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INTRODUCTION 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
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