[petsc-users] SNES with constraint diverges

[Matthew Knepley]

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/ <http://www.cse.buffalo.edu/~knepley/>
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