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The Impact of Flow Cavitation
| Article
# : |
19127 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
1 / 1991 |
2,724 Words |
| Author
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Andrea Prosperetti
Andrea Prosperetti was professor of physics at the University
of Milano, Italy, for 10 years. He is now professor and
chairman of the Department of Mechanical Engineering at Johns
Hopkins University. |
On July 1, 1987, Sugiichiro Watari and Shoichi Saba, president and chairman respectively of the Toshiba Corporation, resigned over a major international blowup caused by the unauthorized sale to the USSR of advanced manufacturing equipment by the Toshiba Machine Company. The machines exported illegally, it was widely reported in the press, would enable the Soviets to produce a new generation of "quiet" submarines and the cost to NATO, for reestablishing a lead in the area of antisubmarine warfare, was estimated to be $30 billion. What was possibly not very clear from these reports was how these machines would have contributed to silencing the Soviet's submarines--by preventing the formation of bubbles on their propellers.
This is but one example of the unsuspected impact that bubbles have on our world, in natural processes as well as in military and civilian technology.
To understand the connection between bubbles and Toshiba it is necessary to say a few words about flow cavitation, that is, the formation of vapor-filled cavities--bubbles--in a flowing liquid.
It is a matter of common knowledge that, at high elevation such as on a mountain, water boils at a lower temperature than at sea level. This is because the boiling temperature of a liquid depends on the pressure: the atmospheric pressure decreases with height, and so does the boiling temperature. One can therefore expect that, if the pressure is decreased sufficiently, water can be "hot" enough to boil even at the temperature of oceanic waters. This of course requires quite a substantial lowering of the external pressure such as the one that results when a liquid flows at high speed (30-50 MPH) past an immersed body such as the propeller blades of submarines or surface ships. Indeed, one of the basic results of Hydrodynamica (fluid mechanics), published in 1738 by the Swiss mathematician and physicist Daniel Bernoulli, establishes a connection between the pressure and velocity of a flowing liquid: the higher the velocity, the lower the pressure, and vice versa. (The actual statement of Bernoulli's theorem is considerably more complex, but this is enough for the present purposes.)
As the propeller blade slices through the water, suitable conditions for this sort of "cold" boiling can then be established, particularly if slight manufacturing imperfections--such as those that the Toshiba machines could avoid--exist. Vapor bubbles will then form at the surface of the propeller just as they form on the bottom of a heated pot. As the bubbles are swept away by the flow, they
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