[petsc-users] KSPMonitorSingularValue

[Klaij, Christiaan]

>> 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