dolfin team mailing list archive

[Patrick Riesen <priesen@xxxxxxxxxxxxxxx>]

Hi

so i corrected my form-file for the new representation of L = F and a = 
(F')u. However the solving still repeats and does not converge, the 
problem remains.

i don't know if it has to with L being expected as a nonlinear form but 
my L = F is actually linear in u?

it is linear in u because i put the nonlinear part ( the computation of 
the variable viscosity ) in another form file, so in the main form file 
(hence in L) i only have something like

eta*dot(D(v),D(u))       (D() is the symmetric part of grad(u))

and eta = trace(mult(D(u),D(u))^(exponent) i get from somewhere else.

patrick




Anders Logg wrote:
> Yes, the bilinear form should be the linearized nonlinear form (not
> minus).
>
> Note that with UFL, it's now possible to just write down the nonlinear
> form
>
>   (v, F(u)) = 0 for all v
>
> and then let UFL compute the linearization.
>
>
>
> ------------------------------------------------------------------------
>
> __> ouuuuu, this might be the lucky guess, i wasn't aware of that...
>
> Ostien, Jakob T wrote:
> > In a totally blind guess, I believe that for a nonlinear problem, the linear
> > form used to require a minus sign (DOLFIN 0.8), but now (DOLFIN 0.9+) the
> > minus sign is built into NewtonSolver.  You might want to check that first.
> >
> > Jake
> >
> >
> > On 5/1/09 3:09 AM, "Patrick Riesen" <priesen@xxxxxxxxxxxxxxx> wrote:
> >
> > > hello!
> > >
> > > i want to update my powerlaw solver from dolfin 0.8 to dolfin 0.9.2.
> > > the solver class is derived from the NonlinearProblem class and i
> > > followed the CahnHillard-demo to do something similar for updating from
> > > 0.8 to 0.9.2.
> > >
> > > However I have a problem which i do not understand:
> > >
> > > If i hand the created forms to VariationalProblem and let it solve, all
> > > works fine and in the second iteration the newton solver converges (if i
> > > initialize a newtonian problem just for checking).
> > >
> > > if i do
> > >
> > > newtonsolver.solve(*this,uu->vector());
> > >
> > > the problem gets solved as well, i.e. all bcs and my own void F and void
> > > J get called but it does not converge, it's like the problem gets solved
> > > over and over again and the solution is added, because in the end i get
> > > a solution but it's the number of iterations \times velocity.
> > >
> > > i tried all kinds of things (with/without pointers, no cast/cast,
> > > different classes for problem/solve ...) but i don't get it, i miss
> > > something.
> > >
> > > maybe somebody has a hint or does see my error and can help me turn on
> > > the light?
> > >
> > > many thanks in advance
> > >
> > > patrick
> > >
> > >
> > > below is a code snippet of the main part:
> > > /-----------------------------------------------------------------------------
> > > --
> > >    _uu = new Function(*_V);
> > >    Function* __uu = dynamic_cast<Function*>(_uu);
> > >
> > >    // create the forms
> > >    a_powstokes = new PowerlawStokesBilinearForm(*_V,*_V);
> > >    PowerlawStokesBilinearForm* _a_powstokes =
> > > dynamic_cast<PowerlawStokesBilinearForm*>(a_powstokes);
> > >    _a_powstokes->w0 = *__uu;
> > >    _a_powstokes->eta1 = eta;
> > >    _a_powstokes->eta2 = d_eta;
> > >
> > >
> > >    L_powstokes = new PowerlawStokesLinearForm(*_V);
> > >    PowerlawStokesLinearForm* _L_powstokes =
> > > dynamic_cast<PowerlawStokesLinearForm*>(L_powstokes);
> > >    _L_powstokes->w0 = *__uu;
> > >    _L_powstokes->eta1 = eta;
> > >    _L_powstokes->f = f;
> > >
> > > //   VariationalProblem pde(*a_powstokes, *L_powstokes, bcs, true);
> > > //   pde.solve(fluid.velocity(),fluid.pressure());
> > >
> > >    newtonsolver.solve(*this,__uu->vector());
> > > _______________________________________________
> > > DOLFIN-dev mailing list
> > > DOLFIN-dev@xxxxxxxxxx
> > > http://www.fenics.org/mailman/listinfo/dolfin-dev
>
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