Sounds of Fluid Days

“Falling bodies and the liquids or gases through which they fall, known as fluids, refuse to interact in a simple manner. The objects move irregularly, perhaps even randomly. The equations that have been developed to describe motions of fluids prove too complex to be solved in the case of falling leaves or even the simpler surrogate materials used in experiments. ‘It seems like such a standard, elementary problem in physics’ that physicists feel as if they should have solved it already, says Franco Nori, a theorists at the University of Michigan at Ann Arbor.

“...Scientists have long noted an element of randomness in the fall of objects in a fluid. In this Principia, published in 1687, Isaac Newton mentions experiments by a Dr. Desaguliers, who formed hogs’ bladders into ‘spherical orbs’ and dropped them from the cupola of St. Paul’s Cathedral in London. ‘The bladders did not always fall straight down, but fluttered a little in the air, and waved to and fro as they were descending,’ Newton reported.

“Scientists came to regard the unpredictability of falling objects paths as a consequence of the complex interplay between the object and the disorderly, often turbulent motions of the fluid. By the late 1980s, theorists in the then-Soviet Union began to suggest that chaos might play a role.

bluff bodies: nonstreamlined bodies, such as a coin or a leaf














“In the July 17, 1997 Nature scientists presented the first [published] experimental evidence... of chaos in the motion of falling bodies.

“They found four modes of motion, three of which are regular and similar to the straight fall, side-to-side sway, and tumbling previously described by theorists. They also discovered a chaotic mode in which the disk sways back and forth, gradually swinging higher and higher. After an unpredictable period, it tips to such a steep angle that it overturns into a tumble. Then after some time, also unpredictable, it recovers the oscillating mode.

“A leaf [or other bluff body tumbling in air] also sheds vortices of air each time it flutters back and forth. ...[R]esearchers must find out more about the interaction between those vortices and the edges where they form.... ‘That’s the big puzzle that needs to be solved.’”

“the new findings have expanded the frontiers of chaos theory by demonstrating a path of sudden transitions in and out of chaotic behavior ... that had been predicted but never before seen experimentally. [the name for these sudden transitions: intermittency]

---excerpted from Peter Weiss, “The Puzzle of Flutter and Tumble: Physicists Reconsider the Fall of Leaves,” Science News 154.18 (Oct. 31 [Halloween] 1998): 285-87.

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