[petsc-users] SNES divergence

[Sang pham van]

Hi Barry,

I want to try the original version of Newton method, no line search nor
trust region...
Using grid sequencing confuses me at the point that I don't know explicitly
the final mesh size, or even if I can estimate the mesh size from the
input -snes_grid_sequence
n it's not so convenient for me when some of my routines need such info
from very first moment. I wish PETSc could allow us to pass in the finest
mesh for grid sequencing.


S,


On Tue, Jun 17, 2014 at 3:46 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> On Jun 17, 2014, at 2:30 PM, Sang pham van <pvsang002 at gmail.com> wrote:
>
> > Thanks Barry,
> >
> > To refine the grid, I just put more points in the direction. For both
> fine and coarse meshes, I used simple initial guess (say constant values in
> whole domain for all variables).
> >
> > By using grid sequencing, is the finest mesh is the one I first input
> the solver?
>
>   No, you pass in the coarse one.
> >
> > Can you let me know what options should I use to have pure Newton method?
>
>    Not sure what you mean by pure Newton method, maybe without grid
> sequencing? It simply may not be possible to get convergence from a “poor”
> initial guess. One should always use grid sequencing if possible.
>
>    Barry
>
> >
> > S.
> >
> >
> > On Tue, Jun 17, 2014 at 3:20 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> >
> > On Jun 17, 2014, at 2:12 PM, Sang pham van <pvsang002 at gmail.com> wrote:
> >
> > > Hi,
> > >
> > > I am using DM structure and SNES to solve a 3D problem. In the problem
> I have 3 variables.
> > >
> > > I got SNES converged with a grid. Obtain result are physically right.
> However when I refine the grid, SNES does not always converge,
> >
> >    With the refined grid how are you starting the solution? Do you use
> the interpolated solution from the coarser grid (called grid sequencing) or
> just some “not good” initial guess?
> >
> > > the reason of divergence is line search fail or linear solver failed.
> (I also tried other type of SNES, line search seems to be the one best fits
> my problem)
> > >
> > > Can you please give me a suggestion to figure out the problem with my
> solver? What options should I use to have pure Newton method in SNES? Is
> there any advance option I can use with line search to improve SNES
> convergence.
> > >
> >
> >    You should use grid sequencing, not only does it get convergence when
> you may not otherwise get it but it will also solve the problem faster.
> With PETSc DM you can use -snes_grid_sequence n  or SNESSetGridSequence()
> in the code to do n levels of grid sequencing.
> >
> >    Barry
> >
> > > Thank you very much.
> > >
> > > Minh.
> > >
> >
> >
>
>
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