"Turbulence is everywhere," says Beverley McKeon—from continent-spanning weather systems down to the swirls of air your car leaves behind itself as you drive. "I think about things like ships, planes, and pipelines," she explains, noting that about half of the energy consumed by each of those three transportation systems goes to counteract turbulence-induced drag. In her Watson Lecture on May 16, 2012, McKeon, a professor of aeronautics at Caltech, notes that finding a way to reduce that turbulence by 30 percent would save the global economy well over $100,000,000 dollars in fuel costs annually.
Unfortunately, says McKeon, turbulent drag or "skin friction" is inevitable. Even the smoothest airplane wing is rough on the atomic level. A thin layer of air molecules sticks to this uneven surface, and they pull their neighbors along with them rather than allowing them to slide smoothly by. Those molecules, in turn, pull their neighbors along after them, although not as hard, and so on and so on until eventually the air flows around the wing undisturbed. Or, looking at it from the aircraft's point of view, the plane has to shoulder its way through a sticky, resistant fluid.
This "boundary layer" between the wing and the free-flowing air is only up to a few handbreadths thick. The turbulent structures within it are chaotic, says McKeon, yet they have their own weird orderliness—a hint that they may be open to manipulation. But controlling them "is like herding cats," she says, noting that her goal is to learn how "to tickle the flow—to make it do what it thinks it wants to do, but is really what I want it to do."
"Taming Turbulence" is available for download in HD from Caltech on iTunesU. (Episode 12)
Written by Douglas Smith