工程流体力学第五章all.ppt

Chapter5BoundaryLayer(BL),5.1Introduction,*DAlembertParadox（佯谬，疑题）:Abodyimmersedinafrictionlessfluidhaszerodrag.,2,or,3,Boundarylayer(边界层),Althoughfrictionaleffectsinslightlyviscousfluidareindeedpresent,theyareconfinedtoathinlayernearthesurfaceofthebody,andtherestoftheflowcanbeconsideredinviscid.(1904,Prandtl),LudwigPrandtl(1875-1953),第三届国际数学大会德国海登堡（论粘性很小的流体运动）,5.2ThreeThicknessesofaBoundaryLayer,2.DisplacementThickness(排移厚度，位移厚度）,1.BLThicknessd(名义厚度，厚度）,Thelocusofpointswherethevelocityuparalleltotheplatereaches99percentoftheexternalvelocityU.,5,3.MomentumThickness（动量厚度，动量损失厚度）,5.3MomentumIntegralRelationforflat-plateBL(平板边界层动量积分方程),U=constp=const,Steady&incompressible,RTT,8,Momentumequation:,Continuity:,速度积分法求阻力,10,BC:,11,12,ThethicknessofaflatplateBL:,Estimate,13,Shapefactor(形状因子),14,Dragcoefficient(阻力系数),Homework:P7.9,P7.10,P7.11,Skin-frictioncoefficient(表面摩擦系数),15,5.4BoundaryLayerEquation,2-D,steady,incompressible,neglectbodyforceN-Sequations,16,EulerEquation,17,Reynoldsstress(雷诺应力),PrandtlsBoundaryLayerEquation,18,5.5BlasiusSolutionofLaminarFlat-PlateBoundaryLayer,Prandtlsstudent,similarity,Coordinatetransformation（坐标变换）,Blasius1908,19,Blasiusequation,（吴望一流体力学下册）,20,21,BlasiusSolutionvsExperimentalresults,22,5.6BoundaryLayersSeparation,s1,23,Criticalpoint,Favorablepressuregradient(顺压梯度),Adversepressuregradient(逆压梯度),Separation(分离),s2,s3,s4,s5,24,5.7TurbulentBoundaryLayer,Inertiaforce/viscousforce,Turbulence,25,26,ReynoldsEquation,Wecananalyzetheturbulentflowbyrecognizingthatthevelocitiesandpressurescompriseofanaverageandafluctuatingpart.ThisrepresentationwasfirstsuggestedbyReynoldsandiscalledthe“Reynoldsdecompositi