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Introduction |
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Cavitation occurs when a ship propeller is rotated so fast that high flow rates are achieved on the surface. The
flowing water gets under low pressure, so that water begins to evaporate. Spontaneously spherical, water-vapor-filled
bubbles are formed, known as "cavitation". If these bubbles come back to areas of higher pressure, they fall together
with sharp bang and produce much audible noise. When the bubbles collapse on the propeller surface, they cause the dreaded
"pitting" on the propeller surface. Therefore cavitation is an extremely unpopular appearance among shipbuilders. We are considering a different phenomenon. If a body falls into water, it has to displace water on its way. This action seperates the fluid from the body and forms a cavity that is filled by air flowing. Everyone has already made this observation. One can see this particularly well in the immersion of a dive from 10 m platform. The "art" of the tower diver is to shape his hands in the immersion phase to form a cavity that is larger than his body size. Only then "splashless" immersion is possible. If a body is accelerateted beneave the watersurface to high velocity fluid seperates from the body and a cavity is formed, that is filled with watervapor and gases formerly solved in water. The higher the velocity the larger the cavity behind the body. Mistakably such a cavity sometimes is qualified as cavitation bubble. Assuming that you are able to calculate such cavities you could design a body that fits into the cavity created by its front face without touching the cavity wall. Such a body has no contact with water beside with its front face. So it has very little surface friction, which usually is the main source of drag. A body that on its trajectory under water is surrounded by a small layer of gas and only in water contact with its front face is called a "supercavitating" body. The phenomenon described is called "supercavitation".
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