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| Shape-shifting skin to reduce drag on planes and subs |
| 2008-04-16 |
| Aircraft or submarines covered with an undulating skin able to change at a flick of a button would experience 50% less drag than conventional vehicles. This trick, which naturally occurs in dolphins, is now being tested by human engineers. Clever dolphins. Turbulence is the bane of engineers' lives. Chaotic air flow sets up unstable vortices and patterns in gases and liquids, increasing friction and drag. Giving craft skin than can tweak its surface to impose order on these currents could dramatically cut the effect of drag, says Dimitris Lagoudas at Texas A&M University, US. Calming the chaotic waves makes them interact less with the skin. "The particles in the fluid stop "speaking" to the craft’s surface," he says. Lagoudas and colleagues have worked out that wrinkling the surface of a craft in the right way can cut problems. The surface must assume the shape of the ideal ordered surface wave it is trying to create, something that changes at different velocities. It might seem counterintuitive to reduce drag by wrinkling the surface of a craft, but nature provides a precedent. "Dolphins induce their skin to wrinkle, so water won’t stick to them," says Lagoudas. After calculating that this approach would work, his team tested designs for an "active skin" that shifts to the shape of an ideal surface wave. One design uses "legs" just beneath the skin that lengthen under the influence of an electric field, bending the skin upwards. By controlling the field around each piezoceramic leg, Lagoudas’ team can deform the skin into corrugations of right wavelength and amplitude to cut down drag. The corrugations can be at most 30 micrometres high. "We measured flow velocities very close to the skin and derived the skin friction drag – we have seen reductions as much as 50%," says researcher Othon Rediniotis. Lagoudas says the shape-shifting skin approach would work best as cladding for submarines. "It would be feasible to use this on aircraft but more challenging," he explains. "The velocities are higher and so the travelling waves must be higher in frequency." "It’s a novel technique that has been demonstrated to work under lab conditions," says Jonathan Morrison at Imperial College London, UK. "But implementing this in something the size of an aircraft would be pretty daunting." The complexity of the morphing skin might deter designers worried about the consequences were it to fail during flight or an underwater mission, says Morrison. But Morrison he adds that the skin does not have to be so complex to be useful - corrugated skin with a fixed shape could also cut drag. "You could design it to be most effective while cruising," he says, for example the speed an aircraft maintains for most of its flight. Static skin like that would also have to be designed not to work under some conditions, he adds. "When you’re coming to land you actually want the drag." |
| Posted by:gorb |
| #4 I read a paper on this years ago, the wrinkles hold pockets of air. Air on air friction is much less than air on skin friction. I don't know if it would be quiet at all though. |
| Posted by: bigjim-ky 2008-04-16 22:35 |
| #3 Welcome to the world of smart materials. |
| Posted by: phil_b 2008-04-16 21:15 |
| #2 Congratulations to Professor Lagoudas and Texas A&M. He has taken a major step forward in an area engineers have been trying to improve for decades. |
| Posted by: Penguin 2008-04-16 19:19 |
| #1 Sort of like golf balls? Or all the hail dents on my car? |
| Posted by: Glenmore 2008-04-16 19:02 |