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. >>>>> >>>> >> >>