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1、7708d_c09_5327/24/018:10AMPage532Impulsivestartofflowpastanarrayofcylinders:Thecomplexstructureoflaminarflowpastarelativelysimplegeometricstructureillustrateswhyitisoftendifficulttoobtainexactanalyticalresultsforexternalflows.1Dyeinwater.21PhotographcourtesyofONER
2、A,France.27708d_c09_5337/24/018:11AMPage5339FlowOverImmersedBodiesInthischapterweconsidervariousaspectsoftheflowoverbodiesthatareimmersedinafluid.Examplesincludetheflowofairaroundairplanes,automobiles,andfallingsnowflakes,ortheflowofwateraroundsubmarinesandfish.In
3、thesesituationstheobjectiscompletelysurroundedbythefluidandtheflowsaretermedexternalflows.Externalflowsinvolvingairareoftentermedaerodynamicsinresponsetotheimpor-tantexternalflowsproducedwhenanobjectsuchasanairplanefliesthroughtheatmosphere.Althoughthisfieldofexte
4、rnalflowsisextremelyimportant,therearemanyotherexamplesthatareofequalimportance.Thefluidforce1liftanddrag2onsurfacevehicles1cars,trucks,bicycles2hasbecomeaveryimportanttopic.Bycorrectlydesigningcarsandtrucks,ithasbecomepossibletogreatlydecreasethefuelconsumptionan
5、dimprovethehandlingcharac-teristicsofthevehicle.Similareffortshaveresultedinimprovedships,whethertheyaresur-facevessels1surroundedbytwofluids,airandwater2orsubmersiblevessels1surroundedcompletelybywater2.Manypracticalsit-Otherapplicationsofexternalflowsinvolveobje
6、ctsthatarenotcompletelysurroundeduationsinvolveflowbyfluid,althoughtheyareplacedinsomeexternal-typeflow.Forexample,theproperde-pastobjects.signofabuilding1whetheritisyourhouseoratallskyscraper2mustincludeconsiderationofthevariouswindeffectsinvolved.Aswithotherarea
7、soffluidmechanics,twoapproaches1theoreticalandexperimental2areusedtoobtaininformationonthefluidforcesdevelopedbyexternalflows.Theoretical1i.e.,analyticalandnumerical2techniquescanprovidemuchoftheneededinformationaboutsuchflows.However,becauseofthecomplexitiesofthe
8、governingequationsandthecom-plexitiesofthegeometryoftheobjectsinvolved,theamountofinformationobtainedfrompurelytheoreticalmethodsislimited.Withcurrentan