Mimicking the bumps on humpback-whale fins could lead to more efficient wind turbines.
Marine scientists have long suspected that humpback whales’
incredible agility comes from the bumps on the leading edges of their
flippers. Now Harvard University researchers have come up with a
mathematical model that helps explain this hydrodynamic edge. The work
gives theoretical weight to a growing body of empirical evidence that
similar bumps could lead to more-stable airplane designs, submarines
with greater agility, and turbine blades that can capture more energy
from the wind and water.
“We were surprised that we were able to replicate a lot of the
findings coming out of wind tunnels and water tunnels using relatively
simple theory,” says Ernst van Nierop,
a PhD candidate at the School of Engineering and Applied Sciences at
Harvard. He coauthored the study with mathematics professor Michael
Brenner and researcher Silas Alben.
The advantage of the humpback-whale flipper seems to be the angle of
attack it’s capable of–the angle between the flow of water and the
face of the flipper. When the angle of attack of a whale flipper–or an
airplane wing–becomes too steep, the result is something called stall.
In aviation, stall means that there isn’t enough air flowing over the
top surface of the wing. This causes a combination of increased drag
and lost lift, a potentially dangerous situation that can result in a
sudden loss of altitude. Previous experiments have shown, however, that
the angle of attack of a humpback-whale flipper can be up to 40 percent
steeper than that of a smooth flipper before stall occurs.
In a paper recently published in Physical Review Letters and highlighted in the journal Nature,
the Harvard research team showed that the bumps on the humpback
flipper, known as tubercles, change the distribution of pressure on the
flipper so that some parts of it stall before others. Since different
parts of the flipper stall at different angles of attack, abrupt
stalling is easier to avoid. This effect also gives the whale more
freedom to attack at higher angles and the ability to better predict
its hydrodynamic limitations.
The researchers also found that the amplitude of the bumps plays a
greater role than the number of bumps along a flipper’s leading edge.
“The idea is, you could make an aircraft that’s much harder to stall
and easier to control,” says van Nierop. For example, fighter jets
could be designed to be more acrobatic without risk of stall-induced
crashes. In the water, naval submarines could be made more nimble.
The Harvard research validates the first controlled wind-tunnel
tests of model flippers, conducted five years ago at the U.S. Naval
Academy, in Annapolis, MD, where it was shown that stall typically
occurring at a 12-degree angle of attack is delayed until the angle
reaches 18 degrees. In these tests, drag was reduced by 32 percent and
lift improved by 8 percent.
That research was detailed in a 2004 study
in collaboration with West Chester University and Duke University.
“This [Harvard work] basically shows that theory and empirical
measurements are close, and adds greater weight to our original
assertion on the function of the tubercles,” says Frank Fish, a biology
professor at West Chester and a lead author of the original study.
Already, attempts are being made to incorporate the tubercle design
into commercial products. Fish is president of a venture based in
Toronto, Ontario, called WhalePower,
which has begun demonstrating the advantages of tubercles when they’re
integrated into the leading edges of wind-turbine and fan blades.
Prototypes of wind-turbine blades (see image below) have shown that
the delayed stall doubles the performance of the turbines at wind
speeds of about 17 miles per hour and allows the turbine to capture
more energy out of lower-speed winds. For example, the turbines
generate the same amount of power at 10 miles per hour that
conventional turbines generate at 17 miles per hour. The tubercles
effectively channel the air flow across the blades and create swirling
vortices that enhance lift.
WhalePower, based in Toronto, Ontario, is testing this
wind-turbine blade at a wind-testing facility in Prince Edward Island.
The bumps, or “tubercles,” on the blade’s leading edge reduce noise,
increase its stability, and enable it to capture more energy from the
Stephen Dewar, director of research and development at WhalePower, says that ongoing tests
at the Wind Energy Institute of Canada, in the province of Prince
Edward Island, have shown the tubercle-lined blades to be more stable,
quiet, and durable than conventional blades. “The turbine has survived
being hit by the edge of a hurricane, and it survived wind-driven snow
and ice,” he says.
WhalePower has also shown in demonstrations that tubercle-lined
blades on industrial ceiling fans can operate 20 percent more
efficiently than conventional blades can, and they do a better job at
circulating air flow in a building. The results were dramatic enough to
convince Canada’s largest maker of ventilation fans to license the
design, which will appear in a new line of products scheduled for
release at the end of April.
“This licensing agreement with the fan company is great,” says Fish.
“It basically shows one of the many potential applications for this
technology. The union of biology and engineering through biomimetics
will make future innovations possible.”
The Harvard study reaches the same conclusion. “It is possible that
the lessons learned from humpback-whale flippers will soon find their
way into the design of special-purpose wings, hydrofoils, as well as
wind turbine and helicopter blades.”