I do believe that based off the results in https://doi.org/10.1137/040608817 we should be able to make LSC, with proper scaling, compare very favorably with PCD
On Tue, Jun 27, 2023 at 10:41 AM Alexander Lindsay <alexlindsay...@gmail.com> wrote: > I've opened https://gitlab.com/petsc/petsc/-/merge_requests/6642 which > adds a couple more scaling applications of the inverse of the diagonal of A > > On Mon, Jun 26, 2023 at 6:06 PM Alexander Lindsay < > alexlindsay...@gmail.com> wrote: > >> I guess that similar to the discussions about selfp, the approximation of >> the velocity mass matrix by the diagonal of the velocity sub-matrix will >> improve when running a transient as opposed to a steady calculation, >> especially if the time derivative is lumped.... Just thinking while typing >> >> On Mon, Jun 26, 2023 at 6:03 PM Alexander Lindsay < >> alexlindsay...@gmail.com> wrote: >> >>> Returning to Sebastian's question about the correctness of the current >>> LSC implementation: in the taxonomy paper that Jed linked to (which talks >>> about SIMPLE, PCD, and LSC), equation 21 shows four applications of the >>> inverse of the velocity mass matrix. In the PETSc implementation there are >>> at most two applications of the reciprocal of the diagonal of A (an >>> approximation to the velocity mass matrix without more plumbing, as already >>> pointed out). It seems like for code implementations in which there are >>> possible scaling differences between the velocity and pressure equations, >>> that this difference in the number of inverse applications could be >>> significant? I know Jed said that these scalings wouldn't really matter if >>> you have a uniform grid, but I'm not 100% convinced yet. >>> >>> I might try fiddling around with adding two more reciprocal applications. >>> >>> On Fri, Jun 23, 2023 at 1:09 PM Pierre Jolivet <pierre.joli...@lip6.fr> >>> wrote: >>> >>>> >>>> On 23 Jun 2023, at 10:06 PM, Pierre Jolivet <pierre.joli...@lip6.fr> >>>> wrote: >>>> >>>> >>>> On 23 Jun 2023, at 9:39 PM, Alexander Lindsay <alexlindsay...@gmail.com> >>>> wrote: >>>> >>>> Ah, I see that if I use Pierre's new 'full' option for >>>> -mat_schur_complement_ainv_type >>>> >>>> >>>> That was not initially done by me >>>> >>>> >>>> Oops, sorry for the noise, looks like it was done by me indeed >>>> in 9399e4fd88c6621aad8fe9558ce84df37bd6fada… >>>> >>>> Thanks, >>>> Pierre >>>> >>>> (though I recently tweaked MatSchurComplementComputeExplicitOperator() >>>> a bit to use KSPMatSolve(), so that if you have a small Schur complement — >>>> which is not really the case for NS — this could be a viable option, it was >>>> previously painfully slow). >>>> >>>> Thanks, >>>> Pierre >>>> >>>> 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 < >>>> alexlindsay...@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 < >>>>> 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. >>>>>>> >>>>>> >>>> >>>>