Re: [petsc-users] Problem with TS and SNES VI

[Abhyankar, Shrirang G via petsc-users]

Moritz,
  I think your case will also work with using TSEvent. I think your problem is 
similar, correct me if I am wrong, to my application where I need to constrain 
the states within some limits, lb \le x. I use events to handle this, where I 
use two event functions:
(i)                 x – lb = 0.      if x > lb &
(ii)              \dot{x} = 0       x = lb

The first event function is used to detect when x hits the limit lb. Once it 
hits the limit, the differential equation for x is changed to (x-lb = 0) in the 
model to hold x at limit lb. For releasing x, there is an event function on the 
derivative of x, \dot{x}, and x is released on detection of the condition 
\dot{x} > 0. This is done through the event function \dot{x} = 0 with a 
positive zero crossing.

An example of how the above works is in the example 
src/ts/examples/tutorials/power_grid/stability_9bus/ex9bus.c. In this example, 
there is an event function that first checks whether the state VR has hit the 
upper limit VRMAX. Once it does so, the flag VRatmax is set by the post-event 
function. The event function is then switched to the \dot{VR}
if (!VRatmax[i]))
    fvalue[2+2*i] = VRMAX[i] - VR;
} else {
     fvalue[2+2*i] = (VR - KA[i]*RF + KA[i]*KF[i]*Efd/TF[i] - KA[i]*(Vref[i] - 
Vm))/TA[i];
}

You can either try TSEvent or what Barry suggested SNESLineSearchSetPreCheck(), 
or both.

Thanks,
Shri


From: "Huck, Moritz" <moritz.h...@rwth-aachen.de>
Date: Wednesday, August 7, 2019 at 8:46 AM
To: "Smith, Barry F." <bsm...@mcs.anl.gov>
Cc: "Abhyankar, Shrirang G" <shrirang.abhyan...@pnnl.gov>, 
"petsc-users@mcs.anl.gov" <petsc-users@mcs.anl.gov>
Subject: AW: [petsc-users] Problem with TS and SNES VI

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?
________________________________________
Von: Smith, Barry F. <bsm...@mcs.anl.gov<mailto:bsm...@mcs.anl.gov>>
Gesendet: Dienstag, 6. August 2019 17:47:13
An: Huck, Moritz
Cc: Abhyankar, Shrirang G; 
petsc-users@mcs.anl.gov<mailto:petsc-users@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.h...@rwth-aachen.de<mailto:moritz.h...@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. <bsm...@mcs.anl.gov<mailto:bsm...@mcs.anl.gov>>
Gesendet: Dienstag, 6. August 2019 15:51:16
An: Huck, Moritz
Cc: Abhyankar, Shrirang G; 
petsc-users@mcs.anl.gov<mailto:petsc-users@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@mcs.anl.gov<mailto:petsc-users@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.abhyan...@pnnl.gov<mailto:shrirang.abhyan...@pnnl.gov>>
Gesendet: Montag, 5. August 2019 17:21:41
An: Huck, Moritz; petsc-users@mcs.anl.gov<mailto:petsc-users@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-boun...@mcs.anl.gov<mailto:petsc-users-boun...@mcs.anl.gov>> on 
behalf of "Huck, Moritz via petsc-users" 
<petsc-users@mcs.anl.gov<mailto:petsc-users@mcs.anl.gov>>
Reply-To: "Huck, Moritz" 
<moritz.h...@rwth-aachen.de<mailto:moritz.h...@rwth-aachen.de>>
Date: Monday, August 5, 2019 at 5:18 AM
To: "petsc-users@mcs.anl.gov<mailto:petsc-users@mcs.anl.gov>" 
<petsc-users@mcs.anl.gov<mailto:petsc-users@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