intermediate fluid mechanics [me563 course notes]

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1、ME563-IntermediateFluidDynamics-SuLecture0-VisualfluidsexamplesFluiddynamicsisauniquesubjectbecauseit'sveryvisual.ThebookAnAlbumofFluidMo-tion"byMiltonvanDyke(onreserveatWendt)isacollectionoffascinatingimagesfromfluidsexperiments.Youreallycan'tsayyouunderstandfluidsify

2、ouonlythinkofitintermsofequations.Figure1isatopviewofatriangularwingimmersedinaflowofwater(movingfromlefttorightintheimage).Coloredfluid,appearingaswhite,isintroducedneartheleadingedge.ThewingFigure1:Turbulenttransitioninflowoverawing.isinclinedata20angleofattack.Initi

3、allythefluidpatternisverysmooth,thenthelamentsofcoloredfluidmakeaveryabrupttransitiontoturbulence.Theabruptnessoftheturbulenttransitionisinterestingformanyreasons,notleastofwhichisthatit'snotreallypredictedbytheequationsoffluidflow.Anotherinterestingpropertyoffluidflowsis

4、thattheorganizationofitisverypersistent.TheupperleftphotoinFig.2isofthewakeofacircularcylinderinawaterflow.Thecylinderisattheleftedgeofthephoto.Themeanflowisveryslow{aboutonecylinderdiameterpersecond.TheReynoldsnumberis105.Thealternatingpatternofeddies(vortices)iscalle

5、dthe`Karmanvortexstreet.'Intuitively,itkindofmakessensethataslow,laminarflowwouldbeveryorganized.TheupperrightphotoinFig.2,showingthewakeofaplateata45angleofattack,istakenataflowthatis,relatively,about40timesfaster(Reynoldsnumber4300).Theflowinthiscaseisturbulent,but

6、evenso,thealternatingpatternofeddiesisvisibleabovetherandomness.Todrivehomethispointfurther,thelowerphotointhegureisofthewakeofatankerinclinedatroughly45tothemeancurrent.Thepatternassumedbytheoilslickisamazinglysimilartothatintheupperrightphoto,eventhoughtheReynold

7、snumberoftheshipwakeisontheorderof107.Thesheerrangeoflengthscalesthatareinterestinginfluidsproblemsisprettyremarkabletoo.Activeresearchrangesfromflowinmicrochannels(bloodflowincapillaries,forexample)allthewayuptocosmicsystemslikenebulae.TheupperleftphotoinFig.3showstheK

8、elvin-Helmholtzinstability,whicharisesintheinterfacebetweenflowstreamsatdierentvelocities.Thephotoshowsarectangulartube(the18inchru