Yes, you are right, it's an issue with the null space due to boundary conditions.
Thomas Am 03.02.2012 10:57, schrieb Jed Brown: > On Fri, Feb 3, 2012 at 10:48, Thomas Witkowski > <thomas.witkowski at tu-dresden.de > <mailto:thomas.witkowski at tu-dresden.de>> wrote: > > Shouldn't be, but it seems that is is close to singular in > computer arithmetic. I would like to understand we it's so. The > matrix is a 2x2 block matrix with no coupling between the main > blocks. I know that this does not make much sense but its for > tests only and I would like to add some couplings later. Both > blocks are nonsingular and easy solvable with direct solvers. But > when adding both together, the condition number rise to something > around 10^23. Is it only a question of scaling both matrices to > the same order? > > > If it's *very* poorly scaled, then yes, it could be. You can try to > correct it with -ksp_diagonal_scale -ksp_diagonal_scale_fix. > > It seems more likely to me that it's a null space issue. How many > near-zero eigenvalues are there? Perhaps you effectively have an > all-Neumann boundary condition (e.g. incompressible flow with all > Dirichlet velocity boundary conditions leaves the pressure > undetermined up to a constant). -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20120203/58dcf776/attachment.htm>
