Fluid Mechanics Fundamentals and Applications - ( Chapter ONE Introduction AND Basic Concepts) PDF
| Title | Fluid Mechanics Fundamentals and Applications - ( Chapter ONE Introduction AND Basic Concepts) |
|---|---|
| Author | Muhammad Sultan Shahid |
| Course | Fluid Mechanics |
| Institution | Edith Cowan University |
| Pages | 70 |
| File Size | 3.8 MB |
| File Type | |
| Total Downloads | 68 |
| Total Views | 155 |
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Download Fluid Mechanics Fundamentals and Applications - ( Chapter ONE Introduction AND Basic Concepts) PDF
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1
CHAPTER
1 INTRODUCTIONANDBASIC CONCEPTS OBJECTIVES Whenyoufinishreadingthischapter,youshouldbeableto Understandthebasicconceptsoffluidmechanics Recognizethevarioustypesoffluidflowproblemsencounteredinpractice Modelengineeringproblemsandsolvetheminasystematicmanner
Copyright © 2017. McGraw-Hill Higher Education. All rights reserved.
Have a working knowledge of accuracy, precision, and significant digits, and recognize the importanceofdimensionalhomogeneityinengineeringcalculations
Inthisintroductorychapter,wepresentthebasicconceptscommonlyusedintheanalysiso fluidflow.Westartthischapterwithadiscussionofthephasesofmatterandthenumerous waysofclassificationoffluidflow,suchasviscousversusinviscidregionsofflow,interna versusexternalflow,compressibleversusincompressibleflow,laminarversusturbulentflow natural versus forced flow, and steady versus unsteady flow. We also discuss the no-sli condition at solid–fluid interfaces and present a brief history of the development of fluid mechanics. Afterpresentingtheconceptsof systemand controlvolume, wereview the unit system that willbe used. We thendiscuss how mathematicalmodels for engineering problemsare preparedandhowtointerprettheresultsobtainedfromtheanalysisofsuchmodels.Thisi followedby apresentationofanintuitivesystematicproblem-solvingtechnique thatcan be usedasamodelinsolvingengineeringproblems.Finally,wediscussaccuracy,precision,and
Çengel, Yunus A., and John M. Cimbala. Fluid Mechanics : Fundamentals and Applications, McGraw-Hill Higher Education, 2017. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/ecu/detail.action?docID=5662567. Created from ecu on 2020-02-23 20:06:16.
significantdigitsinengineeringmeasurementsandcalculations.
SchlierenimageshowingthethermalplumeproducedbyProfessorCimbalaashewelcomesyoutothefascinatingworldof fluidmechanics. CourtesyofMichaelJ.HargatherandJohnCimbala.
Copyright © 2017. McGraw-Hill Higher Education. All rights reserved.
2
1–1 INTRODUCTION Mechanicsistheoldestphysicalsciencethatdealswithbothstationaryandmovingbodie undertheinfluenceofforces.Thebranchofmechanicsthatdealswithbodiesatrestiscalled statics,whilethebranchthatdealswithbodiesinmotionundertheactionofforcesiscalled dynamics. The subcategory fluid mechanics is defined as the science that deals with th behavioroffluidsatrest(fluidstatics)orinmotion(fluiddynamics),andtheinteractiono fluids with solids or other fluids at the boundaries. Fluid mechanics is also referred to a fluiddynamicsby consideringfluids atrestas aspecialcase ofmotionwith zerovelocity (Fig.1–1).
Çengel, Yunus A., and John M. Cimbala. Fluid Mechanics : Fundamentals and Applications, McGraw-Hill Higher Education, 2017. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/ecu/detail.action?docID=5662567. Created from ecu on 2020-02-23 20:06:16.
FIGURE1–1 Fluidmechanicsdealswithliquidsandgasesinmotionoratrest. ©Goodshoot/FotosearchRF
Fluidmechanicsitselfisalsodividedintoseveralcategories.Thestudyofthemotiono fluids that can be approximated as incompressible (such as liquids, especially water, an gases at low speeds) is usually referred to as hydrodynamics. A subcategory o hydrodynamicsishydraulics,whichdealswithliquidflowsinpipesandopenchannels.Ga dynamicsdealswiththeflowoffluidsthatundergosignificantdensitychanges,suchasthe flow of gases through nozzles at high speeds. The category aerodynamics deals with th flowofgases(especiallyair)overbodiessuchasaircraft,rockets,andautomobilesathigho low speeds. Some other specialized categories such as meteorology, oceanography, an hydrologydealwithnaturallyoccurringflows.
Copyright © 2017. McGraw-Hill Higher Education. All rights reserved.
WhatIsaFluid? Youwillrecallfromphysicsthatasubstanceexistsinthreeprimaryphases:solid,liquid,and gas. (At very high temperatures, it also exists as plasma.) A substance in the liquid or ga phaseisreferredtoasafluid.Distinctionbetweenasolidandafluidismadeonthebasiso thesubstance’sabilitytoresistanappliedshear(ortangential)stressthattendstochangeit shape. A solid can resist an applied shear stress by deforming, whereas a fluid deform continuouslyundertheinfluence ofa shearstress,no matterhow small.In solids,stressi proportional to strain, but in fluids, stress is proportional to strain rate. When a constan shearforceisapplied,asolideventuallystopsdeformingatsomefixedstrainangle,wherea afluidneverstopsdeformingandapproachesaconstantrateofstrain. Considerarectangularrubberblocktightlyplacedbetweentwoplates.Astheupperplat is pulled with a force F while the lower plate is held fixed, the rubber block deforms, a shown in Fig. 1–2. The angle of deformation α (called the shear strain or angula displacement) increases in proportion to the applied force F. Assuming there is no sli betweentherubberandtheplates,theuppersurfaceoftherubberisdisplacedbyanamoun equal to thedisplacement of the upper platewhile the lower surface remains stationary. In equilibrium,thenetforceactingontheupperplateinthehorizontaldirectionmustbezero
Çengel, Yunus A., and John M. Cimbala. Fluid Mechanics : Fundamentals and Applications, McGraw-Hill Higher Education, 2017. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/ecu/detail.action?docID=5662567. Created from ecu on 2020-02-23 20:06:16.
andthusaforceequalandoppositetoFmustbeactingontheplate.Thisopposingforcetha developsattheplate–rubberinterfaceduetofrictionisexpressedas F= τA,where τis th shearstressand Aisthecontactareabetweentheupperplateandtherubber.Whentheforc is removed, the rubber returns to its original position. This phenomenon would also b observed with other solids such as a steel block provided that the applied force does no exceedtheelasticrange.Ifthisexperimentwererepeatedwithafluid(withtwolargeparalle platesplacedinalarge 3
bodyofwater,forexample),thefluidlayerincontactwiththeupperplatewouldmovewith theplatecontinuouslyatthevelocityoftheplatenomatterhowsmalltheforce F.Thefluid velocitywoulddecreasewithdepthbecauseoffrictionbetweenfluidlayers,reachingzeroa thelowerplate.
Copyright © 2017. McGraw-Hill Higher Education. All rights reserved.
FIGURE1–2 Deformation ofa rubber blockplaced between two parallelplates under theinfluence of a shearforce.Theshearstressshownisthatontherubber—anequalbutoppositeshearstres actsontheupperplate. Youwillrecallfromstaticsthatstressisdefinedasforceperunitareaandisdetermined bydividingtheforcebytheareauponwhichitacts.Thenormalcomponentofaforceacting onasurfaceperunitareaiscalledthenormalstress,andthetangentialcomponentofaforc actingonasurfaceperunitareaiscalledshearstress(Fig.1–3).Inafluidatrest,thenorma stressiscalledpressure.Afluidatrestisatastateofzeroshearstress.Whenthewallsar removedoraliquidcontaineristilted,asheardevelopsastheliquidmovestore-establish horizontalfreesurface.
Çengel, Yunus A., and John M. Cimbala. Fluid Mechanics : Fundamentals and Applications, McGraw-Hill Higher Education, 2017. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/ecu/detail.action?docID=5662567. Created from ecu on 2020-02-23 20:06:16.
Copyright © 2017. McGraw-Hill Higher Education. All rights reserved.
FIGURE1–3 The normal stress and shear stress at the surface of a fluid element. For fluids at rest, th shearstressiszeroandpressureistheonlynormalstress. Inaliquid,groupsofmoleculescanmoverelativetoeachother,butthevolumeremain relativelyconstantbecauseofthestrongcohesiveforcesbetweenthemolecules.Asaresult a liquid takes the shape of the container it is in, and it forms a free surface in a large container in a gravitationalfield. A gas, on the other hand, expands until it encounters the wallsofthecontainerandfillstheentireavailablespace.Thisisbecausethegasmolecule arewidelyspaced,andthecohesiveforcesbetweenthemareverysmall.Unlikeliquids,aga inanopencontainercannotformafreesurface(Fig.1–4). Althoughsolidsandfluidsareeasilydistinguishedinmostcases,thisdistinctionisnotso clearinsomeborderlinecases.Forexample, asphaltappearsandbehavesasasolidsincei resists shear stress for short periods of time. When these forces are exerted over extended periods of time, however, the asphalt deforms slowly, behaving as a fluid. Some plastics lead, and slurry mixtures exhibit similar behavior. Such borderline cases are beyond th scopeofthistext.Thefluidswedealwithinthistextwillbeclearlyrecognizableasfluids.
Çengel, Yunus A., and John M. Cimbala. Fluid Mechanics : Fundamentals and Applications, McGraw-Hill Higher Education, 2017. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/ecu/detail.action?docID=5662567. Created from ecu on 2020-02-23 20:06:16.
FIGURE1–4 Unlikealiquid,agasdoesnotformafreesurface,anditexpandstofilltheentireavailabl space. Intermolecular bonds are strongest in solids and weakest in gases. One reason is tha molecules in solids are closely packed together, whereas in gases they are separated b relativelylargedistances(Fig.1–5).Themoleculesinasolidarearrangedinapatternthati repeated throughout. Because of the small distances between molecules in a solid, th attractive forces of molecules on each other are large and keep the molecules at fixe positions.ThemolecularspacingintheliquidphaseisnotmuchdifferentfromthatofFree surface
Copyright © 2017. McGraw-Hill Higher Education. All rights reserved.
FIGURE1–5 Thearrangementofatomsindifferentphases:(a)moleculesareatrelativelyfixedpositions in a solid, (b) groups of molecules move about each other in the liquid phase, and (c individualmoleculesmoveaboutatrandominthegasphase. 4
thesolidphase,exceptthemoleculesare nolongerat fixedpositionsrelativeto eachothe and they can rotate and translate freely. In a liquid, the intermolecular forces are weake relative to solids, but still strong compared with gases. The distances between molecule generallyincreaseslightlyasasolidturnsliquid,withwaterbeinganotableexception. Inthegasphase,themoleculesarefarapartfromeachother,andmolecularorderingi nonexistent.Gasmoleculesmoveaboutatrandom,continuallycollidingwitheachotherand the walls of the container in which they are confined. Particularly at low densities, th intermolecularforcesareverysmall,andcollisionsaretheonlymodeofinteractionbetween themolecules.Moleculesinthegasphaseareataconsiderablyhigherenergylevelthanthey areintheliquidorsolidphase.Therefore,thegasmustreleasealargeamountofitsenergy beforeitcancondenseorfreeze. Gasand vaporareoftenusedassynonymouswords.Thevaporphaseofasubstancei customarilycalleda gaswhenitisabovethecriticaltemperature. Vaporusuallyimpliestha
Çengel, Yunus A., and John M. Cimbala. Fluid Mechanics : Fundamentals and Applications, McGraw-Hill Higher Education, 2017. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/ecu/detail.action?docID=5662567. Created from ecu on 2020-02-23 20:06:16.
Copyright © 2017. McGraw-Hill Higher Education. All rights reserved.
thecurrentphaseisnotfarfromastateofcondensation. Anypracticalfluidsystemconsistsofalargenumberofmolecules,andthepropertieso thesystemnaturallydependonthebehaviorofthesemolecules.Forexample,thepressureo agasinacontaineristheresultofmomentumtransferbetweenthemoleculesandthewall ofthecontainer.However,onedoesnotneedtoknowthebehaviorofthegasmoleculesto determine the pressure in the container. It is sufficient to attach a pressure gage to th container(Fig.1–6).Thismacroscopicorclassicalapproachdoesnotrequireaknowledgeo the behavior of individual molecules and provides a direct and easy way to analyz engineeringproblems.Themoreelaboratemicroscopicor statisticalapproach,basedonthe averagebehavior of large groups ofindividual molecules,is ratherinvolved and isused in thistextonlyinasupportingrole.
FIGURE1–6 On a microscopic scale, pressure is determined by the interaction of individual ga molecules. However, we can measure thepressure on a macroscopic scale with a pressur gage.
ApplicationAreasofFluidMechanics It isimportant to developa good understanding ofthe basic principlesof fluid mechanics sincefluidmechanicsiswidelyusedbothineverydayactivitiesandinthedesignofmodern engineering systems from vacuum cleaners to supersonic aircraft. For example, fluid mechanicsplaysavitalroleinthehumanbody.Theheartisconstantlypumpingbloodtoal partsofthehumanbodythroughthearteriesandveins,andthelungsarethesitesofairflow in alternating directions. All artificial hearts, breathing machines, and dialysis systems ar designedusingfluiddynamics(Fig.1–7).
Çengel, Yunus A., and John M. Cimbala. Fluid Mechanics : Fundamentals and Applications, McGraw-Hill Higher Education, 2017. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/ecu/detail.action?docID=5662567. Created from ecu on 2020-02-23 20:06:16.
FIGURE1–7 Fluiddynamicsisusedextensivelyinthedesignofartificialhearts.ShownhereisthePenn StateElectricTotalArtificialHeart.
Copyright © 2017. McGraw-Hill Higher Education. All rights reserved.
CourtesyoftheBiomedicalPhotographyLab,PennStateBiomedicalEngineeringInstitute.Usedbypermission.
Anordinaryhouseis,insomerespects,anexhibitionhallfilledwithapplicationsoffluid mechanics. The piping systems for water, natural gas, and sewage for an individual hous andtheentirecityaredesignedprimarilyonthebasisoffluidmechanics.Thesameisalso true for the piping and ducting network of heating and air-conditioning systems. A refrigeratorinvolvestubesthroughwhichtherefrigerantflows,acompressorthatpressurizes therefrigerant,andtwoheatexchangerswheretherefrigerantabsorbsandrejectsheat.Fluid mechanics playsa major rolein the designof all thesecomponents. Even the operationo ordinaryfaucetsisbasedonfluidmechanics. We can also see numerous applications of fluid mechanics in an automobile. Al componentsassociatedwiththetransportationofthefuelfromthefueltanktothecylinder —thefuelline,fuelpump,andfuelinjectorsor 5
carburetors—aswellasthemixingofthefuelandtheairinthecylindersandthepurgingo combustiongasesinexhaustpipes—areanalyzedusingfluidmechanics.Fluidmechanicsi alsousedinthedesignoftheheatingandair-conditioningsystem,thehydraulicbrakes,the powersteering,theautomatictransmission,thelubricationsystems,thecoolingsystemofth engine block including the radiator and the water pump, and even the tires. The slee streamlined shape of recent model cars is the result of efforts to minimize drag by usin extensiveanalysisofflowoversurfaces.
Çengel, Yunus A., and John M. Cimbala. Fluid Mechanics : Fundamentals and Applications, McGraw-Hill Higher Education, 2017. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/ecu/detail.action?docID=5662567. Created from ecu on 2020-02-23 20:06:16.
Copyright © 2017. McGraw-Hill Higher Education. All rights reserved.
On a broader scale, fluid mechanics plays a major part in the design and analysis o aircraft, boats,submarines, rockets, jet engines, windturbines, biomedical devices, cooling systemsforelectronic components,andtransportation systemsfor movingwater,crude oil andnaturalgas.Itisalsoconsideredinthedesignofbuildings,bridges,andevenbillboard tomake surethat thestructures canwithstand windloading.Numerous naturalphenomena suchastheraincycle,weatherpatterns,theriseofgroundwatertothetopsoftrees,winds oceanwaves,andcurrentsinlargewaterbodiesarealsogovernedbytheprinciplesoffluid mechanics(Fig.1–8).
FIGURE1–8 Someapplicationareasoffluidmechanics. 6
1–2 ABRIEFHISTORYOFFLUIDMECHANICS1 One of the first engineering problems humankind faced as cities were developed was th Çengel, Yunus A., and John M. Cimbala. Fluid Mechanics : Fundamentals and Applications, McGraw-Hill Higher Education, 2017. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/ecu/detail.action?docID=5662567. Created from ecu on 2020-02-23 20:06:16.
Copyright © 2017. McGraw-Hill Higher Education. All rights reserved.
supplyofwaterfordomesticuseandirrigationofcrops.Oururbanlifestylescanberetained onlywithabundantwater,anditisclearfromarcheologythateverysuccessfulcivilizationo prehistory invested in the construction and maintenance of water systems. The Roma aqueducts,someofwhicharestillinuse,arethebestknownexamples.However,perhapsth mostimpressiveengineeringfromatechnicalviewpointwasdoneattheHellenisticcityo Pergamoninpresent-dayTurkey.There,from283to133bc,theybuiltaseriesofpressurized leadandclaypipelines(Fig.1–9),upto45kmlongthatoperatedatpressuresexceeding1. MPa(180mofhead).Unfortunately,thenamesofalmostalltheseearlybuildersarelostto history.
FIGURE1–9 SegmentofPergamonpipeline.Eachclaypipesectionwas13to18cmindiameter. CourtesyofGuntherGarbrecht.Usedbypermission.
The earliest recognized contribution to fluid mechanics theory was made by the Greek mathematicianArchimedes(285–212bc).Heformulatedandappliedthebuoyancyprinciple inhistory’sfirstnondestructivetesttodeterminethegoldcontentofthecrownofKingHiero II.TheRomansbuiltgreataqueductsandeducatedmanyconqueredpeopleonthebenefitso clean water, but overallhad a poor understandingof fluids theory. (Perhaps they shouldn’ havekilledArchimedeswhentheysackedSyracuse.) DuringtheMiddleAges,theapplicationoffluidmachineryslowlybutsteadilyexpanded Elegantpistonpumpsweredevelopedfordewateringmines,andthewatermillandwindmil wereperfectedtogrindgrain,forgemetal,andforothertasks.Forthefirsttimeinrecorded humanhistory,significantworkwasbeingdonewithoutthepowerofamusclesuppliedbya person or animal, and these inventions are generally credited with enabling the late
Çengel, Yunus A., and John M. Cimbala. Fluid Mechanics : Fundamentals and Applications, McGraw-Hill Higher Education,...
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