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Jed,<br>
<br>
thank you very much!<br>
I will try with <i> </i><i>-mg_levels_ksp_type chebyshev
-mg_levels_pc_type sor</i> and report back.<br>
Yes, I removed the nullspace from both the system matrix and the
rhs.<br>
Is there a way to have something similar to Dendy's multigrid or the
deflated conjugate gradient method with PETSc?<br>
Thank you,<br>
<br>
Michele<br>
<br>
<br>
<br>
<i></i>
<div class="moz-cite-prefix">On 05/19/2014 03:30 PM, Jed Brown
wrote:<br>
</div>
<blockquote cite="mid:8761l1qu4x.fsf@jedbrown.org" type="cite">
<pre wrap="">Michele Rosso <a class="moz-txt-link-rfc2396E" href="mailto:mrosso@uci.edu"><mrosso@uci.edu></a> writes:
</pre>
<blockquote type="cite">
<pre wrap="">Jed,
thanks for your reply.
By using the options you suggested, namely /-mg_levels_ksp_type
richardson -mg_levels_pc_type sor/, I was able to
solve without bumping into the DIVERGED_INDEFINITE_PC message.
Nevertheless, the number of iterations increases drastically as the
simulation progresses.
</pre>
</blockquote>
<pre wrap="">
What about SOR with Chebyshev? (A little weird, but sometimes it's a
good choice.) If the solve is expensive, you can add a few more
iterations for eigenvalue estimation.
</pre>
<blockquote type="cite">
<pre wrap="">The Poisson's equation I am solving arises from a variable-density
projection method for incompressible multi-phase flows.
At each time step the system matrix coefficients change as a consequence
of the change in location of the heavier phase; the rhs changes
in time because of the change in the velocity field. Usually the
black-box multigrid or the deflated conjugate gradient method are used
to solve efficiently this type of problem: it is my understanding -
please correct me if I am wrong - that AMG is a generalization of the
former.
</pre>
</blockquote>
<pre wrap="">
Dendy's "black-box MG" is a semi-geometric method for cell-centered
discretizations. AMG is not a superset or subset of those methods.
</pre>
<blockquote type="cite">
<pre wrap="">The only source term acting is gravity; the hydrostatic pressure is
removed from the governing equation in order to accommodate periodic
boundary conditions: this is more a hack than a clean solution. Could it
be the reason behind the poor performances/ DIVERGED_INDEFINITE_PC
problem I am experiencing?
</pre>
</blockquote>
<pre wrap="">
If you have periodic boundary conditions, then you also have a pressure
null space. Have you removed the null space from the RHS and supplied
the null space to the solver?
</pre>
</blockquote>
<br>
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