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<div class="moz-cite-prefix">On 09/23/2013 09:24 AM, Mark F. Adams
wrote:<br>
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<div>On Sep 23, 2013, at 11:55 AM, Michele Rosso <<a
moz-do-not-send="true" href="mailto:mrosso@uci.edu">mrosso@uci.edu</a>>
wrote:</div>
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<div bgcolor="#FFFFFF" text="#000000"> Hi,<br>
<br>
<font face="Ubuntu">I am successfully using PETSc to solve a
3D Poisson's equation with CG + MG </font>. Such
equation arises from a projection algorithm for a multiphase
incompressible flow simulation.<br>
I set up the solver <font face="Ubuntu">as I was suggested
to do in a previous thread</font> (title: "GAMG speed")
and run a test case (liquid droplet with surface tension
falling under the effect of gravity in a quiescent fluid). <br>
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.<br>
Since the solution of the Poisson equation is
mission-critical, I need to speed it up as much as I can.<br>
Could you please help me out with this?<br>
<br>
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<div>Just to add, it is not unusual as structures develop in
your simulation to see an increase in iterations. If you plot
the material coefficients I suspect that you will see sharper
structures developing in time. Can you look at this in some
way or generate some statistics like (max) gradient of
material coefficients or even just max & min coefficients?</div>
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Yes, I suspected that this may have been a reason. Initially both
phases are at rest, so basically no fluid structures are present,
thus the convergence is very fast(6 iterations maximum). Then
vortices start developing both inside and outside the droplet and at
that point the number of iteration increases. I noticed that the
convergence rate becomes almost steady at a certain point, I guess
when the flow is fully developed. I would like to speed up the solve
for this last scenario.<br>
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<blockquote cite="mid:F1C2C139-0470-414F-884B-55687E561AAD@lbl.gov"
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<div bgcolor="#FFFFFF" text="#000000"> I run the test case
with the following options: <br>
<br>
-pc_type mg -pc_mg_galerkin -pc_mg_levels 5
-mg_levels_ksp_type richardson -mg_levels_ksp_max_it 1 <br>
-mg_coarse_pc_type lu
-mg_coarse_pc_factor_mat_solver_package superlu_dist <br>
-log_summary -ksp_view -ksp_monitor_true_residual
-options_left <br>
<br>
Please find the diagnostic for the final solve in the
attached file "final.txt'. <br>
Thank you, <br>
<br>
Michele<br>
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<span><final.txt></span></blockquote>
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