[petsc-users] how to control snes_mf_operator

[Matthew Knepley]

On Tue, Dec 11, 2012 at 1:34 PM, Zou (Non-US), Ling <ling.zou at inl.gov> wrote:
> Dear All,
>
> I have recently had an issue using snes_mf_operator. I've tried to figure it
> out from PETSc manual and PETSc website but didn't get any luck, so I submit
> my question here and hope some one could help me out.
>
> (1)
> =================================================================
> A little bit background here: my problem has 7 variables, i.e.,
>
> U = [U0, U1, U2, U3, U4, U5, U6]
>
> U0 is in the order of 1.
> U1, U2, U4 and U5 in the oder of 100.
> U3 and U6 are in the order of 1.e8.
>
> I believe this should be quite common for most PETSc users.
>
> (2)
> =================================================================
> My problem here is, U0, by its physical meaning, has to be limited between 0
> and 1. When PETSc starts to perturb the initial solution of U (which I
> believe properly set) to approximate the operation of J (dU), the U0 get a
> perturbation size in the order of 100, which causes problem as U0 has to be
> smaller than 1.
>
> From my observation, this same perturbation size, say eps, is applied on all
> U0, U1, U2, etc. <=== Is this the default setting?
> I also guess that this eps, in the order of 100, is determined from my
> initial solution vector and other related PETSc parameters.  <=== Is my
> guessing right?
>
> (3)
> =================================================================
> My question: I'd like to avoid a perturbation size ~100 on U0, i.e., I have
> to limit it to be ~0.01 (or some small number) to avoid the U0 > 1
> situation. Is there any way to control that?
> Or, is there any advanced option to control the perturbation size on
> different variables when using snes_mf_operator?

Here is a description of the algorithm for calculating h. It seems to
me a better way to do this
is to non-dimensionalize first.

   Matt

>
> Hope my explanation is clear. Please let me know if it is not.
>
>
> Best Regards,
>
> Ling
>



--
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