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

[Matthew Knepley]

On Fri, Jul 26, 2013 at 9:11 AM, Bishesh Khanal <bisheshkh at gmail.com> wrote:

>
>
>
> 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).
>

These are all iterative solvers. You have to make sure everything
converges. I do this problem in the tutorial with
a constant viscosity.

    Matt


>     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
>>
>
>


-- 
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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