A Groovy Way to Beat Drag
Roughing up a surface sounds like an unpromising approach to making it glide more easily though air and water. But sometimes intuition can be completely wrong. In tomorrow's issue of Nature, researchers present evidence that a randomly grooved surface actually reduces the drag generated when a fluid flows past it at high speeds--a finding that, if confirmed, might smooth the passage of everything from cars to planes.
It's not the first claim that a grooved surface can reduce drag. In the 1980s, a winner of the America's Cup sailing race attributed his victory to several precisely etched grooves called riblets in the hull of the boat, but physicists never agreed on exactly how the grooves worked. One idea was that the grooves stabilized so-called roll modes, spinning cylinders of fluid that form next to a moving surface and are a major source of drag. Lawrence Sirovitch and Sture Karlsson of Brown University, however, thought that destroying rather than stabilizing the rolls might reduce drag even further. Earlier work had revealed another kind of wave that might be used to destroy the roll modes: regular slow moving waves called oblique plane waves, that can be created with grooves.
To test this idea, they compared the friction coefficient of air blowing through channels with a smooth floor, a floor with aligned grooves, and one with randomly placed grooves. The aligned grooves increased the production of the rolls and their energy, and drag increased by 20% compared to the smooth channel. In the channel with random grooves, however, the energy of the rolls was reduced, and drag dropped by 13%.
If the strategy could be translated into low drag surfaces for aircraft, says Sirovitch, "Fuel savings could run into many billions of dollars per year." But it is unlikely that airplanes, automobiles, or boats with grooved surfaces will appear overnight. "Drag reduction is a tricky business," says Katepalli Sreenivasan, an engineer at Yale . "These are very interesting results, but I would like to see them repeated and then start looking for explanations and applications."