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[Kristian B. Ølgaard <question154628@xxxxxxxxxxxxxxxxxxxxx>]

Question #154628 on DOLFIN changed:
https://answers.launchpad.net/dolfin/+question/154628

Kristian B. Ølgaard proposed the following answer:
On 29 April 2011 00:25, Anders Logg <logg@xxxxxxxxx> wrote:
> On Thu, Apr 28, 2011 at 09:48:05PM -0000, Marcos Samudio wrote:
>> New question #154628 on DOLFIN:
>> https://answers.launchpad.net/dolfin/+question/154628
>>
>> Hello, I'm a beginner on Fenics, and I'm having some trouble understanding the examples.
>> The particular issue I'm dealing with is the definition of Dirichlet boundary conditions.
>>
>> In the first demo:
>>
>> def u0_boundary(x, on_boundary):
>>       return on_boundary
>> bc = DirichletBC(V, u0, u0_boundary)
>>
>> The function u0_boundary returns on_boundary, but what value does
>> on_boundary take??? It acts like a boolean, right? But what
>> influence has the 'x' variable on it? I tried to track the path
>> along debugging, but got lost...
>
> x and on_boundary are inputs and your task should be to return a value
> (True or False) that tells DOLFIN whether the point x is inside the
> domain you consider as your boundary. The on_boundary argument is a
> piece of extra information (to you from DOLFIN) that tells you whether
> the point x is on the actual boundary of the domain. (Note that you may
> very well specify a boundary condition inside the domain.)
>
> Here are a couple of examples:
>
>  return on_boundary
>
>  return x[0] < 0.5
>
>  return x[0] < 0.5 and on_boundary
>
> This is discussed in some detail in an upcoming book which you can get
> a copy of by

... and it should probably also be in the docstring of
AutoSubDomain/DirichletBC in dolfin.fem.bcs.py :)

Kristian

>  bzr branch lp:fenics-book
>
> --
> Anders
>
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