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

[Barry Smith]

  This seems completely reasonable. We should have a simple mechanism for multi-step methods to save the appropriate multisteps after an integration and to load them into a TS before a new integration.

   Barry


> On Mar 10, 2022, at 11:05 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. 
> 
> 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 <mailto: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 <mailto: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 <mailto: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

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