[petsc-users] SNES with constraint diverges

[Josh L]

Barry,

Yes, you are right, the entire FormFunction norm doesn't necessarily
decreases as SNES converges to the DVI solution.

Thanks,
Josh


2018-09-12 16:41 GMT-05:00 Smith, Barry F. <bsmith at mcs.anl.gov>:

>
>
> > On Sep 12, 2018, at 4:31 PM, Josh L <ysjosh.lo at gmail.com> wrote:
> >
> > Barry,
> >
> > 0 represents fully damaged and 1 is intact. if I unload it, the crack
> shouldn't heal(solution goes back to 1), so I set the constraint as
> >     0 <= u^{i+1} <= u^{i}
> >     i is either load step or time step.
>
>    Ok, then how do you know that the entire FormFunction() norm
> will/should even decrease as SNES converges to the DVI solution?
>
>     Barry
>
> >
> >
> >
> > Matt,
> >
> > Yes, that is what I am going to do next. a linear penalty gives a
> quadratic distribution instead of the exponential distribution given by
> quadratic penalty term.
> >
> >
> > Thanks,
> > Josh
> >
> >
> >
> > 2018-09-12 16:23 GMT-05:00 Matthew Knepley <knepley at gmail.com>:
> > On Wed, Sep 12, 2018 at 5:16 PM Smith, Barry F. <bsmith at mcs.anl.gov>
> wrote:
> >
> >    The function norm given by the -snes_monitor is the function norm for
> the DVI problem (which depends on where the solution is constrained) while
> FormFunction() norm you compute is for the original unconstrained problem.
> >
> >     I am still not sure why you solve this as a DVI. If you solve it
> without the constraints do you get the "wrong" answer?
> >
> > If this is a variational crack problem, you can solve it unconstrained
> if you use a quadratic penalty term. However,
> > that induces spurious long range communication between crack tips. If
> you bound the phase field between 0 and 1,
> > you can use a linear penalty which has physical behavior.
> >
> >    Matt
> >
> >     Barry
> >
> >
> > > On Sep 12, 2018, at 3:32 PM, Josh L <ysjosh.lo at gmail.com> wrote:
> > >
> > > Hi,
> > >
> > > my solution is mostly very close to 1. only  for a very small area
> where solution goes from 0 to 1(a smeared crack).
> > >
> > > I set -snes_atol 1e-7 and it is converging.
> > >
> > > I've noticed the following:
> > >
> > > There is a difference between the function norm.
> > >
> > > I calculate the function norm in FormFunction, so every time it is
> called it gives the function norm
> > > , and the result is different from the function norm given by
> -snes_monitor if i set
> > >
> > >      SNESSetType(snes,SNESVINEWTONRSLS,ierr)
> > >      SNESVISetVariableBounds(snes,xl,xu,ierr)
> > >
> > > The function norm calculated in FromFunction is NOT reducing, however,
> the function norm given by -snes_monitor is reducing
> > > They are the same if I just use regular SNES without setting variable
> bounds.
> > >
> > >
> > > Thanks,
> > > Josh
> > >
> > > 2018-09-12 12:02 GMT-05:00 Smith, Barry F. <bsmith at mcs.anl.gov>:
> > >
> > >      You have too tight a convergence tolerance for your problem.  You
> can't expect to get more than 1.e-12 as the minimum residual norm or even
> less.
> > >
> > >       How close is your solution to 1 and -1?
> > >
> > >       If you really need much higher convergence you can try
> ./configure --with-precision=__float128
> > >
> > >
> > >       Barry
> > >
> > > > On Sep 11, 2018, at 11:53 PM, Josh L <ysjosh.lo at gmail.com> wrote:
> > > >
> > > > Yes, I initialize all u_i to 1.0
> > > >
> > > >
> > > >
> > > > 2018-09-11 23:37 GMT-05:00 Smith, Barry F. <bsmith at mcs.anl.gov>:
> > > >
> > > >    Do you start with initial conditions of  0 <= u_i <= 1 ?
> > > >
> > > >     Run with -snes_monitor -snes_converged_reason
> -ksp_monitor_true_residual -info -snes_linesearch_monitor and send all the
> output
> > > >
> > > >   Barry
> > > >
> > > >
> > > > > On Sep 11, 2018, at 11:33 PM, Josh L <ysjosh.lo at gmail.com> wrote:
> > > > >
> > > > > Hi,
> > > > >
> > > > > I am using SNES to solve an nonlinear equation f(u), and I know
> all the u_i should be 0 and 1.
> > > > >
> > > > > First, I use SNES without constraint, and it converges.
> > > > >
> > > > > But, If I set
> > > > >      SNESSetType(snes,SNESVINEWTONRSLS,ierr)
> > > > >      SNESVISetVariableBounds(snes,xl,xu,ierr)
> > > > >
> > > > > where xl and xu is vector, and xl_i=0 and xu_i=1
> > > > >
> > > > > then SNES fails to converge, because linesearch fails(snes reason
> = -6), and the norm of residual is not reducing(the norm of incremental
> solution is reducing)
> > > > >
> > > > > The reason to add constraint is that I want to implement some
> irreversibility.
> > > > >
> > > > >
> > > > > Thanks,
> > > > > Josh
> > > > >
> > > >
> > > >
> > >
> > >
> >
> >
> >
> > --
> > What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> > -- Norbert Wiener
> >
> > https://www.cse.buffalo.edu/~knepley/
> >
>
>
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