[petsc-users] snes options for rough solution

[Praveen C]

Sorry for being incomplete. I use backward euler and snes. The figure I
sent shows solution changes by a large amount in each time step. The major
change is at extrema. The change in one time step

u^{n+1} - u^n

which must come from snes is not a smooth function of x.

If I use dt = dx, then snes does not converge even after 50 iterations,
here is output (Fenics code)

  0 SNES Function norm 1.977638959494e+00

  1 SNES Function norm 1.924169835496e+00

  2 SNES Function norm 1.922201608879e+00

  3 SNES Function norm 1.920237421814e+00

  4 SNES Function norm 1.918277062381e+00

  5 SNES Function norm 1.916320289472e+00

  6 SNES Function norm 1.914366865403e+00

  7 SNES Function norm 1.912416585240e+00

  8 SNES Function norm 1.910469283868e+00

  9 SNES Function norm 1.899960770375e+00

 10 SNES Function norm 1.879131065459e+00

 11 SNES Function norm 1.857531063656e+00

 12 SNES Function norm 1.836809521483e+00

 13 SNES Function norm 1.816709863124e+00

 14 SNES Function norm 1.797014998190e+00

 15 SNES Function norm 1.777737697197e+00

 16 SNES Function norm 1.758825541543e+00

 17 SNES Function norm 1.740232061718e+00

 18 SNES Function norm 1.721929885464e+00

 19 SNES Function norm 1.703895519687e+00

 20 SNES Function norm 1.686113465512e+00

 21 SNES Function norm 1.668566528915e+00

 22 SNES Function norm 1.651247832992e+00

 23 SNES Function norm 1.634150402758e+00

 24 SNES Function norm 1.617265971731e+00

 25 SNES Function norm 1.600589248992e+00

 26 SNES Function norm 1.584114929900e+00

 27 SNES Function norm 1.567836662164e+00

 28 SNES Function norm 1.551748332761e+00

 29 SNES Function norm 1.535845822400e+00

 30 SNES Function norm 1.520125060009e+00

 31 SNES Function norm 1.504582049738e+00

 32 SNES Function norm 1.489213340181e+00

 33 SNES Function norm 1.474015969067e+00

 34 SNES Function norm 1.458987020278e+00

 35 SNES Function norm 1.444123487154e+00

 36 SNES Function norm 1.429422455304e+00

 37 SNES Function norm 1.414881258532e+00

 38 SNES Function norm 1.400497521029e+00

 39 SNES Function norm 1.386269021843e+00

 40 SNES Function norm 1.372193573931e+00

 41 SNES Function norm 1.358269024537e+00

 42 SNES Function norm 1.344493290602e+00

 43 SNES Function norm 1.330864378208e+00

 44 SNES Function norm 1.317380313100e+00

 45 SNES Function norm 1.304039060314e+00

 46 SNES Function norm 1.290838655798e+00

 47 SNES Function norm 1.277777248327e+00

 48 SNES Function norm 1.264853054978e+00

 49 SNES Function norm 1.252064339134e+00

 50 SNES Function norm 1.239409403964e+00


Function norm is decreasing but very slowly. The initial guess for newton,
which is just solution from old time, is too far from the solution.

With dt = dx^2, it works fine, but this time step is too small.

Best
praveen

On Mon, Dec 12, 2016 at 11:50 AM, Matthew Knepley <knepley at gmail.com> wrote:

> On Sun, Dec 11, 2016 at 11:23 PM, Praveen C <cpraveen at gmail.com> wrote:
>
>> Dear all
>>
>> I am solving a nonlinear parabolic problem with snes. The newton update
>> is rather non-smooth and I have convergence problems when using default
>> options.
>>
>> Attached figure shows how solution changes in two time steps.
>>
>
> It is not clear what you mean here. Newton does not solve timestepping
> problems. Maybe you are using it
> with an implicit timestepper, but its still not clear what you mean by
> non-smooth updates. Did you try with
> TS?
>
>    Matt
>
>
>> Are there any special algorithms/options in snes that I can use for such
>> problem ?
>>
>> Thanks
>> praveen
>>
>
>
>
> --
> 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
>
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