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
On Oct 13, 2016, at 13:12, David Knezevic <david.kneze...@akselos.com<mailto:david.kneze...@akselos.com>> wrote: On Thu, Oct 13, 2016 at 12:34 PM, Rossi, Simone <sro...@email.unc.edu<mailto: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<mailto:david.kneze...@akselos.com>> wrote: On Thu, Oct 13, 2016 at 11:57 AM, Rossi, Simone <sro...@email.unc.edu<mailto: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
