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 <alexlindsay...@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 <j...@jedbrown.org> wrote: > >> Alexander Lindsay <alexlindsay...@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. >> >