[petsc-users] Implementing TS routine

Manuel Valera mvalera-w at sdsu.edu
Mon Jul 8 18:53:02 CDT 2019


Hi Zhang,

Thanks to your help i have implemented the TS routine for my temperature
DMDA array, and it works correctly including in MPI.

The breakthrough for me was realizing that TS steps and max time steps are
all set up once, and used/advanced all at once when invoked with
TSSolution. So what i did was to add all of the TSCreate / initialization
into the main loop so i am basically creating and destroying the TS objects
every time step. I hope the overhead time is not to bad when i scale to
bigger problems.

Next step would be to apply the same to advance the velocities, but problem
now is that they are in a 4D dmda array of 3 degrees of freedom, any
suggestions on how to implement this? does TS support multiple degrees of
freedom arrays?

Thanks,

On Thu, Jul 4, 2019 at 9:18 PM Zhang, Hong <hongzhang at anl.gov> wrote:

>
>
> On Jul 3, 2019, at 3:10 PM, Manuel Valera <mvalera-w at sdsu.edu> wrote:
>
> Thanks Zhang for your answer,
>
> I ended up getting a compiling and running TS routine... that does not
> give me the answers, and i am not sure what i am doing wrong,
>
> My TS code so far looks like this:
>
> (...initialization...)
>
> call TSCreate(PETSC_COMM_WORLD,ts,ierr)
> call TSSetProblemType(ts,TS_NONLINEAR,ierr)
> call TSSetSolution(ts,gTemperature,ierr)
> call
> TSSetRHSFunction(ts,PETSC_NULL_VEC,FormRHSFunction,PETSC_NULL_VEC,ierr)
>
>
> The second last argument should be the user context. If you pass NULL, the
> ctx variable in your FormRHSFunction will be NULL.
>
> call TSSetType(ts,TSSSP,ierr)
> call TSSetTimeStep(ts,dt,ierr)
> call TSSetDM(ts,daScalars,ierr)
> call TSSetMaxTime(ts,TotalTime,ierr)
> call TSSetExactFinalTime(ts,TS_EXACTFINALTIME_STEPOVER,ierr)
> call TSSetFromOptions(ts,ierr)
> call TSSetUp(ts,ierr)
>
> (... then inside main calculations loop i have...)
>
> call TSSolve(ts,gTemperature,ierr)
>
> (...and my RHSFunction looks like this...)
>
> subroutine FormRHSFunction(ts,t,t_loc,rhs_loc,ctx,ierr)
>
>   use petscvec
>   use petscts
>   use petscdmda
>   use petscdm
>
>   USE utils
>   USE dmdaobjs
>   USE probsize
>   USE modelparams
>
>   implicit none
>
>   TS            :: ts
>   PetscReal     :: t
>   Vec           :: ctx,t_loc,rhs_loc
>   PetscErrorCode    :: ierr
>
>
>
>         call TSGetDM(ts,daScalars,ierr)
>
>         call DMGetLocalVector(daScalars,t_loc,ierr);
>
>
> t_loc is the input, it should be not modified.
>
>         call
> DMGlobalToLocalBegin(daScalars,gTemperature,INSERT_VALUES,t_loc,ierr);CHKERRQ(ierr)
>         call
> DMGlobalToLocalEnd(daScalars,gTemperature,INSERT_VALUES,t_loc,ierr);CHKERRQ(ierr)
>
>
> FormRHSFunction is supposed to implement rhs_loc = f(t,t_loc). So you want
> to scatter ghost points for t_loc, not gTemperature.
>
>         call DMDASolveTExplicit(3)
>
>         call DMGetLocalVector(daScalars,rhs_loc,ierr);
>         call
> DMGlobalToLocalBegin(daScalars,TRHSS_ts,INSERT_VALUES,rhs_loc,ierr);CHKERRQ(ierr)
>         call
> DMGlobalToLocalEnd(daScalars,TRHSS_ts,INSERT_VALUES,rhs_loc,ierr);CHKERRQ(ierr)
>
>
> There is no need to scatter ghost points for rhs_loc.
>
>
>
>         call DMRestoreLocalVector(daScalars,t_loc,ierr);CHKERRQ(ierr)
>         call DMRestoreLocalVector(daScalars,rhs_loc,ierr);CHKERRQ(ierr)
>
> end subroutine FormRHSFunction
>
> Where DMDASolveTExplicit(3) is the old function to calculate time
> integration with runge kutta, modified to only generate the f(t,u) which in
> this case is rhs_loc,
>
> I still have several doubts:
>
>
>    - Will this explicit RHS calculation work with TS routines? my time
>    integration is explicit so far and it would involve a fair deal of extra
>    work to make it implicit.
>
> For explicit time integration, one needs to provide only RHSFunction.
>
>
>    - This 't' parameter in the RHSFunction is controlled by PETSC? i am
>    not providing anything for it directly, where is it coming from?
>
>
> It is controlled by PETSc. If your problem is autonomous (the RHS does not
> depend on t), it can be simply ignored.
>
>
>
>    - Do i need to provide a Jacobian or the TS routine will try to guess
>    one? This is related to the first point where my time scheme being explicit
>    does not use a jacobian.
>
> For explicit time integration, Jacobian is not needed.
>
> Hong
>
>
> Thanks, any help is appreciated, if you see any errors or need more
> information please let me know,
>
>
> Regards,
>
> Manuel
>
> On Wed, Jun 26, 2019 at 9:54 PM Zhang, Hong <hongzhang at anl.gov> wrote:
>
>>
>>
>> On Jun 26, 2019, at 4:17 PM, Manuel Valera via petsc-users <
>> petsc-users at mcs.anl.gov> wrote:
>>
>> Hi PETSc,
>>
>> I am trying to implement the Time stepping routines in my model, i have a
>> working runge-kutta time scheme that goes to the following steps:
>>
>>
>>    - Interpolate u,v,w to the time advancing variable position.
>>    - Calculate nonlinear coefficients and advect velocities with a
>>    forward-backward shock capturing scheme.
>>    - Calculate the variable laplacian
>>    - Sum terms to create RHS (nonlinear advected velocities + laplacian)
>>    - Finally, the runge kutta integration is done in a typical way that
>>    looks like:
>>
>>                    newtemp(t+1) = prevtemp(t) + dt*RHS
>>
>>
>> So my questions are:
>>
>>    - I think my problem is nonlinear, but is being made linearized by
>>    the advecting scheme, is this correct? this is to know if i should use the
>>    linear or nonlinear routines for TS.
>>
>> TSComputeRHSFunctionLinear is just a convenience function for linear ODEs
>> in the form udot=Au. Using it won’t buy you much. So for TS starters, it is
>> fine to assume your problem is nonlinear and think of the form udot=f(t,u)
>> where f is the RHS function.
>>
>>
>>    - How do i know what are the appropriate routines i should be using
>>    here? so far i think i should use the following:
>>
>> call TSCreate(PETSC_COMM_WORLD,ts,ierr)
>> call TSSetProblemType(ts,TS_LINEAR,ierr)
>> call TSSetTimeStep(ts,dt,ierr)
>>
>> call TSSetFromOptions(ts,ierr)
>>
>> call TSSetRHSFunction(ts,NULL,TSComputeRHSFunctionLinear,NULL,ierr)
>> call TSSolve(ts,loctemperature,ierr)
>>
>> Should i use call TSSetRHSJacobian for the temperature jacobian in this
>> case?
>>
>>
>> I would suggest to write your own RHSFunction and set it to TS with
>> TSSetRHSFunction().
>>
>>
>>
>> I am using
>> https://www.mcs.anl.gov/petsc/petsc-current/src/ts/examples/tutorials/ex4.c.html as
>> a  general guide, is there a more appropriate example?
>>
>>
>> ex4 is a good example. In addition, ex9 uses finite volume method with
>> slope limiters to solve a variety of problems such as advection equation,
>> burgers equation and shallow water equation. It might be an overkill, but
>> it seems to be close to the problem you are trying to solve. Note that you
>> might want to use the SSP methods (-ts_type ssp) instead of the classic
>> Runge-Kutta methods for problems with shocks.
>>
>> Hong (Mr.)
>>
>>
>> The dt value and total timesteps are controlled by the model,
>>
>> Thanks for your help,
>>
>>
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20190708/975e2f4f/attachment.html>


More information about the petsc-users mailing list