<html><head><meta http-equiv="Content-Type" content="text/html charset=iso-8859-1"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><br><div><div>On Sep 23, 2013, at 12:27 PM, Michele Rosso <<a href="mailto:mrosso@uci.edu">mrosso@uci.edu</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite">
<meta content="text/html; charset=ISO-8859-1" http-equiv="Content-Type">
<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></div></blockquote><div><br></div><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><br><blockquote type="cite"><div bgcolor="#FFFFFF" text="#000000">
The system matrix is built so to remain symmetric positive defined
despite the coefficients.<br>
<br>
Michele<br>
<br>
<br>
<div class="moz-cite-prefix">On 09/23/2013 09:11 AM, Matthew Knepley
wrote:<br>
</div>
<blockquote cite="mid:CAMYG4G=iy3OLnFsFx33afmA5KmAyoj9aJBxuHvnh-mRrk7KUKA@mail.gmail.com" type="cite">
<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>
<div class="gmail_extra">
<div class="gmail_quote">
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<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>
</div>
</blockquote>
<div><br>
</div>
<div>First, what does the coefficient look like?</div>
<div><br>
</div>
<div>Second, what are the boundary conditions?</div>
<div><br>
</div>
<div>
Matt</div>
<div> </div>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<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>
</div>
</blockquote>
</div>
<br>
<br clear="all">
<div><br>
</div>
-- <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>
</div>
</blockquote>
<br>
</div>
</blockquote></div><br></body></html>