dolfin team mailing list archive

["Garth N. Wells" <g.n.wells@xxxxxxxxxxxxxxx>]

Now that I'm thinking properly, a function like FEM::assembleBCresidual should
return the residual for both Neumann and Dirichlet BCs (since non-zero Neumann
BCs are not yet implemented, I'm starting to forget that they even exist!)

I'll revert to the old applyBC to keep things consistent and add
FEM::assembleBCresidual which we can discuss if needed.

Garth  



Quoting Johan Hoffman <jhoffman@xxxxxxxxxxx>:

> I think that would be a good idea. And the name is an accurate description
> of what is going on.
> 
> /Johan
> 
> 
> > A possibility would be a function "FEM::assembleBCresidual".
> >
> > Garth
> >
> > On Fri, 2005-12-02 at 09:11 +0100, Garth N. Wells wrote:
> >> Quoting Anders Logg <logg@xxxxxxxxx>:
> >>
> >> > This looks strange to me, but maybe I don't understand it.
> >> >
> >> > What if you don't even have a vector x? What vector should x should
> >> > you then give as an argument?
> >>
> >> If you don't have a vector x, you don't want to use this function :-).
> >> It's
> >> basically returning the Dirichlet boundary condition "residual".
> >>
> >> >
> >> > Maybe it is better to just call the standard version of applyBC()
> >> > without x and then do
> >> >
> >> > b *= -1.0;
> >> > b += x;
> >>
> >> This would add x to all elements of b, not just to elements where
> >> Dirichlet
> >> boundary conditions are applied. I originally used the old applyBC for a
> >> vector
> >> to use an analogous procedure to the above, but this required two
> >> temporary
> >> vectors to be created, initialised and have boundary conditions applied
> >> at each
> >> Newton iteration (in addtion to the usual assemble of b) - not very
> >> nice.
> >>
> >> >
> >> > This would be two lines of code extra (one if you put them on the same
> >> > line... :-). It's probably better to put this in the Newton solver
> >> > since the Newton solver knows why this operation should be performed.
> >>
> >> The Newton solver knows nothing about the boundary coniditons. The BC's
> >> need to
> >> be applied by the user in the resdiudal vector b. This is done in the
> >> class
> >> NonlinearFunction.
> >>
> >> A possibility is a function that inserts components of x into b (rather
> >> than the
> >> BC value), where Dirichlet BCs are applied. PETSc has a function to the
> >> set the
> >> vector R, where F(u) = R is being solved, so R would contain the
> >> Dirichlet BCs.
> >>
> >>
> >> Garth
> >>
> >> >
> >> > /Anders
> >> >
> >> >
> >> > On Thu, Dec 01, 2005 at 02:16:12PM +0100, dolfin@xxxxxxxxxx wrote:
> >> > > Commit from garth (2005-12-01 14:16 CET)
> >> > > -----------------
> >> > >
> >> > > Functions for applying boundary conditions to the RHS vector only
> >> > now accept the solution vector x. Where Dirichlet boundary conditions
> >> > are applied, b = x - bc.The is useful for applying boundary conditions
> >> > within Newton iterations.
> >> > >
> >> > > This is not consistent with the other applyBC functions which return
> >> b = bc
> >> > (no minus sign). This is due to the definition of the nonlinear
> >> function
> >> > F(u)=0.
> >> > > For Poisson's equation, I've been using the format F(u) = (grad
> >> v,grad u) -
> >> > (v,f), hence the need for the minus. Is there a preference as to how
> >> we
> >> > set-up nonlinear problems?
> >> > >
> >> > > Would it be better to use a different name for funtions that return
> >> the
> >> > difference between the current approximate solution and the Dirichlet
> >> BC?
> >> > >
> >> > >   dolfin  src/kernel/fem/FEM.cpp       1.43
> >> > >   dolfin  src/kernel/fem/dolfin/FEM.h  1.23
> >> > >
> >> > > _______________________________________________
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> >> > > DOLFIN-dev@xxxxxxxxxx
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> >> > >
> >> >
> >> > --
> >> > Anders Logg
> >> > Research Assistant Professor
> >> > Toyota Technological Institute at Chicago
> >> > http://www.tti-c.org/logg/
> >> >
> >> > _______________________________________________
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> >> >
> >>
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> > --
> > Dr. Garth N. Wells
> > Faculty of Civil Engineering and Geosciences
> > Delft University of Technology
> > Stevinweg 1
> > 2628 CN Delft
> > The Netherlands
> >
> > tel.     +31 15 278 7922
> > fax.     +31 15 278 6383
> > e-mail   g.n.wells@xxxxxxxxxxxxxxx
> > url      http://www.mechanics.citg.tudelft.nl/~garth
> >
> >
> >
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> >
> 
> 
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