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Supercavitation


Published on Jan 28, 2016

Abstract

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

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 centimeter 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: 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 "race" and spin ineffectively. When a propeller induces significant cavitation, it is pushing against a combination of liquid water and water vapor. Since water vapor is much less dense than liquid water, the propeller can exert much less force against the water vapor bubbles. With the problems it causes, it is no wonder maritime engineers try to avoid cavitation.

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: vapor or ventilated. Vapor cavities are the pure type of supercavity, formed only by the combination of a number of smaller cavities. In a ventilated cavity, however, gases are released into the bubble by the supercavitating object or a nearby water surface

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 traveling 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 vapor, significantly lowering the drag on the supercavitating object.

Supercavitation
With an appropriate nose shape and a speed over 180 km per hour, the entire projectile may reside in a vapor 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.

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.

(I) 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 vapor 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.

(II) 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 inorder to withstand such high stresses nose must be made of materials hard as well as light weight. Light weight 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.








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