supercavitation

{{Short description|Use of a cavitation bubble to reduce skin friction drag on a submerged object}}

Supercavitation is the use of a cavitation bubble to reduce skin friction drag on a submerged object and enable high speeds. Applications include torpedoes and propellers, but in theory, the technique could be extended to an entire underwater vessel.

Physical principle

Cavitation 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.

{{cite journal

| journal=Scientific American

| title=Warp Drive Underwater

| date=May 2001

| first=Steven

| last=Ashley

| volume=284

| issue=5

| pages=70–79| doi=10.1038/scientificamerican0501-70

| bibcode=2001SciAm.284e..70A

}} 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{{cite thesis|url=http://www.aem.umn.edu/research/supercavitation/documents/thesis_eric.pdf|title=thesis_erc.pdf |page=22}}
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,{{Cite web|url=http://www.articlesextra.com/supercavitation-torpedoes.htm|title=Shkval Torpedoes (Barracudas) and super-cavitation – A loophole in physics alarms submarine crew|website=www.articlesextra.com|date=27 July 2023 }}{{Cite web|url=http://www.periscope.ucg.com/mdb-smpl/weapons/minetorp/torpedo/w0004768.shtml#pictures%7B%7Bdead+link%7Cdate=January+2018+%7Cbot=InternetArchiveBot+%7Cfix-attempted=yes+%7D%7D|title=Ucg.com|website=www.periscope.ucg.com|url-status=dead|archive-url=https://web.archive.org/web/20031215221055/http://www.periscope.ucg.com/mdb-smpl/weapons/minetorp/torpedo/w0004768.shtml#pictures%7B%7Bdead+link%7Cdate=January+2018+%7Cbot=InternetArchiveBot+%7Cfix-attempted=yes+%7D%7D|archive-date=2003-12-15|access-date=2010-03-23}} 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).{{Citation needed|date=December 2014}}

In 2006, German weapons manufacturer Diehl BGT Defence announced their own supercavitating torpedo, the Barracuda, now officially named {{lang|de|Superkavitierender Unterwasserlaufkörper}} ({{langx|en|supercavitating underwater projectile}}). According to Diehl, it reaches speeds greater than {{convert|400|km/h}}.{{Cite web |url=http://www.diehl-bgt-defence.de/index.php?id=550 |title=Diehl BGT Defence: Unterwasserlaufkörper |access-date=2006-10-07 |archive-url=https://web.archive.org/web/20090825164316/http://www.diehl-bgt-defence.de/index.php?id=550 |archive-date=2009-08-25 |url-status=dead }}

In 1994, the United States Navy 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 {{convert|12.7|mm|in|abbr=}}, {{convert|20|mm|in}}, and {{convert|30|mm|in}}.{{Cite web|url=https://www.globalsecurity.org/military/systems/munitions/mk258.htm|title=MK258 Armor Piercing, Fin Stabilized, Discarding Sabot-Tracer (APFSDS-T) Hydroballistic Ammo Anti-Mine Projectile|website=www.globalsecurity.org}} The projectile's terminal ballistic design enables the explosive destruction of sea mines as deep as {{convert|45|m|ft|sp=us}} with a single round.{{Cite web|url=http://www.ctechdefense.com/projects.html|title=C Tech Defense Projects: airborne laser targeting and super cavitating projectile technologies|website=www.ctechdefense.com}} In 2000 at Aberdeen Proving Ground, RAMICS projectiles fired from a hovering Sea Cobra gunship successfully destroyed a range of live underwater mines. As of March 2009, Northrop Grumman completed the initial phase of RAMICS testing for introduction into the fleet.{{Cite web|url=https://news.northropgrumman.com/news/releases/northrop-grumman-navy-team-exceeds-expectations-during-mine-clearing-weapon-test|title=Northrop Grumman-Navy Team Exceeds Expectations During Mine-Clearing Weapon Test|website=Northrop Grumman Newsroom|language=en|access-date=2019-09-20}}

Iran 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.{{cite web |title=International Assessment and Strategy Center > Research > China's Alliance with Iran Grows Contrary to U.S. Hopes |url=http://www.strategycenter.net/research/pubID.109/pub_detail.asp |url-status=dead |archive-url=https://web.archive.org/web/20080704124105/http://www.strategycenter.net/research/pubID.109/pub_detail.asp |archive-date=2008-07-04 |access-date=2008-08-06}} [https://web.archive.org/web/20060518151541/http://cns.miis.edu/pubs/other/wmdi060504a.htm] [http://www.foxnews.com/story/0,2933,190219,00.html] {{Webarchive|url=https://web.archive.org/web/20070228010531/http://www.foxnews.com/story/0,2933,190219,00.html|date=2007-02-28}} Russian Foreign Minister Sergey Lavrov denied supplying Iran with the technology.{{cite web|url=http://www.irna.ir/en/news/view/line-22/0604055724212153.htm|title=Irna|url-status=dead|archive-url=https://web.archive.org/web/20070311170151/http://www.irna.ir/en/news/view/line-22/0604055724212153.htm|archive-date=2007-03-11|access-date=2007-02-28}}

In 2004, DARPA 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 in late 2006.{{Citation needed|date=September 2009}} In 2009, DARPA announced progress on a new class of submarine:

{{blockquote|The submarine's designer, Electric Boat, is working on a one-quarter scale model for sea trials off the coast of Rhode Island. If the trials are successful, Electric Boat will begin production on a full-scale 100-foot submarine. Currently, the Navy's fastest submarine can only travel at 25 to 30 knots while submerged. But if everything goes according to plan, the Underwater Express will speed along at 100 knots, allowing the delivery of men and materiel faster than ever.{{Cite web|url=https://www.popsci.com/military-aviation-amp-space/article/2009-07/darpa-readies-ultra-fast-mini-sub/|title=DARPA Readies an Ultra-Fast Mini-Sub|website=Popular Science|date=29 July 2009 }}|sign=|source=}}

A prototype ship named the Ghost, 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.{{cite web|url=http://www.businessweek.com/articles/2014-08-21/juliet-marines-ghost-boat-will-be-hard-sell-to-u-dot-s-dot-navy#r=read|archive-url=https://web.archive.org/web/20140822103341/http://www.businessweek.com/articles/2014-08-21/juliet-marines-ghost-boat-will-be-hard-sell-to-u-dot-s-dot-navy#r=read|url-status=dead|archive-date=August 22, 2014|title=This Stealth Attack Boat May Be Too Innovative for the Pentagon|publisher=Bloomberg BusinessWeek|author=Caroline Winter|date=2014-08-21}}

The Chinese Navy{{Cite web|url=https://www.extremetech.com/extreme/188752-chinas-supersonic-submarine-which-could-go-from-shanghai-to-san-francisco-in-100-minutes-creeps-ever-closer-to-reality|title=China's supersonic submarine, which could go from Shanghai to San Francisco in 100 minutes, creeps ever closer to reality - ExtremeTech|website=www.extremetech.com|date=27 August 2014 }}{{Cite web|url=https://www.scmp.com/news/china/article/1580226/shanghai-san-francisco-100-minutes-chinese-supersonic-submarine|title=Shanghai to San Francisco in 100 minutes by Chinese supersonic submarine|date=August 24, 2014|website=South China Morning Post}}{{Cite web|url=http://www.defensereview.com/chinese-military-developing-supercavitating-supersonic-submarine-for-high-speed-naval-warfare/|title=Chinese Military Developing Supercavitating Supersonic Submarine for High-Speed Naval Warfare|last=Crane+|first=David|website=DefenseReview.com (DR): An online tactical technology and military defense technology magazine with particular focus on the latest and greatest tactical firearms news (tactical gun news), tactical gear news and tactical shooting news.}} and US Navy{{Cite web|url=http://www.defense-aerospace.com/articles-view/release/3/175085/us-navy-looks-to-supercavitating,-supersonic-submarines.html|title=US Navy Is Developing 'Supersonic Submarines' That Could Cut Through the Ocean At the Speed of Sound Using A Bubble|website=www.defense-aerospace.com}} are reportedly working on their own supercavitating submarines 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 boats, 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{{cite web|url=http://world.guns.ru/assault/as69-e.htm|title=Modern Firearms - APS underwater assault rifle|url-status=dead|archive-url=https://web.archive.org/web/20041025183045/http://world.guns.ru/assault/as69-e.htm|archive-date=2004-10-25|access-date=2004-11-07}} underwater firearms, and other similar weapons.{{Cite web |date=2019-12-02 |title=DSG's supercavitating underwater bullets annihilate ballistics tests |url=https://newatlas.com/military/dsg-cavx-supercavitating-underwater-bullets/ |access-date=2024-01-26 |website=New Atlas |language=en-US}}

Alleged incidents

The Kursk submarine disaster was initially thought to have been caused by a faulty Shkval supercavitating torpedo,{{cite news|title=Russian book sheds light on missile|first=Bill |last=Gertz|work=Washington Times|date=August 23, 2001|page=A.4}} though later evidence points to a faulty 65-76 torpedo.

References

External links

[http://cav.safl.umn.edu/ Supercavitation Research Group at the University of Minnesota]
[https://web.archive.org/web/20100418195217/http://www.diehl-bgt-defence.de/index.php?id=550&L=1 Diehl BGT Defence's "Barracuda" - a German supercavitating Torpedo]
[https://web.archive.org/web/20061015182506/http://www.darpa.mil/sto/solicitations/underwaterexpress/ DARPA Underwater Express Program]
[http://www.globalsecurity.org/military/systems/ship/systems/hsuw.htm Global Security.org on Supercavitation]
[https://archive.today/20130201230609/http://www.sciam.com/article.cfm?id=how-to-build-a-supercavit&ref=sciam How to Build a Supercavitating Weapon, Scientific American]

Category:Fluid dynamics