multi-point constains in fenics

2 Answers

Can you describe in more detail what you want to do?

answered Dec 1, 2016 by Kent-Andre Mardal FEniCS Expert (14,380 points)

Thank you, Kent-Andre.

The simplest one is to enforce the displacements at two nodes to be identical during the deformation process. For instance, an initially vertical edge remains vertical (i.e., the horizontal displacements along that vertical edge being equal).

commented Dec 2, 2016 by jywu FEniCS Novice (510 points)
edited Dec 2, 2016 by jywu

Are you constraining points on the boundary or in the interior of the domain. The former can be done by DirichletbBC, the latter requires some work on the user side.

commented Dec 2, 2016 by MiroK FEniCS Expert (80,920 points)

One option is to use the "R" space and a Lagrange multiplier approach.
This would be similar to the "neumann-poisson" demo. But you would need
to localize the "R" functions (which are global). One way of localizing them
is to multiply them with functions that are zero away from the points
you are interested in.

commented Dec 2, 2016 by Kent-Andre Mardal FEniCS Expert (14,380 points)

hi, MireK,

would you please give more details? I am going to constrain some nodes on the boundaries, with theirs displacements coincident but unknown (to be solved).

I know that in numerical implementation of FEM, the linear constraint can be done by modifying the stiffness and load vector. But for fenics, this might be rather cumbersome.

Thanks.

commented Dec 3, 2016 by jywu FEniCS Novice (510 points)
edited Dec 3, 2016 by jywu

...