[petsc-users] PetscFE and TS

[Matthew Knepley]

On Wed, Nov 9, 2016 at 7:52 AM, Julian Andrej <juan at tf.uni-kiel.de> wrote:

> I tried the example with different signs for the forcing, so this
> might be a mistake by me, yes. it doesn't change anything though. I
> also have to correct myself, the time for the callback in
> PetscDSAddBoundary changes correctly! Although u_t in all other
> callbacks seems to remain zero.
>
> I am out of ideas on my side though :)


The bug is that the boundary conditions are not being properly enforced. I
am fixing it now.
Thanks for the simple example demonstrating this. It made it much much
easier to debug.

    Matt


> On Wed, Nov 9, 2016 at 2:33 PM, Matthew Knepley <knepley at gmail.com> wrote:
> > On Wed, Nov 9, 2016 at 12:32 AM, Julian Andrej <juan at tf.uni-kiel.de>
> wrote:
> >>
> >> I tested a few other things and it turned out that the function added
> >> as a dirichlet boundary condition via PetscDSAddBoundary doesn't
> >> receive the time values correctly (PetscReal time is always 0.0).
> >
> >
> > I will check out your example and fix this. However, I wonder if there
> is a
> > bug in your implementation:
> >
> > void f0_temp(PetscInt dim, PetscInt Nf, PetscInt NfAux,
> >           const PetscInt uOff[], const PetscInt uOff_x[], const
> PetscScalar
> > u[], const PetscScalar u_t[], const PetscScalar u_x[],
> >           const PetscInt aOff[], const PetscInt aOff_x[], const
> PetscScalar
> > a[], const PetscScalar a_t[], const PetscScalar a_x[],
> >           PetscReal t, const PetscReal x[], PetscScalar f0[])
> > {
> >   f0[0] = u_t[0] - 2.0;
> > }
> >
> > I get
> >
> >   du/dt - \Delta u = -2
> >
> > so that if u = x^2 + y^2 + 2t, then
> >
> >   2 - (2 + 2) = -2
> >
> > so you would want u_t + 2.
> >
> > If you give the Laplacian the opposite sign, this is an unstable
> > formulation.
> >
> >   Thanks,
> >
> >      Matt
> >
> >>
> >> I also see no effect adding DMTSSetIJacobianLocal vs. not setting the
> >> jacobian function. The analytic solution i used in my attached example
> >> is the same as in the src/ts/examples/tutorials/ex32.c.
> >>
> >> Am i doing anything obviously wrong here? My next step would be to try
> >> if assembling the matrices myself and writing a custom
> >> IFunction/IJacobian which assembles the different parts of the matrix
> >> like M and J for the stiff ODE with nontrivial mass matrix (see manual
> >> section of TS -> F = M u' - f) but i think this should be obsolete
> >> right?
> >>
> >> Appreciate your help.
> >>
> >> Regards
> >> Julian
> >>
> >> On Mon, Nov 7, 2016 at 10:49 AM, Julian Andrej <juan at tf.uni-kiel.de>
> >> wrote:
> >> > Hello,
> >> >
> >> > i'm using PetscFE in combination with SNES and a hand written backward
> >> > euler solver right now and like to hand that over to TS. I have been
> >> > fiddling quite a while getting TS to work with PetscFE and have
> >> > encountered a few unclarities.
> >> >
> >> > I have looked at examples which are outdated i guess
> >> > (src/ts/examples/tutorials/ex32.c) and i am confused on how i have to
> >> > formulate the discretization of the jacobian and the residual. It
> >> > seems obvious to use the DMTSSetIFunctionLocal and
> >> > DMTSSetIJacobianLocal functions because there are prepared wrappers
> >> > from DMPlexTSComputeIFunctionFEM and DMPlexTSComputeIJacobianFEM.
> >> >
> >> > I need a Mass matrix for my problem, is that formed from the function
> >> > space information or do i have to form it myself? Is there any working
> >> > example which uses PetscFE and TS to form the DMTSSetIFunctionLocal
> >> > and DMTSSetIJacobianLocal? (grepping the tree tells me "no"
> >> > unfortunately).
> >> >
> >> > Regards
> >> > Julian
> >
> >
> >
> >
> > --
> > What most experimenters take for granted before they begin their
> experiments
> > is infinitely more interesting than any results to which their
> experiments
> > lead.
> > -- Norbert Wiener
>



-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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