dolfin team mailing list archive

[Patrick Riesen <priesen@xxxxxxxxxxxxxxx>]

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

> --
> Anders