[petsc-users] Call KSP routine before each timestep

[Matthew Knepley]

On Mon, May 1, 2017 at 4:27 PM, Scott Dossa <doss0032 at umn.edu> wrote:

> Hi All,
>
> Matt:
> Thank you! Using the application context is a good approach to pass the
> vector information. Can you also direct me to which command allows TSSolve
> to be only called for one timestep / start at the correct timestep? When
> TSSolve() is called, it always resets to timestep 0.
>

You should not need that since PreStep will be called at the beginning of
each step, but just in case

  http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/TS/TSStep.html

although using this is tricky so I do not recommend it.


> Barry:
> Yes, this is a pressure projection method where one needs the pressure
> field at each timestep to solve for the velocity field.
>

If it was me, I would not do it this way, but its somewhat a matter of
taste. It makes more sense to me to formulate the whole
system as a DAE, meaning time derivatives on some things (v) and not others
(p). Then use a DAE timestepper and your
fluid solver can be formulated as pressure projection using PCFIELDSPLIT.
This way, if you want to use another kind of fluid
solver, you can, whereas now you are stuck with the alternation of
projection of momentum update.

  Thanks,

    Matt


> I will likely have more follow up questions as I quick write this up.
> Thank you both for your input.
> -Scott Dossa
>
> On Mon, May 1, 2017 at 3:32 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
>>
>>    Scott -  Are you doing some kind of pressure projection method?
>>
>>    PETSc-developers - should this functionality be directly added to TS
>> since it comes up fairly often?
>>
>>    Barry
>>
>>
>>
>> > On May 1, 2017, at 3:24 PM, Matthew Knepley <knepley at gmail.com> wrote:
>> >
>> > On Mon, May 1, 2017 at 3:13 PM, Scott Dossa <doss0032 at umn.edu> wrote:
>> > Hi All,
>> >
>> > I'm looking to pass a vector between a KSP and TS routine. The KSP
>> routine must be called before each timestep, and the solution vector is
>> needed for the TS routine. Normally, TSSolve() runs over all timesteps, but
>> in my case, I'd like to be able to add a routine before each timestep.
>> >
>> > Can someone direct me to an example script or briefly explain a case
>> which shows how to control time stepping such that one could achieve
>> something along the lines of:
>> >
>> > while (step < maxsteps+1){
>> >         KSPSolve(ksp, v, p); /* solves for Vec p and passes this info
>> onto TS */
>> >         TSSolve(ts, u); /* only iterate for 1 timestep */
>> > }
>> >
>> > The function TSSetPreStep() seemed promising, but it can only take TS
>> as arguments which may not be sufficient to pass a global vector.
>> >
>> > Yes, this is the correct thing. You can
>> >
>> >   a) Just attach a Vec to the TS using PetscObjectCompose(), but that
>> is ugly so you can
>> >
>> >   b) Make a context structure, and stick it in the TS using
>> >
>> >   http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages
>> /TS/TSSetApplicationContext.html
>> >
>> >       That is also where the KSP should go.
>> >
>> >   Thanks,
>> >
>> >     Matt
>> >
>> > Thank you in advance.
>> > Scott Dossa
>> >
>> >
>> >
>> >
>> >
>> > --
>> > 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/20170501/c43b4ac4/attachment-0001.html>