Dolphins zoom with ‘delta-wing’ flippers

DUKE (US)—Much like the wings of a Concorde jet, a dolphin’s swept-back flippers help it lift at low speeds and move efficiently when zooming through the sea.

Engineers from Duke University and researchers from the U.S. Naval Academy and West Chester University conducted a study to systematically compare the hydrodynamic properties of the flippers of several kinds of dolphins, porpoises, and whales.

“We found that flippers of similar shape had similar hydrodynamic performance properties,” says Paul Weber, a graduate student in mechanical engineering and materials science at Duke’s Pratt School of Engineering working with senior researcher Laurens Howle, Duke associate professor of mechanical engineering and materials science.

For their experiments, Weber and Howle collected flippers from dead, stranded animals and performed complete computed tomography (CT) scans on them. The scans were then turned into 3-D renderings, which became the basis for the creation of scale-model flippers.

“The CT scans allowed us to recreate as closely as possible the shape, structure and surface of  each of the flippers,” Weber explains.

The researchers focused on two important forces experienced by flippers during movement—lift, the upward force exerted on the flipper, and drag, the rearward force.

The flippers were categorized as being triangular, swept-pointed or swept-rounded and were then sent to the U.S. Naval Academy where they were put through their paces in water tunnels. Researchers measured the hydrodynamic forces as the flippers’ orientation and water speeds were changed.

When the researchers plotted the results of their experiments in graph form, they found that the lift and drag curves exhibited by the flippers were quite similar to those of hydrofoil surfaces designed by engineers.

“Unexpectedly, we also found a unique lift curve for animals with swept-wing-like flippers,” Weber says. “The same phenomenon occurs in aircraft with delta wings.”

A delta wing is basically a large triangle, named after the uppercase Greek letter “delta,” which in the case of the Concorde generates sufficient lift at low speeds and is highly efficient at high speeds.

By creating models based on real flippers and testing them in water tunnels, the researchers were able to calculate the characteristics of flippers from seven different animals—the Amazon River dolphin, striped dolphin, harbor porpoise, Atlantic white-sided dolphin, bottlenose dolphin, common dolphin, and pygmy sperm whale.

The research proved to be a first step in understanding the association between a flipper’s form and each animal’s ability to exist in its own environment, get food, escape from predators, and mate, Howe explains.

For example, the Amazon River dolphin has larger flippers, since maneuverability—not speed—is essential in its world of rivers and flooded forests. On the other hand, the bottlenose dolphin has smaller, swept flippers for speed in the open ocean.

Howle says that while some studies have focused on flippers of individual species, this is the first comparative study.

“Many factors, including ecology, body shape, and performance requirements, have influenced the evolution of cetacean flippers, and these factors are all linked to hydrodynamic characteristics of the flippers we see today,” Howle adds. “As we continue to evaluate more animals, we will be better able to link these factors together.”

The research was supported by the National Science Foundation. The results appear  in the June 26 issue of the Journal of Experimental Biology.

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