Fluid Dynamics & the Wet-Dog Shake

Some would look at a wet dog shaking off water and think only of running away to avoid the spray. For a group at the Georgia Institute of Technology, however, watching a dog do 'The Twist' reminded them of the motion of a washing machine during a spin cycle. They moved in closer for more.

Andrew Dickerson and four teammates spent some time considering the fluid dynamics behind a wet, furry animal shaking itself dry. They garnered animal volunteers from Georgia Tech's research labs, the local zoo and from friends. The hairy and furry animals were appropriately doused and then filmed with high-speed cameras as they shed water from their coats.

In order to represent mathematically what was going on in their videos, the team created a formula. Three variables were included: Surface tension, which the team decided was responsible for keeping the water droplets attached to the fur, centripetal force, which would be needed to release the droplets, and the animal's radius, a required variable when calculating centripetal force. They came up with the formula R^0.5, with R equal to the animal's radius. They then watched their videos to see if their math was correct.

The team observed that larger animals shake more slowly when shedding water than smaller animals do. A grizzly bear shakes off water with a frequency of 4 Hz (or four back-and-forth shakes a second) while a mouse shakes at 27 Hz.

When they plugged their numbers back in to their formula, however, they found that the correct approximation for what was going on was R^0.75. Dickerson, the team leader, hypothesized that this was because when measuring an animal's radius, they used the distance from the animals' center to its skin. Including the length of the fur, he thought, could be the difference.

The team also noticed that having loose skin helped to fling water away because loose skin can travel further on either side of the spine than tight skin as an animal shakes. (You can see this in the video: The team attached a pink straw to one dog and the straw can be seen passing 90 degrees on either side of the dog's spine as he shakes.)

The Georgia Tech team will present their findings at the American Physical Society's 63rd Annual Meeting of the APS Division of Fluid Dynamics this November in Long Beach, Calif. There, over 2000 graduate students, postdoctoral researchers, university faculty and government and industry researchers will meet to share the latest and greatest in the field of fluid dynamics.

Being able to shake itself dry is critical for a dog, Dickerson said. Otherwise, it would get hypothermia and die every time it got wet. Unfortunately, the formula created to represent this behavior doesn't have any useful applications yet, but there's always hoping on the innovation of tomorrow. In the meantime, check out the cute little mice and big fluffy dogs shaking themselves dry in the video up top. If it doesn't inspire you to create a new invention, at least it will make you smile.

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