On Thu, Jun 14, 2012 at 9:01 PM, Thomas Witkowski < thomas.witkowski at tu-dresden.de> wrote:
> I try to implement an efficient solver for my FEM based unsteady Navier > Stokes code. The scenario I consider is realy simple: 2D FEM with > Taylor-Hood element, equi-spaced grid, simple channel flow with prescribed > inflow boundaries for the velocity, no boundary conditions for the > pressure. Because I want to With no pressure BC, you need to specify the null space. Look at SNES ex62. > solve unsteady flows, at each timepoint an Ossen problem is solved > (implicit Euler time discretization, "trivial" linearization). For using > PETSc, I created a fieldsplit preconditioner, that is configured in the > following way: > > mpirun -np 2 ./navier_stokes init/channel_parallel.dat.2d \ > -ksp_type fgmres \ > -pc_type fieldsplit \ > -pc_fieldsplit_type SCHUR \ > -pc_fieldsplit_schur_fact_type FULL \ > -ksp_converged_reason \ > -ksp_monitor_true_residual \ > -fieldsplit_velocity_ksp_type preonly \ > -fieldsplit_velocity_pc_type lu \ > -fieldsplit_velocity_pc_**factor_mat_solver_package mumps \ > -fieldsplit_pressure_ksp_type gmres \ > -fieldsplit_pressure_ksp_max_**it 10 \ > -fieldsplit_pressure_ksp_rtol 1.0e-2 \ > -fieldsplit_pressure_pc_type lsc \ > This makes no sense unless you specify an auxiliary operator. Just leave it at jacobi. When you use LU for velocity, it will converge in 1 iterate. Since it doesn't, it means you have a problem with the null space. Matt > -fieldsplit_pressure_lsc_ksp_**type bcgs \ > -fieldsplit_pressure_lsc_ksp_**max_it 10 \ > -fieldsplit_pressure_lsc_ksp_**rtol 1.0e-2 \ > -fieldsplit_pressure_lsc_pc_**type hypre > > The direct solver for the velocity part is just for debugging and should > be replaced when everything else works fine. The point is, that I found > this constellation to be not very efficient. It takes around 20 to 30 > iterations, which takes around 30 seconds on a very small problem size > (around 20000 global unknows for each velocity component and 5000 global > unknowns for the pressure) on a very fast desktop CPU (some new Intel Xeno > with 4 core). Any hints for improvements? > > Thomas > -- 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 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20120614/889d7f34/attachment.html>
