Report 2

SUPERCAVITATION SEMINAR REPORT 2010

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.

1.1 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.

1.2 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.

DEPT. OF MECHANICAL ENGG. 1 SBCE, Pattoor

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

http://en.wikipedia.org/wiki/Drag_(physics)

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

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

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

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

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

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

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

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

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

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


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.

Fig. 1.1: Different stages of cavitation

Fig 1.2: A valve after cavitation effects


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


2. 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.

2.1 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

2.2 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

2.3 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.


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

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


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

2.4 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 contacting the water, eliminating

almost all drag and friction and allowing the projectile to slide seamlessly through the

water at great velocity.


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

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

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

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


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



3. 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 contact 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 high-speed 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 contact with the fluid, and states including

contacts with the fluid cavity walls. See Fig.3.1.



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 contacts may be of long-duration (planing), or intermittent

(impacts). In this initial study, we consider intermediate speed regimes where long-

duration cavity contact (planing) does not occur, and where vapor dynamic forces are

negligible.

3.1 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 well-approximated 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



4. 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 contact 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



5. 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 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.

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



6. 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 contact 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 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 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.



7. 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.


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

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

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



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

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

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

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


Fig 7.1: A supercavitating propeller



8. CONCLUSION

Supercavitation is an upcoming 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.



9. REFERENCE

WEBSITES

1. www.wikipedia.org/supercavitation

2. www.wikipedia.org/supercavitating_propeller

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 Feedback Control of High-speed Supercavitating Vehicles Ziyao Cao College of Marine Engineering Northwestern Poly technical University Xi’an 710072, China


http://www.wikipedia.org/supercavitating_propeller

http://www.wikipedia.org/supercavitation

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