[petsc-users] Scalable Solver for Incompressible Flow

[Alexander Lindsay]

Ah, I see that if I use Pierre's new 'full' option for
-mat_schur_complement_ainv_type that I get a single iteration for the Schur
complement solve with LU. That's a nice testing option

On Fri, Jun 23, 2023 at 12:02 PM Alexander Lindsay <alexlindsay239 at gmail.com>
wrote:

> 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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