Supercavitation Seminar Report

Dept. Of Mechanical Engineering S.S.E.T

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CHAPTER 1:

INTRODUCTION

Supercavitation is a phenomenon which is used in underwater objects to

decrease their drag force. Before we study about supercavitation we should have a brief

knowledge on cavitation, as supercavitation uses the concept of cavitation.

The velocity of conventional manned or unmanned underwater vehicles is limited by the

drag introduced by skin friction, interaction of liquid with the vehicle surface. Since drag

increases exponentially with velocity, the amount of thrust propelling an underwater vehicle

has to increase exponentially to achieve increase in speed, as shown on Figure 1.1. Since

new technologies in underwater and rocket propulsion does not offer radical increase in

specific impulse, the only way to increase propelling force by increasing the rate of fuel burnt

in the engine, which becomes highly inefficient if we consider the decrease in range

associated with the higher consumption. Due to this reason, current underwater vehicles are

limited to approximately 50 m/s. Instead it is more beneficial to reduce the surface area in

contact with water. Since the conventional ways of improving propulsion and streamlining

the vehicle did not lead to significant speed increase, Russian designers in the 1970s

proposed a radically different approach, the surface area of the vehicle in contact with the

liquid phase from the vehicle hull was reduced, eliminating skin friction by enveloping the

vehicle with a gas bubble. The water vapour cavity generated by supercavitation led to the

Skhval underwater rocket which can reach speeds up to 100 m/s. Supercavitation can

drastically reduce the wetted surface area by enveloping the vehicle with gaseous water

vapour, leading to an order of magnitude reduction in drag if the body is shaped properly.

Research in cavitation was active in the Unites States since the mid 1950's led my Marshall

Tulin, and the US Navy funded research programs focused on development of

supercavitating propellers.

CHAPTER 2:

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 centimetre 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:


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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 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. With

the problems it causes, it is now under maritime engineers try to avoid cavitation.

CHAPTER 3:

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. 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. Supercavitation is the use of cavitation effects to create a large bubble of gas

inside a liquid. 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. Current applications are mainly

limited to very fast torpedoes. Cavitation happens when water pressure is lowered below its

vapour pressure or vapour pressure increases to equal water pressure. This often happens at

extremely high speed although it can happen at any speed and even when not moving.

Cavitation occurs inside a pump or around an obstacle, such as a rapidly spinning propeller or

in a body of liquid (such as a kettle) due to temperature/pressure change. The pressure of the

fluid can drop due to its high speed(Bernoulli's principle) and when the pressure drops below

the vapour pressure of the water or the temperature increases. When vapour pressure

increases to water pressure, it vaporizes typically forming small bubbles of water vapour

(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, for instance spalling

damage to badly designed rotating propellers, pumps, and piping. 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. As an example, a conventional

rocket engine is used to propel the Russian Shkval supercavitating torpedo.

The main principal behind supercavitation is Bernoulli’s theorem which is as follow:-


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Thus if potential energy remain constant for a body then with increase in dynamic head i.e

kinetic energy, pressure will decrease and if this decreasing pressure falls below or upto

vapour pressure of the surrounding liquid , as a result the surrounding liquid forms air

bubbles (having vapour and dissolve air) if this cavities do not collapse and they grow such

that they surround the whole body then the body is called as supper cavitating body.

CHAPTER 4:

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. 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. 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 vapours is much less

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

bubbles. With the problems it causes, it is no wonder maritime engineers try to avoid

cavitation. Cavitation is a problem for most of the engineering application where as

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

propellers. Whereas boiling does not create much problems in engineering applications and

can be easily controlled.

CHAPTER 5:

DISADVANTAGES OF CONVENTIONAL UNDER-WATER

PROPULSION SYSTEM

The problem holding back conventional propulsion systems from high speeds is drag. No

matter how streamlined an object is, it suffers resistance as it moves through a fluid. One

source of friction is skin friction, the force that is required to shear the thin layers of fluid

lying against the moving body’s surface. This happens in air too, but water being a thousand

21constant

2P v gh


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times as dense as air generates a thousand times as much drag. Moreover the power required

to overcome drag is proportional to the cube of its velocity. So each incremental

improvement in propulsion technology produces only a small increase in speed. Torpedoes

are mostly the fastest propelled objects moving under water. It is drag which is the main

factor that limits the speed of conventional torpedoes. At high speeds drag is so enormous

that efficiency of propulsion is so low. Modern torpedoes can reach speeds below180 km/hr.

As is the case of most bizarre ideas, the idea of an entirely new under water propulsion

system owes its birth to the cold war. In the early 60’s Russian torpedoes were inferior to

those of Americans in speed. Rather than push conventional technology, the Russians decided

to try to leapfrog the Americans with a radical solution.

CHAPTER 6:

A SUPERCAVITATING PROJECTILE

For a start the body has to be cruising very fast at least 180km/hr, which is far faster than

ordinary torpedoes. The nose rather than being streamlined should be flat. Thus at high

speeds water is forced to flow off the edge of the nose at such an angle that it cannot wrap

around the surface of the body. As it passes over the edge it vaporizes due to high

velocity. Thus a big cavity is formed which encloses the front part of the object. If we could

make this cavity enclose the entire body most of the drag could be eliminated. This is

possible by two ways. If the body is fast enough so that the entire length of the body passes

through before the cavity collapses, it will appear as if the cavity is travelling along with the

body. If the object is not fast enough to travel through the vapour cavity before it collapses,

then artificial ventilation into the cavity can keep it open until the object moves past. Once a

super cavity is formed which completely encloses the object, the drag force is nearly

eliminated as the only portion in contact with liquid is the nose. Only the leading edge of

the object actually contacts liquid water. The rest of the object is surrounded by low-pressure

water vapour, significantly lowering the drag on the supercavitating object. With an

appropriate nose shape and a speed over 180 km per hour, the entire projectile may reside in a

vapour cavity. Since drag is proportional to the density of the surrounding fluid, the drag on a

supercavitating projectile is dramatically reduced, allowing supercavitating projectiles to

attain higher speeds than conventional projectiles. In water, a rough approximation predicts

that a supercavitating projectile has 200,000 times less skin friction than a normal projectile.

The potential applications are impressive.

CHAPTER 7:

MAKING A SUPERCAVITATING PROJECTILE

Although the idea may seem simple, making a supercavitating projectile is a daring

challenge. The technological hurdles to be overcome are many. The most important question

is how to propel the body if no other part except the nose is in contact with the surrounding

fluid. Also the enormous drag exerted on the blunt nose would literally crush any material.


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7.1. PROPELLING THE OBJECT

When a supercavitating projectile is enclosed by a cavity conventional propulsion techniques

cannot be used. A rocket engine is a solution. As the cavity encloses the vessel it is similar as

flying in the air. Therefore by using a rocket engine high speeds can be attained which in turn

helps for retaining the cavity. When the projectile is fired from above water it pulls along

with it a ventilated cavity which is unstable but as supercavitation starts this ventilated cavity

is converted to vapour cavity. Then the rocket motor is fired and using the exhaust the cavity

can be stabilized. A rocket motor also provides an immensely powerful thrust, enabling the

object to achieve high velocities. The overall drag reduces enormously once you reach the

supercaviting regime and then increases only linearly with speed. An aluminium burning

rocket is an answer to a compact and efficient propulsion system. It would use water as its

oxidiser and so would not need to carry oxygen. The problem with aluminium has been that

unreacted fuel quickly becomes coated with aluminium oxide, inhibiting any further reaction.

To avoid this, powdered aluminium can be injected to a vortex of water, which keeps

the molten drops apart. Using a rocket motor has another advantage. The exhaust from the

motor can be used to ventilate the cavity and stabilize it. The exhaust can be ducted round

from just behind the nose which strengthens the existing cavity and expands it to a bigger

one. Thus the cavity can be retained much longer.

7.2. THE NOSE

The nose being the only part in contact with water it is subjected to extremely high stresses.

Ordinary materials under these conditions will buckle and eventually crush. So in order to

withstand such high stresses nose must be made of materials hard as well as light weight.

Lightweight materials like carbon composites in honey comb structure can be used. Unlike

conventional noses, a supercavitating body has a rather blunt nose. Water is forced to flow

off the edge of the nose at such an angle that it cannot wrap around the surface of the body. If

the projectile is of the correct shape, a bubble of air starts to form around the object... This

extends to cover the entire projectile, and hence the cavitating object is no longer moving

through water, but through air which creates but a fraction of the friction! Hence

supercavitating projectiles can travel as fast as above the surface.

CHAPTER 8:

OTHER CHALLENGES

At the present time only supercavitating weapons are under development. Supercavitation

occurs when an object moving though water reaches speeds in excess of 100 knots. The speed

at the initial state is currently unattainable using the conventional methods. For firing the

missile from above the water a mechanical catapult offers a simple solution. A Mechanical

catapult is the one which is used in the aircraft carrier for launching aircrafts airborne. It is

powered with the pressure exerted by the compressed fluid in two long cylinders. It will take

the aircraft to about 250 km/hr through less than 50 m path. Using a modified version of this

type of catapult we can attain the initial velocity for the projectile. By this same type

of catapult we can attain a higher initial velocity underwater for the projectiles firing from

submarines etc. The major challenges in the implementation of this technology are the

following. Inside the cavity the projectile is very unstable. A projectile dropped into water

draws a column of air down with it, creating a temporarily ventilated cavity that reduces drag


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on the torpedo. The air eventually leaks out, but if the torpedo is moving fast enough the

collapsing ventilated cavity is replaced by a vapour cavity. However, the behaviour of the

cavity’s tail end becomes a problem. The supercavity’s tail end may splash violently around

the projectile’s rear, causing significant structural damage to control and propulsive surfaces.

The splashing tail of a sphere dropped into water. This splashing tail problem can be solved

by making the cavity by ventilating it by the exhaust of the rocket engine from just behind the

nose at the front and from the rear. But my making the cavity bigger we are increasing the

instability. Since only the nose touches the water the maneuvering is so tough. Another big

challenge is how to steer a supercavitating vehicle. Specially designed retractable control fins

that come in contact with water only when required to steer are a solution.

However there are technological hurdles yet to be overcome, towards realizing this. The

advanced thrust vectoring technology is another possible solution at the present time. The

pressure that the nose has to withstand at high speeds will be very high. So the right selection

of the material is another challenge. The use of composite light weight materials like graphite

epoxy or aluminium honeycomb will be effective

CHAPTER 9:

TYPES OF SUPERCAVITATION

Supercavities are classified as one of two types on the basis of the method of formation of

cavities :

9.1. 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.

9.2. 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.

These two types of supercavitation as discussed above, both separately are difficult to achieve

so the method of creating the supercavity around a body is midway between these types. That

is to achieve the Best of Both Worlds!!


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1) Pressurized gas is injected during Launch.

2) The pressurized gas fills the separated flow region, resulting in a Fully developed

cavity

3) Velocity required to initiate supercavitation in drastically reduced

Allows a state of supercavitation to be prematurely attained until pure hydrodynamic

supercavitation can ensured.

Thus there are various method of creating supercavitation around an immersed body.

CHAPTER 10:

ARTIFICIAL METHODS OF CREATING SUPERCAVITATION

10.1. Gas gun method

This method is used for body travelling at greater speed. The main points of this method are

given below :

a) The projectile leaves the launcher under the force of expanding gases (i.e. gun launch).

These gases are provided by compressors. They directly create the impact on the object to be

surrounded by supercavitation.

b) The driving gases are the source of cavitation nuclei. These gases get fitted in between the

interface of gas and object that is the low pressure space.

10.2. Gas injection method

This is one of the another artificial method of creating a supercavitation around a body. Uses

for body travelling at lower speed.

a) Pressurized gas is injected in the trajectory of the projectile prior to launch.

b) The supercavity is fully developed before the projectile interacts with the surrounding

fluid.


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Fig.1

CHAPTER 11:

PARTS OF A SUPERCAVITATING VEHICLE

Fig. 2


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The main parts of a supercavitating vehicle are:

11.1. Body

The body is assumed to be built by three sections, a conical first section holding the

cavitator and providing low drag coefficient in the initial fully wetted phase, for simplicity.

A slender cylindrical body houses the main components of the vehicle, while a smaller

diameter conical section accounts for the nozzle.

11.2. Cavitator

The cavitator is the most critical part of the supercavitating vehicle. It generates the cavity,

provides control forces, and supports the ventilation gas flow rate. The cavitator is also

the only part of the vehicle which is continuously in contact with water. The selection of

disk cavitator might not be the ideal choice for all requirements, especially due to limited

surface area for sensors and low lift coefficient at moderate angles-of-attack. However the

current study does not consider these additional requirements and only focuses on control

design, hence the disk cavitator is sufficient for control purposes. It is important to notice

that the cavitator actuator has rate and deflection limits, which restricts the achievable

level of performance in closed loop control.

The two types of cavitators are:

Disk type cavitator

Fig.3


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Conical type cavitator

Fig. 4

11.3. Fins

Four swept back, wedge shaped _ns at the aft end of the vehicle are used for control. The

importance of these surfaces are obvious, since supercavitating vehicles have to provide

the necessary control forces with the small portions of body in contact with the liquid.

Conventional underwater vehicles operate under the influence of buoyancy, and vortex

shedding helps providing forces during maneuvers. The unique features influencing the

forces acting on the fins include not just only the presence of cavity during normal flight, but

also the transition from fully wetted to supercavitating condition during launch. The fins

are individually actuated around rotation axis, no elevator or rudder coupling is enforced

among them to be able to account for asymmetric immersion on different sides.

11.4. Cavity

Behaviour of the cavity plays a central role in the vehicle dynamics. It causes the dynamical

system to be highly nonlinear, and depend on the history of the vehicle motion. The

interaction of the vehicle with the cavity is via the fins and via after body contact with

the liquid phase. The offset between the cavity centre line and the body x-axis determines

the immersion of the _ns, which changes as the vehicle follows non-steady maneuvers,

continuously influencing the control authority on the fins. Afterbody planing have a hybrid

nature, it does not always appear, it is present when the tail-cavity offset is sufficiently

high and the surface of the body is in contact with the water. Then planing exerts a high

impulsive force to direct the tail back to the cavity.

The cavity closure zone influences both planing and fin immersion is the most difficult

problem when describing the cavity shape. Cavity self oscillation can happen in the zone

where the gas cavity transitions to liquid even when the vehicle is assumed to travel straight

and level with constant speed. These oscillations become more significant when the cavity is


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ventilated, as high gas flow rates can destabilize the cavity, making fin immersion and

planing rapidly changing and highly uncertain.

CHAPTER 12:

ISSUES WITH SUPERCAVITATION

12.1. Advantages of supercavitation:-

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

Thus damage of material is avoided.

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

12.2. Disadvantages of supercavitation:-

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

force is not acting on the body.

2) 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.

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

CHAPTER 13:

APPLICATIONS

The phenomenon of supercavitation is of great importance in underwater automobiles,

missiles and torpedo.

Many countries are currently working on this technology to improve their war resources

under water.


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Most popular applications are:-

13.1. 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 target sat relatively long range. Projectiles fired

from underwater guns can effectively travel long distances by making

u s e o f s u p e r c a v i t a t i o n . 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 contact with the water ( e x c l u d i n g a t t h e t i p a n d o cc a s i o n a l

c o l l i s i o n s w i t h t h e c a v i t y w a l l , " t a i l s l a p " ) , t h e 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 500meters/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.

P r e s s u r e a l s o i n f l u e n c e s t h e s i z e o f t h e c a v i t y . T h e s i z e o f t h e

c a v i t y i s i n v e r s e l y 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


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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. 5

13.2. SUPERCAVITATING TORPEDO

The nose of a supercavitating torpedo uses gas nozzles that continually expel an envelope of

water vapour around the torpedo as it speeds through the ocean. This bubble of

gas—a 'super cavity'--prevents the skin of the torpedo from contacting 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.

T h e U S n a v y i s s e e k i n g t o b u i l d i t s o w n v e r s i o n o f t h e S h k v a l , b u t

o n e w i t h a m u c h 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 vapour

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 dragon 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,


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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 vapour 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 vapour 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 counter measures. A drawback

that had been pointed out in several articles is that the Shkval and its peers only

have ranges of several kilometres, 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 w i l l u s e t h e i r

s u p e r c a v i t a t i n g w e a p o n s t o f u l l e s t p o s s i b l e e f f e c t . A l s o , i t i s a l m o s t 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

s o m e b a l l a s t a n d q u i c k l y a s c e n d i n g . H o w e v e r , a n e n e m y s u b m

a r i n e c a p t a i n m a y anticipate this, and may launch a second or even a third

Shkval simultaneously, aimed a b o v e t h e t a r g e t s u b m a r i n e , i n o r d e r t o

k e e p t h e e n em y v e s s e l f r o m a t t e m p t i n g t h i s maneuver.


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Fig. 6

13.3. 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 vapour pressure of water. If the speed of the

object is not fast enough to travel through the vapour 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 o n l y p a r t o f t h e

o b j e c t i n c o n s t an t c o n t a c t w i t h t h e w a t e r t h r o u g h w h i c h t h e v e h i c l e

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 e f f e c t o f t h e p r o p u l s i o n p l u m e ,

t o t h e r e b y m i n i m i z e l o s s o f a i r a n d t o i n c r e a s e l i f e expectancy of a

reservoir of ventilation air on-board the vehicle.

A s u p e r c a v i t a t i n g v e h i c l e i s a n a d v an c e d c o n c e p t

f o r a c h i e v i n g v e r y h i g h s p e e d s 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


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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.

Depending on the design, the gas could be water vapour, 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. 7


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13.4. 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 ru

nning 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 perf

ormance 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

isthus used, and the surface piercing propeller generally enjoys lower drag than thesupercavit

ating 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. 8

http://en.wikipedia.org/wiki/Propeller

http://en.wikipedia.org/wiki/Supercavitation

http://en.wikipedia.org/wiki/Boat_racing

http://en.wikipedia.org/w/index.php?title=Model_boat_racing&action=edit&redlink=1

http://en.wikipedia.org/wiki/Skin_friction

http://en.wikipedia.org/wiki/Skin_friction

http://en.wikipedia.org/wiki/Erosion

http://en.wikipedia.org/wiki/Surface_piercing_propeller

http://en.wikipedia.org/wiki/Entrainment_(engineering)


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CHAPTER 14:

FUTURE OF SUPERCAVITATION

Far from now this simple cavitating theory could bring us the ultimate fighting machines. Sub

fighters racing around beneath the waves at thousands of kilometres per hour... Massive

underwater sub fighter carriers silently gliding through the deep blue... It may seem like

fantasy, but it's the future...

Future doesn’t end with underwater sub fighters, there will be supersonic fighters cruising

underwater at thousands of kilometres per hour maneuvering like aircrafts performing

dogfights and when nearing land they can be airborne and be a fighter aircrafts and can again

be back to the water. They can be launched either from a subcarrier or from land. They rules

air, land and water. They are the ultimate future.

CHAPTER 15:

CONCLUSION

As is the case of most cutting edge technologies, supercavitation is largely concentrated

around military developments and applications. Very little is known outside about the recent

advancements in detail, as they are closely guarded military affairs. However this technology

is sure to revolutionize underwater weaponry and travel. Under water bullets have already

broken the sound barrier in water. The day is not far away when pencil shaped, rocket

powered vehicles break the sound barrier underwater.


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REFERENCE:

- WWW.SEMINARPROJECTS.COM

- GOOGLE (FOR IMAGES)

- RESEARCH PAPER FROM UNIVERSITY OF MINNESOTA

- STUDIES ON DYNAMICS OF SUPERCAVITATING

PROJECTILE BY S.KILKARNI, R.PRATAP, APPLIED

MATHEMATICAL MODELLING

http://www.seminarprojects.com/

Documents

Supercavitation Seminar Report