In hydrodynamic engineering, supercavitation is the artificial generation of a cavitation bubble to reduce skin friction drag on a submerged object and enable high-speed travel. Applications include torpedoes and propellers, but in theory, the technique could be extended to an entire underwater vessel.
Physical principle Cavitation is the internal boiling of a liquid caused by rapid flow around an object. Fluid flow around sharp corners requires very large pressure gradients, and in particular very low pressures "past the corner". In those areas, the pressure can drop below the vapor pressure, at which point the liquid boils. Cavitation potential is measured by the nondimensional cavitation number, which is equal to the difference between local pressure and vapor pressure, divided by dynamic pressure. At increasing depths (or pipe pressures), the potential for cavitation is lower because the local pressure is much further from the vapor pressure. Cavitation is typically considered a nuisance in hydrodynamic engineering, as cavitation bubbles released from the surface subsequently implode. The implosion generates small concentrated impulses that may damage surfaces like ship propellers and pump impellers. A supercavitating object is a high-speed submerged object that is designed to initiate and maintain a cavitation bubble at its nose. The bubble extends (either naturally or augmented with internally generated gas) past the aft end of the object and prevents contact between the sides of the object and the liquid. This separation substantially reduces the skin friction drag on the supercavitating object. A key feature of the supercavitating object is the nose, which typically has a sharp edge around its perimeter to form the cavitation bubble. The nose may be articulated and shaped as a flat disk or cone. The shape of the supercavitating object is generally slender so the cavitation bubble encompasses the object. If the bubble is not long enough to encompass the object, especially at slower speeds, the bubble can be enlarged and extended by injecting high-pressure gas near the object's nose. The very high speed required for supercavitation can be temporarily reached by underwater-fired projectiles and projectiles entering water. For sustained supercavitation, rocket propulsion is used, and the high-pressure rocket gas can be routed to the nose to enhance the cavitation bubble. The key engineering difficulty in supercavitation design is stability: because a supercavitating vehicle fully encased in bubble is no longer submerged, it experiences no buoyant force. One alternative only partially contains the vehicle in the bubble, supported by a submerged rear, but such situations trade off between support and increased drag. In principle, supercavitating objects can be maneuvered using various methods, including the following:
Drag fins that project through the bubble into the surrounding liquid A tilted object nose Gas injected asymmetrically near the nose to distort the cavity's geometry Vectoring rocket thrust through gimbaling for a single nozzle Differential thrust from multiple nozzles
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