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<div class="moz-cite-prefix">On 09/23/2013 09:48 AM, Mark F. Adams
wrote:<br>
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<div>On Sep 23, 2013, at 12:27 PM, 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"> The boundary conditions
are periodic. <br>
The equation I am solving is:<br>
<br>
div(beta*grad(u))= f<br>
<br>
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.<br>
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<div>This is a pretty big jump for geometric MG. You might try
AMG. I suspect that the geometry is getting more complex as
the simulation progresses. Does the simulation start with
both phases? Also this problem is singular. You might try
projecting out the constant. It could be that as the geometry
gets more complex floating point errors are creeping in and
you are getting an effective constant component to your RHS.</div>
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The simulation does start with both phases and the geometry is
supposed to become more complex as the simulation progresses.<br>
But so far the run is stopped before there are significant changes
in the shape of the droplet. <br>
I can give a shot to AMG: which options would you suggest to use.<br>
Also, how can I project out the constant from the rhs? Thanks a lot!<br>
<br>
Michele<br>
<blockquote cite="mid:D6E1D3CE-5365-42AF-9668-F20085EEF6F6@lbl.gov"
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<div bgcolor="#FFFFFF" text="#000000"> The system matrix is
built so to remain symmetric positive defined despite the
coefficients.<br>
<br>
Michele<br>
<br>
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<div class="moz-cite-prefix">On 09/23/2013 09:11 AM, Matthew
Knepley wrote:<br>
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cite="mid:CAMYG4G=iy3OLnFsFx33afmA5KmAyoj9aJBxuHvnh-mRrk7KUKA@mail.gmail.com"
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<div dir="ltr">On Mon, Sep 23, 2013 at 8:55 AM, Michele
Rosso <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:mrosso@uci.edu" target="_blank">mrosso@uci.edu</a>></span>
wrote:<br>
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<blockquote class="gmail_quote" style="margin:0 0 0
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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>
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<div>First, what does the coefficient look like?</div>
<div><br>
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<div>Second, what are the boundary conditions?</div>
<div><br>
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<div> Matt</div>
<div> </div>
<blockquote class="gmail_quote" style="margin:0 0 0
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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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-- <br>
What most experimenters take for granted before they
begin their experiments is infinitely more interesting
than any results to which their experiments lead.<br>
-- Norbert Wiener </div>
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