dolfin team mailing list archive

["Johan Hoffman" <jhoffman@xxxxxxxxxx>]

> On Tue, Jun 3, 2008 at 11:31 PM, Anders Logg <logg@xxxxxxxxx> wrote:
>> On Tue, Jun 03, 2008 at 11:24:54PM +0200, Evan Lezar wrote:
>>> The problem is that I am not quite sure how to use it ...
>>> The code I have at the moment is as follows:
>>>     mesh = UnitSquare(1,1)
>>>     mesh.refine()
>>>     mesh.refine()
>>>     element = FiniteElement("Nedelec", "triangle", order);
>>>     v = TestFunction(element)
>>>     u = TrialFunction(element)
>>>     # assemble the mass and stiffness matrices
>>>     # the bilinear form is defined as the transverse curl since it is
>>> a two dimensional problem
>>>     # a = dot(curl(v), curl(u))
>>>     a = (vec(v.dx(0))[1] - vec(v.dx(1))[0])*(vec(u.dx(0))[1] -
>>> vec(u.dx(1))[0])
>>>     (S) = assemble(a*dx, mesh)
>>>     (T) = assemble(dot(v, u)*dx, mesh)
>>> I then use S and T to solve the eigenvalue system S x = lambda T x The
reason I am having trouble is that this does not use the pde class and
I have not found an example that uses the DirichletBC class to specify
the boundary conditions for matrix assembly.
>>> Thanks for the quick reply
>>> Evan
>> Hmm... I realize now I don't know how to set boundary conditions for an
eigenvalue problem (other than by eliminating unknowns). For
homogeneous boundary conditions, I guess one could zero out a row in S,
insert a 1 on the diagonal and zero out the corresponding row in T.
There is an example of applying boundary conditions in
>>  demo/pde/convection-diffusion/python/demo.py
>> but it is for a linear system Ax = b. There is also a function zero()
in DirichletBC which can be used to zero out a row in a matrix. --
>> Anders
>> _______________________________________________
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>
> Well, eliminating unknowns is exactly what I need to do.  Typically in
my own code I would not even create matrix entries for the edges on the
boundary, thus eliminating the rows and columns associated with those
degrees of freedom.  I think I did come across the zero()
> function in my search, but how would I actually remove the rows or not
have them assembled in the first place?
>
> I will have a look at the demo and see where that takes me.

Maybe you can solve this by using the subDomain classes?
See the thread: "FFC implementation over a subdomain of \Omega" on the
dolfin-dev mailing list.

/Johan

>
> Thanks
> Evan
>
> --
> visit http://randomestrandom.blogspot.com
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