[petsc-users] Problem with TS and SNES VI

[Smith, Barry F.]

> On Aug 7, 2019, at 7:45 AM, Huck, Moritz <Moritz.Huck at rwth-aachen.de> wrote:
> 
> Thank you for your response.
> The sizes are only allowed to go down to a certain value.
> The non-physical values do also occur during the function evaluations (IFunction).
> 
> I will try to implment your suggestions with SNESLineSearchSetPreCheck. This would mean I dont have to use SNESVISetVariableBounds at all, right?

   Yes

   Let us know how it goes. If the solutions to the underlying "analytic/continuum" model don't go to zero size then this approach has a good chance of working, since the non-physical values are just an artifact of the nonlinear solver algorithm. But if the "true" solutions do go to zero than likely this approach will fail because it is unable to find the zeros of the nonlinear function needed to evolve in time, and this makes sense since the discrete model should mimic the behavior of the "analytic/continuum" model.

  Barry

> ________________________________________
> Von: Smith, Barry F. <bsmith at mcs.anl.gov>
> Gesendet: Dienstag, 6. August 2019 17:47:13
> An: Huck, Moritz
> Cc: Abhyankar, Shrirang G; petsc-users at mcs.anl.gov
> Betreff: Re: [petsc-users] Problem with TS and SNES VI
> 
>   Thanks, very useful.
> 
>   Are the non-physical values appearing in the nonlinear solver ? Or just at the time-step?
> 
>   Do you check for non-physical values each time you do a function evaluation needed by SNES/TS?
> 
>   If the non-physical values are an artifact of the steps taken in the nonlinear solver in SNES then the correct solution is to use
> SNESLineSearchSetPreCheck() what you do is change the step so the resulting solutions are physical.
> 
>   For you case where the sizes go negative I am not sure what to do. Are the sizes allowed to go to zero? If so then adjust the step so that the sizes that go to negative values just go to zero. If they are suppose to be always positive then you need to pick some tolerance (say epsilon) and adjust the step so they are of size epsilon. Note you don't scale the entire step vector by a small number to satisfy the constraint you change each entry in the step as needed to satisfy the constraints.
> 
>   Good luck and let us know how it goes
> 
>   Barry
> 
> 
> 
>> On Aug 6, 2019, at 9:24 AM, Huck, Moritz <Moritz.Huck at rwth-aachen.de> wrote:
>> 
>> At the moment I output only the values at the actual time-step (with the poststep functionality), I dont know the values during the stages.
>> Unphysical values are e.g. particle sizes below zero.
>> 
>> My model as no explicit inequalities, the only handling of the constraints is done by setting SNES VI.
>> 
>> The model does not change in the senes that there are new equations. If have put in an conditional that xdot is calculated to be positive of x is on or below the lower bound.
>> ________________________________________
>> Von: Smith, Barry F. <bsmith at mcs.anl.gov>
>> Gesendet: Dienstag, 6. August 2019 15:51:16
>> An: Huck, Moritz
>> Cc: Abhyankar, Shrirang G; petsc-users at mcs.anl.gov
>> Betreff: Re: [petsc-users] Problem with TS and SNES VI
>> 
>> Could you explain in a bit more detail what you mean by "some states go to unphysical values" ?
>> 
>> Is this within a stage or at the actual time-step after the stage?
>> 
>>  Does you model explicitly have these bounds on the solution; i.e. it is imposed as a variational inequality or does the model not explicitly have the constraints because its "analytic" solution just naturally stays in the physical region anyways? But numerical it can go out?
>> 
>> Or, is your model suppose to "change" at a certain time, which you don't know in advance when the solution goes out of some predefined bounds" (this is where the event is designed for).
>> 
>>  This information can help us determine what approach you should take.
>> 
>>  Thanks
>> 
>>  Barry
>> 
>> 
>>> On Aug 6, 2019, at 2:12 AM, Huck, Moritz via petsc-users <petsc-users at mcs.anl.gov> wrote:
>>> 
>>> Hi,
>>> I think I am missing something here.
>>> How would events help to constrain the states.
>>> Do you mean to use the event to "pause" to integration an adjust the state manually?
>>> Or are the events to enforce smaller timesteps when the state come close to the constraints?
>>> 
>>> Thank you,
>>> Moritz
>>> ________________________________________
>>> Von: Abhyankar, Shrirang G <shrirang.abhyankar at pnnl.gov>
>>> Gesendet: Montag, 5. August 2019 17:21:41
>>> An: Huck, Moritz; petsc-users at mcs.anl.gov
>>> Betreff: Re: [petsc-users] Problem with TS and SNES VI
>>> 
>>> For problems with constraints on the states, I would recommend trying the event functionality, TSEvent, that allows detection and location of discrete events, such as one that you have in your problem.
>>> https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/TS/TSSetEventHandler.html.
>>> 
>>> An example using TSEvent functionality:  https://www.mcs.anl.gov/petsc/petsc-current/src/ts/examples/tutorials/ex40.c.html
>>> 
>>> A brief intro to TSEvent can be found here<https://www.mcs.anl.gov/petsc/petsc-20/conference/Abhyankar_S.pdf>.
>>> 
>>> Thanks,
>>> Shri
>>> 
>>> 
>>> From: petsc-users <petsc-users-bounces at mcs.anl.gov> on behalf of "Huck, Moritz via petsc-users" <petsc-users at mcs.anl.gov>
>>> Reply-To: "Huck, Moritz" <Moritz.Huck at rwth-aachen.de>
>>> Date: Monday, August 5, 2019 at 5:18 AM
>>> To: "petsc-users at mcs.anl.gov" <petsc-users at mcs.anl.gov>
>>> Subject: [petsc-users] Problem with TS and SNES VI
>>> 
>>> Hi,
>>> I am trying to solve a DAE with the ARKIMEX solver, which works mostly fine.
>>> The problem arises when some states go to unphysical values. I try to constrain my states with SNESVISetVariableBounds (through the petsc4py interface).
>>> But TS seems not respect this e.g. I have a state with is usually between 1 and 1e3 for which I set a lower bound of 1, but the state goes t0 -0.8 at some points.
>>> Are there some tolerances I have to set for VI or something like this?
>>> 
>>> Best Regards,
>>> Moritz
>>> 
>> 
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