dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Thu, May 06, 2010 at 08:42:19PM -0000, Marie Rognes wrote:
> Question #109900 on DOLFIN changed:
> https://answers.launchpad.net/dolfin/+question/109900
>
> Marie Rognes proposed the following answer:
> Anders Logg wrote:
> > Question #109900 on DOLFIN changed:
> > https://answers.launchpad.net/dolfin/+question/109900
> >
> > Anders Logg proposed the following answer:
> > On Thu, May 06, 2010 at 02:52:33PM +0200, Marie Rognes wrote:
> >
> >> Achim Schroll wrote:
> >>
> >>> New question #109900 on DOLFIN:
> >>> https://answers.launchpad.net/dolfin/+question/109900
> >>>
> >>> for a plain Poisson eqn with pure Neumann b.c., how to specify the scaling condition u*dx = 0 ?
> >>>
> >>
> >> If you have a recent dolfin, you can introduce a constant c acting
> >> as a Lagrange multiplier corresponding to the constraint. See
> >> example below.
> >>
> >> from dolfin import *
> >>
> >> mesh = UnitSquare(32, 32)
> >> V = FunctionSpace(mesh, "CG", 1)
> >> Q = FunctionSpace(mesh, "R", 0)
> >> M = V * Q
> >>
> >> (u, c) = TrialFunctions(M)
> >> (v, d) = TestFunctions(M)
> >>
> >> f = Expression("x[0]*x[1]*sin(pi*x[0])")
> >>
> >> a = dot(grad(v), grad(u))*dx + d*u*dx + c*v*dx
> >> L = v*f*dx
> >>
> >> pde = VariationalProblem(a, L)
> >> u_h = pde.solve()
> >>
> >> plot(u_h[0])
> >> interactive()
> >>
> >
> > But make sure to not name that space 'M' if you try this with a UFL
> > file or you will get a strange error (since 'M' is reserved in form
> > files).
> >
> > Anyway, I'm wondering about the dofmap for the "R" element. It sets
> > _global_dimension to m.num_entities[2] which is the global number of
> > cells, but then it maps all dofs to 0. Is that right? Shouldn't the
> > global dimension be 1?
> >
> >
>
>
> Yes, it is a bug for non-mixed elements(!). I noticed but didn't really
> want to start fixing before
> the weekend. It's on the todo-list though.

ok! I thought I might have missed the point but then I think I know
what's going on. Very cool feature.

Btw, you can add that demo now. ;-)

--
Anders

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