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

[Mark Lohry]

>
> There are more efficient split methods than SIMPLE, particularly for
> steady state or large time steps, but they are still low Mach solvers.


I only mention SIMPLE as a canonical segregated/pressure-based method;
segregated methods aren't really my area.

Are you doing steady state or transient solves?  RANS?


Varies -- I wrote a high order unstructured parallel DG code for coupled
compressible NS that does fantastic with explicit time DNS, and I'm trying
to get any kind of good implicit time or steady state solutions to attack
the majority of problems where explicit is useless. Basically anything in
the AIAA high order workshops is of interest to me. RANS, LES, and steady
state are all of interest; I think steady state solutions in this area are
probably doomed to use a pseudo-transient method to get there anyway so
it's the same problem. I don't know if anyone in high order cfd has
actually gotten a purely steady state formulation to converge on something
nontrivial.


 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.


Hence me using petsc as a solver library for research =), it's the best
option for trying various kinds of algebraic solvers (seriously you devs
have done fine work). Relatively generic AMG solvers have had massive
success in the same problems for FV discretizations, so it seems hopeful
they could extend to DG.

I recall you working in this area, so any suggestions welcome.

On Wed, Nov 8, 2017 at 11:12 PM, Jed Brown <jed at jedbrown.org> wrote:

> 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.
>
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