[petsc-users] how to speed up convergence?

[Matthew Knepley]

On Thu, Nov 10, 2011 at 4:25 PM, Konstantinos Kontzialis <
ckontzialis at lycos.com> wrote:

> Dear all,
>
>  I use the DG method for simulationg the flow over a cylinder at M = 0.2
> and Re=250. I use and implicit scheme.
>
> I run my code as follows:
>
> mpiexec -n 8 ./hoac cylinder -snes_mf_operator -llf_flux -n_out 2
> -end_time 0.4 -implicit -pc_type asm -sub_pc_type ilu
> -sub_pc_factor_mat_ordering_**type rcm -sub_pc_factor_reuse_ordering
> -sub_pc_factor_reuse_fill -gll -ksp_type fgmres -sub_pc_factor_levels 0
> -snes_monitor -snes_converged_reason -ksp_converged_reason -ts_view
> -ksp_pc_side right -sub_pc_factor_nonzeros_along_**diagonal -dt 1.0e-3
> -ts_type arkimex -ksp_gmres_restart 100 -ksp_max_it 500 -snes_max_fail 100
> -snes_max_linear_solve_fail 100
>
> and I get:
>
> **************************************************************************
>  METIS 4.0.3 Copyright 1998, Regents of the University of Minnesota
>
> Graph Information ------------------------------**---------------------
>  Name: mesh.graph, #Vertices: 1680, #Edges: 3280, #Parts: 8
>
> Recursive Partitioning... ------------------------------**-------------
>  8-way Edge-Cut:     177, Balance:  1.01
>
> Timing Information ------------------------------**--------------------
>  I/O:                     0.000
>  Partitioning:            0.000   (PMETIS time)
>  Total:                   0.000
> **************************************************************************
>
>
> Approximation order = 2
> # DOF = 115200
> # nodes in mesh = 1680
> # elements in mesh = 1600
> Navier-Stokes solution
> Using LLF flux
>
>
> Linear solve converged due to CONVERGED_RTOL iterations 1
>
>
> Timestep   0: dt = 0.001, T = 0, Res[rho] = 0.966549, Res[rhou] = 6.11366,
> Res[rhov] = 0.507325, Res[E] = 2.44463, CFL = 0.942045
>    0 SNES Function norm 3.203604511352e+03
>    Linear solve did not converge due to DIVERGED_ITS iterations 500
>    1 SNES Function norm 3.440800722147e+02
>    Linear solve did not converge due to DIVERGED_ITS iterations 500
>    2 SNES Function norm 2.008355246473e+02
>    Linear solve did not converge due to DIVERGED_ITS iterations 500
>    3 SNES Function norm 1.177925999321e+02
>
> as you may see the step size is quite small for this problem and I use and
> inexact solution for the linear part of the newton iterations. Furthermore,
> I compute numerically the jacobian of the matrix using coloring.
>
> Is there a tuning parameter I should set differently or use something else?
>

ASM really stinks for incompressible flow. Consider using PCFIELDSPLIT.

  Matt


> Thank you,
>
> Kostas
>



-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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