[petsc-users] discontinuous viscosity stokes equation 3D staggered grid

[Bishesh Khanal]

On Wed, Jul 17, 2013 at 9:48 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> Bishesh Khanal <bisheshkh at gmail.com> writes:
>
> > Now, I implemented two different approaches, each for both 2D and 3D, in
> > MATLAB. It works for the smaller sizes but I have problems solving it for
> > the problem size I need (250^3 grid size).
> > I use staggered grid with p on cell centers, and components of v on cell
> > faces. Similar split up of K to cell center and faces to account for the
> > variable viscosity case)
>
> Okay, you're using a staggered-grid finite difference discretization of
> variable-viscosity Stokes.  This is a common problem and I recommend
> starting with PCFieldSplit with Schur complement reduction (make that
> work first, then switch to block preconditioner).


Ok, I made my 3D problem work with PCFieldSplit with Schur complement
reduction using the options:
-pc_fieldsplit_type schur -pc_fieldsplit_detect_saddle_point
-fieldsplit_1_ksp_constant_null_space


You can use PCLSC or
> (probably better for you), assemble a preconditioning matrix containing
> the inverse viscosity in the pressure-pressure block.  This diagonal
> matrix is a spectrally equivalent (or nearly so, depending on
> discretization) approximation of the Schur complement.  The velocity
> block can be solved with algebraic multigrid.  Read the PCFieldSplit
> docs (follow papers as appropriate) and let us know if you get stuck.
>

Now,  I got a little confused in how exactly to use command line options to
use multigrid for the velocity bock and PCLS for the pressure block. After
going through the manual I tried the following:

-pc_type fieldsplit -pc_fieldsplit_type schur
-pc_fieldsplit_detect_saddle_point -fieldsplit_0_pc_type mg
-fieldsplit_0_pc_mg_levels 2 -fieldsplit_0_pc_mg_galerkin
-fieldsplit_1_ksp_type fgmres -fieldsplit_1_ksp_constant_null_space
-fieldsplit_1_ksp_monitor_short -fieldsplit_1_pc_type lsc
-ksp_converged_reason

but I get the following errror:

[0]PETSC ERROR: --------------------- Error Message
------------------------------------
[0]PETSC ERROR: Null argument, when expecting valid pointer!
[0]PETSC ERROR: Null Object: Parameter # 2!
[0]PETSC ERROR:
------------------------------------------------------------------------
[0]PETSC ERROR: Petsc Release Version 3.4.2, Jul, 02, 2013
[0]PETSC ERROR: See docs/changes/index.html for recent updates.
[0]PETSC ERROR: See docs/faq.html for hints about trouble shooting.
[0]PETSC ERROR: See docs/index.html for manual pages.
[0]PETSC ERROR:
------------------------------------------------------------------------
[0]PETSC ERROR: src/AdLemMain on a arch-linux2-cxx-debug named edwards by
bkhanal Fri Jul 26 14:23:40 2013
[0]PETSC ERROR: Libraries linked from
/home/bkhanal/Documents/softwares/petsc-3.4.2/arch-linux2-cxx-debug/lib
[0]PETSC ERROR: Configure run at Fri Jul 19 14:25:01 2013
[0]PETSC ERROR: Configure options --with-cc=gcc --with-fc=g77
--with-cxx=g++ --download-f-blas-lapack=1 --download-mpich=1
-with-clanguage=cxx --download-hypre=1
[0]PETSC ERROR:
------------------------------------------------------------------------
[0]PETSC ERROR: MatPtAP() line 8166 in
/home/bkhanal/Documents/softwares/petsc-3.4.2/src/mat/interface/matrix.c
[0]PETSC ERROR: PCSetUp_MG() line 628 in
/home/bkhanal/Documents/softwares/petsc-3.4.2/src/ksp/pc/impls/mg/mg.c
[0]PETSC ERROR: PCSetUp() line 890 in
/home/bkhanal/Documents/softwares/petsc-3.4.2/src/ksp/pc/interface/precon.c
[0]PETSC ERROR: KSPSetUp() line 278 in
/home/bkhanal/Documents/softwares/petsc-3.4.2/src/ksp/ksp/interface/itfunc.c
[0]PETSC ERROR: KSPSolve() line 399 in
/home/bkhanal/Documents/softwares/petsc-3.4.2/src/ksp/ksp/interface/itfunc.c
[0]PETSC ERROR: PCApply_FieldSplit_Schur() line 807 in
/home/bkhanal/Documents/softwares/petsc-3.4.2/src/ksp/pc/impls/fieldsplit/fieldsplit.c
[0]PETSC ERROR: PCApply() line 442 in
/home/bkhanal/Documents/softwares/petsc-3.4.2/src/ksp/pc/interface/precon.c
[0]PETSC ERROR: KSP_PCApply() line 227 in
/home/bkhanal/Documents/softwares/petsc-3.4.2/include/petsc-private/kspimpl.h
[0]PETSC ERROR: KSPInitialResidual() line 64 in
/home/bkhanal/Documents/softwares/petsc-3.4.2/src/ksp/ksp/interface/itres.c
[0]PETSC ERROR: KSPSolve_GMRES() line 239 in
/home/bkhanal/Documents/softwares/petsc-3.4.2/src/ksp/ksp/impls/gmres/gmres.c
[0]PETSC ERROR: KSPSolve() line 441 in
/home/bkhanal/Documents/softwares/petsc-3.4.2/src/ksp/ksp/interface/itfunc.c
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20130726/7f3cca8b/attachment.html>