On Thu, Feb 5, 2015 at 6:15 PM, Fabian Gabel <gabel.fab...@gmail.com> wrote:
> On Do, 2015-02-05 at 16:45 -0600, Matthew Knepley wrote:
> > On Thu, Feb 5, 2015 at 4:15 PM, Fabian Gabel <gabel.fab...@gmail.com>
> > wrote:
> > Thank you for your feedback.
> >
> > > -coupledsolve_pc_type fieldsplit
> > > -coupledsolve_pc_fieldsplit_0_fields 0,1,2
> > > -coupledsolve_pc_fieldsplit_1_fields 3
> > > -coupledsolve_pc_fieldsplit_type schur
> > > -coupledsolve_pc_fieldsplit_block_size 4
> > > -coupledsolve_fieldsplit_0_ksp_converged_reason
> > > -coupledsolve_fieldsplit_1_ksp_converged_reason
> > > -coupledsolve_fieldsplit_0_ksp_type gmres
> > > -coupledsolve_fieldsplit_0_pc_type fieldsplit
> > > -coupledsolve_fieldsplit_0_pc_fieldsplit_block_size
> > 3
> > > -coupledsolve_fieldsplit_0_fieldsplit_0_pc_type ml
> > > -coupledsolve_fieldsplit_0_fieldsplit_1_pc_type ml
> > > -coupledsolve_fieldsplit_0_fieldsplit_2_pc_type ml
> > >
> > > Is it normal, that I have to explicitly specify the
> > block size
> > > for each
> > > fieldsplit?
> > >
> > >
> > > No. You should be able to just specify
> > >
> > >
> > > -coupledsolve_fieldsplit_ksp_converged
> > > -coupledsolve_fieldsplit_0_fieldsplit_pc_type ml
> > >
> > >
> > > and same options will be applied to all splits (0,1,2).
> > >
> > > Does this functionality not work?
> > >
> > >
> > It does work indeed, but what I actually was referring to, was
> > the use
> > of
> >
> > -coupledsolve_pc_fieldsplit_block_size 4
> > -coupledsolve_fieldsplit_0_pc_fieldsplit_block_size 3
> >
> > Without them, I get the error message
> >
> > [0]PETSC ERROR: PCFieldSplitSetDefaults() line 468
> > in /work/build/petsc/src/ksp/pc/impls/fieldsplit/fieldsplit.c
> > Unhandled
> > case, must have at least two fields, not 1
> >
> > I thought PETSc would already know, what I want to do, since I
> > initialized the fieldsplit with
> >
> > CALL PCFieldSplitSetIS(PRECON,PETSC_NULL_CHARACTER,ISU,IERR)
> >
> > etc.
> > >
> > >
> >
> > > Are there any guidelines to follow that I could use
> > to avoid
> > > taking wild
> > > guesses?
> > >
> > >
> > > Sure. There are lots of papers published on how to construct
> > robust
> > > block preconditioners for saddle point problems arising from
> > Navier
> > > Stokes.
> > > I would start by looking at this book:
> > >
> > >
> > > Finite Elements and Fast Iterative Solvers
> > >
> > > Howard Elman, David Silvester and Andy Wathen
> > >
> > > Oxford University Press
> > >
> > > See chapters 6 and 8.
> > >
> > As a matter of fact I spent the last days digging through
> > papers on the
> > regard of preconditioners or approximate Schur complements and
> > the names
> > Elman and Silvester have come up quite often.
> >
> > The problem I experience is, that, except for one publication,
> > all the
> > other ones I checked deal with finite element formulations.
> > Only
> >
> > Klaij, C. and Vuik, C. SIMPLE-type preconditioners for
> > cell-centered,
> > colocated finite volume discretization of incompressible
> > Reynolds-averaged Navier–Stokes equations
> >
> > presented an approach for finite volume methods. Furthermore,
> > a lot of
> > literature is found on saddle point problems, since the linear
> > system
> > from stable finite element formulations comes with a 0 block
> > as pressure
> > matrix. I'm not sure how I can benefit from the work that has
> > already
> > been done for finite element methods, since I neither use
> > finite
> > elements nor I am trying to solve a saddle point problem (?).
> >
> >
> > I believe the operator estimates for FV are very similar to first
> > order FEM,
>
> Ok, so you would suggest to just discretize the operators differently
> (FVM instead of FEM discretization)?
>
I thought you were using FV.
> > and
> > I believe that you do have a saddle-point system in that there are
> > both positive
> > and negative eigenvalues.
>
> A first test on a small system in Matlab shows, that my system matrix is
> positive semi-definite but I am not sure how this result could be
> derived in general form from the discretization approach I used.
>
You can always make it definite by adding a large enough A_pp. I thought
the penalization would be small.
Thanks,
Matt
> >
> > Thanks,
> >
> >
> > Matt
> >
> > >
> > > > Petsc has some support to generate approximate
> > pressure
> > > schur
> > > > complements for you, but these will not be as good
> > as the
> > > ones
> > > > specifically constructed for you particular
> > discretization.
> > >
> > > I came across a tutorial
> > (/snes/examples/tutorials/ex70.c),
> > > which shows
> > > 2 different approaches:
> > >
> > > 1- provide a Preconditioner \hat{S}p for the
> > approximation of
> > > the true
> > > Schur complement
> > >
> > > 2- use another Matrix (in this case its the Matrix
> > used for
> > > constructing
> > > the preconditioner in the former approach) as a new
> > > approximation of the
> > > Schur complement.
> > >
> > > Speaking in terms of the PETSc-manual p.87, looking
> > at the
> > > factorization
> > > of the Schur field split preconditioner, approach 1
> > sets
> > > \hat{S}p while
> > > approach 2 furthermore sets \hat{S}. Is this
> > correct?
> > >
> > >
> > >
> > > No this is not correct.
> > > \hat{S} is always constructed by PETSc as
> > > \hat{S} = A11 - A10 KSP(A00) A01
> >
> > But then what happens in this line from the
> > tutorial /snes/examples/tutorials/ex70.c
> >
> > ierr = KSPSetOperators(subksp[1], s->myS,
> > s->myS);CHKERRQ(ierr);
> >
> > It think the approximate Schur complement a (Matrix of type
> > Schur) gets
> > replaced by an explicitely formed Matrix (myS, of type
> > MPIAIJ).
> > >
> > > You have two choices in how to define the preconditioned,
> > \hat{S_p}:
> > >
> > > [1] Assemble you own matrix (as is done in ex70)
> > >
> > > [2] Let PETSc build one. PETSc does this according to
> > >
> > > \hat{S_p} = A11 - A10 inv(diag(A00)) A01
> > >
> > Regards,
> > Fabian
> > >
> > >
> >
> >
> >
> >
> >
> >
> > --
> > 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
