Supersonic Air Flow due to Pebble Splash

Who hasn’t taken a rock and thrown it in a pond just to see the splash and ensuing ripples? Stephan Gekle and his colleagues have discovered that when you take stone and throw it into a lake, you get the familiar crown splash. Just behind the stone, a large air cavity is formed. This can be seen in the first frame of the image to the right. As the fluid fills in behind the stone, this air cavity collapses (second frame of image to right. In the third frame, the shape of that cavity is in the form of a converging-diverging nozzle, which is the same shape that is found in rocket engines. From the NewScientist article on the paper:

Using high-speed photography, the team spotted a cavity of air forming in an hourglass shape – 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. Even though their camera took 15,000 frames per second, they still couldn’t measure the fastest speeds directly, so they simulated the behaviour they had observed.

My question is does this also happen when you perform a cannonball off the diving board into the pool?

via NewScientist : Pebble splashes break the speed of sound

Download of the paper : Supersonic Air Flow due to Solid-Liquid Impact

This story written by Paul Adams

Paul Adams leads an award-winning, diverse contractor team that runs a federal high performance computing facility where he has worked for 17 years. He loves getting his hands on the latest visualization and computer hardware, astronomy, aerospace engineering, working with the poor, and ringing cowbells.