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The Tao of pee
Date:11/7/2013

WASHINGTON D.C. Nov. 7, 2013 -- Although we don't often think about it, fluid dynamics touches almost every aspect of our lives, from a billowing breeze that buffets a flag, to swirling river currents that shape canyons to the surging blood that sustains our lives. One of the basest of bodily functions -- urination -- is governed primarily by the equations of fluid motion.

Later this month, at the American Physical Society (APS) Division of Fluid Dynamics (DFD) meeting in Pittsburgh, Pa., two teams of researchers reveal new insight into the physics of peeing.

In the laboratory of Georgia Tech's David Hu, scientists and engineers look to nature for engineering ideas. In work that could help in the design of scalable hydrodynamic systems, researchers from the Hu lab recently filmed the urination habits of 16 animals of varying sizes five mice, five rats, one dog, two goats, two cows, and one elephant. The results? Size matters. Although small animals such as mice and rats take about 2 seconds to pee, urination "events" in animals larger than about 5 kilograms consistently clocked in at an average of 21 seconds.

"An elephant has a large bladder and a urethra with dimensions comparable to a household pipe," says graduate student and study leader Patricia Yang. As gravity pulls fluid down to the bottom of the urethra, Yang explains, the flow speed increases, causing urine to be eliminated more quickly than in a medium-sized animal, like a dog, which has a shorter urethra and gets less of a boost from gravity. The dog, however, has a smaller bladder, and "this is why an elephant and a dog empty their bladder in the same time," she says.

When it comes to urination accuracy, however, speed and size are less important than angle, says fluid dynamicist Randy Hurd of Brigham Young University, who will present a study of the dynamics of urinal use. Hurd and his graduate advisor, Tadd Truscott, got the idea for the work during a caffein
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Contact: Jason Socrates Bardi
dfdmedia@aps.org
240-535-4954
American Physical Society's Division of Fluid Dynamics
Source:Eurekalert  

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