快猫短视频

COULD rising bubbles really sink a ship? It sounds far-fetched, but lab tests
have proved that bubbles can sink floating objects. The findings add weight to
suggestions that methane bubbles escaping from methane reserves in the seabed
might have been to blame for vessels disappearing in the Bermuda Triangle and
the North Sea.

The Greek mathematician Archimedes realised that for something to float, the
density of the liquid has to be greater than the density of the object. So a
simple argument is that if you mix enough bubbles into a liquid to lower its
average density, an object floating on its surface should sink. People have
suggested that this process is behind the mysterious demise of many ships that
sank for no obvious reason
(快猫短视频, 2 December 2000, p 20).

However, Bruce Denardo at the Naval Postgraduate School in Monterey,
California, was sceptical. He points out that rising bubbles often carry
currents of water up with them, exerting an upwards force on the floating
object. For all but the most violent bubbles, this upward drag might be enough
to keep an object afloat.

Denardo and his colleagues decided to test the theory. They filled a
four-litre glass beaker with water, then fed in air at the bottom at varying
speeds. Then they dropped in steel balls filled with varying amounts of water
and air to see how easily they would sink. If, in the absence of rising bubbles,
the ball only just floated on the surface, switching on the bubbles made it
sink.

鈥淲e were surprised that the theory was confirmed,鈥� says Denardo. 鈥淭his is
just what one might naively expect, but we expected that an upward drag would
辞肠肠耻谤.鈥�

Even so, the case isn鈥檛 closed, Denardo says. Because the experiment was
carried out in a closed container, he thinks upward currents might not have had
room to form. In the open sea, upwellings would form more easily in the region
of the bubbles, while the water would flow downwards again a short distance
away.

Initially this would help a boat to stay afloat. But if the vessel were swept
slightly to one side, it might just hit the down currents and sink.

Denardo concludes that we can鈥檛 rule out the methane theory for ships lost in
the Bermuda Triangle. 鈥淚f a phenomenon can be made to occur in a lab, it
probably occurs somewhere in the natural Universe,鈥� he says.

If bubbles can indeed sink ships, the military might want to use them as a
weapon. Michael Stumborg, a researcher at the US Naval War College in Rhode
Island, has proposed building 鈥渂uoyancy bombs鈥� that would collect and release
bubbles. An underwater vehicle could extract methane from a deposit in the
seabed, then transport it to a point underneath a target ship. 鈥淭he release of
the methane will reduce the buoyancy of the ship and could in principle sink
it,鈥� says Denardo.