流体力学2-流体静力学.ppt

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1、02PressureandFluidStatics02-2PressurePressurePressureisdefinedasanormalforceexertedbyafluidperunitarea.DimensionofPressure:{FL2}={ML1T2}Dimension&UnitsSIUnit:N/m2=Pa(Pascal).BGUnit:lbf/ft2=psf(notcommon).Otherunits:bar,atm,kgf/cm2,lbf/in2=psiSinceth

2、eunitPaistoosmallforpressuresencounteredinpractice,kilopascal(1kPa=103Pa)andmegapascal(1MPa=106Pa)arecommonlyused.AtmosphericpressureSincetheseslidesarepreparedusingseveralbooks,wewillinterchangeablyusepatmorpaastheatmosphericpressure.1atm=101,325Pa=14.

3、696psi02-3Absolute,Gage,andVacuumPressuresAbsolutepressureisusedinthiscourseunlessotherwisestated.AbsoluteActualpressureatagivepointiscalledtheabsolutepressure.GagePressureVacuumPressureMostpressure-measuringdevicesarecalibratedtoreadzerointheatmospher

4、e,andthereforeindicategagepressure,pgage=pabspatmPressurebelowatmosphericpressureiscalledvacuumpressurepvac=patmpabs02-4DAsinqDAcosqpnDAWpxDAsinqpzDAcosqqqrg(DAcosqDz)/2DzConsidertheequilibriumofanarbitrarywedgeelement.PressureataPointinaStaticFluidFl

5、uidatrest(nomotion)–cannotsupportshearstress(onlynormalstresses)Normalstressonanyplaneispressure(+forcompression)NopressurechangeinthehorizontaldirectionVerticalpressurechangeproportionaltodensity,gravityanddepthchange.Forapoint,Dz0,Pressureatanypointi

6、nafluidisthesameinalldirections.Ascalarvariable.02-5Ignoringtheviscousforce,considertheequilibriuminthezdirectionSFz=maz:Engineeringapplicationrequiresthedistributionofpressureinafluidflow.EquationofMotionofaFluidElementAssumethatthefluidisundergoingana

7、cceleration:a=axi+ayj+azkzxydxdydzW=rg(dxdydz)p+dppWdxdydzForageneral3Dproblem,OrinavectorformConsideraninfinitesimalfluidelementasshown.02-6HydrostaticPressureDistributionPressureinacontinuouslydistributeduniformstaticfluidvariesonlywithverticaldistanc

8、eandisindependentoftheshapeofthecontainer.Thepressureisthesameatallpointsonagivenhorizontalplaneinthefluid.Thepressureincreaseswiththedepthinthefluid.EquationofMotionForstaticproblems,ax=ay=az=0StaticProblemsHydrostaticPressureEq