[petsc-users] PetscFE and TS

[Matthew Knepley]

On Wed, Nov 9, 2016 at 8:14 AM, Matthew Knepley <knepley at gmail.com> wrote:

> 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.
>

I think this works. Let me know if you agree.

There is the business of u_t on the boundary, which is not handled right
now. I am putting that
in, but the homogeneous values for that is still alright here.

  Thanks,

     Matt


>     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
>



-- 
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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20161109/efd10c2c/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: ex1.c
Type: text/x-csrc
Size: 7947 bytes
Desc: not available
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20161109/efd10c2c/attachment.c>