[petsc-users] How to speed up geometric multigrid

[Michele Rosso]

The boundary conditions are periodic.
The equation I am solving is:

        div(beta*grad(u))= f

where beta is 1 inside the gas phase, 0.001 inside the liquid phase and 
a value in between for the nodes close to the interface.
The system matrix is built so to remain symmetric positive defined 
despite the coefficients.

Michele


On 09/23/2013 09:11 AM, Matthew Knepley wrote:
> On Mon, Sep 23, 2013 at 8:55 AM, Michele Rosso <mrosso at uci.edu 
> <mailto:mrosso at uci.edu>> wrote:
>
>     Hi,
>
>     I am successfully using PETSc to solve a 3D Poisson's equation
>     with CG + MG . Such equation arises from a projection algorithm
>     for a multiphase incompressible flow simulation.
>     I set up the solver as I was suggested to do in a previous thread
>     (title: "GAMG speed") and run a test case (liquid droplet with
>     surface tension falling under the effect of gravity in a quiescent
>     fluid).
>     The solution of the Poisson Equation via multigrid is correct but
>     it becomes progressively slower and slower as the simulation
>     progresses (I am performing successive solves) due to an increase
>     in the number of iterations.
>     Since the solution of the Poisson equation is mission-critical, I
>     need to speed it up as much as I can.
>     Could you please help me out with this?
>
>
> First, what does the coefficient look like?
>
> Second, what are the boundary conditions?
>
>    Matt
>
>     I run the test case with the following options:
>
>     -pc_type mg  -pc_mg_galerkin  -pc_mg_levels 5 -mg_levels_ksp_type
>     richardson -mg_levels_ksp_max_it 1
>     -mg_coarse_pc_type lu -mg_coarse_pc_factor_mat_solver_package
>     superlu_dist
>     -log_summary -ksp_view  -ksp_monitor_true_residual -options_left
>
>     Please find the diagnostic for the final solve in the attached
>     file "final.txt'.
>     Thank you,
>
>     Michele
>
>
>
>
> -- 
> 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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