[petsc-users] troubleshooting AMG, coupled navier stokes, large eigenvalues on coarsest level

[Jed Brown]

Mark Lohry <mlohry at gmail.com> writes:

>>
>>
>>>> You want to use the Schur complement PCs for NS. We have some support
>>>> for PC where you give us a mass matrix.
>>>
>>>
>>> I'll look into it, but I'm not aware of Schur complement being used for
>>> compressible/coupled NS, only for incompressible/segregated.
>>>
>>
>>
>> Really? OK, you've looked into it more than me.
>>
>
> Correct me if I'm wrong, I haven't looked into it much at all. I just
> haven't encountered Schur complement in CFD outside of segregated
> SIMPLE-type algorithms (like in the ex70/PCFIELDSPLIT example.) Those
> segregated pressure-correction methods tend to get progressively worse as
> you become less elliptic / non-zero Mach.

There are more efficient split methods than SIMPLE, particularly for
steady state or large time steps, but they are still low Mach solvers.
Are you doing steady state or transient solves?  RANS?  There aren't
good AMG solvers available for this kind of system; not for lack of
trying but it remains a worthy area of research.