On Tue, 26 Jan 2010, Vijay S. Mahadevan wrote: > The issue is that I get negative temperatures at the first element > during assembly, immaterial of the refinement or the resolution of the > domain. I do not understand this behavior and was wondering if there > was a requirement with continuous FEM (Lagrange basis) that was not > satisfied in these problems with non-smooth solutions. I am certain > that there should be a fix to make sure that negative solutions are > avoided and that energy was conserved. But all my literature search > has not led to any decent solutions yet.
Not sure about this particular problem (example wasn't attached, and I wouldn't have time to read it if it was), but I've managed to avoid similar "out-of-bounds solution" failures just by being careful about the time discretization. For example the Galerkin H^2 formulation of Cahn-Hilliard can blow up with Crank-Nicholson integration (where each timestep can effectively extrapolate the solution out of the domain of the equations) but works with trapezoidal integration (where the residual terms at the end-of-timestep smoothly prevent that class of error). Not sure what the best way to go is when you don't have that same level of feedback in the residual, though. Oscillations in convection-dominated transport can lead to negative concentrations, which can then crash later physics despite being "reasonable" solutions to the transport equations. --- Roy ------------------------------------------------------------------------------ The Planet: dedicated and managed hosting, cloud storage, colocation Stay online with enterprise data centers and the best network in the business Choose flexible plans and management services without long-term contracts Personal 24x7 support from experience hosting pros just a phone call away. http://p.sf.net/sfu/theplanet-com _______________________________________________ Libmesh-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/libmesh-users
