[petsc-users] PetscFE and TS

[Julian Andrej]

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 :)

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