[petsc-users] Rigid body nullspace for Stokes operator

[Amneet Bhalla]

I set the rigid body null vectors but PETSc errors out that these are not
orthogonal. Is there a canned routine in PETSc to orthogonalize a bunch of
Vecs?

[0]PETSC ERROR: Invalid argument

[0]PETSC ERROR: Vector 0 must be orthogonal to vector 2, inner product is
0.612391

[0]PETSC ERROR: See https://urldefense.us/v3/__https://petsc.org/release/faq/__;!!G_uCfscf7eWS!e0btVnW2NyDoumIgYMZpmOJ8Vvkh18HjVCEY3LyPW9fAsr4jD7MqRxxBLgh-q49bw3jvM0FZc2A_s2U6RxXpfK06Dg$  for trouble shooting.

[0]PETSC ERROR: Petsc Release Version 3.17.5, unknown

[0]PETSC ERROR: ./acoustic_streaming_hier_integrator_2d on a darwin-dbg
named APSB-MacBook-Pro-16.local by amneetb Mon Nov  4 18:01:09 2024

[0]PETSC ERROR: Configure options --CC=mpicc --CXX=mpicxx --FC=mpif90
--PETSC_ARCH=darwin-dbg --with-debugging=1 --download-hypre=1 --with-x=0
-download-mumps -download-scalapack -download-parmetis -download-metis
-download-ptscotch

[0]PETSC ERROR: #1 MatNullSpaceCreate() at
/Users/amneetb/Softwares/PETSc-Gitlab/PETSc/src/mat/interface/matnull.c:271

[0]PETSC ERROR: #2 resetMatNearNullspace() at
../../../IBAMR/ibtk/lib/../src/solvers/impls/PETScKrylovLinearSolver.cpp:697

P=00000:Program abort called in file
``../../../IBAMR/ibtk/lib/../src/solvers/impls/PETScKrylovLinearSolver.cpp''
at line 697

P=00000:ERROR MESSAGE:

P=00000:
Abort trap: 6

On Mon, Nov 4, 2024 at 10:11 AM Jed Brown <jed at jedbrown.org> wrote:

> Unless the problem is entirely floating (the true null space is all six
> rigid body modes), then they will be different, so yes, you'll typically
> have two MatNullSpace objects.
>
> Amneet Bhalla <mail2amneet at gmail.com> writes:
>
> > Hi Jed,
> >
> > Do I need to create two separate MattNullSpace objects if I want to use
> > both MatSetNullSpace() and MatSetNearNullSpace()?
> >
> > Thanks,
> >
> >
> > On Thu, Oct 31, 2024 at 8:18 AM Jed Brown <jed at jedbrown.org> wrote:
> >
> >> Pierre Jolivet <pierre at joliv.et> writes:
> >>
> >> >> On 31 Oct 2024, at 2:47 PM, Mark Adams <mfadams at lbl.gov> wrote:
> >> >>
> >> >> Interesting. I have seen hypre do fine on elasticity, but do you know
> >> if boomeramg (classical) uses these vectors or is there a smoothed
> >> aggregation solver in hypre?
> >> >
> >> > I’m not sure it is precisely “standard” smoothed aggregation, see
> bottom
> >> paragraph of
> >>
> https://urldefense.us/v3/__https://hypre.readthedocs.io/en/latest/solvers-boomeramg.html*amg-for-systems-of-pdes__;Iw!!G_uCfscf7eWS!fdj4AzKhAcDmM1x_ZQ8gxqWeX6BAKY9urnvATMpT7hC8lw77ak7tqxqXGIX3PMg2wYA5PGu7EzyCW0yzixutBg$
> >> > I’ve never made it to work, but I know some do.
> >> > A while back, Stefano gave me this pointer as well:
> >>
> https://urldefense.us/v3/__https://github.com/mfem/mfem/blob/17955e114020af340e9a06a66ebef43e05012d9c/linalg/hypre.cpp*L5245__;Iw!!G_uCfscf7eWS!fdj4AzKhAcDmM1x_ZQ8gxqWeX6BAKY9urnvATMpT7hC8lw77ak7tqxqXGIX3PMg2wYA5PGu7EzyCW0wVEi33Pw$
> >>
> >> It's still classical AMG, and in my experience, struggles on very thin
> >> structures (e.g., aspect ratio 1000 cantilever beams) when compared to
> SA.
> >> However, it can be quite competitive for many structures. I found that
> the
> >> "MFEM elasticity suite", which is based on Baker et al 2010, gave rather
> >> poor results. This is a configuration that works on GPUs and gives good
> >> convergence and performance for elasticity:
> >>
> >>
> >>
> https://urldefense.us/v3/__https://github.com/hypre-space/hypre/issues/601*issuecomment-1069426997__;Iw!!G_uCfscf7eWS!arUVBVKKcYs1M5OhNqqRZl2b2o0NIUkG7fV_22qBbg-ssHhhHazhkpMbYNjCOTN66Sfbk-VZilfox9bxDf0$
> >>
> >> In the above issue, I was only using BoomerAMG as a coarse level for
> p-MG
> >> so all the options have a `-mg_coarse_` prefix; here are those options
> >> without the prefix:
> >>
> >> -pc_hypre_boomeramg_coarsen_type pmis
> >> -pc_hypre_boomeramg_interp_type ext+i
> >> -pc_hypre_boomeramg_no_CF
> >> -pc_hypre_boomeramg_P_max 6
> >> -pc_hypre_boomeramg_print_statistics 1
> >> -pc_hypre_boomeramg_relax_type_down Chebyshev
> >> -pc_hypre_boomeramg_relax_type_up Chebyshev
> >> -pc_hypre_boomeramg_strong_threshold 0.5
> >> -pc_type hypre
> >>
> >
> >
> > --
> > --Amneet
>


-- 
--Amneet
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