Mark, This is a flow solver Poisson equation. Is the coordinate functionality applicable?
John On Wed, Mar 20, 2013 at 6:04 PM, Mark F. Adams <mark.adams at columbia.edu>wrote: > > On Mar 20, 2013, at 6:14 PM, John Mousel <john.mousel at gmail.com> wrote: > > Can you comment on a GASM type approach to find a solution for the null > space? I notice that the null vectors that successfully make the true > residual drop are only complicated in a very thin band around the > interface. This band is easy to identify using a level set. Other than > that, the null space vector has a low frequency variation. My thought was > to break the matrix into two sub-matrices, and somehow apply GAMG as a > preconditioner on the far matrix, and ILU on the interface-adjacent matrix. > Is this dumb or a complete misunderstanding of GASM? > > > The null space in GAMG is not a projection, it does not have to be exact. > Do you try using the 6 RBM or giving GAMG coordinates? > > Also, you might try not smoothing the (-pc_gamg_nsmooths 0). Unsymetric > matrices can work better this way. > > > > On Wed, Mar 20, 2013 at 5:08 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote: > >> >> On Wed, Mar 20, 2013 at 5:04 PM, John Mousel <john.mousel at gmail.com>wrote: >> >>> I've wanted to scrap this approach for a long time, but moving away from >>> these GFM-type treatments is not a choice that I've been allowed to follow >>> through on for various reasons which are out of my control. >> >> >> Unless there are some clever tricks to characterize the null space or to >> keep preconditioners compatible with the null space, the folks making the >> decisions might have to reconsider. It doesn't matter how sexy a method >> looks if it requires a solve and that solve cannot be done efficiently. >> > > > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20130320/e2a6233a/attachment-0001.html>
