Ok, I'll keep that in mind. thank you very much for the explanations. Domenico.
> On Mon, Apr 11, 2011 at 13:32, > <domenico.borzacchiello at univ-st-etienne.fr>wrote: > >> Still it's unclear to me why this happens and whether or not defining >> the >> Jacobian instead of computing it by FD may possibly fix it. I'd like to >> stick with FD approximation of Jacobian because I'll add complex >> rheology >> models for which computing the Jacobian analitically won't be an easy >> task. >> > > To use FD, you have to make sure that your equations are well scaled. You > should be able to volume-scale the residual (most PETSc examples do this) > and choose units such that the system is well-scaled independent of grid > resolution. You should do this regardless of whether you use FD, but with > FD, you have half the number of digits to work with before running into > rounding error problems. >
