[petsc-users] Rigid body nullspace for Stokes operator

[Amneet Bhalla]

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
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