HYPERSONIC AERODYNAMICS Prepared by Mohammad Fazlur Rahman Asst. Professor (AERO) B. S. Abdur Rahman University This document contains the basic information regarding the subject matter HypersonicAerodynamics.Theeffortismadetohelpthestudents gettingexposuretothesubjectaswellasunderstandthebasicand fundamental behaviour of the fluid when the flow takes place at very high speed in the hypersonic regime. It must be noted that this document in no waycanavoidtheuseoftextbooks.Forthedetailedanddeep understandingofthesubjectmatterstudentsmustreferthetextbooks. WhileprovidinginformationthesyllabusoftheB.S.AbdurRahman University has been targeted. Introduction to Hypersonic Aerodynamics Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 1 Contents Motivation ............................................................................................................................... 2 Introduction ............................................................................................................................ 6 Some historical facts ....................................................................................................... 8 Comparison of Supersonic and Hypersonic Vehicles .......................................... 16 Types of hypersonic vehicles ..................................................................................... 28 Hypersonic Atmosphere ................................................................................................... 35 Definition of Hypersonic Flow .................................................................................... 35 Hypersonic flow characteristics ................................................................................. 36 Thin Shock Layers ........................................................................................................... 36 Entropy Layer .................................................................................................................. 37 Viscous Interaction .......................................................................................................... 38 High Temperature Flow ................................................................................................... 40 Low Density Flow............................................................................................................ 43 Recapitulation .................................................................................................................. 48 Extra thinking .................................................................................................................. 49 Hypersonic Studies .......................................................................................................... 50 Summary ............................................................................................................................ 52 Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 2 Motivation Travellingandreachingsomewhereasfastaspossibleistheveryfascinatingideaforany one.Evenforthecargotransportation,wenormallyliketomakethecargosreachtoits destinationsassoonaspossible.Andthishasledtodiscovernewtechnologiesandinvent newconceptvehiclescontinuouslywhichcantravelwithveryhighspeed.Highspeedhas beenfascinatingthehumankindsincethebeginningthatshowsprinters100mrunis inspiredandpeopletakeanyavailablemeanstoimprovetheexistingrecordevenbythe fraction of seconds. No doubt speed thrills. Whenwetalkaboutspeed,inthecaseoftravelingontheground,wenormallygoforhigh speedsuptonotmorethan0.3Machnumber.Whenwetravelinthesky,weevenliketo cross the sonic barrier and would like to go for supersonic speeds. But thats more than 100 years old story. Now we are in the space age and we talk about space travels and supersonic doesnt charm the space scientists any more. Now we talk about the hypersonic speeds. Travellinginthespacewithsupersonicspeedsgivelittlecharmandeventheslowest traveling spacecraft is very near to hypersonic speeds. Speed of moon going round the Earth is 1023 m/s!!! Taking the low temperature there in the space, this speed will be more than even5M.soanormalspeedinthespacegoesinthehypersonicrange.Takingthecaseof EarthsrevolutionaroundSun;itrevolveswithanaveragespeedof30,000m/s.Students shouldfindthemoreaccurateresultsbytakingtheorbittobeellipticalandrealisethe veracity of the figures. (Note: The average distance of moon from earth is3.85 108 m and assuming moon completes its revolution around the earth 27.32 days on the average, average speed of the moon around the Earth is 1023 m/s.) Humanity is seeking great hope in the space science and it is said the future of the humanity liesinthespace.Ifthesciencefictionistobebelieved,spacetravelandinterplanetary transportation is going to be a reality sooner or later. Space is so wide apart that distances are measuredinlightyears,notinkilometerormiles.Insuchasituation,onlythevehicles travellingwithhypersonicspeedsisgoingtohelpus.Soinoneworditcanbesaidthat hypersonicvehiclesisthefutureoftransportationandstudyingthehypersonicspeedinthe aerospace science is a necessity. Hypersonic is not only related with the space travel. Even in the case of aircraft it is tried to make some vehicle which can travel at the hypersonic speeds.Hypersonic vehicle travelling Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 3 intheskycanmakeaquicktransportationforthehumanbeingaswellascargo.Sointhe aeronautics also, hypersonic speed vehicles make important subject.In1913theLondonnewspaperTheDailyMailofferedaprizeof10,000to"theaviator who shall first cross the Atlantic in an aeroplane in flight from any point in the United States ofAmerica,CanadaorNewfoundlandandanypointinGreatBritainorIreland"in72 continuous hours". Ajetlinertakesmorethan13hours40minutestocrossoverPacificrouteandreachfrom Kuala Lumpur to Heathrow. A supersonic passenger aircraft which used to take only half the timetakenbyitscontemporaryjetlinersinitsregularflights,frequentlyflewinthe transatlantic route form London and Paris to New York and Washington and Barbados. Speed hasbeenthrillingandattractingthehumankindsincethebeginningandraceisstillon. Peoplehavenotrestedandnowstartingfromlowsubsonicspeedandgoingthroughhigh supersonic speeds, now the era of hypersonic has begun. A hypersonic plane will hardly take nearabout 2 hours to go to Heathrowfrom KualaLumpur. A hypersonic vehicle whichcan travel Mach 7 to 12 can take man from New York to Tokyo in less than two hours. Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 4 Concorde taking off Distance between Kuala Lumpur, Malaysia to Heathrow, London Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 5 Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 6 Introduction Now we are already know that humanity is opting for higher and higher speed which not only gives thrilling effect rather it also helps in the growth of humanity in so many ways. We are living in the space age so hypersonic is the only option we must go for. We have already seen thateventhesimplespeedassociatedwiththespacecraftarehavinghypersonicspeed associated with it. So hypersonic speed and objects associated with the hypersonic speed are goingtobetheprimeareaofconcerninthenextfutureofhumanity.Questionis;whatis hypersonic speed? What is the definition of it? Whenwestudythedynamics,baseduponthevariousrangesofspeedswecanclassifythe study of motion in following ways: Sl. No. RangeDescription 1. Low subsonic speed Inthisregionobjecttravellingwithverylowsubsonicspeeds arestudiedinwhichcompressibilityeffectisneglectedand viscosityisthemaincriteriawhichisconsideredasitaffects mostoftheflowphenomenaspeciallytheappearanceof boundary layer. This ranges from 0.0 to 0.3 M 2. High subsonic speed In this region, though speed is still subsonic, it is high enough tocausethecompressibilityeffectandhenceitmustbe consideredwhileanalysingtheflow.Viscosityoftheflow graduallystartlosingeffectinthisregion.Thisnormally ranges from 0.3 M to 0.85 M. 3. Transition speed In this region flow over any body is partly sub sonic and partly supersonic extra care has to be taken while analysing the flow. Theflowbehavesinboththeway,subsonicandsupersonic. This type of behavior normally starts at 0.8 M and sustains till 1.2 M. After the upper limit flow becomes fully supersonic. 4. Supersonic speed In this region flow speed is more than 1.0 M and flow is mostly characterisedbycompressibilityandappearanceofshocks whicharecompressionwavesinnature.Duetotheexistence ofshocksflowbecomesdiscontinuousandsuddenrisein temperature,pressure,densityandentropyisfeltintheflow. Hypersonic Aerodynamics (Introduction to Hypersonic Aerodynamics) Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 7 Sl. No. RangeDescription Shockeffectissoprominentinthisregionthatflowcan actually be taken as inviscid for all practical purposes. 4. Hypersonic speed In this region even the supersonicconcepts are not very much applicable.Thisregionischaracterisedbythespecific phenomenawhichtakeplaceatveryhighspeedswhichisthe main topic of discussion of the following session. What is hypersonic speed and at which Mach number the vehicle can be said to be travelling at hypersonic speed? A very interesting and intriguing type of question it is. P. L. Roe made a comment in his lecture at Von Karman Institute, Belgium; January, 1970: Almosteveryonehastheirowndefinitionofthetermhypersonic.Ifweretoconductsomethinglikean opinion poll among thosepresent,and askedeveryoneto namea Mach number above which theflow a gas should properly be described as hypersonic, there would be a majority of answers round about five or six, but it would be quite possible for someone to advocate, and defend, numbers as small as three or as high as 12.Before we actually get into the deep discussion for proper comprehension of hypersonic and itsbehaviour,letusknowalittleaboutthehistoricalfactsassociatedwiththehypersonic speeds and event. Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 8 Some historical facts OnFebruary24,1949rocketV2/WACCorporalwaslaunchedfromtheWhiteSand Proving Ground test range. This rocket was brought from Germany after the WWII and it was an attempt to demonstrate the use of use of multistage rocket to achieve high speed and high velocity.ThiseventwaspartofabigprogrammelabeledBumperbytheU.S.Army.Till then all the previous rockets launched either in America or in Europe had utilised the single stage.V2itselfwasasinglestagerocketandWACCorporalwasaddedtoittoprovidean extra stage. V2 took the flight to 100 miles of altitude and attained the velocity of 3,500 mph, atthispointWACCorporalwasignitedandtheslenderupperstage(WACCorporal) accelerated to maximum velocity of 5150 mph and reached an altitude 244 miles. This broke the earlier record of 130 miles set by V2 rocket alone. After reaching the peak altitude, WAC Corporal noses over and careers back into the atmosphere at about 5000 mph.In doing so, it became the first object of human origin to achieve hypersonic flight. It was the first time any vehicle has flown at a speed five times faster than the speed of sound. An interesting part of thisstoryis,despitethepenplottercharteditscoursebackontheearth,andtheWAC Corporal was never found back in the desert. However the charred electric switch and part of the tail section were found later after almost one year in April, 1950. V2/WAC Corporal lift off on February 24, 1949, the first object of human origin to achieve hypersonic flight. Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 9 Anothereventoftheinterestinthepresentdiscussiontookplaceinasmallvillagecalled SmelookaintheTrenovdistrict,SaratovregionofRussia.OnApril12,1961astrange looking object landed just now under the canopy of parachute. The surface of the capsule is charred by high temperature and it contained three windows covered with heat resisting glass. Inside the capsule is Flight Major Yuri Gagarin who has went in space by sitting on the top ofarocketandjustnowreturnedwhilehavingafreefallintheearthsatmosphere.Yuri GagarinwentinspaceinhisspaceshipcalledVostokIandenteredintotheorbittogo around the earth. The orbit of this space has minimum distance from earth atperigeeequal to175kmandmaximumdistancefromtheearthatitsapogeehas302km.Itenteredinto theatmospherebyfiringtheretro-rocketaftergoingroundtheearthintheorbitwith specification mentioned above. During re-entry the capsule entered into the atmosphere with aspeedinexcessof25timesthespeedofsound.YuriGagarinbecamethefirstmanin history to experience the hypersonic flight. Entire flight of the Yuri Gagarin from takeoff to landingtookalmost108min.Theseprecious108min.madeamangoinspace,goaround the earth and come back on earth. Vostok I, in which Russian Major Yuri Gagarin became the first human to fly at hypersonic speed, during the world first manned, orbital flight, April 12, 1961. Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 10 Some pictures of interest: V2 Rocket Model Big Bertha Rocket Model V2 firing Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 11 Vostok 1 Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 12 Vostok 1 Capsule (Schematic view) Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 13 Vostok 1 Capsule (The recovered one) Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 14 Latertheyear1961becamethebumperyearforthehypersonicflights.OnMay5,AlanB. Shepherdbecomesthesecondmaninthespace.HetakesasuborbitalflightoverAtlantic Ocean reaching an altitude of 115.7 miles, entering the atmosphere at a speed above Mach 5. ThenonJune23,U.S.AirForcetestpilotMajorRobertWhitefliestheX-15airplaneat Mach 5.3. This record was bettered by White on November 9, by flying the X-15 at Mach 6. Air Force Major Joe Eagle with X-15 X-15 Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 15 In the military application, many vehicles like rockets, missiles and the space shuttle during re-entry travel at very high speed in the hypersonic range. The urge for the high speed travel can be understood by this fact that starting from the Write Brothers flyer which flew 35 mph atsealevelin1903,wealreadyhaveaircraftwhichflewat400mphat30,000ft.during theWWIIandthentransitioningto1200mphsupersonicflightsat60,000ft.altitudein the 1960. Not to forget the flights of experimental X-15 at the hypersonic speed. On August 23,1963X-15flewat Mach7atanaltitudeof354,200 ft.Thisraceofflyinghigherand higherspeedwascappedwiththeeventofspaceshuttlere-entrywithMach25intothe Earthsatmospherefrom200milelowearthorbit.Ifweplotthegraphagainstthetime linethenweshallfindthatthespeedaswellasaltitudebothhaveincreasedexponentially over the time in the last 100 years. Above were the examples of manned vehicles in the civilian uses. Taking into account of the militaryapplicationsandmissilesandrocketsfurtherconfirmsthepassionofhumankind towardshighspeed.IntercontinentalballisticmissilesweredesignedtoflyatMach25 during1950s.When thesafetyconcernisnotthereandsafelandingisnotexpected,speed limitautomaticallybecomeshighforthevehicle.Intheraceofhighspeedformanned mission,historiceventofApollospacecraftleadstherace,foritssuccessfulreturningthe men from Moon while reentering at Mach 36 in 1969 followed by the Mach 25 Mercury, Gemini and Vostok manned orbital space missions of 1960s. FlightatthisendofspectrumiscalledHypersonic Flightsandtheaerodynamicsandgas dynamics characteristics of such flights are classified as Hypersonic Aerodynamics. Onceitstartedthereisnoendtoitandeffortsaremadetoreachhigherandhigherspeed which will enable the interplanetary travel a reality one dayIN SHAA ALLAH. Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 16 Comparison of Supersonic and Hypersonic Vehicles Hypersonic aerodynamics is different from the more conventional and experienced regime of supersonicaerodynamics.Toknowdetailsofthesedifferences,wemustwaittillwe comprehend and the terms and basic ideas of hypersonic aerodynamics. Some features which are related to vehicle shape and other features can be discussed here. PicturebelowshowsasupersonicairplaneLockheedF104.Thisaircraftisdesignedfor sustainedsupersonicflightatMach2.Itsbodyandoutershapeemploysagooddesign basedupontheprinciplesofsupersonicaerodynamicdesign.Asharp needlelikenoseand slenderfuselage,verythinwingsandtailsurfaceswithsharpleadingedgeswith3.36 percent of thickness to chord ratio and aspect ratio as low as equal to 2.45 for the straight wing itself. All these are efforts to make the wave drag as low as possible as we know that in thesupersonicrangeofflightsurfacefrictiondragissupersededbythewavedragwhich becomeverymuchprominentduetoappearanceofsupersonicwaves.A3viewfigureof LockheedF104isgivenbelowforthebettercomprehensionofthesupersonicdesignshape of the aircraft. Lockheed F104 Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 17 3View design of Lockheed F104 Thedesignofsupersonicvehiclewasatemptingreplicaforthehypersonicvehiclesalso duringtheearlydesignofhypersonicvehiclesin1953.ThatswhyRobertCarmanand HubertDrakeofNACAdesignedtheirhypersonicplanewithsharpnoseasseenbelow. Supersonicdesigninfluenceisclearlyevidentinthedesignitself.Atthattimeideaof hypersonic aerodynamics was in its infancy stage. Drake-Carman hypersonic aircraft (Proposed in 1953) Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 18 Just seven years later a new spacecraft X20A Dynasoar was designed with complete contrast and a completely different look has emerged. X-20A Dyna Soar Space Orbital X-20A Dynasoar with its powerful engine Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 19 X-20A Dynasoar utilises a sharply swept delta wing with blunt, rounded leading edge and ratherthickfuselage.Fuselagewasplacedonthetopofthewingsothatentireunder surfaceofthevehiclewasflat.Itwasdesignedtobeanexperimentalaircraftforrocket powerd flight at Mach 20. Eclipsed by the Mercury, Gemini and Apollo manned space-flight, Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 20 X-20 project was cancelled in 1963 without production of any single vehicle. Nonetheless the design features were uniquely hypersonic and were later contained in the design of space shuttle. Space Shuttle Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 21 Space Shuttle in Action Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 22 Space Shuttle Block Components Sowefindthathypersonicvehicledesignsarequitedifferentfromthesupersonicvehicle designs.Theimmediatequestionwhichcomesinourmindis,ishypersonicaerodynamics alsodifferentformthesupersonicaerodynamics?TheanswerisYES. Belowisthepicture of hypersonic vehicle which, was designed to return the humans from the Moon and enter the Earths atmosphere at extreme hypersonic speed of Mach 36. Artistic view of Apollo space craft re-entry Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 23 Artistic View of Apollo re-entry Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 24 ThemainproblemapartfromAerodynamics,aerospacevehicledesignisgoingtofaceis extremeamountofpropulsiveforcerequiredtopropelthevehicletohypersonicspeed. ThisideaswasfirstseriouslyexaminedbytheU.S.AirForceintheearly1960sandthe combinationofair-breathingandrocketpropulsionwasintendedtopowerthevehicle. WorkonearlyaerospacevehiclewascancelledinOctober1963,mainlyduetothedesign requirement exceeding the state of art at that time. The idea was resurrected in the mid-1980s by both NASA and Department of Defense as well as by the aerospace companies in England and Germany. Current thinking of manned aerospace plane are shown in the picture below.

Artists impression of hypersonic aerospace vehicles Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 25 These trans-atmospheric aerospace planes will relyheavily on thesupersonic combustion RAMjet(SCRAMJet)engineforthepropulsion.Whatdistinguishesthehypersonic transportandaerospaceplaneconceptsfromtheconventionalsubsonicandsupersonic airplane design philosophy? It is a great area of interest Forthesupersonicandsubsonicaircraft,thecomponentsforprovidinglift(thewing), propulsion (the engine and nacelles) and the volume (the fuselage) are not strongly coupled to each other. They are separate and distinct components and easily identifiable while looking at theairplane.Moreovertheyallareaerodynamicbodiesseparatelywhicharecombined togetherwithamoderateinteractionwhichaffectstheircombinedperformancemildly. Modern hypersonic aircraft has entirely opposite aerodynamics. Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 26 IHSTDV (Indian Hypersonic Technology Demonstration Vehicle) Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 27 Someofthehypersonicaircraftare shown above in the pictures. They give clear idea about the mixed components involvedinthedesign.Theentire undersurfaceofthevehicleisthepart oftheSCRAMJetengine.Initial compressiontakesplacethroughthe bowshockfromthenoseofthe aircraft.Furthercompressionand supersoniccombustiontakesplace inside a series of modules near the rear of the aircraft and then the expansion of the burned gases is partially realised through nozzle intheenginemodulebutmainlyoverthebottomrearsurfaceoftheaircraftwhichis sculpturedtoanozzlelikeshape.Hencethepropulsionmechanismisintimatelyintegrated over the air-frame. Moreover,mostoftheliftisproducedbythehighpressurecreatedbehindthebowshock which,isexertedovertherelativelyflatundersurfaceofthevehicle,sothelarge,distinct wingsarenotnecessaryfortheproductionofthehighlift.Alsohighliftcoefficientisnot required to lift the plane because high velocity will take care of the net lift generation for this typeofvehicles.Alsothefuelfortheairbreathingenginetobeusedinthehypersonic aircraftwilllikelytobeLiquidH2(LH)whichoccupiesalargevolumeowingtoitslow molecularweightandhencedensity.Alltheseconsiderationscombineinahypersonic vehicle in such a fashion that the components to generate lift, propulsion, and volume are not Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 28 separate from each other; rather they are closely integrated in the same overall lifting shape in direct contrast to conventional subsonic and supersonic vehicle design. Sohypersonicisphysicallydifferentfromthesupersonicanditisgoingtodictatethe forthcoming age of humanity which can be easily termed as Space Age Types of hypersonic vehicles Bylookingintomissionrequirementsthehypersonicvehiclescanbeclassifiedinvarious categories. Some of the missions are designed for high deceleration in the outer atmosphere during re-entry. Hence, those flight vehicles experience longer flight duration at high angle of attacks due to which, blunt nosed configuration are generally preferred for such aircrafts. On the contrary, some missions are centered on low flight duration with major deceleration shouldtakeplaceclosertoearthsurface;hencethesevehicleshavesharpnoseandlow angleofattackflights.Reentryflightpathofhypersonicvehicleisthusgovernedbythe parameterscalledasballisticparameterandliftingparameter.Theseparametersare obtained by applying momentum conservation equation in the direction of the flight path andnormaltoit.Velocity-altitudemapoftheflightisthusmadefromtheknowledgeof these governing flight parameters, weight and surface area. Ballistic parameter is considered for non-lifting reentry flights like flight path of Apollo capsule, however lifting parameter is considered for lifting reentry trajectories like that of space shuttle. Therefore hypersonic flight vehicles are classified in four different types based on the design constraints imposed from mission specifications. 1.Re-entryVehicle(RV):Thesevehiclesaretypicallylaunchedusingrocket propulsionsystem.Re-entryofthesevehiclesiscontrolledbycontrolsurfaces.Large angleofattackflightofbluntnosedconfigurationsistheneedoftheseflights.Space shuttle(US),BURAN(Russian),HOPE(Japan)andHERMES(European)aresome examples of these kind vehicles. 2.CruiseandAccelerationVehicle(CAV):Inthesetypes,highspeedismore important and slender configurations with low angle of attack flights are main features of theseflights.Thesevehiclesarepreparedforhighheatingloadswithablativecooling system. Air breathing propulsion system of ramjet or scramjet type is generally preferred for these vehicles. Sanger, which is a two stage (TSTO) hypersonic vehicle, has first stage with air breathing propulsion and second stage is propelled with rocket. Hence first stage of Sanger falls in CAV category for which separation takes place at Mach 7. Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 29 3.Ascent and Reentry Vehicle (ARV): These vehicles have opposite requirements for theirdesignduetodualduty.Theirascentflightisdominatedbythefuelrequirement andthereentryflightisdominatedbyaero-braking.Rocketorair-breathingpropulsion canbepreferredfortheseflights.NASPorNationalAerospacePlaneofUS,Space PlanebyJapanandHOTOLaresomeexampleofthesevehicles.Intercontinental ballisticmissilesfallinthesamecategorywhichareusedinthebattlefieldduringwar time. 4.Aero-assisted Orbital Transfer Vehicle (AOTV): Ionisation and hence presence of plasma in the vicinity of the spacecraft is the major concern of these vehicles. This new hypersonic vehicle concept is the aero-assisted orbital transfer vehicle (AOTV) which will be employed to transfer material and people between the space shuttle in low earthorbitcalledLEO(about300kmabovethesurfaceofearth)andsatellitein geosynchronousorbits(35,000kmabovethesurfaceofearth).WhentheAOTVleaves thegeosynchronousorbitandreturnstolowearthorbit,itwilldipintotheearths atmosphereanduseaerodynamicdragtoreduceitsvelocity.Thusenablingrendezvous with the space shuttle. AOTV will be the high-flying hypersonic flying designed to fly at no less than Mach 30 and at an altitude no less than 250,000 ft. altitude. Each of these four types of vehicles face different flight challenges based upon their missions andflightconfigurations.Thesechallengesformthetopicofresearchinthefieldof hypersonic aerodynamics. Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 30 Challenger and Buran Hope of Japan and Hermes of Europe Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 31 Sanger II NASP of NASA Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 32 NASP of NASA HOTOL Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 33 HOTOL ICBM, Intercontinental Ballistic Missile Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 34 Cassini inter-planet trajectory Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 35 Hypersonic Atmosphere It is a new term we aregetting introduced with, hypersonic atmosphere.What is it? A good intriguing question. In all the other regime of speeds, we normally associate the body and its speed with the regime. Like we call subsonic speed, or supersonic speed. This speed may be the speed of the object flying with the particular speed or it may be the speed at which flow is passing over the body. But in both the cases, we normally associate the speed with the speed regime. Why talk about the atmosphere in the case of hypersonic? Theanswerliesintheverydefinitionofthehypersonicitself.Definitionofflowregimeis based on the Mach number of the flow. If Mach number is below unity then the flow is called as subsonic. Sonic flowhas Mach number exactlyequal to one howeverflow in the narrow rangeofMachnumberbetween0.8-1.2iscalledastransonicflow.WhentheflowMach number exceeds beyond 1 then flow is called as supersonic flow. Definition of Hypersonic Flow Forallthesethreeregimes,itisappropriatetotaketheMachnumberasthebasisof definition of the flow regime, but in the case of hypersonic flow regime it is not the specific Machnumberwhichmakesaflowregimetobehypersonic,rathersomeofthecertain physicalphenomenawhichstartexhibitingintheatmosphere,tellthattheflowisactually hypersonic. And these certain physical phenomena start showing upon as early as Mach 3 or Mach4,butinaverybeginningmanner.TheygraduallygrowwithMachnumberand becomefullydevelopedatnearaboutMach10orMach12.Sobylookingatthedefinition two things are clear right now. 1.There is not fixed Mach or sharp boundary of Mach number which will make the flow hypersonicononesideandnon-hypersonicontheotherside.So,asperthethumbrules, when flow speed exceeds five times the sound speed, we start treating it as hypersonic flow. Howeverhypersonicflowhascertaincharacteristicswhich,whenexperiencedintheflow, should then only be termed as hypersonic. So the thumb rule is just a thumb rule as it is said to be, and there is not great change in the flow regime and flow behaviour takes place when theflowMachnumberchangesfrom4.99to5.01likewhatweseeinthecaseflowgoing from subsonic at Mach 0.99 to supersonic Mach 1.01. Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 36 2.Itisalsoclearthatitisthecharacteristicoftheatmospherewhichdefinesand hypersonicnatureoftheflowratherthantheMachnumberoftheflow.Hencetherecomes the term hypersonic atmosphere in use. Hypersonic flow characteristics Theatmosphericcharacteristicswhichdefinethehypersonicflowregimearementioned below. Thin Shock Layers The region between the shock and the body (flight vehicle) is known as shock layer. The picturebelowgivestheideaabouttheshocklayerregioninthecaseofanobliqueshock appearinginthecaseofflowovera wedge with some wedge angle. For the samewedgeangleasthefreestream Machnumberisincreasedthewave angles becomes smallergradually then thereisverythinregionavailable betweenthesolidboundaryandthe shock wave for the after stream flow to takeplaces.Sothesameflowwhichbeforetheshockwastakingplaceinamorewideand openregion,isnowforcedtoflowinacomparativelynarrowregionaftertheshock.This decrement of thewave angle with the increasingMach number is also evident from the relation.When theflowMachnumber isveryhighinthe hypersonicregion,the thicknessoftheshock layercomesdown drasticallyandthis phenomenonisalso markedbythehigh densityoftheflowin thethinshocklayer region.Thisisrequired tocompensatethedecrementinarea,sothatcontinuitycanbemaintainedasthelawof Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 37 conservationofmasscannotbeviolatedinanycase.Forexampleweintheabovecaseof flow over a wedge of 200 half wedge angle when the Mach number is of the order of 20 to 25, theweaveanglebecomessomewhat220.Ifthewedgeangleissmallerlike150thewave angle becomes very close to 180. This is correct for the calorically perfect gases which have constant specific heat capacities. This interpretation of thinness of shock layer for calorically perfect gas, is also applicable for the thermally perfect gas and chemically reacting gas, although their specific heat capacities may not remainconstant at that level.As a matter of fact inthe case of chemicallyreacting gaseous flow when the temperature is very high, this thin region can be expected to be further thin. So this is a basic characteristic of the hypersonic flow that the shock layer is very thin and shock lies very close to the solid body surface. Whenthethinshocklayersbecomesthinenoughtobecomparablewiththethickboundary layer growing over the solid surface, the situation becomes little bit complicated, because the thinshocklayerstartsinteractingwiththethickboundarylayer.Thisproblembecomes importantforthelowReynoldsnumberflow.However,whentheReynoldsnumberishigh, the flow in the shock layer is practically inviscid, the thinness of the shock layer becomesa theoreticaladvantage,andageneralanalyticalapproachcalledthinshock-layertheory becomesapplicable.Thisthinkshocklayertheorybecomesahandytoolandisfrequently usedinthehypersonicaerodynamicsforapproximatecalculations.Thistheorywas postulatedbytheIsaacNewtonin1687;weshallstudythistheoryinmuchdetailsalittle later. Entropy Layer When we have a conical leading edge, the shock remains attached with the body and strength oftheshockremainsalmostsameforaconsiderablelength.Inthecaseofhypersonic vehicles,wenormallyhavebluntleadingedgeforwewantadetachedshockhere.This detached shock varies in its strength form normal shock at the leading edge to strong oblique shockandthenweakobliqueshockandthenfinallyaMachwave.Wealsoknowthatthe Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 38 entropy change in the flow depends upon the strength of the shock. Stronger the shock, larger willbethechangeintheentropy.Alsotheshocklayerforthestrongershocknearthenose will be very thin, with a small shock detachment distance, d. So the entropy gradient will be verylargeintheregionnearnose,i.e.largeentropychangewilltakeplaceinaveryshort region.Nowastheshockstrengthdecreasesdownstreamflow,alayerofentropyvariation getsformeddownstreamoftheshockanditiscalledentropylayer.Thisstrongentropy layer flows downstream and wets the body for a large distance from nose. Now the boundary laterwhichgrowsnearthesurface,growsinsidetheentropylayerandisaffectedbyit. Boundary layer is essentially a region of non-zerovorticityduetohighviscous effectavailableintheregion.Since entropylayerisalsoaregionofstrong vorticityaccordingtotheCroccos theoremfromclassicalcompressible flow;thisinteractionbetweenthetwo regionsisalsocalledasvorticity interaction.Thepresenceofentropy layercausesanalyticalprobleminthe standardboundarylayercalculationonthesurface,becausethereisnotcertainideanow aboutwhatwillbeproperconditionattheouteredgeoftheboundarylayer.Thisentropy layer is essentially a property of hypersonic flow which also takes part in its definition. Viscous Interaction Weknowthatformationofboundarylayertakesplacenearthesolidsurfaceduetonoslip conditionintheviscousfluidflow.Nowletusrefertotheabovefigureagainandconsider theboundarylayerflowovertheflatplateinthehypersonicregime.Highvelocity hypersonicflowcontainshugeamountofkineticenergy,butinsidetheboundarylayer, owingtotheeffectofviscosity,thefluidhastocometorestinthelayeradjacenttothe surfaceandmustincreasegraduallytoattainthefinalflowspeed.Sowithintheboundary layer,thelostkineticenergyistransformedintointernalenergyofthegas.Thiseffectis called viscous dissipation. We already know from the ideas we gathered during early classes of heat transfer that; increased internal energy actually increases the temperature of the gas. A typical temperature profile in the boundary layer in the hypersonic regime has been shown in the figure below. Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 39 Itisverymuchevidentthatvelocityvariationisalmostsameasinanormalhighspeed boundary layer profile but temperature variation is a little bit typical here. Near the boundary temperature increases and becomes even larger than the flow temperature. This is due to the transformationofhighkineticenergyintointernalenergyofthegas.Nowthisincreased temperaturedoestwoworks.Firstlythisincrementoftemperatureinturn,affectsthe viscosity and increases it (we know this by the knowledge of the effect of temperature on the viscosity of gases). This increased viscosity affects the growth of boundary layer and there is arapidincreaseintheboundarylayerthickness.Inadditiontoit;becausethepressure remainsconstantintheboundarylayerinthedirectionnormaltosurface(weknowthis throughourknowledgeduringtreatmentofforcedconvectionoveraflatplateinboundary layertheory),theincreaseintemperature ,resultsindecreaseindensity throughthe equationofstate = ,where isthespecificgasconstant.Soinordertopassthe required mass through the boundary layer, at decreased density, the boundary layer thickness mustbelarger.Soagrowthinboundarylayerthicknessisexpected.Bothofthese phenomenacombinetomaketheboundarylayerthicknessgrowfasterthanthatatthelow speed flows. Indeed, over a flat plate, boundary layer thickness grows essentially as below:

2

SoastheMachnumberincreases,boundarylayerthicknessincreasestooanditincrease accordingtothesquarecurve.Soitwillbecomelargeinthehypersonicregimeandthe reason is that explained above. Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 40 Thisthickboundarylayerinhypersonicflowcanexertamajordisplacementeffectonthe outerinviscidflowregion.Thiswillcauseabodytoappearmuchthickerthanitreallyis. Now due to this extreme thickness of the boundary layer flow, outer inviscid flow is greatly changed,andthechangeintheinviscidflowinturnfeedbacktoaffectthegrowthofthe boundarylayer.Thismajorinteractionbetweentheouterinviscidflowandtheboundary layer iscalledviscous interaction which takesplace in a loop. This viscous interaction can have important effect on the surface layer pressure distribution, hence lift, drag and stability of the hypersonic vehicle, all are affected by it. Moreover, skin friction and heat transfer are also increased by the viscous interaction. Thefigurebelowillustratestheeffectof viscousinteractiononthepressurevariation on the surface of a sharp right circular cone at zero degree angle of attack. In the absence of viscousinteractionpressurewillbe constant

throughout the length but it varies duetointeractionofboundarylayerwiththe outer flow which practically remains inviscid. It is clear that the effect of viscous interaction isgreaternearthenoseareaanditseffect diminishesfurtherdownstreamandfardownstreamitapproachesthevalueequaltothe inviscid value. (Note:Theboundarylayerattimecanbecomesothickthattheshocklayercompletelymergedwithinitand then shock layer must be treated fully viscous and in this case theconventional boundary layer analysis is not valid.) High Temperature Flow We have already discussed about the high temperature generation within the boundary layer inahypersonicflowduetoviscousdissipation.ButdowerealisehowHIGHthis temperature might be which is generated due to hypersonic speed? Actually this temperature sometimes can be so high that it can excite the vibrational energy inside the molecules, and to cause dissociation and even ionization within the gas. If the surface of the hypersonic vehicle isprotectedbyanablativeheatshield,theproductofablationarealsopresentinthe boundarylayer,givingrisetocomplexhydrocarbonchemicalreactions.Sowhetherbeit dissociationandionizationorthehydrocarbonchemicalreaction,onbothaccounts,surface Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 41 ofahypersonicvehiclewillbewettedbyachemicallyreactingboundarylayer.Thisisthe caseofboundarylayerinthehypersonicflow.Sothechemicalequilibriumhastobe accountedwhiledoingtheseestimations.Thishappenssobecause,forthehypersonicflow, not only the boundary layer will be chemically reacting, rather entire shock layer too can be dominated by the chemically reacting flows. Boundarylayerisnottheonlyregionofhigh temperatureflowoverahypersonicvehicle.At the nose where blunt shape is given to detach the shockandprotectthestructurefromexcessive temperature,thebowshockhasanatureof normalshock.Thisstrongnormalshockatvery highMachnumbercangenerateveryhigh temperaturebehindtheshock.Thisveryhigh temperaturesometimescanbegoodenoughto causethemoleculardissociationoftheglowing gas. This gives rise to a complex situation that at theseelevatedtemperaturetheconceptof caloricallyperfectgasdoesntremainvalidand hence assumptions of constant thermodynamic properties gives unrealistic estimations. Hence itbecomesessentialtotakeintoaccountthetemperaturedependenceofspecificheatsand theirratioasfunctionoftemperatureforrational estimates. TakingthecaseofApollosreentryatMach36, temperaturebehindtheshockandintheboundary layer can be estimated to be near about 11,000 K by considering the chemically reacting gas flow at that hightemperature.Takingconstant willgivevery high value which is unrealistic. Thegraphbelowillustratestheideaabouthow unrealistic the temperature estimation can become at very high reentry vehicle velocity if we assume the gas to be calorically perfect. Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 42 Thedependenceof thermodynamicpropertieson temperaturemainlycomesfrom microscopic changes in the fluid duetoincreaseininternal energyofthefluidbythevirtue oflossofkineticenergy. Increasedinternalenergyleads initiallytovibrationalexcitation followedbydissociationand finallyionizationaccordingto theextentofincreaseininternal energy.Thiscausesthespecific heats

and

tobecomefunctionoftemperature.Sotheratioofspecificheats =

alsobecomesafunctionoftemperature.Forairthiseffectbecomesimportantabovea temperature of 800 K. As per the order of magnitude estimate, vibrational excitation of air takes place at around 800 K. Oxygen dissociation starts at around 2000 K and completes at 4000 K. At around 4000 K nitrogendissociationcommencesandcompletesat9000K.Ionizationofthishigh temperature air or mixture of gases starts from 9000 K temperature. Hence the initial air with atmosphericcompositionbecomesplasmaafter9000K.Asaresultofallthesereactions, hypersonicvehiclegetsengulfedbyreactingboundarylayerandhightemperatureplasma. Therefore treatment of air or any fluid flowing with hypersonic speed over any configuration should be done properly by incorporating all the microscopic changes which essentially leads tochangeinthermodynamicpropertieswithtemperature.Thisdependenceishighlynon-linear,henceanalysisorpredictionofflowfieldbecomestougherinthisflowregime. Thereforetwotypesofassumptionsaregenerallymadeabouttheflowconditionsforhigh temperaturefluidasequilibriumflowandnon-equilibriumflow.Ifthemicroscopicchanges or reactions are at faster rate than the movement of the fluid, then it is treated as equilibrium flow otherwise it is treated asnon-equilibrium flow which is difficult toanalyze. All these difficultiesarecollectivelytermedashightemperatureeffectandarealsofrequently referred as real gas effects although there are technical reasons to discourage the use of this term for this effect. Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 43 Someconsequencesofpresenceofhightemperaturereactingfluidorplasmainthevicinity of the flight vehicle include, influence on aerodynamic parameters, aerodynamic heating andcommunicationblock-out.Flightparameterslikepitch,roll,drag,lift,deflectionof control surfaces get largely deviated from their usual estimate ofcalorically perfect gas. Presence of hot fluid near the cold vehicle surface induces heat transfer not only through convectionbutalsothroughradiation.Communicationwaveswhicharenecessarilyradio wavesgetabsorbedbyfreeelectronsformedfromionizationofatmosphericfluid.This phenomenoniscalledascommunicationblock-outwhereonboardandground communication gets terminated. Low Density Flow Inanormalenvironmentwhichwenormally encounterinourdailylives,aircontainsmillionsof particleswhicharecontinuouslyinrandommotion. Asingleparticletravelstosomedistancetillit collideswithanotherparticleandthenitsdirection getschanged.Thesecollisionsarepractically100% elasticsothereisnolossofenergy.Thedistance moleculetravelbeforetheycollide,isnotalways constantratheritvariesfromcollisiontocollision, butontheaveragetheytraveladistancebetween two collisions which we call as mean free path and is denoted by . So by definition, mean free path is thedistancemoleculesofagastravelonthe averagebetweenany twocollisions.At standardsealevelcondition, = . m.Itispractically averysmalldistance.Thisimpliesthatwhensomeonemovesat thesealevelcondition,thenumberofimpactsofairparticleson his body will be so large and continuous that individual impact of theparticleswillbehardlyfeltbythebody.Thiswillfeellikea continuous medium and the flow of medium too can be treated as continuous and this concept of accepting the flow to be continuous in the airflow in the technology, is called continuum. Most aerodynamic problems are dealt with by assuming this concept to be very much valid. Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 44 Now as we move at higher and higher altitude the possibility of the existence of huge number ofparticlesinasmallspacekeepsdiminishingandparticleskeepgettingapartfromeach otherandtheirmeanfreepathkeepsincreasinggradually.Atanaltitudeabout105km,the mean free path of the air particles becomes as high as equal to 1 ft. Now if any one moves in thisenvironmentthenthepossibilityofairparticlehittinghisbodywillbetoolowand numberofparticleshittingperunitareawillalsobe low.Sotheindividualhittingoftheparticlewillbe feltandthenthemediumwillnolongerbefeltas continuous. It will feel like an open region punctuated byindividual,widelyspacedparticlesofmatter. Undertheseconditions,theaerodynamicconcepts, equations,andresultsbasedupontheassumptionof continuumbeginstobreakdown;andwhenthis happenswehavetoapproachaerodynamicsfroma different point of view, using concepts from kinetic theory. This regime of aerodynamics is called low-density flow. In most of the hypersonic applications low density flow is involved specially at high altitudes flights. For example, at about 95 km altitude, the flow in the nose region cannot be treated as continuum.Withthegradualincreaseinaltitude,assumptionofcontinuumflowbecomes tenuous.Soanaltitudecanbereachedwheretheconventionalviscousflowno-slip conditionbeginstofail.Soatlowdensities,theflowvelocityatthesurface,whichis normallyassumedtobezeroduetofriction,takesonafinitevalue.Thisiscalledvelocity slipcondition.Intheanalogousfashionthe,thegastemperatureonthesurface,whichis normallythetemperatureofthesurfaceitself,nowbecomessomethingdifferent.Thisis calledtemperatureslipconditionortemperaturejumpcondition.Ontheonsetofthese Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 45 slipeffect,governingequationstillassumingthecontinuumconceptoftheflow,canbe utilisedwiththeproperconsiderationofvelocity-slipconditionandtemperature-slip conditionsattheboundary.Howeverifthealtitudeincreasesfurther,therecomesapoint where the continuum equation no longer remain valid and methods fromkinetic theory must be used to predict the aerodynamic behaviour of the flow. Finally, the air density can become so low that only a few molecules impact the surface per unit time and after the reflection they never collide with the incoming molecule. Such regime is called asfree molecule flow. For spaceshuttlethisfreemoleculeflowregimebeginsatabout150km.Soduringreentrythe space shuttle faces a flight from a much rarified atmosphere to denser atmosphere. This will shiftfromfreemoleculeregime,whereindividualmoleculesimpactonthesurfaceis important,tothetransitionregimewherenoslipconditionisimportanttofinallynormal continuum regime where continuum of the flow becomes more important. Thesimilarityparameterwhichgovernstheseregimeoffreemolecule,transitionand continuumregimesistheKnudsennumber.Knudsennumberisdefinedbytheratioof meanfreepathoftheparticlestothecharacteristiclengthoftheflowproblem.By definition =

.Theapplicabilityoftheflowregimeandtheirrespectivegoverning equations which is controlled by the Knudsen number is shown in the picture shown below. It is clear that for the < 0.2 the continuum concept is very much valid and the continuum Navier-Stokesequationsareverymuchapplicablethere.Howevertheslipconditionstarts takingplaceasfortheKnudsennumberaslowas0.03.SoforlargervaluesofKnudsen numberslipconditionstartstakingplaceandthismustbeaccountedforanyregime with > 0.03.Thefreemoleculeflowbecomeeffectivethemomentmeanfreepath Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 46 becomes comparable to the characteristic length i.e. =

or = 1 and then the range for theapplicabilityofthefreemoleculeregimeextendstoinfinity.Sothetransitionregimeis essentially contained within the range of Knudsen number 0.03 to 1 i.e. (0.03 < < 1). NowitmustbenotedtherethatKnudsennumberactuallydependsupontwoparameters1. Meanfreepath ()and2.Characteristiclength (

).Sobothhavetheirowneffect.Inthe space where mean free path is very large, effectively any type of vehicle actually deals with the free molecule regime. In the nearby space altitude where mean free path is not very large, theapplicabilityofthefreemoleculeregimewilldependupontheeffectivecharacteristic length.VehicleswithlowcharacteristiclengthwillstillhavethehighKnudsennumber effect. So the free molecule regime effect will still be applicable there, however for the large characteristiclengthvehicles,theregimemayhavechangedtocontinuumortransition regime.Sothesetwoparameterscombinedlydeterminewhichwhattypeofequationsare applicable in the flow regime. Inthecaseofflowoveraflatplatenormallywetakethedistanceofthelocationfrom leading edge as the characteristic length. So near the leading edge where characteristic length is very small, the value of Knudsen number may go equal to unity. Hence for any vehicle at any altitude, the flow immediately at the leading edge is governed bylow density flow. For most practical cases in the aerodynamics, this leading edge region is very small and is usually ignored. However for high altitude hypersonic vehicles, the proper treatment of leading edge flow by low density flow methods is important. Another point to be noted here is, in all the other hypersonic characteristics we took the Mach numberasbasisofdefinition.Likehightemperaturephenomenonorthinshocklayer phenomenon, it is the Mach number which forms the basis and at which we define that these phenomenonbecomesignificantatsoandsoMachnumberwhichfallinthehypersonic regime, but in thecase of low densityflow andfree molecule flow this is not thecase.It is basicallyKnudsennumberbasedphenomenonwhichbecomesimportantforanyMach numberdependingupontheKnudsennumber.Soathighaltitudewheremeanfreepathis very large, Knudsen number too will be large and hence a vehicle even if it travels at Mach number, not in the hypersonic range; low density flow methods treatment is necessary. Thats whythisphenomenonthough,notrelatedtohypersonicMachnumber,stillimportantto know because most of the hypersonic vehicles normally navigate in the space where Knudsen number is very large and more or less flow regime falls in the low density flow regime. Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 47 On concluding the low density regime part, up to the altitude of 90 km from Earths surface, whereKnudsennumberremainsbelow0.2,theapplicabilityofthecontinuumassumption remains intact. However applicability of no slip condition and no temperature jump condition becomesapplicableatthevalueaslowas0.3.Abovethisaltitudetill150kmfromthe surface of the Earth, density becomes very low and the assumption of no-slip condition starts losing its validity. So the transition range lies within 0.03 to 1.0 where continuum assumption can be applied with the no slip condition correction. Beyond this altitude of 150 km, density of the atmosphere become so low that a need for change in governing equations arise in this regime.Hencekinetictheoryofgasesfindsitsapplicationforhypersonicflightsatsuch altitudes. Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 48 Recapitulation So to recall and collect all the ideas we gathered about hypersonic regime, hypersonic flow is the flow where all or some of the specific phenomenon become more important as the Mach number is increased to high values. These phenomenon are (just repeating): 1.Thin shock layer 2.Entropy layer 3.Viscous interaction 4.High temperature flow 5.Low density flow. Thefigurebelowsummarisestheentirediscussionofhypersonicspecificphenomenonina nutshell. Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 49 Extra thinking One question may arise this time in mind that, are these specific phenomenon available only inthehypersonicregime?Arethesephenomenanotapplicabletotheotherflowregimes? The answer is, YES. These phenomena are actually available even for the low speed regime, buttheirsignificanceisnotthere.Likeweknowandunderstandthatflowbecomes compressible beyond the Mach 0.3. And gradually becomes more and more compressible and bythetimeitbecomeshighsubsonicregime,wedontcountviscositythatimportantand compressibilitybecomesmoreimportantforus.Thefactis,alltheseeffectsof compressibility and viscosity are available in every regime, but depending upon their relative significance, we treat them important or non-important in the various flow regimes. So at low speed the effect of viscosity is more important than compressibility and as the speed increases gradually,effectofcompressibilitykeepsbecomingmoreimportantandeffectofviscosity keeps becoming less and less important relatively. Similar kind of concept is applicable in the caseofhypersonicphenomenaalso.Theseeffectsareavailableinalmosteveryregimebut their significance is very low as their effect on the behaviour of flow is insignificant. So they areignoredinthenon-hypersonicflowregimes.Inthehypersonicregimes,these phenomenon become highly effective and their treatment becomes very much important. So they are not ignored here, rather they are used in the definition of hypersonic aerodynamics. Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 50 Hypersonic Studies Hypersonicaerodynamicscanbedividedinthreeblocksandtheycomprisethreedifferent sub regions of the fluid flow mechanics. These are: 1.Inviscid flow 2.Viscous flow 3.High temperature flow Inviscid Flow:In the inviscid flow we normally analyse the dynamiceffects of largeMach number and any viscous effect is intentionally ignored for the time being. In this section we analyse the flowat verylarge Mach number andtry to observe what happens when the free streamMachnumberbecomeslarge,andhowthisinfluencestheaerodynamictheoriesat highMachnumber.Inthis;compressibilityisthemaintopicwediscuss,andthen compressibleflowphenomenalikeshocksandexpansionfansbecomemoreimportant.The studiescanbefurtherdividedintolocalsurfaceinclinationmethodsandflowfield consideration method. The other typical aspects followed in each method is listed below: Localsurfaceinclinationmethod:Newtonianflowmethod,Tangentwedgemethod, Tangent cone method, shock-expansion method Flow-fieldconsideration:Machnumberindependence,hypersonicsimilarity,Blast-wave theory, Thin-shock layer theory, Method of characteristics, the blunt-body problem, Modern computational approach (Eulers equations). ViscousFlow:HerethecombinedeffectofthehighMachnumberandfiniteReynolds numberisexamined.Fluiddynamiceffectofhypersonicflowwithfrictionandthermal conduction is the main issue in this section, but high temperature effect is excluded here for the time being. This normally deals with the flow in the very vicinity of the solid surface and treatmentofboundarylayerinthehypersonicregimeisdoneinthissection.Thetechnical aspects which are dealt with in this regime are as follows: Basicaspects,Hypersonicboundarylayertheory,Hypersonictransition,Hypersonic turbulentboundarylayer,ReferenceenthalpyandotherApproximateengineeringmethods, Strongandweakviscousinteractions,Moderncomputationalmethod(Navier-Stock equations) Hightemperatureflow:Itthissection,importantaspectsofthehightemperaturegas dynamicsisdiscussed.Chemicallyreactinggasesinthevicinityofthevehiclearethemain Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 51 issue of discussion and also how they affect the viscous and inviscid flow are discussed. High temperaturenormallycausesdissociationandionizationofthefluidmolecules.Itcanalso cause the exothermic chemical reaction if some hydrocarbon are available in the near vicinity ofthefluidflow.Sobasicallyhightemperatureflowsectionstudiesachemicallyreacting flow which may containplasma instead of neutral gases. High temperatureflow finds many applicationinthefieldsadditionaltohypersonicflows,suchascombustionprocess, explosion, plasmas, high energy lasers etc. The basic aspects dealt with in this section are: BasicphysicalchemistryincludingStatisticalThermodynamicandKineticTheory, Chemicallyreactinginviscidflow(equilibriumandnon-equilibrium),Chemicallyreacting viscous flow (equilibrium and non-equilibrium), Catalytic wall effects, Shock layer radiation. Notes on Hypersonic Aerodynamics prepared by Asst. Professor MohammadPage 52 Summary Beforeweactuallygoaheadletussummarisethehypersonicphenomena;thephenomena whicharecharacteristicsofahypersonicregime.Thewholestorycanbetoldbytheflow chart given below.