[petsc-users] Call KSP routine before each timestep

[Emil Constantinescu]

On 5/1/17 4:42 PM, Matthew Knepley wrote:
> On Mon, May 1, 2017 at 4:27 PM, Scott Dossa <doss0032 at umn.edu 
> <mailto: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.

If it's a projection you may need to set the PostStep and (or) PostStage 
if using multistage methods 
(http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/TS/TSSetPostStage.html#TSSetPostStage); 
otherwise the last step may not be div free.

>     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.

Yes, formulating it as a DAE is desirable; however, if you project it 
separately you have access to significantly more time steppers.

Emil


>    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
>     <mailto: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 <mailto:knepley at gmail.com>> wrote:
>          >
>          > On Mon, May 1, 2017 at 3:13 PM, Scott Dossa <doss0032 at umn.edu
>         <mailto: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
>         <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