[petsc-users] non linear solver options

[Matthew Knepley]

On Sat, Jan 18, 2014 at 2:52 PM, Dharmendar Reddy
<dharmareddy84 at gmail.com>wrote:

> Hello,
>                  I am solving a set of equations with SNES
> F1 (x1,x2,x3) = 0
> F2 (x1,x2,x3) = 0
> F3 (x1,x2,x3) = 0
>
> The system of equations is shown on page 1 of pdf here
> http://dunham.ee.washington.edu/ee531/notes/SemiRev.pdf
>
> F1 = equation 1
> F2 = equation 2
> F3 = equation 5
>
> x1 = n, X2=p and X3 = psi,
> X1 and X2 have an exponential dependance on X1
>

This is important, and is why Gummel's method was invented. These guys
discuss
it (http://www.iue.tuwien.ac.at/phd/pourfath/node89.html), and point out
that Newton
can be hard to converge. I suspect that preconditioning Newton with
something that
looks like Gummel is the way to go.

If you get this coded up, we can help you experiment with nonlinear
preconditioning.
Gummel is really a particular kind of nonlinear FieldSplit.

   Thanks,

     Matt


> after i scale the variables, X3 typically varies between say +/- 100
> where as X1 and X2 vary between 0 to 2. norm(X) then may usually
> dominated by solution values of X3.
>
> Can you suggest me the snes options that i need to use to achieve the
> following:
>
> 1. X1 > 0 and X2 > 0  (as per previous emails, i can use
> SNESSetVariableBounds)
> 2. I want the updates to solution to have an adaptive stepping based
> on norm of (F) or norm(X). If norm(F) is decreasing as the iteration
> progresss, larger stepping others wise reduce the step size..
> Similarly for Norm of X.
>
> Can i get the work done with existing petsc routines/ options ?
>
> Do i need to impliment the update method via SNESPOSTCheck  ?
>
>
> Thanks
> Reddy
>



-- 
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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