Supercavitation
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Supercavitation is the phenomenon of a cavitation bubble reducing skin friction drag on a submerged object and enabling high speeds. Applications include torpedoes and
propeller A propeller (often called a screw if on a ship or an airscrew if on an aircraft) is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon a working flu ...
s, but in theory, the technique could be extended to an entire underwater vessel.


Physical principle

Cavitation Cavitation in fluid mechanics and engineering normally is the phenomenon in which the static pressure of a liquid reduces to below the liquid's vapor pressure, leading to the formation of small vapor-filled cavities in the liquid. When sub ...
is the formation of vapour bubbles in liquid caused by flow around an object. Bubbles form when water accelerates around sharp corners and the pressure drops below the vapour pressure. Pressure increases upon deceleration, and the water generally reabsorbs the vapour; however, vapour bubbles can implode and apply small concentrated impulses that may damage surfaces like ship propellers and pump impellers. The potential for vapour bubbles to form in a liquid is given by the nondimensional cavitation number. It equals local pressure minus vapour pressure, divided by dynamic pressure. At increasing depths (or pressures in piping), the potential for cavitation is lower because the difference between local pressure and vapour pressure is greater. A supercavitating object is a high-speed submerged object that is designed to initiate a cavitation bubble at its nose. The bubble extends (either naturally or augmented with internally generated gas) past the aft end of the object and prevents contact between the sides of the object and the liquid. This separation substantially reduces the skin friction drag on the supercavitating object. A key feature of the supercavitating object is the nose, which typically has a sharp edge around its perimeter to form the cavitation bubble. The nose may be articulated and shaped as a flat disk or cone. The shape of the supercavitating object is generally slender so the cavitation bubble encompasses the object. If the bubble is not long enough to encompass the object, especially at slower speeds, the bubble can be enlarged and extended by injecting high-pressure gas near the object's nose. The very high speed required for supercavitation can be temporarily reached by underwater-fired projectiles and projectiles entering water. For sustained supercavitation, rocket propulsion is used, and the high-pressure rocket gas can be routed to the nose to enhance the cavitation bubble. In principle, supercavitating objects can be maneuvered using various methods, including the following: * Drag fins that project through the bubble into the surrounding liquid * A tilted object nose * Gas injected asymmetrically near the nose to distort the cavity's geometry * Vectoring rocket thrust through gimbaling for a single nozzle * Differential thrust from multiple nozzles


Applications

The Russian Navy developed the VA-111 Shkval supercavitation torpedo, which uses rocket propulsion and exceeds the speed of conventional torpedoes by at least a factor of five. NII-24 began development in 1960 under the code name "Шквал" (Squall). The VA-111 Shkval has been in service (exclusively in the Russian Navy) since 1977 with mass production starting in 1978. Several models were developed, with the most successful, the M-5, completed by 1972. From 1972 to 1977, over 300 test launches were conducted (95% of them on Issyk Kul lake). In 2006, German weapons manufacturer Diehl BGT Defence announced their own supercavitating torpedo, the Barracuda, now officially named ' (). According to Diehl, it reaches speeds greater than . In 1994, the
United States Navy The United States Navy (USN) is the naval warfare, maritime military branch, service branch of the United States Department of Defense. It is the world's most powerful navy with the largest Displacement (ship), displacement, at 4.5 millio ...
began development of the Rapid Airborne Mine Clearance System (RAMICS), a sea mine clearance system invented by C Tech Defense Corporation. The system is based on a supercavitating projectile stable in both air and water. RAMICS projectiles have been produced in diameters of , , and . The projectile's terminal ballistic design enables the explosive destruction of sea mines as deep as with a single round. In 2000 at
Aberdeen Proving Ground Aberdeen Proving Ground (APG) is a U.S. Army facility located adjacent to Aberdeen, Harford County, Maryland, United States. More than 7,500 civilians and 5,000 military personnel work at APG. There are 11 major commands among the tenant units, ...
, RAMICS projectiles fired from a hovering Sea Cobra gunship successfully destroyed a range of live underwater mines. As of March 2009,
Northrop Grumman Northrop Grumman Corporation is an American multinational Aerospace manufacturer, aerospace and Arms industry, defense company. With 97,000 employees and an annual revenue in excess of $40 billion, it is one of the world's largest Arms industry ...
completed the initial phase of RAMICS testing for introduction into the fleet.
Iran Iran, officially the Islamic Republic of Iran (IRI) and also known as Persia, is a country in West Asia. It borders Iraq to the west, Turkey, Azerbaijan, and Armenia to the northwest, the Caspian Sea to the north, Turkmenistan to the nort ...
claimed to have successfully tested its first supercavitation torpedo, the Hoot (Whale), on 2–3 April 2006. Some sources have speculated it is based on the Russian VA-111 Shkval supercavitation torpedo, which travels at the same speed. Russian Foreign Minister Sergey Lavrov denied supplying Iran with the technology. In 2004,
DARPA The Defense Advanced Research Projects Agency (DARPA) is a research and development agency of the United States Department of Defense responsible for the development of emerging technologies for use by the military. Originally known as the Adva ...
announced the Underwater Express program, a research and evaluation program to demonstrate the use of supercavitation for a high-speed underwater craft application. The US Navy's ultimate goal is a new class of underwater craft for littoral missions that can transport small groups of navy personnel or specialized military cargo at speeds up to 100 knots. DARPA awarded contracts to Northrop Grumman and
General Dynamics Electric Boat General Dynamics Electric Boat (GDEB) is a subsidiary of General Dynamics Corporation. It has been the primary builder of submarines for the United States Navy for more than 100 years. The company's main facilities are a shipyard in Groton, Co ...
in late 2006. In 2009, DARPA announced progress on a new class of submarine: A prototype ship named the ''
Ghost In folklore, a ghost is the soul or Spirit (supernatural entity), spirit of a dead Human, person or non-human animal that is believed by some people to be able to appear to the living. In ghostlore, descriptions of ghosts vary widely, from a ...
'', uses supercavitation to propel itself atop two struts with sharpened edges. It was designed for stealth operations by Gregory Sancoff of Juliet Marine Systems. The vessel rides smoothly in choppy water and has reached speeds of 29 knots. The Chinese Navy and
US Navy The United States Navy (USN) is the naval warfare, maritime military branch, service branch of the United States Department of Defense. It is the world's most powerful navy with the largest Displacement (ship), displacement, at 4.5 millio ...
are reportedly working on their own supercavitating
submarine A submarine (often shortened to sub) is a watercraft capable of independent operation underwater. (It differs from a submersible, which has more limited underwater capability.) The term "submarine" is also sometimes used historically or infor ...
s using technical information obtained on the Russian VA-111 Shkval supercavitation torpedo. A supercavitating propeller uses supercavitation to reduce water skin friction and increase propeller speed. The design is used in military applications, high-performance
racing In sports, racing is a competition of speed, in which competitors try to complete a given task in the shortest amount of time. Typically this involves traversing some distance, but it can be any other task involving speed to reach a specific g ...
boat A boat is a watercraft of a large range of types and sizes, but generally smaller than a ship, which is distinguished by its larger size or capacity, its shape, or its ability to carry boats. Small boats are typically used on inland waterways s ...
s, and model racing boats. It operates fully submerged with wedge-shaped blades to force cavitation on the entire forward face, starting at the leading edge. Since the cavity collapses well behind the blade, the supercavitating propeller avoids spalling damage caused by cavitation, which is a problem with conventional propellers. Supercavitating ammunition is used with German and Russian underwater firearms, and other similar weapons.


Alleged incidents

The ''Kursk'' submarine disaster was initially thought to have been caused by a faulty Shkval supercavitating torpedo, though later evidence points to a faulty 65-76 torpedo.


See also

* Supercavitating torpedo * "Shkval" supercavitating torpedo * APS amphibious rifle * SPP-1 underwater pistol * Supercavitating propeller


References

{{Reflist


Further reading

* Office of Naval Research (2004, June 14). Mechanics and energy conversion: high-speed (supercavitating) undersea weaponry (D&I). Retrieved April 12, 2006, fro
Office of Naval Research Home Page
* Savchenko Y. N. (n.d.). CAV 2001 - Fourth Annual Symposium on Cavitation - California Institute of Technology Retrieved April 9, 2006, archived a
Wayback Machine
* Hargrove, J. (2003). Supercavitation and aerospace technology in the development of high-speed underwater vehicles. In ''42nd AIAA Aerospace Sciences Meeting and Exhibit''. Texas A&M University. * Kirschner et al. (2001, October) Supercavitation research and development. ''Undersea Defense Technologies'' * Miller, D. (1995). Supercavitation: going to war in a bubble. ''Jane's Intelligence Review''. Retrieved Apr 14, 2006, fro
Defence & Security Intelligence & Analysis , Jane's 360
* Graham-Rowe, & Duncan. (2000). Faster than a speeding bullet. ''NewScientist'', 167(2248), 26–30. * Tulin, M. P. (1963). Supercavitating flows - small perturbation theory. Laurel, Md, Hydronautics Inc. * Niam J W (Dec 2014), Numerical Simulation Of Supercavitation


External links


Supercavitation Research Group at the University of Minnesota

Diehl BGT Defence's "Barracuda" - a German supercavitating Torpedo

DARPA Underwater Express Program



How to Build a Supercavitating Weapon, Scientific American
Fluid dynamics