IS IT a bird, is it a plane? No, it鈥檚 a rock falling into a pool of water, but the jet of air it produces flies faster than a speeding bullet.
When an object such as a pebble drops into water, an air-filled cavity is created which ejects air at supersonic speeds, discovered Stephan Gekle at the University of Twente in the Netherlands, and colleagues.
Using high-speed photography, the team spotted a cavity of air forming in an hourglass shape 鈥� with the top of the hourglass at the surface of the water and its base at the sinking object. To measure the speed of air rushing out upwards, they marked the air with smoke before the splash.
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Even though their camera took 15,000 frames per second, they still couldn鈥檛 measure the fastest speeds directly, so they simulated the behaviour they had observed. They found that shortly before the cavity closes, the pressure of the air at the bottom of the hourglass becomes higher relative to the 鈥渘eck鈥�. This difference pushes the air out at speeds faster than sound (Physical Review Letters, ).
of Princeton University is impressed by the research. The presence of supersonic flow was far from obvious, he says.
