Where does cavitation collide?

In fluid mechanics, cavitation is the formation of vapour-filled cavities in a liquid wherever the local pressure falls below the liquid's vapour pressure, typically on fast-moving surfaces such as pump impellers, ship propellers, and valves, followed by the violent collapse of those cavities when they sweep back into higher-pressure regions. Rayleigh's 1917 On the pressure developed in a liquid during the collapse of a spherical cavity derived the enormous local pressures produced by that collapse, which explains the erosion, noise, and vibration cavitation causes in hydraulic machinery. The referent is a pressure-driven phase change and implosion in a flowing liquid, and it shares only its name with a plant's broken water column or a cracking knuckle.[1]

Unit

The onset and intensity of vapour-cavity formation, indexed by the cavitation number

Recovery means

Cavities collapse and the vapour recondenses when pressure recovers, though repeated collapse pits and erodes nearby solid surfaces

Measured by

Quantified by the cavitation number, and detected through the noise, vibration, and surface pitting that collapse produces

In joint biomechanics, cavitation is the sudden formation of a gas bubble in the synovial fluid of a synovial joint when the joint is pulled apart and the fluid pressure drops, producing the audible crack of a knuckle. Unsworth, Dowson, and Wright's 1971 'Cracking joints': a bioengineering study of cavitation in the metacarpophalangeal joint showed that the sound accompanies this rapid cavity formation rather than the bursting of an existing bubble. The referent is a tension-induced bubble in the fluid of a living joint, measured by the joint-separation force and the acoustic click, and it is a wholly different phenomenon from an eroding propeller or a drought-stressed tree, joined to them only by the shared word.[2]

Unit

The joint-distraction force at which a synovial gas cavity forms

Recovery means

The gas gradually redissolves into the synovial fluid over minutes, after which the joint can crack again

Measured by

Studied by the force needed to separate the joint, the accompanying acoustic click, and imaging of the transient cavity

In plant physiology, cavitation is the abrupt breaking of a water column held under tension inside a xylem conduit, which nucleates a gas bubble that expands to fill and block the conduit, an air embolism that stops water transport through that pathway. Tyree and Sperry's 1989 Vulnerability of xylem to cavitation and embolism set out how drought and freezing raise xylem tension to the point of cavitation and how a plant's conductivity collapses as embolisms accumulate. The referent is the failure of metastable water under negative pressure in a plant's plumbing, tracked by loss of hydraulic conductivity, and it is unrelated to the imploding vapour cavities of a pump or the gas bubble behind a cracking joint.[3]

Unit

The xylem tension at which conduits embolise, and the resulting loss of conductivity

Recovery means

Some plants refill embolised conduits by root pressure or overnight relaxation of tension, restoring conductivity

Measured by

Assessed by vulnerability curves relating xylem tension to percent loss of hydraulic conductivity, and by acoustic emissions from cavitation events