[petsc-users] SNES divergence

Sang pham van pvsang002 at gmail.com
Tue Jun 17 14:30:11 CDT 2014

```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?

Can you let me know what options should I use to have pure Newton method?

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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