Adrian Bejan | Lifetime of Turbulent Eddies, from Design in Nature
In this video, Adrian Bejan takes on turbulence, a phenomenon he says people treat as a statement of admitting defeat, and explains it through the life of a single eddy, a rolling blob of water rolling in water, the fluid equivalent of the rolling stone. Starting from a jet discharged into a swimming pool, he shows how eddies are born where moving water meets still water, how friction with the surroundings slows each eddy until it fades away, and why the bigger eddies live longer, travel farther, and complete more turns before disappearing. He closes by arguing that turbulence is no longer an enigma, and that it is time to close the book and move on.
Where eddies come from. A jet discharged into a pool spreads predictably as a cone, but a closer look reveals a meandering flow that thickens and travels forward like a wave. Eddies have to form because the moving water interacts with the surrounding water that is not moving.
Every eddy lives and dies. Friction between the rolling blob and its slower surroundings retards its turning, making it slower and slower as it travels downstream. Eventually the eddy becomes indistinguishable from its surroundings, which means it dies.
A story about rolling stones. As a junior at MIT, Bejan took a psychology course whose professor, opposed to students experimenting with LSD, ran a controlled experiment asking a sober student and a student who was high the same question: what happens to a rolling stone? The high student answered, do not throw rolling stones into a glass house because they gather no moss, two sayings interbred into nonsense, and the lesson is simple, do not play with your brain.
Bigger live longer and travel farther. By comparing an eddy’s inventory of kinetic energy with the rate at which friction dissipates it, Bejan predicts that bigger eddies should live longer and travel farther. In a population of many sizes, this predicts a diversity of lifetimes, a few long-lived movers among many short-lived ones, and those words apply to every diverse group of movers.
The number of rolls is set at birth. Every roll of an eddy is like the tick of a clock, and the number of rolls until death is dictated from the eddy’s birth. Bigger and faster-rolling eddies make more turns before they disappear.
The birth of turbulence has a threshold. Each eddy’s size and rotational speed define its own identity, what Bejan calls the local Reynolds number, like the brand name of a particular eddy. When this number reaches roughly one hundred, eddies are born and the flow shifts from laminar to turbulent, and below that threshold no eddy appears.
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