Hi Lawrence,

Thanks for the responses (and same to you Fabio). I am attempting to experiment with slip condition dirichlet boundary conditions for velocity with these lines:

bcs1 = DirichletBC(W.sub(0).sub(0), Expression(("2*pi*cos(2*pi*x[0])*sin(2*pi*x[1])*sin(2*pi*x[2])")), (1,2))
bcs2 = DirichletBC(W.sub(0).sub(1), Expression(("2*pi*sin(2*pi*x[0])*cos(2*pi*x[1])*sin(2*pi*x[2])")), (3,4))
bcs3 = DirichletBC(W.sub(0).sub(2), Expression(("2*pi*sin(2*pi*x[0])*sin(2*pi*x[1])*cos(2*pi*x[2])")), (5,6))

but I am getting these errors:

Traceback (most recent call last):
File "Compare_P2P0.py", line 66, in <module>
    solver.solve()
File "<string>", line 2, in solve
File "/home/justin/Software/firedrake-deps/PyOP2/pyop2/profiling.py", line 203, in wrapper
    return f(*args, **kwargs)
File "/home/justin/Software/firedrake-deps/firedrake/firedrake/variational_solver.py", line 165, in solve
    bc.apply(self._problem.u)
File "<string>", line 2, in apply
File "/home/justin/Software/firedrake-deps/PyOP2/pyop2/profiling.py", line 203, in wrapper
    return f(*args, **kwargs)
File "/home/justin/Software/firedrake-deps/firedrake/firedrake/bcs.py", line 199, in apply
    raise RuntimeError("%r defined on incompatible FunctionSpace!" % r)
RuntimeError: Coefficient(MixedElement(*[VectorElement('Lagrange', Domain(Coefficient(VectorElement('Lagrange', Domain(Cell('tetrahedron', 3), label=None, data='<data with id 140068500682768>'), 1, dim=3, quad_scheme=None), 1)), 2, dim=3, quad_scheme=None), FiniteElement('Discontinuous Lagrange', Domain(Coefficient(VectorElement('Lagrange', Domain(Cell('tetrahedron', 3), label=None, data='<data with id 140068500682768>'), 1, dim=3, quad_scheme=None), 1)), 0, quad_scheme=None)], **{'value_shape': (4,) }), 7) defined on incompatible FunctionSpace!

Attached is the code. Any ideas on what this one may be?

Thanks,

Justin

On Fri, Jul 24, 2015 at 5:42 AM, Lawrence Mitchell <lawrence.mitchell@imperial.ac.uk> wrote:

> On 24 Jul 2015, at 11:25, Justin Chang <jychang48@gmail.com> wrote:
>
> Thanks for the responses guys. However, I am running into issues regarding
> Dirichlet bcs for P2/P0 elements. Attached is the code:
>
> I am trying to apply homogeneous boundary conditions in the P0 space, and
> when I looked at the documentation online it said to use DirichletBC(Q,
> <value>, <boundary_id>, method="geometric")
> but I am getting these kinds of errors:

Aha, this is a simple problem. When you define a MixedFunctionSpace and then want to apply boundary conditions on a part of the space, you must define the boundary condition on an indexed subspace, rather than the original space.

That is:

V = ...
Q = ...

W = V*Q

bc = DirichletBC(W.sub(1), ...)

rather than

bc = DirichletBC(Q, ...)

This is in the documentation (http://firedrakeproject.org/variational-problems.html#incorporating-boundary-conditions) but maybe not clearly enough.

Making this change, and your code runs fine.

I note in passing, that the DirichletBC constructor can take a list of boundary ids as well as just a single id. So, since all the boundary condition values are the same you can replace your six separate BC objects by:

bcs = DirichletBC(W.sub(1), 0, (1, 2, 3, 4, 5, 6), method="geometric")

Cheers,

Lawrence

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