[petsc-users] how to speed up convergence?

[Konstantinos Kontzialis]

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?

Thank you,

Kostas