GETTING around can be a real drag. Cars, planes and boats use a lot of energy
just overcoming air or water resistance. Now engineers have come up with a way
of cutting this drag by up to 30 per cent by covering the surface of cars and
ships with tiles that carry millions of tiny jet nozzles.
Any object moving at speed through air or water generates vortices in the
turbulent fluid layers close to its surface. As the vortices collide with each
other, they break up and lift away, causing the drag that slows the object
down.
Slightly disturbing the surface can break up these vortices and reduce drag.
One way to do this is to build smart skins studded with millions of tiny moving
parts that break up the vortices
(快猫短视频, 3 June 2000, p 15).
But while these work fine for a while, the tiny flapping parts can quickly break
off or corrode.
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So Francisco Diez and Werner Dahm at the University of Michigan have taken a
different approach: they use tiny microjets that have no moving parts to break.
The jets, which are spaced less than half a millimetre apart, are
set鈥攗sing microchip manufacturing technology鈥攊n a polymer tile 7
centimetres square.
The jets expel up to 1000 pulses of water or air per second, driven by a tiny
pump that has no moving parts, Diaz and Dahm told the Aerospace Sciences
conference in Reno, Nevada, last week.
The pump uses a phenomenon called the electrokinetic effect to force a tiny
amount of fluid electrolyte up the nozzle jet. Fluid ions, attracted by a pulse
of electricity applied to a polymer electrode, shoot up the nozzle. A thin
plastic membrane stops the electrolyte escaping, but the ions protrude through
it slightly and push a tiny bit of water (in the case of a submarine or boat) or
air (in the case of a sports car or aeroplane) out onto the vehicle鈥檚 surface,
where it disrupts any nearby drag-causing vortices.
To ensure the vortices are destroyed within microseconds, the tiles have
pressure sensors that detect them as soon as they form. The sensors send a
signal to computer chips inside each tile, which choreograph the jets so that
they fire in exactly the pattern needed.
In principle, jet-studded tiling could be used to speed up any fast-moving
vehicle or ship. 鈥淲e had an industry adviser from Ford visiting our lab,鈥� says
Dahm, 鈥渁nd he wants to put the stuff on a race car.鈥�