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Supercavitation

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.

Supercavitation theory was developed by Dr. Marshall Tulin in the US in the 1960s. In the 1980s the Soviets fielded the Shkval underwater rocket with a top speed of over 170 knots. In the 1990s NUWC developed a Mach 1 (1500 m/sec) underwater bullet under DARPA and ONR sponsorship.



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