Saturday, July 1, 2017

Chinese engineers master “Super Cavitation”

Chinese engineers master “Super Cavitation”

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Shanghai to San Francisco in 100 minutes by Chinese supersonic submarine
China has moved a step closer to creating a supersonic submarine that
could travel from Shanghai to San Francisco in less than two hours.
New technology developed by a team of scientists at Harbin Institute of
Technology’s Complex Flow and Heat Transfer Lab has made it easier for a
submarine, or torpedo, to travel at extremely high speeds underwater.
Li Fengchen, professor of fluid machinery and engineering, said the team’s
innovative approach meant they could now create the complicated air “bubble”
required for rapid underwater travel. “We are very excited by its potential,”
he said.
Water produces more friction, or drag, on an object than air, which means
conventional submarines cannot travel as fast as an aircraft. However, during
the cold war, the Soviet military developed a technology called supercavitation,
which involves enveloping a submerged vessel inside an air bubble to avoid
problems caused by water drag.
A Soviet supercavitation torpedo called Shakval was able to reach a speed
of 370km/h or more – much faster than any other conventional torpedoes.
In theory, a supercavitating vessel could reach the speed of sound underwater,
or about 5,800km/h, which would reduce the journey time for a transatlantic
underwater cruise to less than an hour, and for a transpacific journey to
about 100 minutes, according to a report by California Institute of Technology
in 2001.
However, supercavitation technology has faced two major problems. First, the
submerged vessel has needed to be launched at high speeds, approaching 100km/h,
to generate and maintain the air bubble.
Second, it is extremely difficult – if not impossible – to steer the vessel
using conventional mechanisms, such as a rudder, which are inside the bubble
without any direct contact with water.
As a result, its application has been limited to unmanned vessels, such as
torpedoes, but nearly all of these torpedoes were fired in a straight line
because they had limited ability to turn.
Li said the team of Chinese scientists had found an innovative means of addressing
both problems. Once in the water, the team’s supercavitation vessel would constantly
“shower” a special liquid membrane on its own surface. Although this membrane would
be worn off by water, in the meantime it could significantly reduce the water drag
on the vessel at low speed.
After its speed had reached 75km/h or more the vessel would enter the supercavitation
state. The man-made liquid membrane on the vessel surface could help with steering
because, with precise control, different levels of friction could be created on
different parts of the vessel.
“Our method is different from any other approach, such as vector propulsion,” or
thrust created by an engine, Li said. “By combining liquid-membrane technology with
supercavitation, we can significantly reduce the launch challenges and make cruising
control easier.”
However, Li said many problems still needed to be solved before supersonic submarine
travel became feasible. Besides the control issue, a powerful underwater rocket engine
still had to be developed to give the vessel a longer range. The effective range of
the Russian supercavitation torpedoes, for example, was only between 11 km and 15 km.
Li said the supercavitation technology was not limited only to military use. In future,
it could benefit civilian underwater transport, or water sports such as swimming.
“If a swimsuit can create and hold many tiny bubbles in water, it can significantly
reduce the water drag; swimming in water could be as effortless as flying in the
sky,” he said.
Besides Russia, countries such as Germany, Iran and the United States have been
developing vessels or weapons using supercavitation technology.
Professor Wang Guoyu, the head of the Fluid Mechanics Laboratory at Beijing Institute
of Technology who is leading another state-funded research project on supercavitation,
said the global research community had been troubled for decades by the lack of
innovative ideas to address the huge scientific and engineering challenges.
“The size of the bubble is difficult to control, and the vessel is almost impossible
to steer,” he said. While cruising at high speed during supercavitation, a fin could
be snapped off if it touched the water because of the  liquid’s far greater density.
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