09-07-2012, 10:58 AM
CAVITATION & SUPERCAVITATION
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I.1.CAVITATION
Cavitation is the process of formation of vapour bubbles of flowing fluid in a region where the pressure of the liquid falls below its vapour pressure and the sudden collapsing of these vapour bubbles in region of high pressure. At first small vapour filled bubbles are formed that gradually increase in size. As the pressure of the surrounding liquid increases, the cavity suddenly collapses-a centimeter sized cavity collapses in milliseconds. Cavities implode violently and create shock waves that dig pits in exposed metal surfaces. .
At first, the physical characteristics of boiling and cavitation are almost identical. Both involve the formation of small vapour-filled spherical bubbles that gradually increase in size. However, the bubbles produced by the two processes end in very different manners. In boiling, bubbles are stable: the hot gas inside either escapes to the surface or releases its heat to the surrounding liquid. In the latter case, the bubble does not collapse, but instead fills with fluid as the gas inside condenses.
When it acts upon propellers, cavitation not only causes damage but also decreases efficiency. The same decrease in water pressure that causes cavitation also reduces the force that the water can exert against the boat, causing the propeller blades to "race" and spin ineffectively. When a propeller induces significant cavitation, it is pushing against a combination of liquid water and water vapor. Since water vapor is much less dense than liquid water, the propeller can exert much less force against the water vapor bubbles. With the problems it causes, it is no wonder maritime engineers try to avoid cavitation.
I.2.SUPERCAVITATION
The scientists and the engineers have developed an entirely new solution to the cavitation problem. Cavitation becomes a blessing under a condition called supercavitation, i.e., when a single cavity called supercavity is formed enveloping the moving object almost completely. In Supercavitation, the small gas bubbles produced by cavitation expand and combine to form one large, stable, and predictable bubble around the supercavitating object.
This fluid-mechanical effect occurs when bubbles of water vapor form in the lee of bodies submerged in fast-moving water flows. The trick is to surround an object or vessel with a renewable envelope of gas so that the liquid wets very little of the body's surface, thereby drastically reducing the viscous drag. Supercavitating systems could mean a quantum leap in naval warfare that is analogous in some ways to the move from prop planes to jets or even to rockets and missiles.
Supercavities are classified as one of two types: vapor or ventilated. Vapor cavities are the pure type of supercavity, formed only by the combination of a number of smaller cavities. In a ventilated cavity, however, gases are released into the bubble by the supercavitating object or a nearby water surface.
II. SUPERCAVITATION FUNDAMENTALS
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.
III. U.S. SUPERCAVITATION EFFORTS
Although supercavitation research in this country focused on high-speed propeller and hydrofoil development in the 1950s, the U.S. Navy subsequently opted to pursue other underwater technologies, particularly those related to stealth operations, rather than high-velocity capabilities. As a result, experts say, the U.S. Navy currently has no supercavitating weapons and is now trying to catch up with the Russian navy.
PROTOTYPE WEAPON. A future supercavitating torpedo based on U.S. Navy design concepts could feature a range of innovative cavitator, sensing, control and propulsion technologies.
III.1.RAMICS
The first class of weapons is represented by RAMICS (for Rapid Airborne Mine Clearance System); a soon-to-be-requisitioned helicopter-borne weapon that destroys surface and near-surface marine mines by firing supercavitating rounds at them. The 20-millimeter flat-nosed projectiles, which are designed to travel stably through both air and water, are shot from a modified rapid-fire gun with advanced targeting assistance. (The fielded RAMICS projectiles are expected to be enlarged to 30-millimeter caliber.) The U.S. Navy is also considering deploying a surface ship–borne, deck-mounted RAMICS-type close-in weapons system that could destroy deadly wake-following torpedoes.
III.2.AHSUM
The next step in supercavitating projectile technology will be an entirely subsurface gun system using Adaptable High-Speed Undersea Munitions (AHSUM). These would take the form of supercavitating "kinetic-kill" bullets that are fired from guns in streamlined turrets fitted to the submerged hulls of submarines, surface ships or towed mine-countermeasure sleds. The sonar-directed AHSUM system is hoped to be the underwater equivalent of the U.S. Navy's Phalanx weapons system, a radar-controlled rapid-fire gun that protects surface vessels from incoming cruise missiles.
III.3.TORPEDOS
The other supercavitating technology of interest is a torpedo with a maximum velocity of about 200 knots. Substantial technical and system challenges stand in the way of the desired torpedo in the areas of launching, hydrodynamics, acoustics, guidance and control, and propulsion, to name a few.
SUBSEA GUNS. The U.S. Navy is developing underwater launchers for rotating gun turrets that would be fitted below the waterline to fire "kinetic-kill" projectiles at mines, obstacles, surface craft, homing torpedoes - even low-flying airplanes and helicopters.
IV. PROFILE OF SUPERCAVITATING TORPEDO
In general, the weapon consists of a large cylindrical hull containing a solid-rocket motor that tapers to a cone enclosing the warhead. The wide aperture of a rocket nozzle protrudes from the center of the aft end encircled by eight small cylinders, which are said to be small starter rockets. These get the Shkval moving up to supercavitation speed, whereupon the main engine cuts in. Nestled between two of the starter motor nozzles is thought to be a spool of guidance wire that unravels as the torpedo makes its way through the water. The wire would allow submarine personnel to control the weapon's operation and warhead detonation.
Up front, things get a bit more speculative. Experts believe that the nose of the torpedo features what is likely to be a flat disk with a circular or perhaps elliptical shape. This is the all-important cavitator, which creates the gas cavity in which the craft moves. The cavitator disk will be tilted forward at the top, providing an "angle of attack" to generate the lift needed to support the forebody of the device. The cavitator's edge is apt to be sharp, which hydrodynamicists say creates the cleanest or least turbulent gas/water boundary, what they call a "glassy" cavity. Just aft of the cavitator sit several rings of ventilation ducts that inject rocket exhaust and steam into the cavitation bubble to enlarge it. About two thirds of the way back from the nose is four spring-out cylinders angled toward the stern. Although they loosely resemble fins, these spring-tensioned skids actually support the aft end of the torpedo by allowing it to bounce off the inner cavity surface. Western experts believe that the Shkval actually "precesses" slowly around the cavity's circumference, repeatedly ricocheting off the walls as it makes its way through the water.
The Shkval is considered to be somewhat unrefined because it can travel only along a straight trajectory, but future supercavitating vehicles are being designed to maneuver through the water. Steering is possible through the use of cavity-piercing control surfaces such as fins, and thrust-vectoring systems, which are directional nozzles for jet exhaust. Extreme care must be taken to keep the body inside the cavity during turns, however, because should it stray from the cavity, the force of slamming into the surrounding wall of water would abruptly turn it into "a crushed Coke can,"
Supercavitating vehicles could be highly agile if the control surfaces were coordinated correctly, says NUWC's Kuklinsky. The idea is to skew the cavity to one side to create the desired side forces with an articulated nose cavitator or with control surfaces and then track the vehicle in it. If the fore and aft control systems operate in phase so that the "back end keeps up with what the front is doing, very fast turns can be accomplished," he notes.
Part of the solution to the control problem is to install a reliable, real-time feedback control loop that can keep abreast of cavity conditions in the rear of the craft and make the appropriate response to measured changes. As supercavitating systems travel unsupported inside low-density gas bubbles, their afterbodies often bang off the inside wall of cavities. Specialists call this the "tail-slap" phenomenon, which is regularly observed in high-speed test photography of supercavitating devices. The ONR has sponsored the development of a "tail-slap" sensor - a monitoring system based on microelectromechanical components that will track intermittent afterbody contact with the cavity.
V. ADVANCED PROPULSION SYSTEMS
Most existing and anticipated autonomous supercavitating vehicles rely on rocket-type motors to generate the required thrust. But conventional rockets entail some serious drawbacks - limited range and declining thrust performance with the rise of pressure as depth increases. The first of these problems is being addressed with a new kind of high-energy-density power-plant technology; the second may be circumvented by using a special kind of supercavitating propeller screw technology.
Getting up to supercavitation speeds requires a lot of power. For maximum range with rockets; you need to burn high-energy-density fuels that provide the maximum specific impulse. A typical solid-rocket motor can achieve a maximum range of several tens of kilometers and a top speed of perhaps 200 meters per second. After considering propulsion systems based on diesel engines, electric motors, atomic power plants, high-speed diesels, and gas turbines, only high-efficiency gas turbines and jet propulsion systems burning metal fuels (aluminum, magnesium or lithium) and using outboard water as both the fuel oxidizer and coolant of the combustion products have real potential for propelling supercavitating vehicles to high velocities.
Aluminum, which is relatively cheap, is the most energetic of these metal fuels, producing a reaction temperature of up to 10,600 degrees Celsius. One can accelerate the reaction by fluidizing [melting] the metal and using water vapor. In one candidate power-plant design, the heat from the combustion chamber would be used to melt stored aluminum sheets at about 675 degrees C and to vaporize seawater as well. The resulting combustion products turn turbine-driven propeller screws.
This type of system has already been developed in Russia, according to media reports there. The U.S. also has experience with these kinds of systems. Researchers are operating an aluminum-burning "water ramjet" system, which was developed as an auxiliary power source for a naval surface ship. In the novel American design, powdered aluminum feeds into a whirlpool of seawater occurring in what is called a vortex combustor. The rapid rotation scrapes the particles together, grinding off the inert aluminum oxide film that covers them, which initiates an intense exothermic reaction as the aluminum oxidizes. High-pressure steam from this combustion process expands out a rocket nozzle or drives a turbine that turns a propeller screw.
Tests have shown that prop screws offer the potential to boost thrust by 20 percent compared with that of rockets, although in theory it may be possible for screws to double available thrust, Designs for a turbo-rotor propeller system with a single supercavitating "hull propeller," or a pair of counter rotating hull props that encircle the outer surface of the craft so they can reach the gas/water boundary, have been tested. Considerable work remains to be done on how the propeller and cavity must interact before real progress can be made.