The Physics of Swimming

Can these sleek mammals of the sea really defy the rules of hydrodynamics?

By Karen C. Fox|Monday, February 20, 2006
RELATED TAGS: OCEAN, FLOW MODELING

From Aristotle's claim that a dolphin could jump over the mast of a ship to tales of dolphins coming to the aid of drowning sailors, myths about this sea mammal have long overshadowed fact. But now the scientific facts themselves are adding to the legends. For instance, take the idea that dolphins swim faster than they should be able to. A host of physicists and biologists have for decades declared dolphin speeds (nearly 25 miles per hour) impossible, given the density of water and the amount of muscle dolphins have. Then researchers began scurrying to find out how the creatures do it.

"Hope springs eternal," says Frank Fish, a biologist at West Chester University in Pennsylvania. "Dolphins fascinate us—the public and scientists and everyone. And we always hope there is something that nature has figured out, some kind of special drag-reduction mechanism, that people might benefit from." The military, of course, wants to know so they can design faster submarines, Olympic swimmers want more effective swimsuits, and scientists just want to satisfy their own curiosity. So far, there is no shortage of theories: Dolphin tears spread over the body to reduce drag; body heat affects the flow of the water. Recent studies show that the truth is simpler. Dolphins have a shape that's the fastest form possible, their blubber helps make them faster, and they have skin properties that come out of a fluid-dynamics textbook.

The quest for dolphin-speed understanding began with zoologist Sir James Gray in 1936. A colleague had clocked a dolphin at about 23 mph. Gray was stunned. Moving through water, the dolphin was able to attain the average speed of a car driven in the city. Gray made some calculations about how much energy the dolphin needed and compared that with what he knew about muscles. His numbers showed the dolphin needed seven times more muscle than it has.

Like earlier scientific announcements that found horses' legs too weak to support their bodies and bumblebees anatomically unable to fly, Gray's paradox became a problem to unravel. His personal theory was that the powerful motion of the dolphin's tail causes water to attach tightly to the dolphin's skin due to a concept called laminar flow, which eliminates turbulence.

The conviction that Gray must be right remained but was untested. Any moving object—airplane or dolphin—splits the medium through which it passes. In the dolphin's case, a thin layer of water flows along the body until it becomes chaotic, or turbulent. Chaotic waters increase friction, and thus drag. After Gray's pronouncement, scientists searched for the mechanism that stopped the turbulence. As it happens, Gray had made a mistake. His calculations for the amount of muscle mass a dolphin needs were based on sprinting levels that dolphins can't maintain. Once Gray's paradox had been solved, researchers moved on to better understand other factors about a dolphin's speed.

The greatest contributor to the dolphin's speed turns out to be its shape. "Incredibly streamlined," says Jim Rohr, a physicist with the U.S. Navy's Space and Naval Warfare Systems Center in San Diego. "That's probably 90 percent of the mystery right there." Rohr studies bioluminescent plankton that give off a flash of light when disturbed. The phenomenon can be easily seen in the wake of an outboard engine at night. Rohr has spent a lot of time quantifying just how much movement is needed to get the glow going, so he was surprised when he heard anecdotes that dolphins never set off bioluminescence, something he had calculated a dolphin's motion would have to do.

Rohr decided to film trained dolphins in San Diego Bay on a dark night to see if they provoked bioluminescence. As the dolphins trailed behind the boat, Rohr saw the whole animal light up—a thin layer over the snout, a thicker swirl starting at midbody. The light layer corresponded to the thin layer of laminar flow. The brightness suggested an increase in turbulence. This switch from laminar to turbulent flow was expected, as it happens over submarines. On the other hand, if the dolphin's streamlined shape keeps almost half its body in laminar flow, that's enough to give it a helping hand in the water.

A dolphin makes use of what it's got. Touch a dolphin and you can feel the density of muscle—a body taut and powerful, consummately athletic. And yet part of the secret is blubber. Ann Pabst, a biological sciences professor at the University of North Carolina in Wilmington, has shown that blubber is far more than simple fat. It consists of a complex of fat cells and collagen fibers in a crisscross pattern that acts as a spring. If muscle moves the dolphin tail in one direction, blubber can help pull it back, like a Slinky spring snapping back. Thus blubber can conserve energy at various speeds, says biologist Frank Fish.

Fish studies the movement of the tail and its two side fins, known as flukes. He pretty much dismisses the mysterious ability to overcome drag. "Whether it supports a turbulent or laminar boundary layer isn't important," he says. "My experiments say the animal can produce enough thrust that it can support a turbulent layer." What the flukes have is shape. As the tail oscillates up and down, it provides lift that is channeled into forward movement, giving the animal thrust. Fish has placed dolphin flukes through CT scans to examine their shape as they bend. He has found that their geometry can change into an arch. The curve is crucial for one tiny moment of every stroke the dolphin makes—the exact moment it switches between up and down. Were the tail perfectly flat, it would lie in a plane with water flowing over it, and for an instant it would not provide any lift. When the tail arches, it probably never lies perfectly flat, and the dolphin doesn't lose thrust.

Convinced that the key to the dolphin's speed is drag reduction, engineers have turned to the mammal's skin. Much of the research is controversial. There were some fascinating attempts to prove skin can delay the transition from laminar to turbulent flow, including a 1977 Russian experiment in which women volunteered to be dragged naked through the water to see if ripples caused in soft skin would smooth water flow. They didn't. Although most dolphin skin research has not added up to much, Yoshimichi Hagiwara, a professor of mechanical engineering at the Kyoto Institute of Technology, has not given up.

Hagiwara got interested in dolphin drag one day when he visited the Echizen Matsushima Aquarium in Mikuni, Japan. There he learned a curious fact: Dolphins shed the entire outer layer of their skin every two hours. Hagiwara wondered what advantage such extreme "dandruff" offered to justify the additional food needed to produce this shedding, especially because dolphins require far more nourishment than most land mammals, ingesting 4 to 5 percent of their body weight a day.

Hagiwara fed what he knew about dolphins into computer programs that examine water movement across ships' hulls or through pipes. He adjusted his models to map individual flakes of skin as they twirled through the water. Because he wanted to compare those results with live-action motion, he and his students Hiroshi Nagamine and Kenji Yamahata built themselves a dolphin—a long, rectangular Lucite channel supported by what looks a lot like a giant Erector Set. The skin for this dolphin was made of rubber silicon, the dandruff made of silver glitter glued on with water-soluble glue. Once in the water, the glitter flaked off over time, simulating the real thing. Water flowing across the skin formed tiny vortices as expected, but the flakes of dandruff helped disrupt the vortices, damping turbulence. The Lucite dolphin findings have been backed up by computer models. Hagiwara's research is continuing because scientists still know very little about how dolphins actually swim.

"The trouble is the dolphin doesn't give up its secrets very easily," says Fish. "Maybe that's why it's smiling."

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