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

[Dave Lee]

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.

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
> JFNK
> >> 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.
>
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