First of all, yes you are correct that I am trying to solve the
stationary incompressible Navier Stokes equations.
On 02.05.2023 21:33, Matthew Knepley wrote:
On Tue, May 2, 2023 at 2:29 PM Jed Brown <j...@jedbrown.org
<mailto:j...@jedbrown.org>> wrote:
Sebastian Blauth <sebastian.bla...@itwm.fraunhofer.de
<mailto:sebastian.bla...@itwm.fraunhofer.de>> writes:
> I agree with your comment for the Stokes equations - for these, I
have
> already tried and used the pressure mass matrix as part of a
(additive)
> block preconditioner and it gave mesh independent results.
>
> However, for the Navier Stokes equations, is the Schur complement
really
> spectrally equivalent to the pressure mass matrix?
No, it's not. You'd want something like PCD (better, but not
algebraic) or LSC.
I would like to take a look at the LSC preconditioner. For this, I did
also not achieve mesh-independent results. I am using the following
options (I know that the tolerances are too high at the moment, but it
should just illustrate the behavior w.r.t. mesh refinement). Again, I am
using a simple 2D channel problem for testing purposes.
I am using the following options
-ksp_type fgmres
-ksp_gmres_restart 100
-ksp_gmres_cgs_refinement_type refine_ifneeded
-ksp_max_it 1000
-ksp_rtol 1e-10
-ksp_atol 1e-30
-pc_type fieldsplit
-pc_fieldsplit_type schur
-pc_fieldsplit_schur_fact_type full
-pc_fieldsplit_schur_precondition self
-fieldsplit_0_ksp_type preonly
-fieldsplit_0_pc_type lu
-fieldsplit_1_ksp_type gmres
-fieldsplit_1_ksp_pc_side right
-fieldsplit_1_ksp_gmres_restart 100
-fieldsplit_1_ksp_gmres_cgs_refinement_type refine_ifneeded
-fieldsplit_1_ksp_max_it 1000
-fieldsplit_1_ksp_rtol 1e-10
-fieldsplit_1_ksp_atol 1e-30
-fieldsplit_1_pc_type lsc
-fieldsplit_1_lsc_ksp_type preonly
-fieldsplit_1_lsc_pc_type lu
-fieldsplit_1_ksp_converged_reason
Again, the direct solvers are used so that only the influence of the LSC
preconditioner is seen. I have suitable preconditioners for all of these
available (using boomeramg).
At the bottom, I attach the output for different discretizations. As you
can see there, the number of iterations increases nearly linearly with
the problem size.
I think that the problem could occur due to a wrong scaling. In your
docs https://petsc.org/release/manualpages/PC/PCLSC/ , you write that
the LSC preconditioner is implemented as
inv(S) \approx inv(A10 A01) A10 A00 A01 inv(A10 A01)
However, in the book of Elman, Sylvester and Wathen (Finite Elements and
Fast Iterative Solvers), the LSC preconditioner is defined as
inv(S) \approx inv(A10 inv(T) A01) A10 inv(T) A00 inv(T) A01
inv(A10 inv(T) A01)
where T = diag(Q) and Q is the velocity mass matrix.
There is an options -pc_lsc_scale_diag, which states that it uses the
diagonal of A for scaling. I suppose, that this means, that the diagonal
of the A00 block is used for scaling - however, this is not the right
scaling, is it? Even in the source code for the LSC preconditioner, in
/src/ksp/pc/impls/lsc/lsc.c it is mentioned, that a mass matrix should
be used...
Is there any way to implement this in PETSc? Maybe by supplying the mass
matrix as Pmat?
Thanks a lot in advance,
Sebastian
I think you can do a better job than that using something like
https://arxiv.org/abs/1810.03315 <https://arxiv.org/abs/1810.03315>
Basically, you use an augmented Lagrangian thing to make the Schur
complement well-conditioned,
and then use a special smoother to handle that perturbation.
> And even if it is, the convergence is only good for small
Reynolds numbers, for moderately high ones the convergence really
deteriorates. This is why I am trying to make
fieldsplit_schur_precondition selfp work better (this is, if I
understand it correctly, a SIMPLE type preconditioner).
SIMPLE is for short time steps (not too far from resolving CFL) and
bad for steady. This taxonomy is useful, though the problems are
super academic and they don't use high aspect ratio.
Okay, I get that I cannot expect the SIMPLE preconditioning
(schur_precondition selfp) to work efficiently. I guess the reason it
works for small time steps (for the instationary equations) is that the
velocity block becomes diagonally dominant in this case, so that diag(A)
is a good approximation of A.
https://doi.org/10.1016/j.jcp.2007.09.026
<https://doi.org/10.1016/j.jcp.2007.09.026>
Thanks,
Matt
--
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
https://www.cse.buffalo.edu/~knepley/
<http://www.cse.buffalo.edu/~knepley/>
And here is the output of my scaling tests
8x8 discretization
Newton solver: iter, abs. residual (abs. tol), rel. residual (rel. tol)
Newton solver: 0, 1.023e+03 (1.00e-30), 1.000e+00 (1.00e-10)
Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations 38
Newton solver: 1, 1.313e+03 (1.00e-30), 1.283e+00 (1.00e-10)
Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations 76
Newton solver: 2, 1.198e+02 (1.00e-30), 1.171e-01 (1.00e-10)
Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations 74
Newton solver: 3, 7.249e-01 (1.00e-30), 7.084e-04 (1.00e-10)
Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations 74
Newton solver: 4, 3.883e-05 (1.00e-30), 3.795e-08 (1.00e-10)
Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations 74
Newton solver: 5, 2.778e-12 (1.00e-30), 2.714e-15 (1.00e-10)
16x16 discretization
Newton solver: iter, abs. residual (abs. tol), rel. residual (rel. tol)
Newton solver: 0, 1.113e+03 (1.00e-30), 1.000e+00 (1.00e-10)
Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations 62
Newton solver: 1, 8.316e+02 (1.00e-30), 7.475e-01 (1.00e-10)
Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations
141
Newton solver: 2, 5.806e+01 (1.00e-30), 5.218e-02 (1.00e-10)
Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations
119
Newton solver: 3, 3.309e-01 (1.00e-30), 2.974e-04 (1.00e-10)
Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations
118
Newton solver: 4, 9.085e-06 (1.00e-30), 8.166e-09 (1.00e-10)
Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations
120
Newton solver: 5, 3.475e-12 (1.00e-30), 3.124e-15 (1.00e-10)
32x32 discretization
Newton solver: iter, abs. residual (abs. tol), rel. residual (rel. tol)
Newton solver: 0, 1.330e+03 (1.00e-30), 1.000e+00 (1.00e-10)
Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations 98
Newton solver: 1, 5.913e+02 (1.00e-30), 4.445e-01 (1.00e-10)
Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations
183
Newton solver: 2, 3.214e+01 (1.00e-30), 2.416e-02 (1.00e-10)
Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations
152
Newton solver: 3, 2.059e-01 (1.00e-30), 1.547e-04 (1.00e-10)
Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations
151
Newton solver: 4, 6.949e-06 (1.00e-30), 5.223e-09 (1.00e-10)
Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations
149
Newton solver: 5, 5.300e-12 (1.00e-30), 3.983e-15 (1.00e-10)
64x64 discretization
Newton solver: iter, abs. residual (abs. tol), rel. residual (rel. tol)
Newton solver: 0, 1.707e+03 (1.00e-30), 1.000e+00 (1.00e-10)
Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations
198
Newton solver: 1, 4.259e+02 (1.00e-30), 2.494e-01 (1.00e-10)
Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations
357
Newton solver: 2, 1.706e+01 (1.00e-30), 9.993e-03 (1.00e-10)
Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations
266
Newton solver: 3, 1.134e-01 (1.00e-30), 6.639e-05 (1.00e-10)
Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations
261
Newton solver: 4, 4.285e-06 (1.00e-30), 2.510e-09 (1.00e-10)
Linear fieldsplit_1_ solve converged due to CONVERGED_RTOL iterations
263
Newton solver: 5, 9.650e-12 (1.00e-30), 5.652e-15 (1.00e-10)
