[petsc-users] difference between left and right pc

[Klaij, Christiaan]

>   Is the "preconditioner" singular?  How about the matrix?  Is
>   the "preconditioner" a linear operator? Did you try KSPFGMRES
>   on it?

The matrix comes from incompressible Navier-Stokes and the
preconditioner is of Schur complement type (SIMPLE). Both are
shells.  FGMRES (and GCR) give the same result as GMRES with
right PC.

>
> > I'm solving a system with GMRES using the same preconditioner
> > either on the left or on the right.  For left preconditioning I
> > get two orders of reduction for the preconditioned residual in
> > 20 its: The solution is ok for left preconditioning but stuck
> > to zero (the initial guess) for right preconditioning.
>
>    Is it really ok? The true residual norm is much much worse in
>    the left case after 20 iterations than the right case.  How
>    are you judging the solution quality?

I'm judging quality by looking at the physics. With right
preconditioner the flow is standing still. With left
preconditioning, even though it is only the first non-linear
iteration, the flow looks more or less ok (right direction, high
pressure in stagnation point etc...)

>
>    Barry


dr. ir. Christiaan Klaij
CFD Researcher
Research & Development
E mailto:C.Klaij at marin.nl
T +31 317 49 33 44

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