ffc team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Wed, Nov 26, 2008 at 10:07:32AM +0100, Johan Hake wrote:
> On Tuesday 25 November 2008 20:51:58 Anders Logg wrote:
> > On Tue, Nov 25, 2008 at 03:56:19PM +0100, Alessio Quaglino wrote:
> > > There is a function in FFC which seems very useful for my aim but I don't
> > > understand how to use it.  One part of my form is the following:
> > >
> > > n = FacetNormal("triangle")
> > > dsD = Integral("exterior facet", 0)
> > > dsN = Integral("exterior facet", 1)
> > > dsF = Integral("exterior facet", 2)
> > >
> > > a = ( dot(psi,b) - div(psi)*w + phi*div(b) )*dx + ( dot(b,n)*psi[0] +
> > > dot(b,n)*psi[1] )*dsF L = phi*f*dx - dot(psi,g)*dsN
> > >
> > > What I want to do is to assemble the matrix splitting the boundary in 3
> > > parts: dsD (no terms), dsN (1 term in a), dsF (1 term in L). However I
> > > don't understand how to pass to the form the subdomains I defined in
> > > dolfin. For example I have:
> > >
> > > class DirichletBoundary : public SubDomain
> > > {
> > >   bool inside(const double* x, bool on_boundary) const
> > >   {
> > >     return x[0] < DOLFIN_EPS;
> > >   }
> > > };
> > >
> > > which I would like to associate to dsD. Is there a way to do that?
> > >
> > > Thank you,
> > > Alessio
> >
> > Take a look at demo/pde/lift-drag/cpp/main.cpp in DOLFIN.
> 
> I do not know if that is a good example for you case.
> 
> In that example a SubDomain marking the subdomain that shall be assembled is 
> passed to the assemble function. This wont work in your case. As the passed 
> SubDomain will be used to create a MeshFunciton over both facet and cells. I 
> assume that your SubDomain is created to define inside if you are on a 
> special boundary, and the resulting MeshFunction defining the cells, will 
> then be defined as "outside". Please correct me here if I am totaly of 
> Anders.

Correct. You will need to define a MeshFunction.

> For your need I would create the actuall meshfunctions, defining the 
> subdomains. I assume you have defined three SubDomains called sd0, sd1, sd2, 
> all overloading the inside function that defines the your subdomain, as you 
> have above. Also see demo/mesh/subdomains.
> 
>     ....
>     // Creating a dummy cell_domain MeshFunction, maybee an zero pointer
>     // would do the job?
>     MeshFunction<uint> cell_domains(mesh, mesh.topology().dim());
>     cell_domains = 0;
>     
>     MeshFunction<uint> facet_domains(mesh, mesh.topology().dim() - 1);
>     // First mark all subdomains with an "outside" marker, defining
>     // the rest of the domain.
>     facet_domains = 3;
>     sd0.mark(facet_domains,0);
>     sd1.mark(facet_domains,1);
>     sd2.mark(facet_domains,2);
>     
>     Matrix A;
>     assemble(A,form,mesh,cell_domains,facet_domains,facet_domains);   
> 
> I am not a native c++ dolfin user so I might have got some of the syntax wrong 
> above, but I hope you get the point. 
> 
> Note that you send the facet_domains as sub_domain markes for the interiour 
> facet integrals too, see signature of assemble() in assemble.h, but that wont 
> be used as no such integral is defined in your form.

Take a look at demo/mesh/subdomains/cpp/main.cpp which demonstrates
how to use SubDomain to mark sub domains in a MeshFunction.

Note that the order in which you mark sub domains is important (if
they are not disjoint).

-- 
Anders

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