[firedrake] Specify boundary conditions at a corner

[Lawrence Mitchell]

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On 21/08/15 11:52, David Ham wrote:
> class PointDirichletBC(DirichletBC): @utils.cached_property def
> nodes(self): # Find the array of coordinate values. x =
> self.function_space().mesh().coordinates.dat.data_ro # Find the
> location of the zero rows in that return
> np.where(~x.any(axis=1))[0]

I believe David is fixing the error in using this kind of idea.
However, I note in passing that this exact form may not do exactly
what you want.  It assumes that the function space that you're wanting
to apply the boundary condition to is the same space that is holding
the coordinate field (namely piecewise linears, P1 on simplices, Q1 on
quads).

If the space you want to pin is not P1, you'll have to work harder
(this is the subdomain problem David alluded to).

If you want to have a boundary marker that applies to a small subset
of the boundary of a mesh, I recommend, rather than using the built in
utility meshes, to build the mesh using Gmsh.  There you can provide
physical IDs to whatever subset of the boundary you like and then use
these ID numbers in your boundary condition objects.

Lawrence

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