May 2001 issue
By Steven Ashley
Propelling a body through water takes considerable effort, as every swimmer knows. Speeding up the pace makes the task even harder because skin friction rises with increased velocity. Swimming laps entirely underwater is even more difficult, as water produces 1,000 times more drag resistance than air does.
Naval architects and marine engineers vie constantly with these age-old problems when they streamline the shapes of their hull designs to minimize the frictional drag of water and fit their ships with powerful engines to drive them through the waves. It can come as a shock, therefore, to find out that scientists and engineers have come up with a new way to overcome viscous drag resistance and to move through water at high velocities. In general, the idea is to minimize the amount of wetted surface on the body by enclosing it in a low-density gas bubble.
"When a fluid moves rapidly around a body, the pressure in the flow drops, particularly at trailing edges of the body," explains Marshall P. Tulin, director of the Ocean Engineering Laboratory at the University of California at Santa Barbara and a pioneer in the theory of supercavitating flows. "As velocity increases, a point is reached at which the pressure in the flow equals the vapor pressure of water, whereupon the fluid undergoes a phase change and becomes a gas: water vapor." In other words, with insufficient pressure to hold them together, the liquid water molecules dissociate into a gas.
"Under certain circumstances, especially at sharp edges, the flow can include attached cavities of approximately constant pressure filled with water vapor and air trailing behind. This is what we call natural cavitation," Tulin says. "The cavity takes on the shape necessary to conserve the constant pressure condition on its boundary and is determined by the body creating it, the cavity pressure and the force of gravity," he explains. Naval architects and marine engineers typically try to avoid cavitation because it can distort water flow to rob pumps, turbines, hydrofoils and propellers of operational efficiency. It can also lead to violent shock waves (from rapid bubble collapse), which cause pitting and erosion of metal surfaces.
Supercavitation is an extreme version of cavitation in which a single bubble is formed that envelops the moving object almost completely. At velocities over about 50 meters per second, (typically) blunt-nosed cavitators and prow-mounted gas-injection systems produce these low-density gas pockets (what specialists call supercavities). With slender, axisymmetric bodies, supercavities take the shape of elongated ellipsoids beginning at the forebody and trailing behind, with the length dependent on the speed of the body.
The resulting elliptically shaped cavities soon close up under the pressure of the surrounding water, an area characterized by complex, unsteady flows. Most of the difficulties in mathematically modeling supercavitating flows arise when considering what Tulin calls "the mess at the rear" of cavities, known as the collapse or closure region. In reality, the pressures inside gas cavities are not constant, which leads to many of the analysis problems, he says.
However they're modeled, as long as the water touches only the cavitator, supercavitating devices can scoot along the interiors of the lengthy gas bubbles with minimal drag.
From other sources in the Internet:
In supercavitation, the small gas bubbles produced by cavitation expand and combine to form one large, stable, and predictable bubble around the supercavitating object. The bubble is longer than the object, so only the leading edge of the object actually contacts the aqueous medium. The rest of the object is surrounded by low-pressure water vapor. A supercavitating body has extremely low drag, because its skin friction almost disappears. Instead of being encased in water, it is surrounded by the water vapour in the supercavity, which has much lower viscosity and density.
A supercavity can also formed around a specially designed projectile. The key is creating a zone of low pressure around the entire object by carefully shaping the nose and firing the projectile at a sufficiently high velocity. At high velocity, water flows off the edge of the nose with a speed and angle that prevent it from wrapping around the surface of the projectile, producing a low-pressure bubble around the object. With an appropriate nose shape, the entire projectile may reside in a vapor cavity.
One can use supercavitation technology to produce high-speed undersea weaponry.
Scientists at the Naval Undersea Warfare Center in Newport, Rhode Island demonstrated in 1997, a fully submerged launch of a supercavitating projectile (with air injected in its nose) with a muzzle velocity of 5,082 feet (1,549 meters) per second, making it the first underwater weapon to break the sound barrier. More recently the US unveiled supercavitating bullets. That program was inspired by the menace posed by harbor mines during the Gulf War. The slow and dangerous job of disarming mines often falls to divers because bullets lose momentum and direction after traveling a few feet through water, which is thousands of times denser than air. But supercavitating bullets fired from planes or helicopters could pierce and detonate mines from a safe distance. Very important is the gyroscopic stabilisation of the bullet travelling in a supercavitation bubble. Projectiles shot from barrels without a twist are quite unstable within their vapor cavities.
The Naval Undersea Warfare Center in Newport, Rhode Island, USA is now also working on the phenomenon.
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.
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. (
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). The SuperPenetrator
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).