no time....

No time to transfer thoughts to words lately thus an article from Aquanews:

Physics of Fluids

A multi-institutional research team from the United States has made a startling discovery in the field of aerodynamics.

Wind tunnel tests of scale-model humpback whale flippers have revealed that the scalloped, bumpy flipper is a more efficient wing design than the current model used by the aeronautics industry on airplanes.

The tests show that bump-ridged flippers do not stall as quickly and produce more lift and less drag than sleek flippers of a comparable size.
The study, involving researchers from West Chester University, Duke University and the U.S. Naval Academy, required the creation of two 22-inch-tall scale models of humpback pectoral flippers -- one with the characteristic bumps, called tubercles, and one without.

The performance of the sleek flipper was similar to that of a typical airplane wing. But the tubercle flipper exhibited nearly 8 percent better lift properties, and withstood stall at a 40 percent steeper wind angle. The team was particularly surprised to discover that the flipper with tubercles produced as much as 32 percent lower drag than the sleek flipper.

"The simultaneous achievement of increased lift and reduced drag results in an increase in aerodynamic efficiency," explained Laurens Howle, a fluid dynamics engineer at Duke University and co-author of the study.

Hydrodynamic Flippers

As whales move through the water, the tubercles disrupt the line of pressure against the leading edge of the flippers. The row of tubercles sheers the flow of water and redirects it into the scalloped valley between each tubercle, causing swirling vortices that roll up and over the flipper to actually enhance lift properties.

"The swirling vortices inject momentum into the flow," said Howle. "This injection of momentum keeps the flow attached to the upper surface of the wing and delays stall to higher wind angles."

The purpose of the tubercles on the leading edge of humpback whale flippers has been the source of speculation for some time, said biomechanicist Frank Fish of West Chester University, a co-author of the study. "The idea they improved flipper aerodynamics was so counter to our current doctrine of fluid dynamics, no one had ever analyzed them," he said.

Humpback whales maneuver in the water with surprising agility for 44-foot animals, particularly when they are hunting for food. By exhaling air underwater as they turn in a circle, the whales create a cylindrical wall of bubbles that herd small fish inside. Then they barrel up through the middle of the "bubble net," mouth open wide, to scoop up their prey.

Manmade designs

According to Fish, the scalloped hammerhead shark is the only other marine animal with a similar aerodynamic design. The expanded hammerhead shark head may act like a wing.

The trick now is to figure out how to incorporate the advantage of the tubercle flipper into manmade designs, said Fish.

The research team now plans to perform a systematic engineering investigation of the role of scalloped leading edges on lift increase, drag reduction and stall delay.

"This discovery has potential applications not only to airplane wings but also on the tips of helicopter rotors, airplane propellers and ship rudders," said Howle.

Their findings were published in the May 2004 issue of Physics of Fluids.