Dear David,
You add a global term, so that the stiffness matrix is no longer sparse
at all. This is why gmm complains. And the linear system solve will only
be possible for a small number of degrees of freedom. This is avoidable
with a specific solve of the system, writing explicitely the constraint
matrix as a rank one matrix. Of course, a simpler alternative is to
prescribe the value of the unknown at any point of the domain (with a
pointwise constraint brick, this is very easy) and if there is a
specific need for solution to be of zero average, make a
post-translation of the solution.
Yves.
Le 23/02/2017 à 11:17, David Danan a écrit :
> Dear Getfem users,
>
> i wanted to solve a simple laplacian with pure Neumann boundary
> condition on a cube.
> In order to do so, since the problem is ill-posed, i added the
> following constraint on the unknown u
> int_omega u dx=0
> via a Lagrangian multiplier.
>
> In the code, the left-hand side is defined by
> Model.add_fem_variable("u", mf_u);
> getfem::add_linear_generic_assembly_brick(model, mim,
> "(Grad_u.Grad_Test_u)");
> model.add_fixed_size_variable("lambda", 1);
> getfem::add_linear_generic_assembly_brick(model, mim,
> "(lambda*Test_u)");
> getfem::add_linear_generic_assembly_brick(model, mim,
> "(u*Test_lambda)");
>
> If i am not mistaken, it should correspond to the left hand side
> described in
> https://fenicsproject.org/olddocs/dolfin/1.3.0/python/demo/documented/neumann-poisson/python/documentation.html
> for instance
>
> As a result, the mean value is relatively small in comparison with u
> (while not exactly 0).
>
> However, when i run the program, i get the following warning:
> Level 2 Warning in ../../src/gmm/gmm_vector.h, line 592: Inefficient
> addition of element in rsvector with **** non-zero entries
>
> Did i do something wrong? If i didn't, is it possible to disable this
> warning?
>
> Thanks in advance,
> David.
>
>
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--
Yves Renard ([email protected]) tel : (33) 04.72.43.87.08
Pole de Mathematiques, INSA-Lyon fax : (33) 04.72.43.85.29
20, rue Albert Einstein
69621 Villeurbanne Cedex, FRANCE
http://math.univ-lyon1.fr/~renard
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