Numerical dispersion in a simple rotating interface model (GWT+BUY)

[ifariasg]

Together with @dzamrsky we have been investigating how MF6+BUY results compare to SEAWAT due to a general idea that MF6 (GWT specifically) tends to show more numerical dispersion. Since UTVD was added to MF6 v6.7.0 we thought this was acknowledged and improved so we started migrating some existing models to MF6 to compare.

For more advective problems results seem to be quite good, but when looking at more density-driven problems we noticed discrepancies. For example, we tested a simplified version of the rotating interface problem.

We found that when dispersion parameters are set to 0, there area noticeable differences in concentration results, with MF6 showing consistently more numerical (?) dispersion. No matter what setup I used I couldn't improve the results. Some of the things I tried:

• Change solver parameters
• Reduce time step size
• Use ATS with very small time step size
• Change initial head formulation
• Change (or remove) the CHD added for stability
• Use the RHS formulation for BUY
• Change the advection scheme (UTVD,TVD,central)

What's also worrying, the MF6 version was ~10 times slower to run, no matter the settings.

At this stage I can only think of two possibilities, there is an option I didn't implement that makes both models essentially different, or there is something strange going on internally. To spark some discussion, I'm sharing the script used to see if anybody has an idea on what could be causing this. Also it would be interesting to know if anybody has been experiencing similar problems.
benchmark_seawat_vs_mf6.py

This is the comparison with no dispersion

...and strangely enough, with dispersion the results look a lot more alike...