Bathtub physics

Despite their apparent simplicity, the so-called bathtub vortices that form in draining containers exhibit complex flow patterns and are an active area of research. The whirlpools that form above the drains exhibit much of the same physics found in turbine intakes and tornadoes. (For more on tornadoes, see the article by Paul Markowski and Yvette Richardson, Physics Today, September 2014, page 26.) These photos are from experiments carried out by Katrine Haaning in the research group of Anders Andersen and Tomas Bohr at the Technical University of Denmark. The experiments employ a rotating cylindrical tub, 20 cm in diameter and filled with water to a depth of 25 cm, to study the shape, stability, flow, boundary layers, and other aspects of bathtub vortices. The vortices themselves are in a steady state: As water drains through the centered 2.4-mm hole on the bottom (not visible here), it gets recirculated...