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Re: [Question #109900]: L2-scaling

 

Marie Rognes wrote:
Anders Logg wrote:
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.


It is a true hack that I added for testing purposes. But it seems useful, so let's keep it. I'll clean it up a bit (and fix that bug.)


Bug fixed.

--
Marie



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