Iteration Termination Criterion

Hello,

I am working with a large regional model in support of an Environmental Impact Assessment.  I have had an isue with head recovery after cessation of pumping, which was much improved by tightening the iteration termination criterion from the default of 1E-8 to 1E-10.

Hopefully the attachment worked, in which case the original curve is in blue and the new curve is in red.  If it didn't work, not a problem, the curves themselves are not really essential to the issue.

What I need is a good brief explanation for non-modellers as to how this criterion affects the modelling process.  Does anyone have a reference for a good non-technical overview of this topic?  The FEFLOW manual and white papers aren't helpful.  I can explain it to modellers or mathematicians, but I am at a bit of a loss for how to convince a non-technical reader that changing this setting does not change the model itself, without oversimplifying it to the point that I am providing inaccurate or misleading information.  I don't want to write a textbook, which is what my current efforts are beginning to look like.  A reference to literature would be good CYA insurance, I think.

Thanks!

scubohuntr,

FEFLOW has a two-tier termination criteria setup which is similar in some ways to the PCG solver MODFLOW uses (the FEFLOW solvers are much more sophisticated than MODFLOW's PCG solver).  The "inner loop" criterion is the termination threshold which you reduced from 1e-8 to 1e-10 (there are also controls for max iterations of the inner loop).  This allowed FEFLOW to do "more work" in solving your problem.  The "outer loop" criterion is under the "numerical parameters" part of the "problem settings" menu.  It too controls how much work feflow will do to solve the problem.  The outer loop has three controls, the error tolerance, the way error is measured and the number of iterations per timestep.  The inner loop is executed many times for every iteration of the outer loop. 

In the outer loop, if you switch from L2 to L1 or Max error, FEFLOW will do more work solving the problem at each time step.  However, if you do not give it enough iterations to do more work, the solver may terminate prematurely.  Premature termination of either the inner or outer loops may result in significantly large error in calculated total head, pressure, saturation, etc.  This error can propagate and grow with each time step and may result in a very poor result which may show up as obviously incorrect water levels or mass balance, or significant oscillations.  However, the error may not be obvious and the only way to know for sure is to runs a series of tests on solvers using different settings.

When you decreased the inner loop setting, FEFLOW was forced to do more work in solving your model.  This resulted in more complete convergence.  The large difference between the two solutions suggests that more test should be done to better understand the model's performance.

Pete