Figure 1 – Vortices on the Charles River Beneath the Watertown Bridge (c) DE Wolf 2013.

Two physicists were crossing a bridge…  Yes it sounds like the start of one of those physicist jokes that end with the punchline:  “Well first you have to assume that the chicken is spherical.”  Anyway my colleague and I were taking a walk at lunch and on passing over the Watertown Bridge we noticed that there was an excess of sudsy pollution on the river.

This is very foul for the waterfowl (sorry).  But it does highlight the flow patterns of the river.  And what we noticed was that as the water goes under the bridge where it narrows and then releases into the wider river below (right hand side in Figure 1), there is a back-flow (left hand side in Figure 1), resulting from Bernouilli’s principle that causes little whirlpools or vortices to form (center in Figure 1).

I took a couple of pictures and we went on to discuss the Coriolis force and whether water truly goes clockwise down a bathtub drain in the northern hemisphere and counter-clockwise in the southern hemisphere.  Life is good!