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

[Bishesh Khanal]

On Fri, Jul 26, 2013 at 2:32 PM, Matthew Knepley <knepley at gmail.com> wrote:

> On Fri, Jul 26, 2013 at 7:28 AM, Bishesh Khanal <bisheshkh at gmail.com>wrote:
>
>>
>>
>>
>> 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:
>>
>
> You want Algebraic Multigrid
>
> -pc_type fieldsplit -pc_fieldsplit_detect_saddle_point
> -pc_fieldsplit_type schur
>   -fieldsplit_0_pc_type gamg
>   -fieldsplit_1_ksp_type fgmres
>    -fieldsplit_1_ksp_constant_null_space
>    -fieldsplit_1_ksp_monitor_short
>    -fieldsplit_1_pc_type lsc
> -ksp_converged_reason
>
> I tried the above set of options but the solution I get seem to be not
correct. The velocity field I get are quite different than the one I got
before without using gamg which were the expected one.
Note: (Also, I had to add one extra option of
-fieldsplit_1_ksp_gmres_restart 100 , because the fieldsplit_1_ksp residual
norm did not converge within default 30 iterations before restarting).


>    Matt
>
>
>> 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
>>
>>
>>
>
>
> --
> 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
>
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