This was going to be a quick end to the turbulence topic (for now), but I fell down a gnuplot rabbit hole.

Paragraph 2.1.7 briefly covers the Reynolds-averaged Simulation (RAS) turbulence model.  I didn't plan to get into the details of the various turbulence models, so this should have been a quick read-through.   But there is a tiny sentence near the end of the discussion that says, "View the results at consecutive time steps as the solution progresses to see if the solution converges to a steady-state or perhaps reaches some periodically oscillating state."

I thought, 'That sounds interesting....and easy!'  If you are a linux person, it probably is.  But usually I'm not a linux person, so I spent some time figuring out how to gnuplot from the command line, getting data from a changing log file.  In addition to looking up standard linux commands, this was the main article that pointed me in the right direction, https://chrisberkhout.com/blog/log-visualization-with-gnuplot/.  Below are the file contents I used to send to gnuplot.

set title "Residuals"
set xlabel 'time'
set ylabel 'Residual'
set format y "%.1e"
plot "< cat log_new | grep 'Solving for Ux' | cut -d',' -f3 | tr -d ',' | cut -d' ' -f5" title 'Ux' with lines,\
"<cat log_new | grep 'Solving for Uy' | cut -d',' -f3 | tr -d ',' | cut -d' ' -f5" title 'Uy' with lines
pause 1
reread 

That's enough of the cavity flow introduction to openFOAM.  I will next jump to Mesh Generation, which is chapter 5 in the User Guide, but seems like a reasonable next step.