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
