The boundary conditions weren't applied correctly, such that the operator was indeed (slightly) nonsymmetric. Seems to work now. Thanks for the hint!
--Nico On Thu, May 2, 2013 at 4:33 PM, Matthew Knepley <knepley at gmail.com> wrote: > On Thu, May 2, 2013 at 9:01 AM, Nico Schl?mer <nico.schloemer at > gmail.com>wrote: > >> Hi all, >> >> I'm trying to solve a discretization of the PDE in weak form >> >> rho/tau u - mu \Delta u = f >> >> where u is vector-valued (let's say in 2D -- this comes from a >> Navier--Stokes problem). Some Dirichlet-boundary conditions come with it, >> too. >> >> After translation in weak form, >> >> rho/tau * inner(u, v) + mu * inner(grad(u), grad(v)) = inner(f, v) >> >> I'm solving this with PETSc's CG and hypre_amg. What I get is >> >> 0 KSP preconditioned resid norm 4.962223194957e+30 true resid norm >> 2.364095175749e-02 ||r(i)||/||b|| 1.000000000000e+00 >> 1 KSP preconditioned resid norm 7.089043065444e+19 true resid norm >> 2.289113027906e-02 ||r(i)||/||b|| 9.682829402926e-01 >> >> Without preconditioning, I'm getting >> >> 0 KSP preconditioned resid norm 2.364095175749e-02 true resid norm >> 2.364095175749e-02 ||r(i)||/||b|| 1.000000000000e+00 >> 1 KSP preconditioned resid norm 4.415430823612e-02 true resid norm >> 4.415430823612e-02 ||r(i)||/||b|| 1.867704341562e+00 >> 2 KSP preconditioned resid norm 1.077641425707e-01 true resid norm >> 1.077641425707e-01 ||r(i)||/||b|| 4.558367348159e+00 >> >> and DIVERGED_INDEFINITE_MAT. >> >> Does anyone else have experience with this sort of problems? Any obvious >> mistakes? >> > > Do you have any non-symmetries in your discretization? With the standard > P_1 basis, that operator is symmetric. > > Matt > > >> --Nico >> >> >> >> > > > -- > 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 > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20130502/3b67611d/attachment.html>
