Residuals ``bottom out''

The bottoming out may be due to:

• truncation error in finite precision arithmetic. If there are reactive terms in the PDEs and these reactive terms are nearly equal and have a magnitude of about larger than the residual when it bottoms out, then the residual being calculated is dominated by truncation error.
• an incorrect system of equations. To many boundary conditions may lead to an overconstrained system of nonlinearx equations in which you can only hope to minimize the residual.

Suggestions:

• If truncation error is the problem, rather than relaxing the relative and/or absolute tolerances, it is probably better to turn on the update norm (-useUpdateNorm).