Editing Cavitation (section)

== Physics ==

=== Inertial cavitation ===
Inertial cavitation was first observed in the late 19th century, considering the collapse of a spherical void within a liquid. When a volume of liquid is subjected to a sufficiently low [[pressure]], it may rupture and form a cavity. 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. A fast-flowing river can cause cavitation on rock surfaces, particularly when there is a drop-off, such as on a waterfall.{{citation needed|date=July 2023|reason=are these holes in river water not just air bubbles at atmospheric pressure?}}

[[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. Once the conditions which caused the bubble to form are no longer present, such as when the bubble moves downstream, the surrounding liquid begins to implode due its higher pressure, building up momentum as it moves inward. As the bubble finally collapses, the inward momentum of the surrounding liquid causes a sharp increase of pressure and temperature of the vapor within. The bubble eventually collapses to a minute fraction of its original size, at which point the gas within dissipates into the surrounding liquid via a rather violent mechanism which releases a significant amount of energy in the form of an acoustic shock wave and as [[sonoluminescence|visible light]]. At the point of total collapse, the temperature of the vapor within the bubble may be several thousand [[Kelvin]], and the pressure several hundred atmospheres.<ref>{{cite journal|journal=Environmental Health Perspectives|volume=64|pages=233–252|date=1985|title=Free radical generation by ultrasound in aqueous and nonaqueous solutions|last1=Riesz|first1=P.|first2=D.|last2=Berdahl|first3=C.L.|last3=Christman|pmc=1568618|doi=10.2307/3430013|pmid=3007091|jstor=3430013}}</ref>

The physical process of cavitation inception is similar to [[boiling]]. The major difference between the two is the [[thermodynamic]] paths that precede the formation of the vapor. Boiling occurs when the local temperature of the liquid reaches the [[saturation temperature]], and further heat is supplied to allow the liquid to sufficiently [[phase transition|phase change]] into a gas. Cavitation inception occurs when the local pressure falls sufficiently far below the saturated vapor pressure, a value given by the tensile strength of the liquid at a certain temperature.<ref>{{cite web|last1=Brennen|first1=Christopher |title=Cavitation and Bubble Dynamics|publisher=Oxford University Press|pages=21 |url=http://authors.library.caltech.edu/25017/1/cavbubdynam.pdf |archive-url=https://web.archive.org/web/20121004094948/http://authors.library.caltech.edu/25017/1/cavbubdynam.pdf |archive-date=2012-10-04 |url-status=live|access-date=27 February 2015}}</ref>

In order for cavitation inception to occur, the cavitation "bubbles" generally need a surface on which they can [[nucleation|nucleate]]. This surface can be provided by the sides of a container, by [[impurity|impurities]] in the liquid, or by small undissolved microbubbles within the liquid. It is generally accepted that [[hydrophobe|hydrophobic]] surfaces stabilize small bubbles. These pre-existing bubbles start to grow unbounded when they are exposed to a pressure below the threshold pressure, termed Blake's threshold.<ref>{{cite book|vauthors=Postema M, de Jong N, Schmitz G|title=IEEE Ultrasonics Symposium, 2005 |chapter=Shell rupture threshold, fragmentation threshold, blake threshold |date=Sep 2005|volume=3 |location=Rotterdam, Netherlands|pages=1708–1711|doi=10.1109/ULTSYM.2005.1603194|isbn=0-7803-9382-1 |s2cid=5683516 |chapter-url=https://hal.archives-ouvertes.fr/hal-03193373/document}}</ref> The presence of an incompressible core inside a cavitation nucleus substantially lowers the cavitation threshold below the Blake threshold.<ref>{{cite journal|vauthors=Carlson CS, Matsumoto R, Fushino K, Shinzato M, Kudo N, Postema M|title=Nucleation threshold of carbon black ultrasound contrast agent|journal=Japanese Journal of Applied Physics|year=2021|volume=60|issue=SD|pages=SDDA06|doi=10.35848/1347-4065/abef0f|bibcode=2021JaJAP..60DDA06C |s2cid=233539493 |url=https://hal.archives-ouvertes.fr/hal-03192654/document|doi-access=free}}</ref>

The vapor pressure here differs from the meteorological definition of vapor pressure, which describes the partial pressure of water in the atmosphere at some value less than 100% saturation. Vapor pressure as relating to cavitation refers to the vapor pressure in equilibrium conditions and can therefore be more accurately defined as the equilibrium (or saturated) [[vapor pressure]].

Non-inertial cavitation is the process in which small bubbles in a liquid are forced to oscillate in the presence of an acoustic field, when the intensity of the acoustic field is insufficient to cause total bubble collapse. This form of cavitation causes significantly less erosion than inertial cavitation, and is often used for the cleaning of delicate materials, such as [[silicon wafer]]s.

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. These techniques have been used to study the evolution of the bubble that is actually created by locally boiling the liquid with a local increment of temperature.

===Hydrodynamic cavitation===
Hydrodynamic cavitation is the process of vaporisation, bubble generation and bubble implosion which occurs in a flowing liquid as a result of a decrease and subsequent increase in local pressure. Cavitation will only occur if the local pressure declines to some point below the saturated [[vapor pressure]] of the liquid and subsequent recovery above the vapor pressure. If the recovery pressure is not above the vapor pressure then flashing is said to have occurred. In pipe systems, cavitation typically occurs either as the result of an increase in the kinetic energy (through an area constriction) or an increase in the pipe elevation.

Hydrodynamic cavitation can be produced by passing a liquid through a constricted channel at a specific [[flow velocity]] or by mechanical rotation of an object through a liquid.  In the case of the constricted channel and based on the specific (or unique) geometry of the system, the combination of pressure and kinetic energy can create the hydrodynamic cavitation cavern downstream of the local constriction generating high energy cavitation bubbles.

Based on the thermodynamic phase change diagram, an increase in temperature could initiate a known phase change mechanism known as boiling. However, a decrease in static pressure could also help one pass the multi-phase diagram and initiate another phase change mechanism known as cavitation. On the other hand, a local increase in flow velocity could lead to a static pressure drop to the critical point at which cavitation could be initiated (based on Bernoulli's principle). The critical pressure point is vapor saturated pressure. In a closed fluidic system where no flow leakage is detected, a decrease in cross-sectional area would lead to velocity increment and hence static pressure drop. This is the working principle of many hydrodynamic cavitation based reactors for different applications such as water treatment, energy harvesting, heat transfer enhancement, food processing, etc.<ref>{{Cite journal |last1=Gevari|first1=Moein Talebian|last2=Abbasiasl|first2=Taher|last3=Niazi|first3=Soroush|last4=Ghorbani |first4=Morteza|last5=Koşar|first5=Ali|date=2020-05-05|title=Direct and indirect thermal applications of hydrodynamic and acoustic cavitation: A review|journal=Applied Thermal Engineering|volume=171|pages=115065 |doi=10.1016/j.applthermaleng.2020.115065|bibcode=2020AppTE.17115065G |s2cid=214446752|issn=1359-4311}}</ref>

There are different flow patterns detected as a cavitation flow progresses: inception, developed flow, supercavitation, and choked flow. Inception is the first moment that the second phase (gas phase) appears in the system. This is the weakest cavitating flow captured in a system corresponding to the highest [[cavitation number]]. When the cavities grow and becomes larger in size in the orifice or venturi structures, developed flow is recorded. The most intense cavitating flow is known as supercavitation where theoretically all the nozzle area of an orifice is filled with gas bubbles. This flow regime corresponds to the lowest cavitation number in a system. After supercavitation, the system is not capable of passing more flow. Hence, velocity does not change while the upstream pressure increase. This would lead to an increase in cavitation number which shows that choked flow occurred.<ref>{{Cite journal |last1=Gevari|first1=Moein Talebian|last2=Shafaghi|first2=Ali Hosseinpour|last3=Villanueva|first3=Luis Guillermo|last4=Ghorbani |first4=Morteza|last5=Koşar|first5=Ali|date=January 2020|title=Engineered Lateral Roughness Element Implementation and Working Fluid Alteration to Intensify Hydrodynamic Cavitating Flows on a Chip for Energy Harvesting|journal=Micromachines|volume=11|issue=1|pages=49|doi=10.3390/mi11010049|pmid=31906037|pmc=7019874|doi-access=free}}</ref>

The process of bubble generation, and the subsequent growth and collapse of the cavitation bubbles, results in very high energy densities and in very high local temperatures and local pressures at the surface of the bubbles for a very short time.  The overall liquid medium environment, therefore, remains at ambient conditions.  When uncontrolled, cavitation is damaging; by controlling the flow of the cavitation, however, the power can be harnessed and non-destructive.  Controlled cavitation can be used to enhance chemical reactions or propagate certain unexpected reactions because free radicals are generated in the process due to disassociation of vapors trapped in the cavitating bubbles.<ref>{{cite web |last1=STOPAR |first1=DAVID |title=HYDRODYNAMIC CAVITATION |url=https://davidstopar.wixsite.com/home/hydrodynamic-cavitation |access-date=17 January 2020}}</ref>

Orifices and venturi are reported to be widely used for generating cavitation. A venturi has an inherent advantage over an orifice because of its smooth converging and diverging sections, such that it can generate a higher flow velocity at the throat for a given pressure drop across it. On the other hand, an orifice has an advantage that it can accommodate a greater number of holes (larger perimeter of holes) in a given cross sectional area of the pipe.<ref>{{cite journal |first1=Vijayanand S. |last1=Moholkar |first2=Aniruddha B. |last2=Pandit |doi=10.1002/aic.690430628 |year=1997 |title=Bubble Behavior in Hydrodynamic Cavitation: Effect of Turbulence |journal=AIChE Journal |volume=43 |issue=6 |pages=1641–1648 |bibcode=1997AIChE..43.1641M }}</ref>

The cavitation phenomenon can be controlled to enhance the performance of high-speed marine vessels and projectiles, as well as in material processing technologies, in medicine, etc. Controlling the cavitating flows in liquids can be achieved only by advancing the mathematical foundation of the cavitation processes. These processes are manifested in different ways, the most common ones and promising for control being bubble cavitation and supercavitation. The first exact classical solution should perhaps be credited to the well-known solution by [[Hermann von Helmholtz]] in 1868.<ref>{{cite journal |last1=Helmholtz |first1=Hermann von |title=Über diskontinuierliche Flüssigkeits-Bewegungen |journal=Monatsberichte der Königlichen Preussische Akademie des Wissenschaften zu Berlin (Monthly Reports of the Royal Prussian Academy of Sciences at Berlin) |date=1868 |volume=23 |pages=215–228 |url=https://www.biodiversitylibrary.org/item/111036#page/223/mode/1up |trans-title=On discontinuous motions of fluids |language=de}}</ref> The earliest distinguished studies of academic type on the theory of a cavitating flow with free boundaries and supercavitation were published in the book ''Jets, wakes and cavities''<ref>Birkhoff, G, Zarantonello. E (1957) Jets, wakes and cavities. New York: Academic Press.  406p.</ref>  followed by ''Theory of jets of ideal fluid''.<ref>Gurevich, MI (1978) Theory of jets of ideal fluid.  Nauka, Moscow, 536p. (in Russian)</ref> Widely used in these books was the well-developed theory of conformal mappings of functions of a complex variable, allowing one to derive a large number of exact solutions of plane problems. Another venue combining the existing exact solutions with approximated and heuristic models was explored in the work ''Hydrodynamics of Flows with Free Boundaries''<ref>Logvinovich, GV (1969) Hydrodynamics of Flows with Free Boundaries. Naukova dumka, Kiev, 215p. (In Russian)</ref>  that refined the applied calculation techniques based on the principle of cavity expansion independence, theory of pulsations and stability of elongated axisymmetric cavities, etc.<ref>Knapp, RT, Daili, JW, Hammit, FG (1970) Cavitation. New York: Mc Graw Hill Book Company.  578p.</ref> and in ''Dimensionality and similarity methods in the problems of the hydromechanics of vessels''.<ref>Epshtein, LA (1970) Dimensionality and similarity methods in the problems of the hydromechanics of vessels. Sudostroyenie, Leningrad, 208p. (In Russian)</ref>

A natural continuation of these studies was recently presented in ''The Hydrodynamics of Cavitating Flows''<ref>Terentiev, A, Kirschner, I, Uhlman, J, (2011) The Hydrodynamics of Cavitating Flows. Backbone Publishing Company, 598pp.</ref>  – an encyclopedic work encompassing all the best advances in this domain for the last three decades, and blending the classical methods of mathematical research with the modern capabilities of computer technologies. These include elaboration of nonlinear numerical methods of solving 3D cavitation problems, refinement of the known plane linear theories, development of asymptotic theories of axisymmetric and nearly axisymmetric flows, etc. As compared to the classical approaches, the new trend is characterized by expansion of the theory into the 3D flows. It also reflects a certain correlation with current works of an applied character on the hydrodynamics of supercavitating bodies.

Hydrodynamic cavitation can also improve some industrial processes. For instance, cavitated corn slurry shows higher yields in [[ethanol]] production compared to uncavitated corn slurry in dry milling facilities.<ref>Oleg Kozyuk; [http://www.arisdyne.com/ Arisdyne Systems Inc.]; US patent US 7,667,082 B2; Apparatus and Method for Increasing Alcohol Yield from Grain</ref>

This is also used in the mineralization of bio-refractory compounds which otherwise would need extremely high temperature and pressure conditions since free radicals are generated in the process due to the dissociation of vapors trapped in the cavitating bubbles, which results in either the intensification of the chemical reaction or may even result in the propagation of certain reactions not possible under otherwise ambient conditions.<ref>{{cite journal |last1=Gogate |first1=P. R. |last2=Kabadi |first2=A. M. |year=2009 |title=A review of applications of cavitation in biochemical engineering/biotechnology |journal=Biochemical Engineering Journal |volume=44 |issue=1 |pages=60–72 |doi=10.1016/j.bej.2008.10.006 |bibcode=2009BioEJ..44...60G }}</ref>

===Acoustic cavitation and ultrasonic cavitation===
Inertial cavitation can also occur in the presence of an acoustic field. Microscopic gas bubbles that are generally present in a liquid will be forced to oscillate due to an applied acoustic field. If the acoustic intensity is sufficiently high, the bubbles will first grow in size and then rapidly collapse. Hence, inertial cavitation can occur even if the rarefaction in the liquid is insufficient for a Rayleigh-like void to occur.

Ultrasonic cavitation inception will occur when the acceleration of the ultrasound source is enough to produce the needed pressure drop. This pressure drop depends on the value of the acceleration and the size of the affected volume by the pressure wave. The dimensionless number that predicts ultrasonic cavitation is the [[cavitation number|Garcia-Atance number]]. High power ultrasonic horns produce accelerations high enough to create a cavitating region that can be used for [[Homogenization (chemistry)|homogenization]], [[Dispersion (chemistry)|dispersion]], deagglomeration, erosion, cleaning, milling, [[emulsion|emulsification]], extraction, disintegration, and [[sonochemistry]].

===Aerodyamic cavitation===
Although predominant in liquids, cavitation exists to an extent in gas as it has fluid dynamics at high speeds.<ref>{{Cite web|url=https://www.researchgate.net/figure/Asymmetric-electrode-arrangement-of-an-aerodynamic-plasma-actuator_fig1_253041135|title=Asymmetric electrode arrangement of an aerodynamic plasma actuator. &#124; Download Scientific Diagram}}</ref><ref>{{Cite web|url=https://www.researchgate.net/publication/301889798|title=Simultaneous Investigation of Flexibility and Plasma Actuation Effects on the Aerodynamic Characteristics of an Oscillating Airfoil}}</ref> For example, a [[bullet]] with a flat tip moves faster underwater as it creates cavitation compared to a bullet with a sharp tip. A [[dune]] shape is very useful for managing aerodynamic cavitation.  The shape of a dune provides minimal resistance to the wind. With small dunes installed on the surfaces of aircraft and other high speed vehicles, friction against the air decreases by several times. The dune surface pushes the air upwards, underneath and behind areas where the air pressure drops, reducing friction. The dune may increase frontal resistance, but that will be compensated for by a decrease in the total friction area, as also happens with an underwater bullet. As a result, the speed of the aircraft or vehicle will increase significantly.<ref>{{cite web | url=https://contest.techbriefs.com/2019/entries/aerospace-and-defense/9431#:~:text=Contrary%20to%20logic%20and%20general,the%20bullet%20surface%20very%20slightly | title=Aerodynamic Cavitation for Aircraft }}</ref>