快猫短视频

WRAPPING submarines in a cloak of tiny waves created by a smart 鈥渟kin鈥� could
help them slip swiftly and silently through the water, say researchers in
Massachusetts and Rhode Island.

Fluid flowing past a solid object, such as the hull of a submarine, tends to
get twisted into turbulent eddy currents. This turbulence increases drag, so
it鈥檚 harder for the submarine to move through the water. Naval architects wage a
constant war against turbulence, designing hulls that are as streamlined as
possible. But Yiquing Du of the Massachusetts Institute of Technology in
Cambridge and George Karniadakis of Brown University in Providence, Rhode
Island, have hit on a way to stop turbulence developing in the first place.

Their approach is to create tiny waves that travel across the surface of the
solid object at right angles to the fluid flowing past it. These waves
effectively wrap the object in a watery 鈥渆nvelope鈥�, so that the water flowing
past hits this soft, wet buffer, rather than a hard surface. 鈥淚t shields the
surface from the outer flow,鈥� explains Karniadakis. 鈥淵ou cut off the main
mechanism of producing turbulence.鈥�

According to their computer simulation you only need waves a few micrometres
high to achieve a striking effect. 鈥淲e have at least 30 per cent drag reduction
and we think we can double this,鈥� says Karniadakis. This beats the best existing
method of eliminating turbulence, which uses tiny ribs to alter flow near the
surface and only reduces drag by 10 per cent
(快猫短视频, 18 January 1997, p 28).

The team have also tried out their idea in a test tank where electromagnets
push and pull a mosaic of tiles, to make waves travel across their surface.
Ultimately they want to create a 鈥渟mart skin鈥� of tiny wave-making machines on
the hull of a submarine, for example. Piezoceramic materials, which flutter very
rapidly in response to a voltage, would be ideal for the job, according to
Karniadakis.

This smart skin could be applied to existing hulls without any remodelling.
鈥淭hese are very thin materials,鈥� notes Karniadakis, 鈥淭his doesn鈥檛 require
complicated contouring of surfaces.鈥� And since the waves are so tiny, they
shouldn鈥檛 require much energy to make them. 鈥淵ou don鈥檛 need to move a very large
volume of water,鈥� he says. Karniadakis and his colleague Kenneth Breuer at Brown
University are developing the technology with research funding from the US
Department of Defense.

鈥淭he plan is to have a prototype submerged vehicle and see how fast it goes,鈥�
says Karniadakis. As well as increasing speed, eliminating turbulence could let
a submarine move through the water more quietly and leave less telltale
disturbance behind. 鈥淭he Navy are very interested in the noise aspect,鈥� says
Karniadakis.

So far they鈥檝e been able to reduce drag in simulations only at comparatively
low flow speeds, as modelling high speeds needs massive number crunching. 鈥淏ut
there isn鈥檛 any `fudge鈥� in the calculation,鈥� comments Kwing-So Choi of the
University of Nottingham. However, Choi鈥攚ho works on reducing drag in
pipelines using a different kind of wave鈥攚ould like to see experiments
showing the effect at high flow speeds.