CHAPTER 1: INTRODUCTION M.Tech. Project Report 2013

INTRODUCTION Chapter I

1.1 Need of Supercavitation

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 [17]. Cavitation is a problem for most of the engineering application

where as supercavitation is the booming scientific discovery for underwater automobiles, torpedo

and propellers. Cavitation becomes a blessing under a condition called supercavitation. In

supercavitation, the small gas bubbles produced by cavitation expand and combine to form one

large, stable and predictable bubble around the supercavitation object. This fluid-mechanical effect

occurs when bubble of water vapour 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.

1.2 Cavitation and Supercavitation

Supercavitation is a phenomenon which is used in underwater objects to decrease their

drag force. Before studying about supercavitation one should have a brief knowledge on cavitation,

as supercavitation uses the concept of cavitation.

1.2.1 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 the

vapour bubbles in region of high pressure. Cavitation is usually divided into two classes of

behaviour: inertial (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. Cavitation can be

used to increase heat and mass transfer in liquids, to promote crystallization and to enhance

various monochemical reactions such as polymerization and polymer degradation. Important in

applications such as ultrasonic cleaning, homogenization of milk, enhanced chemical processes

through coagulation, formation of suspensions and degassing of liquids. Plays a role in

biomechanics, geomorphology and Biomedical applications include the removal of kidney stones.

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1.2.2 Difference between Boiling and Cavitation

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. As the pressure of the surrounding liquid increases, the cavity suddenly

collapses-a centimetre sized cavity collapses in milliseconds. [20] Boiling takes place mostly when

heat is given to the liquid. While in the process of cavitation pressure drop is the main reason for

the production of vapour of the liquid.

1.2.3 Causes of Cavitation

Cavities implode violently and create shock waves that dig pits in exposed metal

surfaces. Since the shock waves formed by cavitation are strong enough to significantly damage

moving parts, cavitation is usually an undesirable phenomenon. 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 vapour. Since water vapour

is much less dense than liquid water, the propeller can exert much less force against the water

vapour bubbles. 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 [11]. With the problems it

causes, it is no wonder maritime engineers try to avoid cavitation.

Whereas boiling, does not create much problems in engineering applications and can

be easily controlled, Cavitation is a problem for most of the engineering application where as

supercavitation is the booming scientific discovery for underwater automobiles, torpedo and

propellers. In the early 1960’s, Mikhail Merkulov at the Hydrodynamics Institute in Kiev realized

that the solution lies in a phenomenon called cavitation. It was a daring idea because naval

architects usually see cavitation as a menace, rather than something that works to their advantage.

1.2.4 SUPERCAVITATION

The scientists and the engineers have developed an entirely new solution to the

cavitation problem. Supercavitation is an extreme version of cavitation. 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. At velocities about 50mps,

blunted-nose cavitators produce these low density cavities. 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.

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, where in

a body is enveloped in a gas layer in order to reduce skin friction. Depending on the type of

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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. Supercavities are classified as one of two types: vapour or ventilated. Vapour

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. They are briefly described in the next chapter.

1.3 Applications

Supercavitation is receiving more interest from the military, scientific, and engineering

communities; it can significantly reduce viscous drag, thereby multiplying the speed of submerged

projectiles and vessels, which then can move at hundreds of kph underwater. Supercavitation

applications are restricted to underwater objects as cavitation is required for supercavitation to take

place. Supercavitation finds application in military projectiles, such as bullets and torpedoes.

In 1999, supercavitation technology was adopted to hunting projectiles. These

"SuperPenetrator" bullets feature a very stable straight line penetration in aqueous media. In 2004,

German weapons manufacturer Diehl BGT Defence announced their own supercavitating torpedo,

Barracuda, now officially named "Supercavitating Underwater Running Body". According to Diehl, it

reaches more than 400kph. Iran has claimed to have successfully tested its first supercavitation

torpedo. Iran called this weapon the Hoot (Whale). The main applications are given below [20]:

1.3.1 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. 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 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 [28].

1.3.2 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 fully-wetted

underwater vehicle, 70% of the overall drag is skin friction drag; the remainder is pressure drag. 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 contact with liquid

water, but the rest of the surface of the vehicle would remain in contact with gas only (inside the

cavity). The gas is much lower in density and viscosity than the surrounding water.

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1.3.3 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. The underwater gun is designed for neutralization of undersea targets at relatively

long range. U.S Naval undersea warfare centre was successful in firing a supercavitating bullet at

5800 kph (mach 5) to destroy the underwater mines. The bullet fired from a helicopter-mounted

with standard Gatling gun supercavitates through the water and detonates the mines with little cost

and even less risk of human life.

Fig 1.1: An image of a bullet from an underwater gun1.3.4 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.

1.3.5 Supercavitating torpedo (SHKVAL Torpedos)

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. It also has a reported 80% kill rate at

ranges of up to 7000 meters. It can also be used as a countermeasure to an incoming torpedo.

Fig 1.2: A Shkval Torpedo1.3.6 US 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, high-velocity capabilities. The U.S.

Navy is now trying to catch up with the Russian navy.

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a. RAMICS (Rapid Airborne Mine Clearance System)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.

Fig 1.3: A Schematic of RAMICS

b. AHSUM (Adaptable High Speed Undersea Munitions)

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. A radar-

controlled rapid-fire gun that protects surface vessels from incoming cruise missiles.

1.4 Future of Supercavitation

Whatever the years ahead may hold for supercavitating weapons, they have already

exerted a strong influence on military and intelligence communities around the world. Indeed, they

seem to have spurred some revaluation of naval strategy. Future applications may include small,

unmanned transport or experimental vessels such as: Neutralizing Mines, Anti-Mine Projectile.

Supercavitating Ships are at the desk of researchers now-a-days [18]. Use of supercavitation

phenomenon to throttle the ship is carried by installing cavitators at the bottom of the ship. The part

of the ship submerged is supercavitated and the speed of ship increases tremendously.

1.5 Issues With Supercavitation

1.5.1 Advantages of Supercavitation

Reduction of water adhesion to the surface or slippage;

It reduces the skin drag friction acting on the body immersed in water or any other fluid.

Thus damage of material is avoided.

High velocities can be attaining since opposition due to water is reduced

1.5.2 Disadvantages of Supercavitation

Since motion is high frequency due to unbalances forces acting on the body, since buoyant

force is not acting on the body.

Prediction of cavity is difficult since the time at which the formation of cavity takes place and

time at which the cavity is fully grown can’t be calculated.

Once the cavity is created it is difficult to maintain it for further motion.

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