While analyzing sails for racing yachts, a team of aerodynamicists has stumbled across a way in which the downward force from air flowing past an odd-shaped object can suddenly turn into a hefty upward lift. Combining the classic but usually disparate physics of airplane wings and golf balls, the surprising reversal might serve to make a new type of mechanical switch that would flip off or on depending on how fast fluid flows past it or perhaps to stabilize machines like underwater gliders.
"It's intriguing," says G.M. "Bud" Homsy, an expert in fluid dynamics at the University of Washington in Seattle. "I've never seen anything like it."
The effect is striking. Take an asymmetrical blunt object such as a pipe sawed in half lengthwise and place it rounded side up in flowing air. At low wind speeds, the air will deflect upward from the pipe's rounded side and push the pipe down—giving it so-called negative lift. However, if the speed increases beyond a threshold, the object will suddenly experience a large upward positive lift, like an airplane wing, as Patrick Bot of the Naval Academy Research Institute in Brest, France, and Marc Rabaud of the University of Paris-South and colleagues report in a paper in press at Physical Review Letters.
The effect melds the physics of balls and airplane wings. As a ball barges forward, the air must part and flow around it. At low speeds, the flow doesn't wrap all the way around the ball, but separates from it at the ball's widest part, creating a wake of whorls and eddies that stretches out behind the ball like the flapping tail of a kite. That "turbulent wake" pulls on the ball, creating drag and slowing its forward progress.
In contrast, a wing—which is typically 10 times wider than it is thick—produces little drag and a large lift, for two reasons. The wing's upper surface arches upward, whereas its bottom surface is relatively flat. Also, the wing sits at an angle with its leading edge slightly higher than its trailing one. The combination of factors ensures that air flows faster over the wing than under it. According to a fundamental concept in fluid dynamics called Bernoulli's principle, faster moving air has lower pressure than slower moving air. So the pressure below the wing exceeds the pressure above it and the wing is pushed upward.
At first blush, the researchers' half-pipe more closely resembles a ball than a wing. At low flow speeds it suffers high drag and negative lift. But as the air speed increases, the lift suddenly reverses and grows. To understand why, one must return to the physics of balls.
Balls suffer high drag because they produce big turbulent wakes. However, if a ball flies fast enough the drag plummets. Above a certain speed—measured by a parameter called the Reynolds number, which accounts for ball's size and the air's viscosity—the air on the surface of the ball itself becomes turbulent. That thin layer of turbulence acts as a lubricant that allows the rest of the air to flow smoothly all the way around the ball, minimizing the size of the turbulent wake and the drag—a phenomenon known as the drag crisis. By design, a golf ball’s dimples produce just such a turbulent layer and minimize its drag.
The drag crisis also explains the sudden lift on the researchers' asymmetrical half-pipe. As the flow speed exceeds the critical Reynold's number, the air suddenly streams around the entire object. It then follows the contours of the arched shape, so that the air flows faster over the object than below it, producing positive lift.
To demonstrate the effect, researchers put their half-pipe in a tank filled with flowing water instead of air and measured the forces on it. As they ramped up the flow, the drag crisis set in, the drag plummeted, and the lift force reversed and surged. The researchers visualized the flow by lacing the water with tiny plastic beads and tracking them with laser light, showing that as the drag crisis set in, the object's wake shrank and the water flow stretched smoothly around the object.
The scientists tripped over the effect as they modeled rounded sails called spinnakers that balloon out in front of boats racing downwind. They noticed odd reversals in the lift, leading them to investigate further, Bot says. "Maybe one of the reasons [the effect] hasn’t been seen before is that it's a little bit in between two fields of application"—drag and lift—he says.
Christophe Clanet, a physicist at the the École Polytechnique in Palaiseau, France, notes that researchers would never consider the half-pipe a wing because, among other things, it is set horizontally—which is key to observing the switching. A wing, by contrast, is always set at an angle to guarantee it produces lift, he says. Clanet also notes that such overlooked oddities pop up from time to time. "I'm surprised you're surprised," he says.
The effect might serve to make a simple mechanical switch that could be, say, inserted into a pipe to control the flow of fluid through it. Perhaps most speculatively, Bot says, such switches might control and stabilize an aircraft or underwater glider in a completely mechanical way, without the need for electrical sensors and powered control systems. Such a passive system might increase reliability and safety. "You should be able to make a very robust control system that doesn't rely on electricity or power."