Supercavitation 5j3t14

Table of Contents 1. Abstract

2. Cavitation 2.1 Difference between boiling and cavitation

3. Supercavitation 3.1 Supercavitation Fundamentals 3.2 Types of supercavity

4. Forces acting on the body 5.1 Modeling assumptions

5. Applications 4.1 Underwater gun systems 4.2 High Speed Supercavitating Vehicles 4.3 Supercavitating propeller 4.4 Supercavitating torpedo

6. Underwater gun system

7. Supercavitating torpedo

8. High speed Supercavitating vehicles

9. Supercavitating propellers

10. Conclusion

11. Future work on supercavitation

12. Reference

1.Abstract: A newly-developed fine bubble aeration system, by which air is transferred under supercavitation conditions, shows a clearly better performance than traditional, well-known aerators that rely on the jetpump principle (Schmid & Geier, 2003). The performance can be compared to oxygen transfer rates achieved in membrane and foil plate aerators. A prototype supercavitation aerator installed at a sewage treatment plant revealed an air input rate, which was about one third lower than that of the replaced jet pump system. In spite of this low air input rate, the daily demand of pure oxygen for the additionally installed membrane aeration system went down by approximately 49 percent, rom the original level of about 1 200 m3/d to about 600 m3/d – and this over a test period of more than seven months. With this new aerator and during the first three months of test phase already, more than 10 000 Euros had been saved because of the reduced pure oxygen consumption. In this article, a review of the patents on different aspects of a new aeration technology using “supercavitation” is presented. Water limits even nature's strategies, and the fastest bird moves twice as quickly as the fastest fish. The phenomenon holding back the fish is the tremendous resistance that water offers to a moving object, called drag. The same drag acts on the bird as well, but the magnitude is considerably less owing to the lesser density of air. The human being has crossed the sound barrier in air and land, what about underwater? Water is the most challenging environment for an Engineer. Being 1000 times denser than air, it offers resistance roughly 1000 times as high as that in air. Supersonic under Water Travel is the dream of scientists working on a bizarre technology called SUPERCAVITATION. Supercavitation is the state of the art technology that may revolutionize underwater propulsion systems.

The term 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. 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.

2. Cavitation Cavitation is the formation of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapour pressure. Cavitation is usually divided into two classes of behavior: inertial (or transient) cavitation, and non inertial cavitation. Inertial cavitation is the process where a void or bubble in a liquid rapidly collapses, producing a shock wave. Such cavitation often occurs in control valves, pumps, propellers, impellers, and in the vascular tissues of plants. Non-inertial cavitation is the process in which a bubble in a fluid is forced to oscillate in size or shape due to some form of energy input, such as an acoustic field. Such cavitation is often employed in ultrasonic cleaning baths and can also be observed in pumps, propellers, etc.

Since the shock waves formed by cavitation are strong enough to significantly damage moving parts, cavitation is usually an undesirable phenomenon. It is specifically avoided in the design of machines such as turbines or propellers, and eliminating cavitation is a major field in the study of fluid dynamics.

Fig. 1.1: Different stages of cavitation

Fig 1.2: A valve after cavitation effects

2.1 Difference between boiling and cavitation The physical process of cavitation inception is similar to boiling. The major difference between the two is the thermodynamic paths that precede the formation of the vapor. Boiling occurs when the local vapor of the liquid rises above its local ambient pressure and sufficient energy is present to cause the phase change to a gas. Cavitation inception occurs when the local pressure falls sufficiently far below the saturated vapor pressure, a value given by the tensile strength of the liquid at a certain temperature.

3. Supercavitation 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 (the bubble) reduces the drag on the object, since drag is normally about 1,000 times greater in liquid water than in a gas.

It is a means of drag reduction in water, wherein a body is enveloped in a gas layer in order to reduce skin friction. Depending on the type of Supercavitating vehicle under consideration, the overall drag coefficient can be an order of magnitude less than that of a fully-wetted vehicle. Current applications are mainly limited to very fast torpedoes.

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.

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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.

3.1 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.

3.2 Types of supercavity Supercavities are classified as one of two types on the basis of the method of formation of cavities:

1. Pure hydrodynamic cavitation (vapour cavity): Vapour cavities are the pure type of supercavity, formed only by the combination of a number of smaller cavities. Naturally occurring nuclei (small gas bubbles) explosively grow due to fluctuating pressure field in the separated flow region. Extremely High Speeds Required For Pure Hydrodynamic Supercavitation.

2. Artificial Cavitation (ventilated cavity): In a ventilated cavity, however, gases are released into the bubble by the supercavitating object or a nearby water surface.Requires a pressurized source of gas to be carried on-board or plumbing to re-direct exhaust gases.

FIGURE 3 FIGURE 4

4. Forces acting on the body

Underwater vehicles such as torpedoes and submarines are limited in maximum speed by the considerable drag produced by the flow friction on the hull skin. Speeds of 40 m/s (75 knots) are considered very high; most practical systems are limited to less than half this figure. While low speed is advantageous for acoustics and hydrodynamic efficiency, some special applications requiring high speed cannot be realized using conventional hydrodynamics. When a body moves through water at sufficient speed, the fluid pressure may drop locally below a level which sustains the liquid phase, and a low-density gaseous „cavity‟ can form. Flows exhibiting cavities enveloping a moving body entirely are called

„supercavitating‟, and, since the liquid phase does not the moving body through most of its length, skin drag is almost negligible.

Several new and projected underwater vehicles exploit supercavitation as a means to achieve extremely high submerged speeds and low drag (Miller, 1995). The sizes of existing or notional supercavitating highspeed bodies range from that of bullets (for example the Adaptable High-Speed Undersea Munition, AHSUM, or the projectiles of the Rapid Airborne Mine Clearance System, RAMICS) to that of full scale heavyweight torpedoes. Since the forces on a supercavitating body are so different from those on conventional submerged bodies, hydrodynamic stability issues need to be completely reassessed. In particular, since the body is wetted only for a tiny percentage of its length, and since vapor dynamic forces are nearly negligible, the center of pressure will nearly always be ahead of the center of mass, violating a standard principle of hydrodynamic stability. Also, the body dynamics consist of at least two qualitatively different phases: pure supercavitating flight, with only tip with the fluid, and states including s with the fluid cavity walls.

In the case of pure Supercavitating flight, forces produced by the flow of water vapor may be a significant stabilizing effect at very high speeds. In the case that the body touches the cavity walls, these s may be of long-duration (planning), or intermittent (impacts). In this initial study, we consider intermediate speed regimes where long-duration cavity (planning) does not occur, and where vapor dynamic forces are negligible.

4.1 Modeling assumptions Our model is based on the following assumptions: 1. The path of the center of mass of the body is assumed to be wellapproximated by a straight horizontal line L. This assumption neglects gravity, which is justified by experimental work which showed no effect of gravity at speeds greater than 8 m/sec 2. The cavity is assumed to be approximately fixed in an orientation which remains symmetric about the horizontal line L. This assumption represents a simplified model of the real motion of the cavity which traces a serpentine form as the body oscillates about the line of travel. The shape of the cavity is assumed to be a known function of the forward velocity of the body, although the only place this is used is in determining when the tail of the body touches the cavity walls, a condition referred to as „tailslap‟. The diameter of the cavity, and hence the clearance between the tail and the cavity walls, is known to decrease as forward velocity decreases. This clearance is small compared to the length of the body, permitting the assumption that the body axis B always makes a small angle 0 with the cavity axis L. 3. The projectile is assumed to rotate about the nose tip. In fact, the center of rotation in a quasi-inertial coordinate system translating with the body will not in general be at the nose. However, if the wavelength of the disturbances in the fluid caused by tailslap is much greater than the projectile length, then the geometry of tailslap dynamics can be well approximated by assuming that the shape of the translating cavity is frozen and the center of rotation is at the nose. This was the case in previous AHSUM tests, where the tailslap frequency was on the order of 600 Hz when the projectile speed was approximately 600 m/s.

4. In the absence of impacts, we assume that the only force on the body is due to the fluid force at the tip. Laboratory experiments have shown that the net tip force acts approximately along the axis of the body B with zero net applied moment. The magnitude F of the tip force is:

F = ;pAv2k cos 0 (1) where p = density of water, A = cross-sectional area of the tip, 21 = i = forward velocity, k = a non dimensional constant, 6‟ = angle between the body axis B and the cavity axis L.

5. We model the impact of the tail against the cavity walls (tailslap) as occurring instantaneously with coefficient of restitution of unity. 6. In order to simplify the analysis we assume that the body is not spinning about its symmetry axis B.

In view of the foregoing assumptions, the in-flight dynamics may be decomposed into a translatory motion and rotation of the body. The translatory motion is uninfluenced by the rotation of the body. The rotation of the body is influenced by the translatory motion because the size of the cavity is dependent on the forward velocity, and this influences the period of time between impacts.

Fig 3.1: Schematic diagram of a Supercavitating object

5. Applications

Supercavitation applications are restricted to underwater objects. This is because cavitation is required for supercavitation to take place. The main applications are given below.

5.1 Underwater gun systems

Presently, research is ongoing for the use of underwater gun systems as anti-mine and anti-torpedo devices. An underwater gun system is typically composed of a magazine of underwater projectiles, an underwater gun, a ship-mounted turret, a targeting system, and a combat system. Specifically, the targeting system identifies and localizes an undersea target. The combat system provides the control commands to direct the ship-mounted turret to point the underwater gun towards the undersea target. The underwater gun shoots the underwater projectiles in which the underwater gun is designed for neutralization of undersea targets at relatively long range

5.2 High Speed Supercavitating Vehicles

We investigate the control challenges associated with supercavitating vehicles using a low order, longitudinal axis vehicle model. In the first part of the paper, a detailed derivation of the equations of motion for the vehicle has been carried out using Newton‟s Laws. Various forces experienced by different regions of the vehicle have been explained.

This model draws heavily on the benchmark HSSV model proposed by Dzielski and Kurdila (2003. It is observed that the linearization, even for a simple trim, straight-level flight, can be very complicated. Thus, numerical methods are used for this purpose. A controller is synthesized to track pitch angle, angular rate, vertical position and vertical speed for the HSSV vehicle model using the proposed approach. Simulations of the closed-loop vehicle are performed and analyzed in the fourth section

of the paper. Challenges facing the model creator and control designer are highlighted with respect to actuator and sensor requirements, modeling issues, robustness and performance.

Fig 2.1: A Supercavitating Vehicle

5.3 Supercavitating propeller The Supercavitating propeller is a variant of a propeller for propulsion in water, where supercavitation is actively employed to gain increased speed by reduced friction. This article distinguishes a supercavitating propeller from a subcavitating propeller running under supercavitating conditions. In general, subcavitating propellers become less efficient when they are running under supercavitating conditions.

The supercavitating propeller is being used for military purposes and for high performance boat racing vessels as well as model boat racing. The supercavitating propeller operates in the conventional submerged mode,

with the entire diameter of the blade below the water line. The blades of a supercavitating propeller are wedge shaped to force cavitation at the leading edge and avoid water skin friction along the whole forward face. The cavity collapses well behind the blade, which is the reason the supercavitating propeller avoids the erosion damage due to cavitation that is a problem with conventional propellers.

Fig 2.2: A supercavitating propeller

5.4 Supercavitating torpedo The nose of a supercavitating torpedo uses gas nozzles that continually expel an envelope of water vapor around the torpedo as it speeds through the ocean. This bubble of gas--a 'super cavity'--prevents the skin of the torpedo from ing the water, eliminating almost all drag and friction and allowing the projectile to slide seamlessly through the water at great velocity.

Some people have described supercavitating torpedoes as the first true underwater missiles. The first such weapon in this class, the Shkval ("Squall"), was in development by the Soviet Union throughout the latter half of the Cold War but was not recognized in the West until the 1990s. Using powerful solid rocket motors, the Shkval is capable of speeds exceeding 230 mph, over four times the velocity of most conventional torpedoes. The Shkval also has a reported 80% kill rate at ranges of up to 7000 meters.

Fig 2.3: A shkval torpedo

6. Underwater Gun System

Presently, research is ongoing for the use of underwater gun systems as anti-mine and anti-torpedo devices. An underwater gun system is typically composed of a magazine of underwater projectiles, an

underwater gun, a ship-mounted turret, a targeting system, and a combat system.

Specifically, the targeting system identifies and localizes an undersea target. The combat system provides the control commands to direct the ship-mounted turret to point the underwater gun towards the undersea target. The underwater gun shoots the underwater projectiles in which the underwater gun is designed for neutralization of undersea targets at relatively long range.

Projectiles fired from underwater guns can effectively travel long distances by making use of supercavitation. A typical supercavitating projectile is depicted in Fig 4.1. Supercavitation occurs when the projectile travels through water at very high speeds and a vaporous cavity forms at a tip of the projectile. With proper design, the vaporous cavity can envelop an entire projectile. Because the projectile is not in with the water (excluding at the tip and occasional collisions with the cavity wall, "tail slap"), the viscous drag on the projectile is significantly reduced over a fully wetted operation.

Current projectiles lack propulsion in that the projectiles are instead launched from a gun at high speeds (of the order of 1000 meters/second). The projectiles decelerate as they travel downrange toward their targets, striking their target at velocities typically of 500 meters/second. It is possible to reduce the velocity needed for launch if the projectile is provided with an on-board propulsion system and/or a drag reduction system.

If a simple propulsion system is provided, the gun can launch the projectiles at their cruise velocity and the propulsion system can maintain and carry the projectile to its target at approximately the cruise velocity.

A related issue in projectile operation is the problem of speed and depth dependency of a generated cavity. At launch, a cavity is formed, the size of which is a function of the projectile speed and the cavitator size. As the projectile begins to travel down-range, the projectile begins to slow down due to the drag generated at the tip of the projectile and the cavity, that the projectile generates shrinks. The cavity continues to shrink as the projectile decelerates until the cavity can no longer envelop the entire projectile.

Pressure also influences the size of the cavity. The size of the cavity is inversely proportional to the ambient pressure. Consequently, projectiles cannot travel as far when deep beneath the ocean surface as the projectiles can travel at very shallow depths. The high ambient pressure of deep ocean depths can be compensated through the injection of gas into the cavity. If gas is forced into the normally vaporous cavity, the internal pressure of the cavity increases and the cavity grows.

It has been demonstrated that forward-directed jets from moving vehicles can produce supercavities in a manner similar to a physical

cavitator. The jet advances forward of the vehicle to where a moving front is produced. The size and shape of the cavity are related to the diameter of the forward-directed jet and the speed of the advancement of the front.

. Fig 4.1: An image of a bullet from an underwater gun

7. Supercavitating Torpedo

The nose of a supercavitating torpedo uses gas nozzles that continually expel an envelope of water vapor around the torpedo as it speeds through the ocean. This bubble of gas--a 'super cavity'--prevents the skin of the torpedo from ing the water, eliminating almost all drag and friction and allowing the projectile to slide seamlessly through the water at great velocity. Some people have described supercavitating torpedoes as the first true underwater missiles.

The first such weapon in this class, the Shkval ("Squall"), was in development by the Soviet Union throughout the latter half of the Cold War but was not recognized in the West until the 1990s. Using powerful solid rocket motors, the Shkval is capable of speeds exceeding 230 mph, over four times the velocity of most conventional torpedoes. The Shkval also has a reported 80% kill rate at ranges of up to 7000 meters. The US navy is seeking to build its own version of the Shkval, but one with a much higher velocity. This is mostly in response to Russia selling stripped down versions of the Shkval on the open international weapons market. However, a US combat-ready version is not expected for at least another 10+ years. The technology does have one great weakness--maneuverability. The bubble of water vapor generated by the gas nozzles tends to become asymmetrical and breaks up along the outer side of the turn if the torpedo alters its course significantly. At the speeds such a torpedo would typically be travelling, the sudden re-assertion of water pressure and drag on it could not only severely knock it off course, but may even rip the projectile apart. A new, improved version of the Shkval has been reported in use by the Russian Navy, one that can maneuver and track its intended target. However, it was also reported that in order to do so, this improved Shkval had to slow down significantly once in the general area of the target so it could scan and home in on its prey like a normal torpedo. While a genuine improvement, the true goal of current research is to have the torpedo maneuver and home in on a target without the need to decrease its velocity. Both Russian and US Navy researchers are striving toward this end.

One means of making sure the gas bubble does not wear down upon a turn would be by having the gas-ejection nozzles pump more water vapor into the side of the bubble that's on the outside of the turn, to provide the torpedo with a thick enough "buffer" for the turn without any more parts of it exiting the cavity. Another option might be to magnetically charge the vapor used in the torpedo‟s bubble, and use a magnetic field to hold the bubble cohesive while it turns. Another weakness of the technology is that the Shkval is both very noisy and shows up very readily on sonar. Whereas some long-range conventional torpedoes might be able to stealth relatively close to their targets before going active, the target of a supercavitating torpedo will know right away if they're in the bulls-eye. However, the supercavitating torpedo may also be travelling fast enough to give its intended victim much less time to take effective countermeasures. A drawback that had been pointed out in several articles is that the Shkval and its peers only have ranges of several kilometers, whereas a number of modern torpedoes, like the US Mark 48, has a range of over 30 nautical miles. It‟s possible that a US submarine could just sit outside of Shkval-equipped submarine's range and pound on such an enemy with impunity. The downside to that strategy is, of course, that most subs are unlikely to be equipped only with supercavitating projectiles. Like most modern combat subs, they will likely carry a variety of different weapons for different purposes, and the Shkval will just be one of the weapons it has in its arsenal. One can assume at long ranges they will likely employ conventional torpedoes, but once within the effective kill-range of a Shkval, they will use their supercavitating weapons to fullest possible effect. Also, it is almost a certainty that all parties engaging in research are striving to increase the weapon's range as much as possible.

Submarines, even with minimal warning, can evade a supercavitating torpedo by blowing some ballast and quickly ascending. However, an enemy submarine captain may anticipate this, and may launch a second or even a third Shkval simultaneously, aimed above the target submarine, in order to keep the enemy vessel from attempting this maneuver.

Fig 5.1: A shkval torpedo

8. High Speed Supercavitating Vehicles

Recent investigations into high-speed underwater vehicles have focused attention on providing vehicles which ride a cushion of air to achieve high speeds in water. For a nominal prior art streamlined, fully-wetted underwater vehicle, 70% of the overall drag is skin friction drag; the remainder is pressure or blockage drag. Supercavitation allows for much higher speeds to be sustainable by eliminating, or drastically reducing, skin friction drag at the higher speeds. The conditions for supercavitation require that enough energy be put into the water to vaporize a given volume of water through which an object can travel. This is done by accelerating fluid over a sharp edge, usually the nose of a vehicle, such as a torpedo, so that the pressure drops below the vapor pressure of water. If the speed of the object is not fast enough to travel through the vapor cavity before the cavity collapses, artificial ventilation into the cavity can keep the cavity "open" until the object moves past. When a cavity completely encapsulates an object, by vaporous and/or vented cavitation, it is referred to as "supercavitation". The vehicle nose, or "cavitator", is the only part of the object in constant with the water through which the vehicle travels. The cavity closure is positioned behind the vehicle. When the cavitator and artificial ventilation generate the necessary cavity properties, i.e., sufficient length and diameter of air cushion, it results in a larger air gap between the vehicle and water than is otherwise necessary at the after end of the vehicle. The air, or other selected gas, is drawn through the gap by a propulsion jet plume, and escapes into the ambient water. It has been found desirable to minimize the downstream entrainment effect of the propulsion plume, to thereby minimize loss of air and to increase life expectancy of a reservoir of ventilation air on-board the vehicle.

A supercavitating vehicle is an advanced concept for achieving very high speeds underwater with significantly less drag than a conventional vehicle. The idea behind this concept is the enshrouding of a vehicle moving through water in a gas cavity. A vehicle is said to be supercavitating when the cavity extends from around the nose to just beyond the tail of the vehicle. Part of the nose of the vehicle, called the cavitator–and, possibly, some control fins–would be in wetted with liquid water, but the rest of the surface of the vehicle would remain in with gas only (inside the cavity). The gas is much lower in density and viscosity than the surrounding water. Depending on the design, the gas could be water vapor, air, or something else. Due to the lower density and viscosity of the gas, this conceptually results in significantly less drag than a similar, but fully wetted vehicle.

Fig 6.1: Schematic diagram of a High speed supercavitating vehicle.

9. Supercavitating Propellers

The supercavitating propeller is a variant of a propeller for propulsion in water, where supercavitation is actively employed to gain increased speed by reduced friction. This article distinguishes a supercavitating propeller from a subcavitating propeller running under supercavitating conditions. In general, subcavitating propellers become less efficient when they are running under supercavitating conditions. The supercavitating propeller is being used for military purposes and for high performance boat racing vessels as well as model boat racing. The supercavitating propeller operates in the conventional submerged mode, with the entire diameter of the blade below the water line. The blades of a supercavitating propeller are wedge shaped to force cavitation at the leading edge and avoid water skin friction along the whole forward face. The cavity collapses well behind the blade, which is the reason the supercavitating propeller avoids the erosion damage due to cavitation that is a problem with conventional propellers. An alternative to the supercavitating propeller is the surface piercing, or ventilated propeller. These propellers are designed to intentionally cleave the water and entrain atmospheric air to fill the void, which means that the resulting gas layer surrounding the propeller blade consists of air instead of water vapour. Less energy is thus used, and the surface piercing propeller generally enjoys lower drag than the supercavitating principle. The surface piercing propeller also has wedge shaped blades, and propellers may be designed that can operate in both supercavitating and surface piercing mode.

Fig 7.1: A supercavitating propeller

10. Conclusion

Supercavitation is an phenomenon which is used in underwater applications like torpedoes, propellers etc. It is used for reducing the drag force in these objects. The technology has advanced to an extent that it can be used in many other applications. And in coming years we might see underwater vehicles used as a means of efficient transport.

11. Future work on supercavitation

12. Reference

Journals 1. Proceedings of DETC‟97 1997 AS M E Design Engineering Technical Conferences September 14-17, 1997, Sacramento, California

2. Supercavitating propellers by A S Achkinasze Ship Theory Department, Saint-Petersburg State Marine Technical University 3, Lotsmanskaya Street, Saint-Petersburg 190008, Russia

3. Model-Based Control of High-speed Supercavitating Vehicles Ziyao Cao College of Marine Engineering Northwestern Poly technical University Xi‟an 710072, China