Wings and Whales

Fluid dynamics:bump-ridged flippers produce more lift

Wind-tunnel tests of scale-model humpback whale flippers have revealed that the scalloped, bumpy flipper is a more efficient wing design than is currently used by the aeronautics industry on airplanes. The tests show that bump-ridged flippers produce more lift and less drag than comparably sized sleek flippers.

The tests were reported by fluid-dynamics engineer Laurens Howle of the Pratt School of Engineering, along with Frank Fish of West Chester University and David Miklosovic and Mark Murray of the U.S. Naval Academy. They reported their findings in the May 2004 issue of Physics of Fluids.

In the study, the team first created two approximately scale models of humpback pectoral flippers--one with the characteristic bumps, called tubercles, and one without. The models were machined at Duke, from thick, clear polycarbonate. Testing was conducted in a low-speed closed-circuit wind tunnel at the U.S. Naval Academy in Annapolis, Maryland.

The sleek flipper performance was similar to a typical airplane wing. But the tubercle flipper exhibited nearly 8 percent better lift properties, and withstood stalling 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," Howle explains.

The findings could have significant implications for airplane wing and underwater vehicle design. Increased lift (the upward force on an airplane wing) at higher wind angles affects how easily airplanes take off and helps pilots maneuver more easily during landing. And improved resistance to stalling would add a new margin of safety to aircraft flight.

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," says Howle. "This injection of momentum keeps the flow attached to the upper surface of the wing and delays stall to higher wind angles."