dolfin team mailing list archive

[Kent-Andre Mardal <kent-and@xxxxxxxxx>]

On to., 2008-08-21 at 14:14 -0500, Catherine Micek wrote:
> On Aug 20, 2008, at 2:18 AM, Kent-Andre Mardal wrote:
> 
> > On ti., 2008-08-19 at 14:59 -0500, Catherine Micek wrote:
> >> Hi,
> >>
> >> I have been looking at demo3.py in the dolfin demos in "sandbox/la/
> >> trilinos/", which solves the Stokes system using preconditioners and
> >> an iterative solver.  I can follow the code until it gets to applying
> >> the boundary conditions:
> >>
> >> # apply bc
> >> for bc in bcs:
> >>      bc.apply(A00, b0, a00)
> >>      bc.zero(A01, a00)
> >>
> >> Why do you apply the second command as "bc.zero(A01, a00)?"  I would
> >> have guessed something more like "bc.zero(A01, a01)."  Perhaps the
> >> better question is more general: how do the bc.apply and bc.zero
> >> commands work?
> >>
> >> Thanks!
> >> Katy
> >
> > bc.apply(A00, b0, a00)
> > will set the Dirichlet boundary conditions by seting
> > the part of A00 on the boundary to the identity and put
> > the boundary conditions in b0.
> >
> > bc.zero(A01, a00) will zero out the part on the boundary.
> >
> > Together these to commands create an identity matrix for
> > the part on the boundary for the block matrix.
> 
> I have some follow-up questions on applying boundary conditions in  
> the Trilinos demo.
> 
> 1.  To enforce boundary conditions, the code
> 
> # No-slip boundary condition for velocity
> bc0 = DirichletBC(noslip, sub_domains, 0)
> 
> # Inflow boundary condition for velocity
> bc1 = DirichletBC(inflow, sub_domains, 1)
> 
> # Collect boundary conditions
> bcs = [bc0, bc1]
> 
> is used.  I don't understand why we don't tell Dolfin where to apply  
> the boundary conditions with something like
> 
> bc0 = DirichletBC(noslip, sub_domains, 0, velocity)
> bc1 = DirichletBC(inflow, sub_domains, 1, velocity)
> 
> Why isn't it necessary to specify the velocity?

Can someone comment on this, I have only copied the code from the other
stokes demos. 

Kent