[petsc-users] vector-valued Laplace solver (Navier-Stokes): DIVERGED_INDEFINITE_MAT?

[Matthew Knepley]

On Thu, May 2, 2013 at 9:01 AM, Nico Schlömer <nico.schloemer at gmail.com>wrote:

> Hi all,
>
> I'm trying to solve a discretization of the PDE in weak form
>
>     rho/tau u - mu \Delta u = f
>
> where u is vector-valued (let's say in 2D -- this comes from a
> Navier--Stokes problem). Some Dirichlet-boundary conditions come with it,
> too.
>
> After translation in weak form,
>
>     rho/tau * inner(u, v) + mu * inner(grad(u), grad(v)) = inner(f, v)
>
> I'm solving this with PETSc's CG and hypre_amg. What I get is
>
>   0 KSP preconditioned resid norm 4.962223194957e+30 true resid norm
> 2.364095175749e-02 ||r(i)||/||b|| 1.000000000000e+00
>   1 KSP preconditioned resid norm 7.089043065444e+19 true resid norm
> 2.289113027906e-02 ||r(i)||/||b|| 9.682829402926e-01
>
> Without preconditioning, I'm getting
>
>   0 KSP preconditioned resid norm 2.364095175749e-02 true resid norm
> 2.364095175749e-02 ||r(i)||/||b|| 1.000000000000e+00
>   1 KSP preconditioned resid norm 4.415430823612e-02 true resid norm
> 4.415430823612e-02 ||r(i)||/||b|| 1.867704341562e+00
>   2 KSP preconditioned resid norm 1.077641425707e-01 true resid norm
> 1.077641425707e-01 ||r(i)||/||b|| 4.558367348159e+00
>
> and DIVERGED_INDEFINITE_MAT.
>
> Does anyone else have experience with this sort of problems? Any obvious
> mistakes?
>

Do you have any non-symmetries in your discretization? With the standard
P_1 basis, that operator is symmetric.

   Matt


> --Nico
>
>
>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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