[petsc-users] Some hypre settings for 3D fully coupled incompressible NS

[Edoardo alinovi]

Hello petsc friends,

It's been a while since I am trying to find a good setup for my coupled
solver.

Recently, I have run a scan with Dakota (more than 1k simulations) on the
Windsor body case with 7Mln cells on 36 cores on my small home server (Dell
R730 with 2x2496 v4 xeon). I thought it was a good idea to share my results
with the community!

Here is a resume of my finding:

1) Multiplicative is faster than Schur:  I have found out that Schur
preconditioner is rarely faster than multiplicative despite the fact Schur
keeps the number of iterations lower. I think there is a lot of room for
improvement as far as FV matrices are concerned. Probably custom Shat is
the way to go, but not easy to find a good one! Up to now "selfp" looks to
be the only good and "ready to go" choice.

2) Vanilla fbcgs is faster than vanilla fgmres: maybe here we can tune
gmres restart, I have not tried this systematically.

3) Stick with preonly: using bcgs/cg as preconditioner ksp lowers the
number of iterations but it adds up a lot of overhead (even setting few
iterations or mild tolerances).

4) Staging is a good idea: beyond bare iteration performance, I think that
for steady state problems it worth setting a max for outer iterations in
fieldsplit, as starting iterations would cost you a lot and probably you
will be far from convergence anyway at the stage, so it is not a good
investment pushing hard on them.

5) Here my best so far settings:

    # Outer solver settings
     "solver": "fbcgs",
     "preconditioner": "fieldsplit",
     "absTol": 1e-6,
     "relTol": 0.01,

      # Field split KSP and PC
      "fieldsplit_u_pc_type": "bjacobi",
      "fieldsplit_p_pc_type": "hypre",
      "fieldsplit_u_ksp_type": "preonly",
      "fieldsplit_p_ksp_type": "preonly",

       ! HYPRE PC options
       "fieldsplit_p_pc_hypre_boomeramg_strong_threshold": 0.05,
       "fieldsplit_p_pc_hypre_boomeramg_coarsen_type": "PMIS",
       "fieldsplit_p_pc_hypre_boomeramg_truncfactor": 0.3,
       "fieldsplit_p_pc_hypre_boomeramg_no_cf": 0,
       "fieldsplit_p_pc_hypre_boomeramg_agg_nl": 1,
       "fieldsplit_p_pc_hypre_boomeramg_agg_num_paths": 1,
       "fieldsplit_p_pc_hypre_boomeramg_P_max": 0,
       "fieldsplit_p_pc_hypre_boomeramg_max_levels": 30,
       "fieldsplit_p_pc_hypre_boomeramg_relax_type_all":
"backward-SOR/Jacobi",
       "fieldsplit_p_pc_hypre_boomeramg_interp_type": "ext+i",
       "fieldsplit_p_pc_hypre_boomeramg_grid_sweeps_down": 0,
       "fieldsplit_p_pc_hypre_boomeramg_grid_sweeps_up": 2,
       "fieldsplit_p_pc_hypre_boomeramg_cycle_type": "v"

I have a question for Barry/Jed/Matt. I have noted that most of the
commercial solvers use what I define as "SAMG with ILU smoother". I am
wondering if there's a way to reproduce this in Petsc.  I have tried
PCPATCH to test VANKA, but I am not really able to use that PC as I am not
using DMplex. With this recipe I am not miles away from Fluent on the same
problem. Yet, I am wondering why commercial solvers do not use fieldsplit.

Hope this can be helpful and of course I am happy to collaborate on this
topic if someone outhere is willing to!

Cheers,

Edoardo
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