Table of Contents

CAVITATION

In this section the term cavitation is defined as the process of formation of the vapour of liquid when it is subjected to reduced pressure at constant ambient temperature. It is used in the engineering context of liquid flow around bodies generally and, in particular, screw-propellers and hydrofoils.

Acceleration zone

In the sequence of cavitation erosion, the zone of the curve of weight loss versus time in which a rapid increase in weight loss occurs (the region between the incubation zone and the deceleration zone which see). Formerly called the Accumulation zone.

Air content

The term used loosely to describe gas content (which see) when gas content is composed of components of air in the liquid.

Air content ratio

Attached cavities

Term applied to cavitation region with fairly well defined line of attachment to the body about which it is formed. It may be a Fully developed cavity or Partial cavity (which see).

Back cavitation

Cavitation occurring on the suction side (back) of a propeller blade.

Base-vented flow or bodies

Flow in which the body has a fully ventilated, blunt trailing edge while the body itself is fully wetted.

Bubble collapse

The final phase in the life history of a transient cavitation bubble that enters an increasing pressure field collapses and, unless containing considerable foreign gas, disappears. The total life of a transient cavitation bubble is measured in times of the order of milliseconds,

Bubble growth

The initial phase in the life history of a cavitation bubble in which a nucleus become unstable under a pressure reduction and grows explosively (vaporous cavitation) or which grows under quasi-equilibrium conditions by diffusion of gas (gaseous cavitation).

Bubble rebound

Regrowth, after initial collapse, of a transient cavity that contains considerable permanent gas, due to energy storage in the compressed gas. Several growth and rebound cycles have sometimes been observed.

Bubble surface stability

The stability of the bubble surface. Expanding bubbles are stable. Collapsing bubbles are unstable, being subject to Taylor instability (light fluid accelerated toward a heavier fluid) or distortions produced by body forces in a pressure gradient.

Cavitating flow

A two-phase flow composed of a liquid and its vapour is called a cavitating flow when the phase transition is a result of a hydrodynamic pressure change.

Cavitating wakes

Cavitation that occurs in the low pressure cores of the turbulent eddies which make up the wake of a moving body.

Cavitation

In the most engineering contexts, cavitation is defined as the process of formation of the vapour phase of a liquid when it is subjected to reduced pressure at constant ambient temperature. In general, a liquid is said to cavitate when vapour bubbles are observed to from and grow as a consequence of pressure reduction. (See also: Vaporous cavitation and Gaseous cavitation.

Cavitation damage

Deformation and/or erosion of materials in cavitated regions, associated primarily with the high pressures developed during cavity collapse.

Cavitation inception

Inception of cavitation takes place when nuclei subjected to reduced pressure reach critical size and grow explosively. It is generally described by the ambient pressure at which cavitation starts, or more precisely, by the Critical cavitation number (which see).

Cavitation number

<m>(sigma)</m>[-]
The ratio of the difference between absolute ambient pressure p and cavity pressure pC to the free stream dynamic pressure q:
<m>sigma = {p - p_c}/q</m>
When the cavity pressure is assumed to be the vapour pressure pV the term is generally called Vapour cavitation number (which see as Cavitation number, vapour.

Cavitation number, critical

Often used as an alternate to Inception cavitation number (which see as Cavitation number, inception)

Cavitation number, inception

<m>(sigma_I)</m>[-]
The inception cavitation number <m>sigma_I</m> is the value of the cavitation number <m>sigma</m> at which the inception of cavitation occurs in a flowing system. When <m>sigma_I</m> > <m>sigma</m>, cavitation will not occur; thus <m>sigma_I</m> is the characteristic of the flow geometry while <m>sigma</m> is characteristic of the liquid gas system. (In practical system, the definition of <m>sigma</m> is usually based on the vapour pressure.) Sometimes also called Critical cavitation number (which see as Cavitation number, critical).

Cavitation number, vapour

<m>(sigma_V)</m>[-]
The ratio of the difference between absolute ambient pressure p and vapour pressure pV to the free stream dynamcre q:
<m>sigma_V = {p-p_V}/q</m>
See also: Cavitation number.

Cavity drag

(DC) [LMT-2]
The energy expended in forming a fully developed cavity, which cannot be recovered at cavity closure and hence is exhibited as drag on the body. It is equal to the energy in the reentrant jet which is dissipated.

Cavity length

(lC)[L]
The streamwise dimension of a fully developed cavitating region, extending from its leading edge (point of attachment) to the point of closure.

Cavity pressure

(pC) [LMT-1M-2]
Actual pressure within a steady (or quasi-steady) cavity. Approximately equal to the sum of the partial pressure of vapour and other gases diffused and entrained into the cavity.

Cavity thickness

<m>(δ_C)</m>[L]
Maximum dimension of a fully developed cavity normal to the length dimension.

Chemo-luminescence

Visible light produced in the gas vapour of cavities in an ultrasonic field (See: Sonoluminescence) caused by chemical reactions associated with high pressure and/or temperatures.

Choked flow

This is defined as the flow condition in which the drag of a body is directly proportional to the square of the upstream velocity and is not a function of the cavitation number. The pressure coefficient at any point on the body is independent of the cavitation number.

Choking cavitation number

This is defined as that value of σ at which a terminal, minimum value of the drag coefficient is found for a cavitating body.

Collapse pressure

<m>(p_AC)</m> [L-1MT-2]
The pressure produced in the field of a collapsing cavitation bubble estimated to be of the order of thousands of atmospheres at the minimum radius reached before the process stops or rebound begins.

Critical cavitation number

Critical pressure

<m>(p_AI)</m> [L-1MT-2]
The absolute pressure at which cavitation inception takes place, in either a flowing system or an imposed pressure field (as in ultrasonic cavitation). In turbulent flow, the critical pres-sure will be a function of the average hydrodynamic pressure and the pressure fluctuations associated with turbulence. Sometimes also called Inception pressure. (See also: Gaseous and Vaporous cavitation.)

Critical velocity

(UI)[LT-1] In a flowing system (or its equivalent: a body moving through a liquid), the free stream ve-locity at which cavitation inception takes place in a field of constant ambient pressure. In a turbulent flow, the critical velocity is also dependent on the velocity fluctuations associated with turbulence. Sometimes also called Inception velocity.

Deceleration zone

In the sequence of cavitation erosion, the zone of the curve of weight loss versus time in which the rate of weight loss decrease (the region following the acceleration zone, which see). Formerly called the Attenuation zone.

Desinent cavitation

Cavitation under conditions of pressure and velocity such that cavitation will be suppressed by a slight change in the ambient conditions: pressure increase and /or velocity reduction.

Electrolytic effects

Enhancement of cavitation erosion by electrochemical interactions due to local differences in the liquid or metal structure.

Entrained gas content

Face cavitation

Cavitation occurring on the pressure side (face) of a propeller blade. It is generally a result of operation such that the local blade angle of attack is excessively negative.

Foam cavitation

A cavitated region formed entirely of a mass of transient cavities so as to resemble foam (formerly called burbling cavitation).

Free gas content

Free streamline flow

Fully developed cavity flow. For steady flows, the cavity walls are stream surfaces of the flow with the unique feature that the pressure is constant on the free streamlines. The term originates in the mathematical problem that the boundaries are “free” to be determined by the known condition of constant pressure.

Fully developed cavity

A cavity formed on a body which terminates sufficiently far downstream so that the flow at the downstream region does not influence the body itself. For example, the cavity is fully developed when the re-entrant jet formed at the downstream end of the cavity is dissipated without impinging on the body. See also: Supercavitating flows.

Gas content

(α)
The gas content of a liquid may be in either a dissolved or undissolved state. The quantity of dissolved gas will vary according to Henry’s law, but it is now generally agreed that cavitation inception is associated with the gas con-tained in nuclei in an undissolved state (see: Nuclei and Nucleation). Total gas content is equal to both the dissolved and undissolved gas. “Free” and “entrained” gas content are alternate terms for undissolved gas content, but the latter term is preferred.

Gas content of the saturated liquid

<m>(alpha S)</m>
The gas content of the saturated liquid at standard temperature and pressure.

Gas content ratio

(aS)[-]
The ratio of the content (dissolved and undissolved) in a test liquid to the gas content of the saturated liquid at standard temperature and pressure: <m>a_S = alpha/alpha_S</m>

Gas injection, protection by

Small amounts of gas injected into the cavitating region to reduce the pressure through a “cushioning” effect during compression by the collapsing cavitation bubbles.

Gaseous cavitation

Depending upon the magnitude of the pressure reduction and the rate of application, a bubble may grow slowly by diffusion gas into the nucleus (which see) and contain mostly gas rather than vapour. Such bubble growth is defined as gaseous cavitation. Such cavitation may occur at pressure greater or less than vapour pressure aided by the process of Rectified diffusion (which see).

Hub cavitation

Hub vortex cavitation

Cavitation in the vortex produced by the blades of a propeller at the hub.

Hysteresis, cavitation

Difference between critical cavitation numbers for incipient and desinent cavitation. Also, the difference between the angle of attack of a lifting surface for initiation or fully developed cavitation during angle of attack increase and the much lower angle of attack at which a fully developed cavity can still be maintained once it has been formed.

Inception of cavitation

Inception cavitation number

Inception pressure

Inception velocity

Incipient cavitation

Cavitation which just begins with a slight change in ambient conditions: pressure decrease and/or velocity increase.

Incubation zone

In the sequence of cavitation erosion, the initial zone of the curve of weight loss versus time in which the material undergoes changes (e.g. work hardening in ductile metals) due to repeated bubble collapse pressures, but in which the material suffers little or no weight loss.

Intensity damage

The power absorbed per unit eroded area of a specimen undergoing erosion.

Intermittent cavitation

A type of cavitation that respectively originates and disappears from a discrete point on a solid surface.

Internal jets

Jets sometimes formed by the unsymmetrical collapse of transient cavities. Also sometimes called microjets.

Jet cavitation

Cavitation formed in the low pressure eddies associated with the turbulent fluctuations in the high shear region of jet flows.

Laminar cavitation

Microjets

Non-stationary cavities

Free-streamline (cavitating) flows in which the cavity size is a function of time. The cavity surface is a boundary surface, but not necessarily a stream surface. Cavities trailing a body entering a water surface are characteristic of non-stationary cavities.

Nucleation

The process of formation of nuclei in liquid. Also, sometimes used to refer to the process of stabilisation of nuclei to account for their persistence in undersaturated and saturated liquids.

Nucleus, nuclei

Small bubbles, often sub-microscopic in size, containing permanent gas and/or the vapour of the liquid, which are required for inception of cavitation at the pressure near vapour pressure. (See also: Nucleation).

Onset cavitation

Orange peel surface appearance

Description of a surface moderately damaged by the cavitation in which the appearance is that of the surface of the Jaffa or California orange.

Partial cavities

Quasi-steady cavities that extend only partially along the bodies about which they are formed.

Pitted surface appearance

Description of a surface damaged by cavitation in which pits are formed either by crater-like deformation (especially as in lead) without loss of material or by actual loss of material following work hardening or fatigue.

Propeller-hull vortex cavitation

Propeller tip vortex cavitation that extends intermittently to the surface of hull.

Protective coating

Metallic and non-metallic materials applied to reduce surface damage by cavitation. They may be welded, sprayed or bonded to the surface.

Pseudo cavitation

Growth and collapse of gas filled bubbles whose size is at all times in static equilibrium with the surrounding pressure field.

Pulsating cavity

A “pulsating” cavity is a ventilated cavity which exhibits self excited oscillations of the cavity surface as a resonance phenomenon of the gas-liquid (cavity-jet) system; i.e. for self sustained oscillations, the frequency of the volume changes due to travelling surface waves on the cavity wall (and, hence, corresponding pressure changes) must be equal to the natural frequency of the gas liquid system.

Rate of weight loss

The primary criterion for cavitation erosion. The weight loss per unit time from a test specimen.

Rectified diffusion

Term applied to the net mass transport into a bubble of gas dissolved in a saturated liquid when the liquid is subjected to an oscillating pressure field.

Re-entrant jets

The re-entrant (upstream) flow at the trailing edge of steady (quasi-steady) cavities. Also, the re-entrant flow associated with the closure of non stationary cavities formed about missiles entering a water surface.

Root cavitation

Cavitation in the low-pressure region of the blade roots on a marine propeller.

Screening effect

Effect associated with the “screening” of nuclei by the pressure gradient about the body to which the nuclei are being convected, thus determining which nuclei will be repelled from and which nuclei will be swept into regions where the pressure are such as to enable cavita-tion inception to take place.

Sheet cavitation

A term applied to describe relatively thin, steady or quasi-steady cavities. (Also, formerly called laminar cavitation)

Sono-luminescence

Visible light produced in the gas or vapour of cavities generated in the alternating pressure of an ultrasonic field. This phenomenon is believed to be associated with high temperatures resulting from compression of the gases within the bubble.

Spongy surface appearance

Description of a surface badly damaged by cavitation in which erosion has taken place to a considerable depth and has the appearance of a sponge. This description is particularly characteristic of brittle materials and other materials after long exposure.

Spot cavitation

A general term for narrow quasi-steady cavities attached to a surface.

Steady quasi-steady cavities

Cavitating flow may be composed of individual transient cavities or of large cavities attached to the body on which cavitation has been induced (particularly if the detachment point is sharply defined, as for hydrofoil with sharp leading edge). The envelope of the bubbles in the former case and the cavities in the latter case are quasi-steady in the sense that envelope or cavity surface is stationary on a temporal average.

Steady zone

In the sequence of cavitation erosion, the final zone of the curve of weight loss versus time, in which the rate of weight loss is nearly constant. (Also called steady-state zone).

Streak cavitation

Narrow quasi-steady cavities formed about excrescences or isolated roughness near the leading edge of a hydrofoil or other body. Such cavitation may also be associated with pressure variations in unstable laminar boundary layers.

Stream nuclei

Undissolved gas nuclei existing in a stabilised condition (either on dust particles or otherwise) which are convected by the stream into regions of low pressure where they form cavitation sources.

Supercavitating flows

Cavity flows in which attached, fully developed cavities extend beyond the trailing edge of the body about which the cavity is formed. (See also: Attached cavities and Fully developed cavities).

Supercavitation

Term sometimes used as synonymous with Supercavitating flow (which see).

Superventilation; Superventilated flow

Terms analogous to Supercavitating flow to denote a ventilated flow in which the cavity extends beyond the trailing edge of the body about which the cavity is formed.

Thoma number

<m>(Th)</m>[-]
The ratio of the difference between total head and the vapour pressure (upstream of the impeller of rotating machinery) to the total head produced or absorbed by the machine.

Tip cavitation

Surface cavitation which occurs near the tip propeller blade.

Tip vortex cavitation

Cavitation occurring in the low-pressure core of the tip vortex of a hydrofoil or propeller.

Total gas content

Trailing vortex cavitation

Persisting cavitation in the low-pressure core of trailing vortices downstream of hydrofoils or propellers. (See also: Tip vortex cavitation and Hub vortex cavitation).

Transient cavities

Cavitation bubbles that grow from nuclei, sometimes oscillate (if containing a high volume of permanent gas component) and eventually collapse and disappear.

Trapped gas

Undissolved gas trapped in the cavities of foreign particles or the crevices of the boundary under study.

Unsteady cavities

Attached cavities which alternately grow (resembling steady cavities at any instant) – extending downstream from the point of attachment and collapse (i.e. sudden reduction in length), presumably by cyclic filling by the reentrant flow and subsequent re-evaporation.

Vapour cavitation number

See: Cavitation number, vapour.

Vaporous cavitation

A nucleus (which see) that grows explosively (after reaching critical size) contains mostly vapour phase, the diffusion time being too short for any significant increase in gas volume. This process, which depends upon evaporation of the liquid into the growing bubble, is a true cavitation and is called vaporous cavitation, For such cavitation to occur, pressure below vapour pressure are required.

Ventilated flow

A ventilated flow is one in which a “cavity” is formed entirely with air (or other permanent gas).

Ventilation

Process by which a ventilated flow is formed and maintained. Natural ventilation is applied to a ventilated flow which derives a continuous flow of gas by means of the pressure created by the flow itself, as from the free surface in the case of a surface piercing, ventilated strut. Forced ventilation is applied to a ventilated flow in which the permanent gas is continu-ously supplied into the cavity by auxiliary means such as a pump.

Ventilation inception

Ventilation inception is defined as the condition at which air (or permanent gas) is drawn into the low-pressure region in a non-cavitating flow, from an external source, as at the free surface of a liquid.

Ventilation index

The ratio of the volumetric air feed rate to the product of free stream velocity and an area proportional to the cavity cross sectional dimension or to some typical body dimension.

Volume loss

(VL)[L3]
An alternative criterion to weight loss for assessing cavitation damage, often derived from weight loss by using the density of the specimen material.

Vortex cavitation

Wall nuclei

The undissolved gas nuclei which may exist in equilibrium in the crevices of the boundary wall material.

Weight loss

(WL) [LMT-2]
Weight of material actually eroded from a specimen during a specified time while undergoing erosion damage. The most widely used measure of cavitation damage.

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structured_dictionary/cavitation.txt · Last modified: 2012/03/19 12:21 by cyberiad
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