On 09/23/2013 09:48 AM, Mark F. Adams wrote:
On Sep 23, 2013, at 12:27 PM, Michele Rosso <[email protected]
<mailto:[email protected]>> wrote:
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.
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.
The simulation does start with both phases and the geometry is supposed
to become more complex as the simulation progresses.
But so far the run is stopped before there are significant changes in
the shape of the droplet.
I can give a shot to AMG: which options would you suggest to use.
Also, how can I project out the constant from the rhs? Thanks a lot!
Michele
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 <[email protected]
<mailto:[email protected]>> 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
