dolfin team mailing list archive

[Ola Skavhaug <skavhaug@xxxxxxxxx>]

On Mon, Aug 10, 2009 at 5:46 AM, Catherine Micek<mice0012@xxxxxxx> wrote:
>
> On Aug 9, 2009, at 3:24 AM, Kristian Oelgaard wrote:
>
>> Quoting Peter Brune <prbrune@xxxxxxxxx>:
>>
>>> As a quick solution, you could, knowing the coordinates of a node,
>>> constrain
>>> everything within some epsilon smaller than the mesh feature size
>>> of the
>>> node (such as DOLFIN_EPS).
>>
>> Yes, and then make sure to use the "pointwise" method when creating
>> the
>> DirichletBC. See comment in dolfin/fem/DirichletBC.h.
>> You can also look at demo/pde/dg/advection-diffusion/main.cpp for
>> an example on
>> how to use the "geometric" approach. The majority of DOLFIN demos
>> uses the
>> "topological" approach which is the default.
>
> This seems simple enough, but I am having a little difficulty with
> it.  I am coding with the python interface, so I looked at the python
> version of the demo in demo/pde/dg/advection-diffusion/.  In there,
> the Dirichlet boundary is specified as the line x=1 on the boundary,
> i.e.
>
> class DirichletBoundary(SubDomain):
>     def inside(self, x, on_boundary):
>        return (abs(x[0] - 1.0) < DOLFIN_EPS and on_boundary
>
> and I'm unclear as to why it's necessary to specify "pointwise" as
> opposed to "topological" boundary conditions.  So this condition
> would fix all the nodes on the line x=1; it doesn't appear to be
> picking out points.  Is it necessary here because we're using DG
> elements?
>
> The other question is related to syntax.  Say I want to fix the point
> (1,0).  When I set the boundary as
>
> class DirichletBoundary(SubDomain):
>     def inside(self, x, on_boundary):
>        return ( (abs(x[0] - 1.0) and abs(x[1] - 0.0) < DOLFIN_EPS and
> on_boundary)

This is not correct, since abs(x[0]-1.0]) almost always evaluates to True.
Try
return ( (abs(x[0] - 1.0) < DOLFIN_EPS and abs(x[1] - 0.0) <
DOLFIN_EPS and on_boundary)

Ola

> and set the Dirichlet boundary condition with the option set to
> "geometric," I am not getting the correct solution.  Is this not the
> correct way to set the boundary conditions?
>
> Again, any suggestions would be appreciated -- thanks!
>
> Katy
>
>
>>
>> Kristian
>>
>>> - Peter
>>>
>>> On Sat, Aug 8, 2009 at 11:27 PM, Catherine Micek
>>> <mice0012@xxxxxxx> wrote:
>>>
>>>> Hi,
>>>> I'm working with a group that's using Fenics to study the
>>>> equations of
>>>> linear elasticity (with a pure displacement formulation), and I
>>>> have a
>>>> question about boundary conditions.  Our goal is to study the
>>>> pure traction
>>>> problem, and this means we have to be careful in formulating the
>>>> boundary
>>>> conditions.  In order to prevent a singularity in our equations,
>>>> we need to
>>>> constrain certain nodes so as to prevent translations and rigid
>>>> rotations.
>>>>  There are many examples in the fenics demos about how to
>>>> prescribe mixed
>>>> conditions, but I have only seen examples where the Dirichlet
>>>> condition
>>>> occurs along an edge (as opposed to occurring at a few individual
>>>> nodes).
>>>>  So my question is, given a mesh, how does one prescribe Dirichlet
>>>> conditions for individual nodes?  Are there any demos that
>>>> address this?
>>>>
>>>> Any help would be greatly appreciated -- thanks!
>>>>
>>>> Katy
>>>>
>>>> ---
>>>> Catherine (Katy) A. Micek
>>>> Graduate Assistant, U of MN Mathematics Department
>>>> http://www.math.umn.edu/~mice0012 <http://www.math.umn.edu/%
>>>> 7Emice0012>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> DOLFIN-dev mailing list
>>>> DOLFIN-dev@xxxxxxxxxx
>>>> http://www.fenics.org/mailman/listinfo/dolfin-dev
>>>>
>>>>
>>>
>>
>>
>
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-- 
Ola Skavhaug