Supersonic Bathtub Physics

From "Supersonic Air Flow due to Solid-Liquid Impact," by Stephan Gekle, Ivo R. Peters, José Manuel Gordillo, Devaraj van der Meer and Detlef Lohse, in Physical Review Letters, Vol. 104; January 15, 2010 (splash sequence)

Sonic booms in your bathtub? Apparently, a hard object falling into a pool can push a jet of air out of the water so fast that it briefly breaches the sound barrier.

Physicists at the University of Twente in the Netherlands and at the University of Seville in Spain set up an experiment in which they pulled a disk-shaped object flat down into water so that it hit the water at the relatively leisurely speed of one meter per second (roughly equivalent to dropping the disk from a height of a few centimeters). The disk displaced the water and created an air bubble in its wake as it sank.

As the water closed in to form the bubble, it pushed air up through a narrower and narrower neck, accelerating the air. “It’s like a little nozzle which closes,” explains Twente’s Detlef Lohse, similar to what happens in a rocket engine. To track the air’s motion, the team filled it with glycerin droplets produced by a smoke machine of the type used in dance clubs.

Using a high-speed camera and computer simulations, the researchers estimated that the jet reached a speed of 350 meters per second at its peak, or just above the speed of sound. Their report appears in the January 15 Physical Review Letters. Although the details change for objects of different shapes and sizes, “the physics is the same,” Lohse says. “By dropping a stone into the water, you create a supersonic jet.”

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