[petsc-users] testing for and removing a null space using JFNK

Matthew Knepley knepley at gmail.com
Thu Apr 4 21:26:52 CDT 2019

On Thu, Apr 4, 2019 at 9:11 PM Dave Lee <davelee2804 at gmail.com> wrote:

> Hey Matt,
> I'm not solving NS per se, but rather wrapping up a Navier Stokes solver
> within a SNES to iterate over the solution of the Navier Stokes equations
> with a certain time period in order to determine approximate periodic
> solutions to the NS equations (with some corrections).
> My residual vector is basically a difference between the final and initial
> velocity states of the NS solve (with corrections). However since one
> component can be diagnosed from the others via the divergence free
> condition (up to a constant), I suspect that maybe what I should be doing
> is just omit one of the velocity components from the residual vector, and
> then diagnose this from the others via incompressibility, rather than try
> and correct for this after the vectors have already been assembled. This is
> all outside the scope of my PETSc question, and I don't expect you to have
> an answer, just mentioning it since you asked.

Interesting.  It sounds like you can impose this condition the same way we
impose \int p = 0.



> Cheers, Dave.
> On Fri, Apr 5, 2019 at 12:12 AM Jed Brown <jed at jedbrown.org> wrote:
>> Mark Adams via petsc-users <petsc-users at mcs.anl.gov> writes:
>> > On Thu, Apr 4, 2019 at 7:35 AM Dave Lee <davelee2804 at gmail.com> wrote:
>> >
>> >> I already have the Navier Stokes solver. My issue is wrapping it in a
>> >> solver to find the periodic solutions. I will keep reading up on SVD
>> >> approaches, there may be some capability for something like this in
>> SLEPc.
>> >>
>> >
>> > Yes, SLEPc will give you parallel eigen solvers, etc.
>> Even so, computing a null space will be *much* more expensive than
>> solving linear systems.

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/>
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