[petsc-users] TSBDF prr-load higher order solution

[Alfredo J Duarte Gomez]

Hello,

The solutions y_n and y_n-1 are the same ones as those saved internally for
the BDF-2.

The practical problem is for example: run a simulation with a BDF-2 for 10
hrs, see that everything looks good and decide that this simulation is
worth continuing for another 10 hrs. Except now when I start from that last
solution, the TSBDF starts from a single solution and steps its way up to
the BDF-2 (numerical noise appears), instead of simply being able to load
the last two solutions from my HDF5 output file and continue as if it had
not stopped.

Maybe I confused you by referencing the TSBDF_Restart, instead of the
"starting" procedure for higher order BDFs, I thought they were the same
but maybe not.

Thank you,

-Alfredo



On Thu, Mar 10, 2022 at 11:04 AM Zhang, Hong <hongzhang at anl.gov> wrote:

>
>
> On Mar 10, 2022, at 10:05 AM, Alfredo J Duarte Gomez <aduarteg at utexas.edu>
> wrote:
>
> Hello Zhang and Hong,
>
> Thank you for your reply.
>
> As I described, I simply wanted to be able to restart a higher order BDF
> from a previous solution.
>
> For example, if I want to restart a BDF-2 solution I can simply load times
> (n is current time step ) t_n-1, t_n, load solutions y_n, and yn-1 from a
> restart file and continue integration with a BDF-2 formula as if it never
> stopped.
>
>
> Are y_n and yn-1 from the restart file different from the states saved
> internally for BDF-2? Are you trying to modify these states yourself? A
> restart is needed typically when you have discontinuities in the system, so
> the solutions before the discontinuity point have to be discarded. If you
> simply want to modify the states, there should be better ways than using
> checkpoint-restart.
>
> Hong (Mr.)
>
>
>
> This would replace the current default approach, which starts from a
> single time tn, solution yn and uses lower order BDF steps as you build up
> to the selected order.
>
> I am not sure why, but an abrupt change in integration order or time step
> leads to unwanted numerical noise in my solution, which I blame on the high
> nonlinearity of the system (I have tested extensively to rule out bugs).
>
> Thank you and let me know if you have any questions,
>
> -Alfredo
>
> On Wed, Mar 9, 2022 at 4:49 PM Zhang, Hong <hongzhang at anl.gov> wrote:
>
>> TSTrajectory supports checkpointing for multistage methods and can
>> certainly be extended to multistep methods. But I doubt it is the best
>> solution to Alfredo’s problem. Alfredo, can you elaborate a bit on what you
>> would like to do? TSBDF_Restart is already using the previous solution to
>> restart the integration with first-order BDF.
>>
>> Hong(Mr.)
>>
>> > On Mar 9, 2022, at 4:24 PM, Jed Brown <jed at jedbrown.org> wrote:
>> >
>> > Can you restart using small low-order steps?
>> >
>> > Hong, does (or should) your trajectory stuff support an exact
>> checkpointing scheme for BDF?
>> >
>> > I think we could add an interface to access the stored steps, but there
>> are few things other than checkpointing that would make sense
>> mathematically. Would you be up for making a merge request to add
>> TSBDFGetStepVecs(TS ts, PetscInt *num_steps, const PetscReal **times, const
>> Vec *vecs) and the respective setter?
>> >
>> > Alfredo J Duarte Gomez <aduarteg at utexas.edu> writes:
>> >
>> >> Good morning PETSC team,
>> >>
>> >> I am currently using a TSBDF object, which is working very well.
>> >>
>> >> However, I am running into trouble restarting higher order BDF methods.
>> >>
>> >> My problem is highly nonlinear, and when restarted for higher order BDF
>> >> methods (using the TSBDF_Restart function), wiggles appear in a
>> specific
>> >> region of the solution.
>> >>
>> >> Is there any way I can initialize the higher order BDF restart loading
>> >> previous solutions from a data file? I took a look at the code, but
>> there
>> >> is no obvious way to do this.
>> >>
>> >> Thanks,
>> >>
>> >> -Alfredo
>> >>
>> >> --
>> >> Alfredo Duarte
>> >> Graduate Research Assistant
>> >> The University of Texas at Austin
>>
>>
>
> --
> Alfredo Duarte
> Graduate Research Assistant
> The University of Texas at Austin
>
>
>

-- 
Alfredo Duarte
Graduate Research Assistant
The University of Texas at Austin
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