Ben, I'll try the Backward-Euler and see if that eliminates the problems. I'll let you know how this goes.
Also, I do a basic form of inexact newton iteration (look at the linear tolerance being set at the end of nonlinear loop). Maybe I'm getting the wrong residual here and setting the tolerance incorrectly. I'll check this again. Vijay On Tue, Jan 26, 2010 at 12:52 PM, Kirk, Benjamin (JSC-EG311) <[email protected]> wrote: >> I'm solving a pure diffusion problem and there is no convection here. >> But I do understand that time integration makes a big difference and >> even making delt=1e-10 does not seem to help. The negativity occurs on >> the first step, the first call to nonlinear residual. When you say >> trapezoidal rule, are you talking about Implicit midpoint here because >> CN is based on the trapezoidal rule and is not L-stable (spurious >> oscillations are not damped). > > You can test implicit Euler easily enough by changing the -.5 at line 703 to > 0 and the .5 at line 714 to 1. > > The only other thing which could be an issue going forward is that the BCs > appear to be evaluated at the current time, which I think is inconsistent > with CN. I believe the correct approach would be to evaluate them at both > the current and the previous time and apply the average, as CN is built > around a discretization at t_(n+1/2)... (this is not an issue for your > current BCs which have no time dependence.) > > ------------------------------------------------------------------------------ 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
