Editing Cavitation (section)

===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>