On Tue, Dec 11, 2012 at 1:40 PM, Matthew Knepley <knepley at gmail.com> wrote: > 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.
I forgot the URL: http://www.mcs.anl.gov/petsc/petsc-dev/docs/manualpages/Mat/MatCreateMFFD.html#MatCreateMFFD Matt > 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 -- 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
