SUPERCAVITATION

Supercavitation is the use of cavitation effects to create a bubble of gas inside a liquid large enough to encompass an object travelling through the liquid, greatly reducing the skin friction drag on the object and enabling achievement of very high speeds. Current applications are mainly limited to projectiles or very fast torpedoes, and some propellers, but in principle the technique could be extended to include entire vehicles.

In water, cavitation occurs when water pressure is lowered below the water's vapour pressure, forming bubbles of vapour. That can happen when water is accelerated to high speeds as when turning a sharp corner around a moving piece of metal such as a ship's propeller or a pump's impeller. The greater the water depth (or pressure for a water pipe) at which the fluid acceleration occurs, the lesser the tendency for cavitation because of the greater difference between local pressure and vapour pressure. (The non-dimensional cavitation number is a measure of the tendency for vapour pressure bubbles to form in a liquid, calculated as the difference between local pressure and vapour pressure, divided by dynamic pressure.) Once the flow slows down again, the water vapour will generally be reabsorbed into the liquid water. That can be a problem for ship propellers if cavitation bubbles implode on the surface of the propeller, each applying a small impulse that is concentrated in both location and time, causing damage.

A common occurrence of water vapour bubbles is observed in a pan of boiling water. In that case the water pressure is not reduced, but rather, the vapour pressure of the water is increased by means of heating. If the heat source is sufficient, the bubbles will detach from the bottom of the pan and rise to the surface as steam. Otherwise if the pan is removed from the heat the bubbles will be reabsorbed into the water as it cools, possibly causing pitting or spalling on the bottom of the pan as the bubbles implode.

A supercavitating object is a high speed submerged object that is designed to initiate a cavitation bubble at the nose which (either naturally or augmented with internally generated gas) extends past the aft end of the object, substantially reducing the skin friction drag that would be present if the sides of the object were in contact with the liquid in which the object is submerged. A key feature of the supercavitating object is the nose, which may be shaped as a flat disk or cone, and may be articulated, but which likely has a sharp edge around the perimeter behind which the cavitation bubble forms. The shape of the object aft of the nose will generally be slender in order to stay within the limited diameter of the cavitation bubble. If the bubble is of insufficient length to encompass the object, especially at slower speeds, the bubble can be enlarged and extended by injection of high pressure gas near the object's nose.

The great speed required for supercavitation to work can be achieved temporarily by a projectile fired under water or by an airborne projectile impacting the water. Rocket propulsion can be used for sustained operation, with the possibility of tapping high pressure gas to route to the object's nose in order to enhance the cavitation bubble. An example of rocket propulsion is the Russian VA-111 Shkval supercavitating torpedo. In principle, maneuvering may be achieved by various means such as drag fins that project through the bubble into the surrounding liquid, by tilting the nose of the object, by injecting gas asymmetrically near the nose in order to distort the geometry of the cavity, by vectoring rocket thrust through gimbaling for a single nozzle, or by differential thrust for multiple nozzles.

Figure showing different types of cavitators, consisting standard geometries as cone-shaped cavitator, gear-shaped cavitator and flat disk cavitator, and more advanced cavitators as piston moving cavitator, telescopic cavitator and propeller cavitator. Stepped caviator or cavitator steps may have the lowest degree of Logvinovich effect. Further figures showing different methods to provoke or extend the supercavitation, as using leidenfrost effect, ventilated cavitator rather than unventilated cavitator, preferably an active gas supplier where the active part comes in addition to the ventilation effect.

SUPERCAVITATING PENGUINS

Resonance-Free SWATH (RFS), 230 meter long supercavitating containership with wave height tolerance of 9 meter, intended by several Japanese professors and universities to operate over the Pacific.

 
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