Threshold violations

The threshold violation warnings are meant to point out potential problem with the solution. They may not actually indicate a problem with the solution. Alamode checks for violations by default because it is likely that all of the unknowns represent concentrations, and it is not physical to have negative concentrations. Thresholds are adjusted because negatives in the solution of a system of PDEs with reactive terms makes it possible to converge to an incorrect fixed point (i.e. two negative quantities multiplied together is positive).

Threshold violation warnings that are printed can be ignored if they do not occur on the last nonlinear update. These are just overshoot in the nonlinear update, and they do not indicate a problem with the converged solution.

Threshold violation warnings/adjustments that occur on the last nonlinear update may be an indication of a problem with the converged solution.

Suggestions:

• Check the magnitude of the violations (turn on -warnThreshold).
• Change the threshold value (-thresholdValue). The default is small, but positive. Zero will trigger a violation. If the solution of any field can go negative, this should be set to a large negative value.
• Turn off checking of thresholds (-noCheckThreshold).
• If the treshold violations are due to overshoot in the timestepping (the timestepping algorithms are not monotonic, and large timesteps may overshoot), turn on -failIfHadThresholdViolation to allow the backoff code to resolve with a smaller time step.