[petsc-users] KSPMonitorSingularValue

[Jed Brown]

On Thu, Oct 20, 2011 at 10:15, Klaij, Christiaan <C.Klaij at marin.nl> wrote:

> 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?
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