Vapor gases evaporate into the cavity from the surrounding medium thus, the cavity is not a vacuum at all, but rather a low-pressure vapor (gas) bubble. Other ways of generating cavitation voids involve the local deposition of energy, such as an intense focused laser pulse (optic cavitation) or with an electrical discharge through a spark. A fast-flowing river can cause cavitation on rock surfaces, particularly when there is a drop-off, such as on a waterfall. This phenomenon is coined cavitation inception and may occur behind the blade of a rapidly rotating propeller or on any surface vibrating in the liquid with sufficient amplitude and acceleration. ![]() When a volume of liquid is subjected to a sufficiently low pressure, it may rupture and form a cavity. Inertial cavitation was first observed in the late 19th century, considering the collapse of a spherical void within a liquid. ![]() However, it is sometimes useful and does not cause damage when the bubbles collapse away from machinery, such as in supercavitation. It is very often specifically prevented in the design of machines such as turbines or propellers, and eliminating cavitation is a major field in the study of fluid dynamics. Since the shock waves formed by collapse of the voids are strong enough to cause significant damage to parts, cavitation is typically an undesirable phenomenon in machinery (although desirable if intentionally used, for example, to sterilize contaminated surgical instruments, break down pollutants in water purification systems, emulsify tissue for cataract surgery or kidney stone lithotripsy, or homogenize fluids). Such cavitation is often employed in ultrasonic cleaning baths and can also be observed in pumps, propellers, etc. Non-inertial cavitation is the process in which a bubble in a fluid is forced to oscillate in size or shape due to some form of energy input, such as an acoustic field. In manufactured objects, it can occur in control valves, pumps, propellers and impellers. Inertial cavitation occurs in nature in the strikes of mantis shrimp and pistol shrimp, as well as in the vascular tissues of plants. The process in which a void or bubble in a liquid rapidly collapses, producing a shock wave, is called inertial cavitation. Cavitation is usually divided into two classes of behavior: inertial (or transient) cavitation and non-inertial cavitation. The most common examples of this kind of wear are to pump impellers, and bends where a sudden change in the direction of liquid occurs. This results in surface fatigue of the metal, causing a type of wear also called "cavitation". Collapsing voids that implode near to a metal surface cause cyclic stress through repeated implosion. Cavitation is a significant cause of wear in some engineering contexts. These shock waves are strong when they are very close to the imploded bubble, but rapidly weaken as they propagate away from the implosion. ![]() When subjected to higher pressure, these cavities, called "bubbles" or "voids", collapse and can generate shock waves that may damage machinery. Bronze propeller on river barge with an anti-cavitation plate above the prop.Ĭavitation is a phenomenon in which the static pressure of a liquid reduces to below the liquid's vapour pressure, leading to the formation of small vapor-filled cavities in the liquid.
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