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

[Nico Schlömer]

The boundary conditions weren't applied correctly, such that the operator
was indeed (slightly) nonsymmetric. Seems to work now. Thanks for the hint!

--Nico


On Thu, May 2, 2013 at 4:33 PM, Matthew Knepley <knepley at gmail.com> wrote:

> 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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