[petsc-users] Scalable Solver for Incompressible Flow

[Alexander Lindsay]

I guess it is because the inverse of the diagonal form of A00 becomes a
poor representation of the inverse of A00? I guess naively I would have
thought that the blockdiag form of A00 is A00

On Fri, Jun 23, 2023 at 10:18 AM Alexander Lindsay <alexlindsay239 at gmail.com>
wrote:

> 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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