dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Mon, May 17, 2010 at 09:50:22AM +0200, Patrick Riesen wrote:
> Anders Logg wrote:
> >On Fri, May 07, 2010 at 02:39:41PM +0200, Patrick Riesen wrote:
> >>hello,
> >>
> >>i want to zero the normal velocity on a boundary part of my
> >>2D-domain, so i used something like
> >>
> >>n = cell.n
> >>a = ... + inner(w,dot(v,n)*n)*ds(2) + ...
> >>
> >>in my form-file (w=testfunction, v=trialfunction).
> >>
> >>i marked the boundary part using the "exterior facet domains"
> >>meshfunction, which will be picked up in assemble(..):
> >>
> >>MeshFunction< uint >* _ext_facet_domains =
> >>mesh.data().create_mesh_function("exterior facet domains", 1)
> >>
> >>*_ext_facet_domains = 5;
> >>boundary.mark(*_ext_facet_domains, 2)
> >>
> >>can i do this? or what is wrong with the above way? because i don't
> >>get zero velocities...
> >>
> >>thank your for help,
> >>
> >>patrick
> >
> >What if you add a (big) parameter in front of that term? Does it have
> >any effect at all on the solution?
> >
> >I would also suggest writing the term as
> >
> >  dot(w, n)*dot(v, n)*ds(2)
>
> thank you for the help anders,
>
> so i noticed the following:
> without parameter in front, i could see no effect at all. if i add a
> parameter C in front and increase it stepwise until C is about 10^6
> i can observe how the velocities cancel out on that boundary.
>
> i guess this approach works, but there is something else:
> looking at the (time-dependent) solution (*pvd file set or single
> vtu files) in paraview, i ran into several
>
> ERROR: In /home/kitware/Dashboard/MyTests/ParaView-3-8/ParaView-3.8/ParaView/VTK/IO/vtkXMLDataReader.cxx,
> line 509
> vtkXMLUnstructuredGridReader (0x1398ce0): Cannot read point data
> array "U" from PointData in piece 0.  The data array in the element
> may be too short.
>
> I noticed that the reader crashes on some absurd numbers appearing
> in PointData "U"  as
>
> [...]
> -3.949617e-105 9.769857e-104 0.000000e+00
> [...]
>
> If i set those to zero, there is no problem. but the appearance of
> such  'almost-zero' numbers is clearly connected to using the
> parameter C.
>
> do you suggest me another way how to do this?
>
> or do you think that the VTK output writing should check for numbers
> < DOLFIN_EPS to avoid such corrupted output, and the approach is ok?
>
> best regards and thanks for the support,
> patrick

It would be easy to add such a check (zero numbers below DOLFIN_EPS).
I'll let Garth comment first since he wrote most of the VTK output.

--
Anders

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