On Thu, Nov 3, 2016 at 4:49 PM, Rossi, Simone <sro...@email.unc.edu> wrote:
I solved the P2 problem in Ex4 and Ex5. > Everything was fine, but for nu = 0.49995, the linear solver was not > converging. > I did not realize that as I was not printing the output of the linear > solver on screen. > Changing the preconditioner, I was able to solve the nearly-incompressible > P2 elasticity problem, getting finally some instabilities in the pressure. > Thanks for the help, > Simone > OK, that makes sense. FYI, I often like to use a direct solver to check that everything is set up correctly. I usually use MUMPS via PETSc: "-ksp_type preonly -pc_type lu -pc_factor_mat_solver_package mumps". David > > > On Oct 13, 2016, at 13:12, David Knezevic <david.kneze...@akselos.com> > wrote: > > On Thu, Oct 13, 2016 at 12:34 PM, Rossi, Simone <sro...@email.unc.edu> > wrote: > >> Dear David, >> thanks for your answer. In this case (nu=0.49995), first order elements >> typically lock, but second order elements typically do not lock. >> In fact many use second order lagrangian elements for nearly >> incompressible materials. I wanted to use this example just to show that >> second order elements are not inf-sup stable. >> But the results I get running Ex4 are not "bad": in my opinion, they are >> nonsense. >> I wonder if the differences come from a different way of handling the >> boundary conditions or from a bug in the assembly. >> Let me know if you have any insight. >> Thanks, >> Simone >> > > > Not sure why that would be the case, I guess you'll need to do more tests > to figure out what's happening. Feel free to reach out if you have any > specific questions. I doubt there's an issue with the BCs since they use > DirichletBoundary code which is widely used, but it wouldn't hurt to check > the assembly (I normally use 3D elasticity, and I'd say that this 2D > elasticity example has not been widely used so a bug is possible, or > alternatively maybe it's a plane strain vs. plane stress issue). > > David > > > > >> On Oct 13, 2016, at 12:00, David Knezevic <david.kneze...@akselos.com> >> wrote: >> >> On Thu, Oct 13, 2016 at 11:57 AM, Rossi, Simone <sro...@email.unc.edu> >> wrote: >> >>> Dear all, >>> I’m playing around with the elasticity tests in the system of equations >>> examples (more specifically Ex4 and Ex5). >>> In particular I’m trying to set the poisson ratio to nu = 0.49995. >>> With this choice the solution I get using second order lagrangian >>> elements does not make any sense. >>> For first order elements the solution looks more reasonable, but still >>> different from what I get from FreeFEM++. >>> Does it depend on the enforcement of the Dirichlet boundary conditions? >>> Thanks, >>> Simone >>> >> >> >> nu=0.49995 is almost incompressible. Normally people use special >> formulations for that type of problem, e.g. a mixed method to enforce >> almost incompressibility (similar to Stokes in fluids). That probably >> explains why you get bad results by naively using the simple formulation >> from ex4 and ex5. >> >> David >> >> >> > > ------------------------------------------------------------------------------ Developer Access Program for Intel Xeon Phi Processors Access to Intel Xeon Phi processor-based developer platforms. With one year of Intel Parallel Studio XE. Training and support from Colfax. Order your platform today. http://sdm.link/xeonphi _______________________________________________ Libmesh-users mailing list Libmesh-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/libmesh-users
