[petsc-users] Scalable Solver for Incompressible Flow

[Alexander Lindsay]

Hi Jed, I will come back with answers to all of your questions at some
point. I mostly just deal with MOOSE users who come to me and tell me their
solve is converging slowly, asking me how to fix it. So I generally assume
they have built an appropriate mesh and problem size for the problem they
want to solve and added appropriate turbulence modeling (although my
general assumption is often violated).

> And to confirm, are you doing a nonlinearly implicit velocity-pressure
solve?

Yes, this is our default.

A general question: it seems that it is well known that the quality of
selfp degrades with increasing advection. Why is that?

On Wed, Jun 7, 2023 at 8:01 PM Jed Brown <jed at jedbrown.org> wrote:

> Alexander Lindsay <alexlindsay239 at gmail.com> writes:
>
> > This has been a great discussion to follow. Regarding
> >
> >> when time stepping, you have enough mass matrix that cheaper
> preconditioners are good enough
> >
> > I'm curious what some algebraic recommendations might be for high Re in
> > transients.
>
> What mesh aspect ratio and streamline CFL number? Assuming your model is
> turbulent, can you say anything about momentum thickness Reynolds number
> Re_θ? What is your wall normal spacing in plus units? (Wall resolved or
> wall modeled?)
>
> And to confirm, are you doing a nonlinearly implicit velocity-pressure
> solve?
>
> > I've found one-level DD to be ineffective when applied monolithically or
> to the momentum block of a split, as it scales with the mesh size.
>
> I wouldn't put too much weight on "scaling with mesh size" per se. You
> want an efficient solver for the coarsest mesh that delivers sufficient
> accuracy in your flow regime. Constants matter.
>
> Refining the mesh while holding time steps constant changes the advective
> CFL number as well as cell Peclet/cell Reynolds numbers. A meaningful
> scaling study is to increase Reynolds number (e.g., by growing the domain)
> while keeping mesh size matched in terms of plus units in the viscous
> sublayer and Kolmogorov length in the outer boundary layer. That turns out
> to not be a very automatic study to do, but it's what matters and you can
> spend a lot of time chasing ghosts with naive scaling studies.
>
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