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

[Matthew Knepley]

On Thu, Jul 18, 2013 at 12:51 PM, Bishesh Khanal <bisheshkh at gmail.com>wrote:

> Thanks Jed. I implemented a 2D case which worked when running the program
> with the options: -pc_type fieldsplit -pc_fieldsplit_type schur
> -pc_fieldsplit_detect_saddle_point
> But I encountered some problems when using following options (this set of
> options is based on one of the tutorial slides in the website for
> multiphysics problem):
> -ksp_type fgmres -pc_type mg -mg_levels_ksp_type fgmres
> -mg_levels_ksp_max_it 2 -mg_levels_pc_type fieldsplit
> -mg_levels_pc_fieldsplit_detect_saddle_point ‐mg_levels_pc_fieldsplit_type
> schur -mg_levels_pc_fieldsplit_factorization_type full
> -mg_levels_pc_fieldsplit_schur_precondition user
> -mg_levels_fieldsplit_0_ksp_type preonly -mg_levels_fieldsplit_0_pc_type
> sor -mg_levels_fieldsplit_0_pc_sor_forward -mg_levels_fieldsplit_0_ksp_type
> gmres -mg_levels_fieldsplit_0_pc_type none -mg_levels_fieldsplit_ksp_max_it
> 5 -mg_coarse_pc_type svd
>
> The relevant error messages:
> [1]PETSC ERROR: --------------------- Error Message
> ------------------------------------
> [1]PETSC ERROR: Invalid argument!
> [1]PETSC ERROR: Unknown logical value: ‐mg_levels_pc_fieldsplit_type!
> [1]PETSC ERROR:
> ------------------------------------------------------------------------
> [1]PETSC ERROR: Petsc Release Version 3.4.1, Jun, 10, 2013
>  ...
> [1]PETSC ERROR: PetscOptionsStringToBool() line 173 in
> /home/bkhanal/Documents/softwares/petsc-3.4.1/src/sys/objects/options.c
> [1]PETSC ERROR: PetscOptionsGetBool() line 1530 in
> /home/bkhanal/Documents/softwares/petsc-3.4.1/src/sys/objects/options.c
> [1]PETSC ERROR: PCSetFromOptions_FieldSplit() line 1060 in
> /home/bkhanal/Documents/softwares/petsc-3.4.1/src/ksp/pc/impls/fieldsplit/fieldsplit.c
>

These line numbers do not match the source. How did you install?

https://bitbucket.org/petsc/petsc/src/160ea6873d9fa631d01b6f8a2d8b12aece7dfb61/src/ksp/pc/impls/fieldsplit/fieldsplit.c?at=maint#cl-1060

   Matt


> [1]PETSC ERROR: PCSetFromOptions() line 174 in
> /home/bkhanal/Documents/softwares/petsc-3.4.1/src/ksp/pc/interface/pcset.c
> [1]PETSC ERROR: KSPSetFromOptions() line 357 in
> /home/bkhanal/Documents/softwares/petsc-3.4.1/src/ksp/ksp/interface/itcl.c
> [1]PETSC ERROR: PCSetUp_MG() line 677 in
> /home/bkhanal/Documents/softwares/petsc-3.4.1/src/ksp/pc/impls/mg/mg.c
> [1]PETSC ERROR: PCSetUp() line 890 in
> /home/bkhanal/Documents/softwares/petsc-3.4.1/src/ksp/pc/interface/precon.c
> [1]PETSC ERROR: KSPSetUp() line 278 in
> /home/bkhanal/Documents/softwares/petsc-3.4.1/src/ksp/ksp/interface/itfunc.c
> ...
> And the warnings:
>
> WARNING! There are options you set that were not used!
> WARNING! could be spelling mistake, etc!
> Option left: name:-mg_coarse_pc_type value: svd
> Option left: name:-mg_levels_fieldsplit_0_ksp_type value: gmres
> Option left: name:-mg_levels_fieldsplit_0_pc_sor_forward (no value)
> Option left: name:-mg_levels_fieldsplit_0_pc_type value: none
> Option left: name:-mg_levels_fieldsplit_ksp_max_it value: 5
> Option left: name:-mg_levels_ksp_max_it value: 2
> Option left: name:-mg_levels_ksp_type value: fgmres
> Option left: name:-mg_levels_pc_fieldsplit_factorization_type value: full
> Option left: name:-mg_levels_pc_fieldsplit_schur_precondition value: user
>
> Is it that -mg_levels_pc_fieldsplit_type should be followed by some "bool"
> value ? 1 ? true ?
> I tried using "true", then it did not give any error but the following
> warnings:
> WARNING! There are options you set that were not used!
> WARNING! could be spelling mistake, etc!
> Option left: name:-mg_coarse_pc_type value: svd
> Option left: name:-mg_levels_fieldsplit_0_pc_sor_forward (no value)
> Option left: name:-mg_levels_pc_fieldsplit_factorization_type value: full
>
> I ran the program for the small size. Now, before I implement the 3D case
> with this approach instead of trying the implementation of Augmented
> Lagrangian method (M2 in previous email) my question is:
> If I use the Petsc with the method you suggested (PCFieldSplit, multigrid
> for momentum etc), tentatively how much of computing resources and the
> corresponding time would it require to solve the problem of the size I want
> (around 250^3 grid sized domain) ?
>
>
>
> 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).  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.
>>
>
>


-- 
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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