>> The setting is Navier-Stokes, colocated FVM, matrix-free Krylov-Picard >> with SIMPLE preconditioning. It works for most case but stagnates >> for this particular case although SIMPLE as solver does converge nicely. >> > >> How are you applying the action of the linear operator? If you use finite >> differencing, it could be inaccurate. Is this incompressible or a low-Mach >> compressible formulation? Try -ksp_monitor_true_residual, if the true >> residual drifts from the unpreconditioned residual computed by FGMRES, the >> Krylov space could be losing orthogonality. You can try >> -ksp_gmres_modifiedgramschmidt. Are you losing a lot of progress in >> restarts? > > >It's incompressible Navier-Stokes. No finite differencing, the >action is computed directly without approximations. It's right >preconditioning, so preconditioned and true residual should be >the same. I don't get any progress, the residual is stagnating >from the very first iteration way before any restart.
Now that I have the singular value monitor working this is what I get for a converging problem and the stagnating problem. Both problems are quite similar (same type of flow, same type of grid type, same BCs, just slightly different geometry). Any ideas? Converging problem: 0 KSP Residual norm 2.365447773598e+00 % max 1 min 1 max/min 1 1 KSP Residual norm 1.688717750865e+00 % max 2.43352 min 2.43352 max/min 1 2 KSP Residual norm 1.441677051369e+00 % max 3.39561 min 0.658191 max/min 5.15901 3 KSP Residual norm 1.309618066659e+00 % max 3.45429 min 0.477676 max/min 7.23145 4 KSP Residual norm 1.236059684429e+00 % max 3.68325 min 0.418244 max/min 8.80645 5 KSP Residual norm 1.178488235028e+00 % max 4.26371 min 0.37149 max/min 11.4773 Stagnating problem (classical Gram-Schmidt): 0 KSP Residual norm 6.466095059482e-02 % max 1 min 1 max/min 1 1 KSP Residual norm 6.466093783326e-02 % max 1.07631 min 1.07631 max/min 1 2 KSP Residual norm 6.466093747580e-02 % max 3.0223 min 1.06721 max/min 2.83196 3 KSP Residual norm 6.466093697245e-02 % max 6.79697 min 1.0663 max/min 6.37436 4 KSP Residual norm 6.466092195305e-02 % max 8.16759 min 1.06345 max/min 7.68027 5 KSP Residual norm 6.466090166280e-02 % max 8.80407 min 1.06326 max/min 8.28026 Stagnating problem (modified Gram-Schmidt): 0 KSP Residual norm 6.470362245549e-02 % max 1 min 1 max/min 1 1 KSP Residual norm 6.470334363709e-02 % max 1.08156 min 1.08156 max/min 1 2 KSP Residual norm 6.470326556761e-02 % max 3.05856 min 1.07224 max/min 2.8525 3 KSP Residual norm 6.470326223293e-02 % max 6.85267 min 1.0711 max/min 6.39777 4 KSP Residual norm 6.470312640256e-02 % max 8.17917 min 1.06837 max/min 7.65572 5 KSP Residual norm 6.470312619015e-02 % max 8.7711 min 1.06807 max/min 8.21213 dr. ir. Christiaan Klaij CFD Researcher Research & Development E mailto:C.Klaij at marin.nl T +31 317 49 33 44 MARIN 2, Haagsteeg, P.O. Box 28, 6700 AA Wageningen, The Netherlands T +31 317 49 39 11, F +31 317 49 32 45, I www.marin.nl