VOL. 16 May ISSUE YEAR 2015

Off the Beaten Track

in Vol. 16 - May Issue - Year 2015
On a Sea of Bubbles
Comparison between vehicles with standard propulsion and supercavitation

Comparison between vehicles with standard propulsion and supercavitation

The nose cone of the Shkval torpedo

The nose cone of the Shkval torpedo

Anyone who has dipped a hand into the water while sitting in a moving boat knows how much effort is required to keep the hand in the same position. Water is about nine hundred times denser than air and thus offers much greater resistance, or drag. This accounts for the much slower speed of boats and submarines compared to aircraft and land vehicles. To build a boat capable of achieving the same speed as an airplane would require immensely more powerful means of propulsion and would burn up more fuel than the boat could conveniently carry, not to mention that the material and the structure of the boat would have to be much more resistant to mechanical stress.

But what if it were possible to eliminate drag and produce boats that could literally "fly" through the water, or even underwater, at speeds similar to those reached by jet airplanes? This idea is not as far-fetched as it may seem. It involves the application of a hydrodynamic process called supercavitation. To understand how supercavitation works, it's necessary to take a step backwards and examine a physical principle called cavitation, from which supercavitation derives.

Cavitation is the formation of vapor cavities, or more simply bubbles, caused by a sudden drop in pressure in a liquid, typically water. Since the state of water depends as much upon its pressure as upon its temperature, lowering the water's pressure below its vapor pressure causes the water to boil as if it were being heated over an open flame. In other words, it is a process of boiling by pressure reduction rather than by heat addition. The drop in pressure can be caused when the water is accelerated to high speeds and simultaneously makes a sharp change in direction, such as when, for example, the water moves around the edges of spinning propeller blades. The faster the speed of the liquid, the greater the drop in pressure, and the more bubbles formed. As soon as the pressure goes back up, the bubbles implode with great force and send out intense shock waves. If they occur cyclically near a metal surface, these shock waves cause surface fatigue that may ultimately lead to the failure of the component. For this reason, engineers try to limit cavitation damage by designing propellers, pump impellers, turbines, pipes and conduits in order to control the pressure and the speed of the passing liquid. Eliminating the causes of cavitation is a major field in the study of fluid dynamics. So, while cavitation is considered a problem, its derived process, supercavitation, can be put to beneficial use.

Supercavitation is a controlled form of cavitation in which a single bubble forms around an object, thus eliminating surface friction almost completely and greatly reducing viscous drag. Practically speaking, the small bubbles created by cavitation expand and merge to become one large bubble, a supercavity, enveloping the entire object. As the supercavity is longer than the object, only the leading edge of the object enters into contact with the liquid, while the rest of the object is surrounded by low-pressure liquid vapor. The supercavity is created by carefully designing the nose or leading edge of the object and by moving the object at a sufficiently high speed. This low-pressure zone can be enhanced by supplying gas to the cavity at ambient pressure.

The principle of supercavitation was first applied for military purposes in 1960 by the Soviet Union. The Soviets created the rocket-propelled Shkval (Squall) torpedo, with a specially designed flat nose and gas release system capable of speeding underwater at over 200 knots (370 km/h), when other torpedoes barely reached a speed of 40 knots. In recent years, other countries have developed their own supercavitating torpedoes, with Germany claiming to have developed a model capable of moving at 210 knots and an anti-torpedo missile that can move underwater at over 430 knots (¡Ö 800 km/h).

Research into the application of supercavitation has extended to the design of boats and submarines. China recently announced that it was well on the way to building a submarine capable of incredible speeds. In theory, supercavitation could help a submarine travel up to the speed of sound, although engineers would have to overcome some formidable problems concerning maneuverability, as any rudder or directional fin would need to operate in contact with the water and outside of the bubble.

By Giovanni Gregorat, Contributing Editor MFN

Author: Giovanni Gregorat