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 <[email protected]> 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 < > [email protected]> 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 <[email protected]> wrote: >> >>> Alexander Lindsay <[email protected]> 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. >>> >>
