> On Mar 7, 2015, at 5:27 PM, David Knezevic <david.kneze...@akselos.com> wrote: > > OK, thanks for letting me know. > > I've tried GAMG and ML with MatNullSpaceCreateRigidBody and those both work > well for me. (I tried ML after Jed pointed out that ML also uses the near > nullspace.) > > I have also tried hypre but the convergence hasn't been as good for the > elasticity models I've been using, though I may not have been setting the > hypre options in an optimal way for elasticity. > > Another situation I've been meaning to ask about: If the elasticity model > includes some "near rigid" regions (very high Young's modulus) all iterative > solvers I've tried fare poorly. I guess it's because the highly contrasting > stiffnesses give a large condition number. Is there a standard way to get the > preconditioner compensate for this?
Off the top of my head: a "domain decomposition" answer might be to use a sparse direct solver on that region; of course how to tie that into the rest of the regions and the iterative solver is a broad question. Maybe in the context of AMG all the nodes in that region could be retained in the coarser grids (assuming it is a relatively small part of the domain) and so all those points end up in the coarse grid which is solved with a sparse direct solver? We don't currently have an interface for you to indicate nodes you want to keep in the coarse grid; Mark would have know if this makes any sense at all. Barry > > Thanks, > David > > > > On Sat, Mar 7, 2015 at 5:57 PM, Mark Adams <mfad...@lbl.gov> wrote: > FYI, stüben used classical AMG for elasticity but he has articulated his code > for elasticity more than Hypre as I understand it. Hypre can work OK for > elasticity in my experience. Its worth a try. > > Mark > > On Thu, Mar 5, 2015 at 5:27 PM, David Knezevic <david.kneze...@akselos.com> > wrote: > OK, got it, thanks! > > David > > > On Thu, Mar 5, 2015 at 5:08 PM, Jed Brown <j...@jedbrown.org> wrote: > David Knezevic <david.kneze...@akselos.com> writes: > > I was just wondering if its possible to achieve the same sort of thing with > > other AMG solvers (e.g. BoomerAMG)? I assume that MatSetNearNullSpace does > > nothing for external solvers like hypre, right? > > It is used by ML (smoothed aggregation), but not BoomerAMG (classical > AMG) which uses an algorithm that doesn't have a natural place for such > information. To my knowledge, classical AMG is not widely used for > elasticity. It is very robust for M-matrices. > > >
