Abstract

Cavitation is the creation of cavities in a liquid when local pressure becomes sufficiently low for vaporization or for preexisting gas nuclei to expand. A complete cavitation event includes nucleation, growth, convection, deformation, breakup, condensation, and collapse. The lowest pressure can occur at the eye or leading edge of a pump impeller, at a valve vena contracta, behind an orifice, in a leakage vortex, within a marine propeller tip vortex, or on the suction surface of a hydrofoil. The cavities may be isolated bubbles, an attached sheet, a coherent vortex core, or a shed cloud [1–3].

1. Foundations, Definitions, Historical Development, and Generation Modes

1.1 Definition

Cavitation is the creation of cavities in a liquid when local pressure becomes sufficiently low for vaporization or for preexisting gas nuclei to expand. A complete cavitation event includes nucleation, growth, convection, deformation, breakup, condensation, and collapse. The lowest pressure can occur at the eye or leading edge of a pump impeller, at a valve vena contracta, behind an orifice, in a leakage vortex, within a marine propeller tip vortex, or on the suction surface of a hydrofoil. The cavities may be isolated bubbles, an attached sheet, a coherent vortex core, or a shed cloud [1–3].

Hydrodynamic cavitation is generated by a flowing pressure field. It differs from acoustic cavitation, in which an imposed sound field drives pressure oscillations, although the collapse physics can overlap. It also differs from ventilation, which is the ingestion of noncondensable gas from a free surface or external source, and from flashing, in which liquid remains partly vaporized downstream because pressure does not recover above vapor pressure [2], [4], [5], [6].

Table 1.1. Terminology and distinctions. [7]

Term	Defining feature	Engineering implication
Hydrodynamic cavitation	Cavities form because flowing liquid passes through a region of low static pressure.	Appears in pumps, valves, propellers, turbines, jets, and process reactors.
Acoustic cavitation	Pressure oscillations are imposed by an acoustic transducer.	Frequency and sound field govern nuclei growth and collapse.
Vaporous cavitation	Cavity content is dominated by liquid vapor.	Collapse can be rapid and erosive after pressure recovery.
Gaseous cavitation or pseudocavitation	Dissolved or entrained gas expands into a low pressure region.	Collapse is often cushioned, gas release may still cause loss of performance and noise.
Flashing	Pressure remains below saturation pressure downstream.	Vapor persists, damage often occurs through high velocity two phase flow rather than repeated collapse.
Ventilation	External air or gas is drawn into the liquid flow.	Common near free surfaces and marine propulsors may mimic or interact with cavitation.
Supercavitation	A cavity is intentionally extended to envelop most of a body or blade.	Can reduce wetted drag but demands cavity stability and controlled closure.

1.2 Why Hydrodynamic Cavitation Matters

In unwanted service, cavitation can reduce pump head, flow, thrust, and efficiency, produce broadband noise, discrete pressure pulses, and vibration; erode metal and coatings; destabilize control valves, excite piping and structures; contaminate a process with wear debris; and shorten the life of seals, bearings, and nearby components. Cavitation can also constrain the speed, immersion, loading, or pressure ratio of otherwise efficient machinery [1], [1], [8], [6].

In deliberate process service, the same local pressure reduction and collapse can generate intense microscale shear, rapid interfacial renewal, pressure pulses, and, in some liquids, reactive species. These effects have been applied to emulsification, extraction, wastewater treatment, disinfection, reaction intensification, biomass pretreatment, food processing, and cleaning [1,9–15]. The engineering objective is therefore not simply to eliminate vapor. It is to control where cavities form, how long they live, where they collapse, and how much useful effect is obtained per unit energy.

1.3 Historical Development

1.3.1 From Marine Performance Loss to a Defined Phenomenon

Cavitation became an urgent engineering problem during the late nineteenth century as marine propellers and steam turbines reached higher rotational speeds and power densities. Sir Charles Algernon Parsons encountered a sharp loss of propeller performance during development of fast turbine driven vessels. He built an early closed circuit experimental apparatus around 1895 to reproduce the low pressure phenomenon and to photograph its development. His 1911 Rede Lecture presented an apparatus and successive images of cavity inception and growth [16], [17]. These experiments helped establish cavitation as a reproducible hydrodynamic process rather than an unexplained defect of a particular propeller.

Figure 1.1. Selected milestones in cavitation science and engineering

1.3.2 Parsons and Experimental Visualization

Parsons recognized the linked roles of rotational speed, pressure, and vapor formation. His experimental facility allowed pressure to be reduced while a model propeller or blade system was observed. The published images are important because they show not only the existence of vapor, but also its organized attachment to a moving surface. Modern cavitation tunnels, pump cavitation facilities, and transparent valve rigs retain the same core logic: control the pressure and velocity field, observe the cavity, and correlate the pattern with performance and loading [16], [17], [6], [20].

Figure 1.2. Parsons's cavitation apparatus and early photographic evidence. Both images are from The Steam Turbine (1911), public domain [16].

1.3.3 Rayleigh, Plesset, and Bubble Dynamics

Lord Rayleigh provided the first influential analytical description of the pressure generated during the collapse of an ideal spherical cavity in an incompressible liquid [18]. Plesset later generalized the dynamics to a gas or vapor bubble whose radius changes under a time dependent pressure field [19]. The resulting Rayleigh Plesset framework remains the starting point for interpreting bubble growth, collapse time, resonant response, and the high accelerations that occur near final collapse [2], [1].

Mid twentieth century research connected bubble dynamics with machinery testing, cavitation number, erosion, and material response. Knapp, Daily, and Hammitt consolidated experimental and engineering knowledge, while later texts integrated computational fluid dynamics, multiphase flow models, vortex cavitation, and modern analysis [1–3]. In parallel, process engineers began to use hydrodynamic cavitation intentionally, leading to reactor classifications and scale up studies from the late twentieth century onward [1,9–12].

1.3.4 Standardization and Modern Practice

Contemporary practice separates test definitions from design margins. Pump standards define hydraulic acceptance and net positive suction head testing; Hydraulic Institute guidance addresses NPSH margin and preferred operating regions; ISA guidance addresses control valve cavitation; and ITTC procedures address propeller erosion and noise testing [4,21,22], [23], [24]. This distinction is essential because an experimentally defined criterion, such as a 3 percent pump head drop, is not the same as an onset of vapor, an onset of noise, or an onset of erosion.

1.4 Principal Generation Modes

Generation methods are classified by the physical source of the tensile pressure or localized energy deposition. All four methods can produce a cavity that follows the same general stages of nucleation, growth, and collapse, but they differ markedly in active volume, controllability, repetition rate, equipment, and practical scale [2,25].

Figure 1.3. The four traditional cavitation categories are classified by the source of pressure disturbance or nucleation event. [26]

Table 1.2. Comparison of the four principal cavitation types. [26]

Type	Primary driver	Typical equipment	Representative uses	Main limitations
Acoustic	Alternating tensile and compressive pressure from ultrasound, commonly in the power ultrasound range	Horn, bath, plate transducer, flow cell	Laboratory chemistry, cleaning, emulsification, extraction, medical and biological uses	Attenuation, field nonuniformity, transducer heating, and scale up
Hydrodynamic	Local pressure reduction and recovery produced by flow acceleration, vortex motion, or rotating elements	Orifice, Venturi, vortex diode, rotor and stator reactor	Continuous process intensification, water treatment, synthesis, extraction, mixing	Pumping loss, erosion, device specific pressure history, and possible clogging
Optical	Laser induced breakdown, rapid heating, or vaporization at a focused spot	Pulsed laser focused into a liquid	Fundamental bubble studies, laser surgery, precision materials processing	Small active volume, expensive optical system, and limited bulk process scalability
Particle induced	Energy deposition along an ionizing particle track in a superheated liquid	Bubble chamber or superheated liquid detector	Particle and radiation detection	Not normally used as a bulk chemical processing method

1.4.1 Acoustic cavitation

Acoustic cavitation is generated when sound waves impose sufficiently large oscillating pressure on a liquid. The broad traditional frequency range extends from about 16 kHz into the megahertz region [25], although most high power sonochemical reactors operate from approximately 20 kHz to several hundred kilohertz. A piezoelectric or magnetostrictive transducer converts electrical energy into mechanical motion. The motion is coupled through a vessel wall, bath, plate, or horn into the liquid.

The term sonochemistry refers to chemical change caused by the cavitation generated by ultrasound rather than direct interaction of sound with molecular electronic states [27]. Acoustic frequency controls the time available for bubble growth, while acoustic pressure amplitude influences whether nuclei merely oscillate or become inertial. Reactor geometry, liquid depth, standing waves, dissolved gas, and the acoustic impedance of boundaries determine where active zones occur. A nominal electrical power rating therefore does not describe the cavitation field by itself.

1.4.2 Hydrodynamic cavitation

Hydrodynamic cavitation is generated by the pressure history of a flowing liquid. Acceleration through a restriction, nozzle, Venturi, vortex core, valve, or rotor gap creates a low pressure interval in which nuclei grow; pressure recovery then produces collapse. Its principal industrial advantage is compatibility with pipelines, recirculation loops, continuous processing, and conventional pumps.

1.4.3 Optical cavitation

Optical cavitation is commonly initiated by a focused pulsed laser. At sufficiently high irradiance, optical breakdown creates a small plasma, rapid energy deposition, and a vapor or gas cavity. Laser absorption can also produce thermocavitation when localized heating drives rapid vaporization without a breakdown plasma. In both cases, the bubble can be controlled accurately in position and time, which is valuable in single bubble research, ophthalmic procedures, lithotripsy related studies, and precision ablation. The active volume is small, so optical cavitation is not usually selected for bulk fluid processing [2,28].

1.4.4 Particle induced cavitation

Particle induced cavitation occurs when an energetic charged particle, neutron interaction product, or recoil nucleus deposits energy along a track in a superheated liquid. Bubble chambers and superheated droplet detectors are adjusted so that localized energy deposition nucleates a visible bubble. The method is central to radiation and particle detection, but it has little relevance as a large scale liquid processing technology.

Nomenclature

The following key defines the symbols and abbreviations used in this chapter. Unless otherwise stated, quantities are expressed in SI units.

Symbol	Definition	SI unit or status
p	Local absolute static pressure	Pa
$p_v$	Saturation vapor pressure	Pa
R	Bubble or cavity radius	m
t	Time	s
U	Characteristic liquid velocity	m s⁻¹
CFC	Controlled flow cavitation	abbreviation
NPSH	Net positive suction head	m

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