As a stone plows into still water, it plows out a column of air. The column collapses in an hourglass shape, and the escaping air (in this video, the air is filled with smoke for visibility) shoots through the shrinking opening at supersonic speeds.
Credit: Stephan Gekle/Physical Review Letters 2010
Found in: 'Just a stone's throw forms a supersonic jet' online at: sciencenews.org/view/generic/id/54965/title/Just_a_stones_throw_forms_a_supersonic_jet
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