logg wrote:
> > On Fri, Jan 29, 2010 at 10:10:37AM +0000, Garth N. Wells wrote:
> >
> >>> Sub domains seem to be very different, but the other two cases just
> >>> seem to be a matter of some dofs being "active" and the other zeroed
> >>> out. This is what Marie suggested yesterday, that a restricted element
> >>> only considers a subset of the dofs of some given element.
> >>>
> >> Sounds good.
> >>
> >>> The thing I don't understand yet is the selection of which dofs should
> >>> be active. If we think of the case with restriction to facets, then
> >>> the element needs to be restricted to different facets depending on
> >>> which facet we are integrating over, or are we always mapping one
> >>> specific facet of the reference cell to the current facet?
> >>>
> >> It works the same way as the DG elements, just the internal dofs are
> >> thrown away, which is the latter if the above, right?
> >>
> >>> Say we have P1 elements in 2D which have 3 dofs. Then we could
> >>> restrict that element to the dofs on the first facet (facet 0). These
> >>> dofs are then labeled 1 and 2. But sometimes a facet in the mesh will
> >>> correspond to the edge between 0 and 1 or 0 and 2.
> >>>
> >> We don't restrict to individual facets, but to all facts of a cell.
> >
> > That makes sense, but one thing still confuses me. Say that we have a
> > P1 element and restrict it to facets. Then all dofs are on the facets
> > so the result of the restriction is just a new P1 element. Same for P2
> > where the result again is a new P2 element.
>
> Yes.
>
> > For P3, the result is P3
> > element minus just one dof.
>
> Yes.
>
> > So does this make much difference for
> > other than very high degree elements?
> >
>
> It is only needed for k > 2. It's important because everything in
> FFC/UFL works for arbitrary orders.

Yes, it should work for any order. I was just questioning the
usefulness of it if it results in standard P1 and P2 elements for
k = 1,2.

Then I think I understand how it all works.

But does the FFC demo make any sense? Could we simplify it so that it
just defines a finite element restricted to a facet and then some dS
integral?

It currently breaks the code we added for RestrictedElement yesterday
because it has a complex nesting of restriction, then mixed with
another element and then restricted again. It looks like this is not
how you are using it in your solver.

--
Anders

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