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
