dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Fri, Aug 22, 2008 at 09:58:17AM +0200, Kent-Andre Mardal wrote:
> 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

I was hoping you would comment so I wouldn't have to. :-)

In the standard Stokes demo in DOLFIN, the system is solved as one
big system (with three components and two subsystems). Then we need to
specify which subsystem a bc should be applied to.

In your Stokes demo, you have already split the system in two, so then
you just need to apply the velocity bc to the velocity system and the
pressure bc to the pressure system (and make sure to zero out the
offdiagonals correctly). Then solve it as a block system.

-- 
Anders

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