The full story, in PDF format. Animated illustrations of the flow in a pint of Guinness. The mathematics used to solve the problem of a pint of Guinness is the same as that used for weather forecasting - they're both fluid dynamics. One of the interesting aspects of the flow diagrams above is the presence of eddies . These are places where the fluid recirculates - that is, it goes continuously round and round a closed curve.
Eddies turn up in lots of other situations.
If you look at a river just after a waterfall or a weir, you'll often see a region where lots of scum and rubbish accumulates. Rubbish which gets into the eddy stays there because the water just moves it around. Eddies can also often be seen in the corners of buildings during a strong wind. Just in the corner, there are often leaves whirling round and round, which mysteriously don't seem to get blown away in the wind. In fact, they're trapped in a little eddy.