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 supercavitating torpedoes, and some propellers, but in principle the technique could be extended to include entire vehicles. Chinese and the US Navy are reportedly working on supersonic supercavitating submarines.

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.

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Wikipedia