Supercavitation
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Supercavitation is the use of cavitation effects to create a large bubble of gas inside a liquid, allowing an object to travel at great speed through the liquid by being wholly enveloped by the bubble. The cavity (i.e., the bubble) reduces the drag on the object and precisely this makes supercavitation an attractive technology: drag is normally about 1,000 times greater in water than in air.
In 1977, Russian engineers developed the first projectile to use supercavitation: the VA-111 Shkval ("Squall") torpedo. This can travel at 230 mph (100 m/s) underwater, compared to the top speed of about 80 mph (35 m/s) for conventional aquatic craft, but it is reportedly not steerable. Even faster speeds of about 310 mph (ca. 140 m/s) and higher have also been rumored. News of the device reached the West in the 1990s. A malfunctioning Shkval torpedo has been officially alleged to have been the cause of the destruction of the K-141 Kursk submarine.
The Naval Undersea Warfare Center in Newport, Rhode Island, USA is now also working on the phenomenon.
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From cavitation to supercavitation
To hydroengineers, cavitation is a known phenomenon. Cavitation happens when water is forced to move at extremely high speed, e.g. inside of a pump or around an obstacle, such as a rapidly spinning propeller. The pressure of the fluid drops due to its high speed (Bernoulli's principle) and when the pressure drops below the vapor pressure of the water, it vaporizes — typically forming small bubbles of water vapour, i.e. of water in its gas phase. In ordinary hydrodynamics, cavitation is a mostly unintended and undesirable phenomenon: the bubbles are typically not sustained but implode as they and the water around them suddenly slows down again, with a resulting sudden rise in ambient pressure. These small implosions can even lead to physical damage, e.g., to badly designed fast-rotating propellers.
A supercavitating object uses this phenomenon in a much larger (and sustained) manner, hence the name. A supercavitating object's main features are a specially shaped nose, typically flat with sharp edges, and a streamlined, aquadynamical and aerodynamical shape. When the object is traveling through water at speeds of above roughly one-hundred miles per hour, the water — which needs to avoid the object it is being displaced by — is forced to move around the flat, sharp nose so fast that it vaporizes. In other words, cavitation occurs. However, given sufficient speed and a suitable shape of the object, the (intended) cavitation can extend as a single large bubble of water vapour, enveloping the entire object. This generation and utilization of this very large gas bubble is what is called supercavitation. A supercavitating object quite literally 'flies' through the gas it is enveloped by. New gas is constantly being generated at its nose, while the water vapor condenses again to water behind the tail of the object. Various underwater methods of propulsion have been proposed to reach the necessary speed, with a possible concept being a rocket engine burning aluminium with water. The use of conventional propellers or turbines is not an option because the very hydrodynamic effects that make them work are disrupted by cavitation.
Current applications
As of 2004, Russian Shkval torpedoes are the only publicly known existing application of supercavitation technology. It has also been claimed that Russia (formerly the Soviet Union) also possessed underwater firearms discharging supercavitating projectiles, which are said to have been developed prior to the Shkval torpedoes.
In 1999 the supercavitation technology was adopted to hunting projectiles. These "SuperPenetrator" bullets feature a very stable straight line penetration in aqueous media.[1] (http://www.grosswildjagd.de/penetrat.htm)
To date, the main emphasis of research into supercavitation has been into the development of torpedoes, due to the fact that supercavitating torpedoes can give an overwhelming advantage to a navy possessing them in quantity (assuming that the opposing navy doesn't possess them).
Development of supercavitating craft
It has also been proposed by science fiction authors that it may become possible in the future to build manned supercavitating submarine craft. Such underwater vessels would possibly more resemble fighter aircraft than ordinary submarines and it is even relatively plausible that it might one day become possible to construct rocket propelled hybrid craft, which would be able to fly in the air and supercavitate underwater (as also envisaged by science fiction authors).
References
- Scientific American, May 2001, "Warp Drive Underwater"
External link
- Deep Angel (http://www.deepangel.com/) — Science fiction website centered around envisaged supercavitation technology