Dear Wolfgang, Thanks also to you for taking the time, I really appreciate it!
I already tested the integration, increasing up by order+1, +2, +3 ... gave identical results. Now it seems, that I should introduce some switches to a direct solver! This might give me a hint, if I need some rescaling of some rows! I will report back, if that helped! Kind regards, Richard Am Mittwoch, 19. Februar 2020 03:03:57 UTC+1 schrieb Wolfgang Bangerth: > > On 2/14/20 5:34 AM, Richard Schussnig wrote: > > > > Could the observed behaviour be caused by applying an iterative solver*? > > (using ReductionControl and a reduction factor of 1e-13 which is really > low) > > * Block-triangular Schur-complement-based approach, similar as in > > step-55 suggested in the further possibilities for extension, basically > > using S~-(1/viscosity)*Mass_pressure giving 20-40 iterations up to 3mio > > dofs tested. > > > > I have been checking the code for a week now, and i really doubt, that > > the integration of just tau*grad(p)*grad(p) is wrong (again, the laplace > > drops out for a rectangular grid). > > Just to check, i used a manufactured solution from Poisuille flow and > > added the laplacian from PSPG to the rhs, giving me the exact (linear) > > solution in the pressure and quadratic convergence in the velocity, thus > > I assume, that the grad(p)grad(q) term added is correctly implemeted. > > (exact solution meaning, that i get an L2 error of err<1e-9 for any > > number of elements used) > > > > Anyone has ever experienced this or has anyone some tipps for further > > debugging? > > We have all spent much time trying to figure out these sorts of issues > :-) My usual approach is to switch to a direct solver to eliminate the > possibility that the problem lies with the solver. Just speaking about, > say, step-22, there is also the issue that the iterative solver actually > just ignores the pressure-pressure block of the matrix (I think that we > put a mass matrix in there as that is used in the preconditioner -- or > at least that's what we used to do). So it's useful to just remove > everything that could be problematic, and the iterative solvers are > definitely a point in case. > > The other thing I usually check is what happens if I increase the order > of quadrature. > > Best > W. > > -- > ------------------------------------------------------------------------ > Wolfgang Bangerth email: bang...@colostate.edu > <javascript:> > www: http://www.math.colostate.edu/~bangerth/ > -- The deal.II project is located at http://www.dealii.org/ For mailing list/forum options, see https://groups.google.com/d/forum/dealii?hl=en --- You received this message because you are subscribed to the Google Groups "deal.II User Group" group. To unsubscribe from this group and stop receiving emails from it, send an email to dealii+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/dealii/bc74215c-9c89-429f-96ce-732812e1879a%40googlegroups.com.
