<div class="gmail_quote">On Thu, Oct 20, 2011 at 10:15, Klaij, Christiaan <span dir="ltr"><<a href="mailto:C.Klaij@marin.nl">C.Klaij@marin.nl</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
<div id=":1u3">The setting is Navier-Stokes, colocated FVM, matrix-free Krylov-Picard<br>
with SIMPLE preconditioning. It works for most case but stagnates<br>
for this particular case although SIMPLE as solver does converge nicely.</div></blockquote></div><br><div>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?</div>