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| [[File:FA-18 Hornet breaking sound barrier (7 July 1999).jpg|thumb|Aerodynamic condensation evidences of Prandtl–Meyer expansion fan|supersonic expansion fans around a transonic F/A-18 Hornet|F/A-18]]
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| [[File:Sears-Haack.png|thumb|right|The Sears–Haack body presents a cross-sectional area variation that minimises wave drag.]]
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| [[File:Shock wave above airliner wing (7).jpg|thumb|Shock waves may appear as weak optical disturbances above airliners with supercritical airfoil|supercritical wings]]
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| [[File:Transonic flow patterns.svg|right|thumb|Transonic flow patterns on an airfoil showing flow patterns at and above critical Mach number]]
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| '''Transonic''' (or '''transsonic''') flow is air flowing around an object at a speed that generates regions of both subsonic and [[Supersonic speed|supersonic]] airflow around that object.<ref name=":2" /> The exact range of speeds depends on the object's [[critical Mach number]], but transonic flow is seen at flight speeds close to the [[speed of sound]] (343 m/s at sea level), typically between [[Mach number|Mach]] 0.8 and 1.2.<ref name=":2">{{Cite book|last=Anderson|first=John D. Jr. |url=https://www.worldcat.org/oclc/927104254|title=Fundamentals of aerodynamics|date=2017|isbn=978-1-259-12991-9|edition=Sixth|location=New York, NY|pages=756–758|oclc=927104254}}</ref>
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| The issue of transonic speed (or transonic region) first appeared during World War II.<ref name=":0">{{Cite journal|last1=Vincenti|first1=Walter G.|last2=Bloor|first2=David|date=August 2003|title=Boundaries, Contingencies and Rigor|url=http://dx.doi.org/10.1177/0306312703334001|journal=Social Studies of Science|volume=33|issue=4|pages=469–507|doi=10.1177/0306312703334001|s2cid=13011496|issn=0306-3127}}</ref> Pilots found as they approached the sound barrier the airflow caused aircraft to become unsteady.<ref name=":0" /> Experts found that [[shock wave]]s can cause large-scale [[Flow separation|separation]] downstream, increasing drag, adding asymmetry and unsteadiness to the flow around the vehicle.<ref name=":1">{{Cite book|last=Takahashi|first=Timothy|url=http://worldcat.org/oclc/1162468861|title=Aircraft performance and sizing. fundamentals of aircraft performance|date=15 December 2017|isbn=978-1-60650-684-4|pages=107|publisher=Momentum Press |oclc=1162468861}}</ref> Research has been done into weakening shock waves in transonic flight through the use of [[Anti-shock body|anti-shock bodies]] and [[supercritical airfoil]]s.<ref name=":1" />
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| Most modern [[jet engine|jet]] powered aircraft are engineered to operate at transonic air speeds.<ref>{{cite book |last1=Takahashi |first1=Timothy |title=Aircraft Performance and Sizing, Volume I |date=2016 |publisher=Momentum Press Engineering |location=New York City |isbn=978-1-60650-683-7|pages=10–11}}</ref> Transonic airspeeds see a rapid increase in drag from about Mach 0.8, and it is the fuel costs of the drag that typically limits the airspeed. Attempts to reduce wave drag can be seen on all high-speed aircraft. Most notable is the use of [[swept wing]]s, but another common form is a wasp-waist fuselage as a side effect of the [[Whitcomb area rule]].
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| Transonic speeds can also occur at the tips of [[Rotorcraft|rotor]] blades of helicopters and aircraft. This puts severe, unequal stresses on the rotor blade and may lead to accidents if it occurs. It is one of the limiting factors of the size of rotors and the forward speeds of helicopters (as this speed is added to the forward-sweeping [leading] side of the rotor, possibly causing localized transonics).
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| == History==
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| === Discovering transonic airflow===
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| Issues with aircraft flight relating to speed first appeared during the [[Supersonic speed|supersonic]] era in 1941.<ref name=":35">{{Cite web|title=Mach 1: Assaulting the Barrier|url=https://www.airspacemag.com/history-of-flight/mach-1-assaulting-the-barrier-22647052/|access-date=14 March 2021|website=Air & Space Magazine|language=en}}</ref> Ralph Virden, a test pilot, crashed in a fatal plane accident.<ref name=":23">{{Cite book|last=Vincenti |first= Walter G.|url=http://worldcat.org/oclc/1027014606|title=Engineering theory in the making: Aerodynamic calculation "breaks the sound barrier."|date=1997|oclc=1027014606}}</ref> He lost control of the plane when a shock wave caused by supersonic airflow developed over the wing, causing it to stall.<ref name=":23" /> Virden flew well below the speed of sound at Mach 0.675, which brought forth the idea of different airflows forming around the plane.<ref name=":35" /> In the 40s, [[Kelly Johnson (engineer)|Kelly Johnson]] became one of the first engineers to investigate the effect of compressibility on aircraft.<ref name=":35" /> However, contemporary [[wind tunnel]]s did not have the capability to create wind speeds close to Mach 1 to test the effects of transonic speeds.<ref name=":23" /> Not long after, the term "transonic" was defined to mean "across the speed of sound" and was invented by [[National Advisory Committee for Aeronautics | NACA]] director [[Hugh Latimer Dryden|Hugh Dryden]] and [[Theodore von Kármán]] of the California Institute of Technology.<ref name=":35" />
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| === Changes in aircraft===
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| Initially, [[National Advisory Committee for Aeronautics|NACA]] designed "dive flaps" to help stabilize the plane when reaching transonic flight.<ref name=":35" /> This small flap on the underside of the plane slowed the plane to prevent shock waves, but this design only delayed finding a solution to aircraft flying at supersonic speed.<ref name=":35" /> Newer wind tunnels were designed, so researchers could test newer wing designs without risking test pilots' lives.<ref name=":4">{{Cite journal|date=2000–2006|title=From Engineering Science to Big Science: The NACA and NASA Collier Trophy Research Project Winners. Pamela E. Mack|url=http://dx.doi.org/10.1086/384834|journal=Isis|volume=91|issue=2|pages=417–418|doi=10.1086/384834|issn=0021-1753}}</ref> The slotted-wall transonic tunnel was designed by NASA and allowed researchers to test wings and different [[airfoil]]s in transonic airflow to find the best wingtip shape for sonic speeds.<ref name=":4" />
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| After [[World War II]], major changes in aircraft design were seen to improve transonic flight.<ref name=":23" /> The main way to stabilize an aircraft was to reduce the speed of the airflow around the wings by changing the [[Chord (aeronautics)|chord]] of the plane wings, and one solution to prevent transonic waves was swept wings.<ref name=":35" /> Since the airflow would hit the wings at an angle, this would decrease the wing thickness and chord ratio.<ref name=":35" /> Airfoils wing shapes were designed flatter at the top to prevent shock waves and reduce the distance of airflow over the wing.<ref>{{Cite journal|last1=Hicks|first1=Raymond M.|last2=Vanderplaats|first2=Garret N.|last3=Murman|first3=Earll M.|last4=King|first4=Rosa R.|date=1 February 1976|title=Airfoil Section Drag Reduction at Transonic Speeds by Numerical Optimization|url=http://dx.doi.org/10.4271/760477|journal=SAE Technical Paper Series|volume=1|location=Warrendale, PA|publisher=SAE International|doi=10.4271/760477|hdl=2060/19760009938|s2cid=118185921 |hdl-access=free}}</ref> Later on, Richard Whitcomb designed the first [[supercritical airfoil]] using similar principles.<ref name=":4" />
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| === Mathematical analysis===
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| [[File:Streamline Patterns for Flow Regimes.png|thumb|Streamlines for three airflow regimes (black lines) around a nondescript blunt body (blue).<ref name=":13" />]]
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| Prior to the advent of powerful computers, even the simplest forms of the [[Compressible flow|compressible flow equations]] were difficult to solve due to their [[Nonlinear system|nonlinearity]].<ref name=":23" /> A common assumption used to circumvent this nonlinearity is that disturbances within the flow are relatively small, which allows mathematicians and engineers to [[Linearization|linearize]] the compressible flow equations into a relatively easily solvable set of [[differential equation]]s for either wholly subsonic or supersonic flows.<ref name=":23" /> This assumption is fundamentally untrue for transonic flows because the disturbance caused by an object is much larger than in subsonic or supersonic flows; a flow speed close to or at Mach 1 does not allow the [[Streamlines, streaklines, and pathlines|streamtubes]] (3D flow paths) to contract enough around the object to minimize the disturbance, and thus the disturbance propagates.<ref name=":13">{{Cite book|last=Ramm|first=Heinrich J.|url=https://www.worldcat.org/oclc/228117297|title=Fluid dynamics for the study of transonic flow|date=1990|publisher=Oxford University Press|isbn=1-60129-748-3|location=New York|pages=|oclc=228117297}}</ref> Aerodynamicists struggled during the earlier studies of transonic flow because the then-current theory implied that these disturbances– and thus drag– approached infinity as local Mach number approached 1, an obviously unrealistic result which could not be remedied using known methods.<ref name=":23" />
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| One of the first methods used to circumvent the nonlinearity of transonic flow models was the [[hodograph]] transformation.<ref name=":0" /> This concept was originally explored in 1923 by an Italian mathematician named [[Francesco Tricomi]], who used the transformation to simplify the compressible flow equations and prove that they were solvable.<ref name=":0" /> The hodograph transformation itself was also explored by both [[Ludwig Prandtl]] and O.G. Tietjen's textbooks in 1929 and by [[Adolf Busemann]] in 1937, though neither applied this method specifically to transonic flow.<ref name=":0" />
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| Gottfried Guderley, a German mathematician and engineer at [[Technical University of Braunschweig|Braunschweig]], discovered Tricomi's work in the process of applying the hodograph method to transonic flow near the end of World War II.<ref name=":0" /> He focused on the nonlinear thin-airfoil compressible flow equations, the same as what Tricomi derived, though his goal of using these equations to solve flow over an airfoil presented unique challenges.<ref name=":0" /><ref name=":23" /> Guderley and Hideo Yoshihara, along with some input from Busemann, later used a singular solution of Tricomi's equations to analytically solve the behavior of transonic flow over a [[Supersonic airfoils|double wedge airfoil]], the first to do so with only the assumptions of thin-airfoil theory.<ref name=":0" /><ref name=":23" />
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| Although successful, Guderley's work was still focused on the theoretical, and only resulted in a single solution for a double wedge airfoil at Mach 1.<ref name=":0" /> [[Walter G. Vincenti|Walter Vincenti]], an American engineer at [[Ames Research Center|Ames Laboratory]], aimed to supplement Guderley's Mach 1 work with numerical solutions that would cover the range of transonic speeds between Mach 1 and wholly supersonic flow.<ref name=":0" /> Vincenti and his assistants drew upon the work of [[Howard Wilson Emmons|Howard Emmons]] as well as Tricomi's original equations to complete a set of four numerical solutions for the drag over a double wedge airfoil in transonic flow above Mach 1.<ref name=":0" /> The gap between subsonic and Mach 1 flow was later covered by both [[Julian Cole]] and [[Leon Trilling]], completing the transonic behavior of the airfoil by the early 1950s.<ref name=":0" />
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| ==Condensation clouds==
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| At transonic speeds [[Prandtl–Meyer expansion fan|supersonic expansion fans]] form intense low-pressure, low-temperature areas at various points around an aircraft. If the temperature drops below the [[dew point]] a visible cloud will form. These clouds remain with the aircraft as it travels. It is not necessary for the aircraft as a whole to reach [[supersonic]] speeds for these clouds to form. Typically, the tail of the aircraft will reach supersonic flight while the nose of the aircraft is still in subsonic flight. A bubble of supersonic expansion fans terminating by a wake shockwave surround the tail. As the aircraft continues to accelerate, the supersonic expansion fans will intensify and the wake shockwave will grow in size until infinity is reached, at which point the bow shockwave forms. This is Mach 1 and the [[Prandtl–Glauert singularity]].
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| ==Transonic flows in astronomy and astrophysics==
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| In astrophysics, wherever there is evidence of shocks (standing, propagating or oscillating), the flow close by must be transonic, as only supersonic flows form shocks. All black hole [[Accretion (astrophysics)|accretions]] are transonic. Many such flows also have shocks very close to the black holes.
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| The outflows or jets from young stellar objects or disks around black holes can also be transonic since they start subsonically and at a far distance they are invariably supersonic. Supernovae explosions are accompanied by supersonic flows and shock waves. Bow shocks formed in [[solar wind]]s are a direct result of transonic winds from a star. It had been long thought that a bow shock was present around the heliosphere of our solar system, but this was found not to be the case according to data published in 2012.
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| [[Category:Aerodynamics]] | | [[Category:Aerodynamics]] |
| [[Category:Airspeed]] | | [[Category:Airspeed]] |
| [[Category:Aircraft performance]] | | [[Category:Aircraft performance]] |
