[petsc-users] Help with ML/BoomerAMG

[Gaetan Kenway]

It is an SA turbulence model and the discrete adjoint computed exactly with
AD. Certainly the grids are highly stretched in the BL since the grids are
resolving the viscous sublayer (y+ < 1) and the  Reynolds numbers are on
the order of  10's of millions. I tend  only to see this behaviour at
higher mach numbers when stronger shocks start to appear.  For example, the
adjoint  system may solve  fine at M=0.80, and fail to converge at M=0.85.
 For these RANS cases, the non-linear  solution  is solved using only RK
with multigrid.

It is entirely possible a different preconditioner may not help at all, but
there's not much else you can do.

Gaetan

On Mon, Apr 29, 2013 at 10:31 AM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> Gaetan Kenway <gaetank at gmail.com> writes:
>
> > For the forward solve I use ASM+ILU in the same manner as for the adjoint
> > problem.
> > The  ASM not a bottleneck  per se. Typically we see the  adjoint problem
> > taking the same amount of time as the non-linear problem for well-behaved
> > flows, and the adjoint is shorter for  less well-behaved flows.
>
> Sounds reasonable.
>
> > The real problem I am having is for certain RANS cases, the
> > frozen turbulence adjoint is extremely difficult to solve --- requiring
> >  GMRES subspace sizes on the order of 400-500 to converge.
>
> Hmm, which turbulence model are you using?  Is it related to stretched
> grids?  Continuous or discrete adjoint?
>
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