dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Tue, Apr 01, 2008 at 09:52:02PM +0200, Murtazo Nazarov wrote:
> > Quoting Murtazo Nazarov <murtazo@xxxxxxxxxxx>:
> >
> >> Anders Logg wrote:
> >> > On Tue, Apr 01, 2008 at 02:32:17PM +0200, Murtazo Nazarov wrote:
> >> >
> >> >> Anders Logg wrote:
> >> >>
> >> >>> On Mon, Mar 31, 2008 at 07:15:23PM +0200, Murtazo Nazarov wrote:
> >> >>>
> >> >>>
> >> >>>> Anders Logg wrote:
> >> >>>>
> >> >>>>
> >> >>>>> On Mon, Mar 31, 2008 at 03:52:57PM +0200, Murtazo Nazarov wrote:
> >> >>>>>
> >> >>>>>
> >> >>>>>
> >> >>>>>> Hi,
> >> >>>>>>
> >> >>>>>> I want to apply a boundary condition to the vertices on a
> >> boundary.
> >> For that I define:
> >> >>>>>>
> >> >>>>>> // Sub domain for MyBC
> >>
> >> >>>>>> class MyBC_Boundary2D : public SubDomain
> >> >>>>>> {
> >> >>>>>> public:
> >> >>>>>>   bool inside(const real* p, bool on_boundary) const
> >> >>>>>>   {
> >> >>>>>>     return  on_boundary && (p[0] < xmax - bmarg) && (p[0] > xmin
> >> +
> >> bmarg);
> >> >>>>>>   }
> >> >>>>>> };
> >> >>>>>>
> >> >>>>>> Then I initialize:
> >> >>>>>>
> >> //-----------------------------------------------------------------------------
> >>
> >> >>>>>> void MyBC::init(SubDomain& sub_domain)
> >> >>>>>> { ...
> >> >>>>>>   mesh.init(0);
> >> >>>>>>   sub_domains = new MeshFunction<uint>(mesh, 0);
> >> >>>>>> ...}
> >> >>>>>>
> >> >>>>>>
> >> >>>>>>
> >> >>>>> Do you remember to set everything to 1 (number of subdomains)
> >> here?
> >> >>>>>
> >> >>>>>
> >> >>>>>
> >> >>>>>
> >> >>>> Yes I do:
> >> >>>>
> >> >>>>   // Mark everything as sub domain
> >> >>>> 1
> >> >>>>   (*sub_domains) = 1;
> >> >>>>
> >> >>>>
> >> >>>>   // Mark the sub domain as sub domain
> >> >>>> 0
> >> >>>>   sub_domain.mark(*sub_domains, 0);
> >> >>>>
> >> >>>> /murtazo
> >> >>>>
> >> >>>>
> >> >>> Then I have no idea. You just need to dig into the code and see what
> >> >>> goes wrong.
> >> >>>
> >> >>>
> >> >>>
> >> >> I think I found a mistake in Subdomain.mark(). The thing is bool
> >> >> on_boundary should be defined
> >> >>  inside the loop for computing subdomain marker:
> >> >>
> >> >>   // Always false when not marking
> >> >> facets
> >>
> >> >>
> >> >>   //bool on_boundary =
> >> >> false;
> >>
> >> >>
> >> >>
> >> >>   // Compute sub domain
> >> >> markers
> >>
> >> >>
> >> >>   for (MeshEntityIterator entity(mesh, dim); !entity.end(); ++entity)
> >> >>   {
> >> >>     bool on_boundary = false;
> >> >>     ...
> >> >>    }
> >> >>
> >> >> Otherwise, if you want to apply a boundary condition on vertices, it
> >> >> becomes always
> >> >> true after once meeting a vertex on the boundary.
> >> >>
> >> >>
> >> >> It solved problem for me.
> >> >>
> >> >
> >> > I'm not sure. If you are marking vertices, then they will never be on
> >> > the boundary, unless you are doing this for a 1-dimensional mesh
> >> > (which typically has just 2 boundary vertices). Only *facets* will
> >> > ever be marked as being on the boundary, so for a 2-dimensional mesh,
> >> > edges on the boundary will be marked as being on the boundary and for
> >> > a 3-dimensional mesh, faces on the boundary will be marked as being on
> >> > the boundary.
> >> >
> >> >
> >> With the change I wrote I was able to mark vertices. Because,
> >> if I mark facets or faces, I miss some nodes in the boundary
> >> of the subdomain where I am applying my bc. That destroys
> >> the solution. Now, I apply my boundary only for vertices
> >> I need, and I do not miss any nodes. It work perfectly with
> >> the slip BC.
> >
> > Sounds like a similar issue that we have for Discontinuous elements. Have
> > you
> > tried to apply BCs using a geometrical (or pointwise) approach?
> >
> 
> Yes, I have. But as I said I had a problem with some side nodes. The
> boundary condition did not apply to those nodes. Therefore I got incorrect
> result. But, with this approach everything went well.
> 
> /murtazo

I still don't understand what the problem is. What kind of mesh do you
have? Triangles or tetrahedra? What are you marking? Vertices?

If you are marking vertices, then on_boundary should always be
false. It is set to false on line 53 and it will never change since
line 60 will not be reached.

-- 
Anders