Supercavitationseminar Report 2010

SUPERCAVITATIONSEMINAR 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

Cavitationis the formation of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below itsvapour pressure. Cavitation is usually divided into two classes of behavior: inertial (or transient) cavitation, and noninertial cavitation. Inertial cavitation is the process where a void or bubble in a liquid rapidly collapses, producing ashock wave. Such cavitation often occurs incontrol 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 anacoustic field. Such cavitation is often employed inultrasonic cleaningbaths 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 offluid dynamics.

1.2 Supercavitation

Supercavitationis the use ofcavitationeffects to create a large bubble ofgas inside aliquid, allowing an object to travel at great speed through the liquid by being wholly enveloped by the bubble. The cavity (the bubble) reduces thedragon the object, since drag is normally about 1,000 times greater in liquidwaterthan 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 fasttorpedoes.

Fig. 1.1: Different stages of cavitation

Fig 1.2: A valve after cavitation effects

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

Thesupercavitating propelleris a variant of apropellerfor propulsion in water, where supercavitationis 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 performanceboat racing vessels as well asmodel 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 waterskin frictionalong the whole forward face. The cavity collapses well behind the blade, which is the reason the supercavitating propeller avoids theerosiondamage 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.

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 fullscale 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 thanthe 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 nondimensional 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

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.

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

5. SUPERCAVITATING TORPEDO

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.