BUBBLES can be trouble, especially for naval vessels, because air bubbles
make their wakes easy to spot from the air. But the US Navy has just patented a
technique that could make ships harder to find鈥攂y eradicating the telltale
bubbles as soon as they appear.
As a ship pushes its way through the sea, pockets of air get trapped in the
water flowing around it. These bubbles get caught up in the strong
counter-rotating currents created by the propellers. 鈥淏ig bubbles stay in the
wake for a relatively short time because they are more buoyant,鈥� says Robert
Kuklinski at the Naval Undersea Warfare Center in Newport, Rhode Island. 鈥淏ut
smaller bubbles can be caught in the wake for a really long time.鈥�
It鈥檚 those small bubbles that make ship wakes easy to spot from the air
because they scatter light well, says Kuklinski. 鈥淚f there weren鈥檛 any bubbles
in that wake, you wouldn鈥檛 see anything.鈥� In a test to remove bubbles, Kuklinski
set up a series of transducers that pumped 1-megahertz acoustic waves into water
in a tank. These waves interfered with one another, producing a
three-dimensional grid of high and low-pressure pockets. Kuklinski found that
small bubbles鈥攁bout 0.2 millimetres across鈥攄rifted into the
low-pressure regions where they formed bigger bubbles, about 1.5
millimetres across. This gave them enough buoyancy to float to the surface.
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Kuklinski says that the same system could be used at sea
(see Diagram). 鈥淭he
tests I鈥檝e done in big water tanks worked surprisingly well,鈥� says Kuklinski.
鈥淚t would take maybe a minute to make the tank look clear.鈥�
Guy Meadows, whose research group at the University of Michigan in Ann Arbor
has studied ship wakes for the US Navy, says bubbles in the wake can be a
significant problem鈥攖hey are as long as a kilometre for some military
vessels. Radar systems mounted on satellites can also detect ship wakes, but for
pilots hoping to intercept ships, real-time identification is vital.FIG-mg22631801.JPG
While pumping acoustic waves into the ocean can pose dangers for the hearing of sea creatures
(快猫短视频, 24 June, p 15), Dave Goodson of the
underwater acoustics group at Loughborough University says Kuklinski鈥檚 system
works at a frequency way above their hearing thresholds鈥攁nd should be
harmless.