dolfin team mailing list archive

[Jake Ostien <tostien@xxxxxxxxx>]

Nuno David Lopes wrote:
> On Thursday 08 May 2008, Anders Logg wrote:
>   
> > On Thu, May 08, 2008 at 11:33:49AM +0100, Nuno David Lopes wrote:
> >     
> > > But then what is the difference between
> > > a MeshFunction for interior and exterior face_domains?
> > > Shouldn't they be the same only with different markers for the boundary
> > > and for the interior?
> > >
> > > MeshFunction<unsigned int> facets_subdomains(mesh, mesh.topology().dim()
> > > - 1); (it has the information for all the facets, doesn't it?)
> > >       
> > Yes it does.
> >
> >     
> > > Is the problem in the order of the markers:
> > > 0...n-1 in the exterior
> > > 0...n-1 in the interior
> > >
> > > If we dont need the interior could we use the same MeshFunction, knowing
> > > that the interior is marked with 0?
> > >       
> > Are you talking about the facets in the interior? Those are not
> > included when integrating *ds terms anyway.
> >     
>
> Exactly. If I understood it well, only if you have *dS terms you need the 
> information for the interior facets.
> But that information is in the same MeshFunction as for exterior facets. 
>
>   
> > For the cells in the interior, you need to have a MeshFunction defined
> > on the cells, so something like
> >
> >   MeshFunction<unsigned int> cell_domains(mesh, mesh.topology().dim());
> >     
>
>
> Ok thats clear form me. 
> For a 3D mesh:  
> Cells information are in   Meshfunction cell_domains(mesh,'3') 
> Facets information are in a MeshFunction facets_domains(mesh,'2')
>
> And still if you want to use: "assemble" you need 3 MeshFunctions:
>
> assemble(A,a,mesh, cell_domains,int_facet_domains,ext_facets_domains)
>                                ^MeshF 1^      ^^MeshF 2^^          ^^MeshF3^^
>
> I really don't understand the difference between MeshF 2 and MeshF 3?
> And if they are different how to construct them. 
> (That's why I've referred the order of the markers, since that seems important 
> for  FFC)
>  
>   
>   

The MeshFunctions for the interior facets and the exterior facets can be 
constructed in a very similar fashion.  Consider this loop over facets:

 /// loop over facets
 for (FacetIterator facet(mesh); !facet.end(); ++facet) {
              
   /// Check if we have an exterior facet
   if ( facet->numEntities(mesh.topology().dim()) != 2 ) {
   ...mark exterior facets i.e. ext_facet_domains(*facet) = n
   } else {
   ...mark interior facets i.e. int_facet_domains(*facet) = m
   }
  
 }

The difference comes during assembly, where, for example, assembly over 
the exterior facets first checks if the facet is actually on the 
exterior of the mesh, and then it checks if the marker of that facet 
matches an associated integral defined in the form file.  Since assembly 
of exterior facets happens separately from the interior facets, it makes 
sense to have a MeshFunction for both the exterior and interior facets. 
As I see it its also necessary to split them up since FFC wants to order 
subdomains from 0 to n-1.  If you wanted one MeshFunction for both 
interior and exterior domains you might have an exterior domain ds0 and 
an interior domain dS1, but FFC would complain since you don't have a 
dS0 (you mentioned this above).


Jake


In this way you can easily tell if