1、单车空气动力学aerodynamicsofcycling 本课件本课件PPT仅供大家学习使用仅供大家学习使用 学习完请自行删除，谢谢！学习完请自行删除，谢谢！本课件本课件PPT仅供大家学习使用仅供大家学习使用 学习完请自行删除，谢谢！学习完请自行删除，谢谢！本课件本课件PPT仅供大家学习使用仅供大家学习使用 学习完请自行删除，谢谢！学习完请自行删除，谢谢！本课件本课件PPT仅供大家学习使用仅供大家学习使用 学习完请自行删除，谢谢！学习完请自行删除，谢谢！OutlineIntroductionBasic Fluid MechanicsDrag and FrictionBicycle Aerody
2、namicsPositionVelocity&Power OutputReducing DragDraftingCrosswind effectsIntroductionAerodynamics,or wind resistance is an everyday experience to bicyclists.At average speeds aerodynamic drag is the largest resistive force aside from the gravity of a large hillDue to the fluidity of air.Composed of
3、normal(Pressure)force and tangential(frictional)force.Extremely geometry dependent.Fluid Mechanics&DynamicsFluid a material that deforms continuously and permanently under the application of a shearing stress.Important propertiesDensity()Specific weight()Specific Gravity(SG)Viscosity()Shear ForceVel
4、ocity GradientVelocity of fluid is zero along surface due to friction.(No slip condition)Fluid Mechanics&DynamicsStreamlineLines tangent to the velocity vector throughout the flow fieldFigure from Fundamentals of Fluid Mechanics pg.97Fluid Mechanics&DynamicsStagnation PointLargest pressure obtainabl
5、e along a streamlineVelocity is zeroFigure from Fundamentals of Fluid Mechanics pg.108Fluid Mechanics&DynamicsAir as a fluidWhen studying aerodynamics air is treated as a fluid.Follows all laws of motion and all laws of fluid mechanicsF=mConservation of EnergyConservation of MassFluid Mechanics&Dyna
6、micsContinuity equationMass is conservedV1A1=V2A2Bernoulli equationP1+1/2 V12+z1=P2+1/2 V22+z2Relationship between Pressure,Velocity,and ElevationBased on conservation of linear momentum(Kinetic Energy)A1A2V1V2AerodynamicsTwo effective forcesPressureFrictionFor cyclists,pressure effect is much large
7、r than friction due to non-streamlined body.Streamlined bodies incorporate gradual tapering to minimize pressure effect and separation of fluid(a)Normal pressure and friction forces(b)Attached and separated flow around a cylinder(c)Attached flow and pressure recovery along a streamlined bodyFigure f
8、rom Bicycle Science pg.174AerodynamicsDrag CoefficientCD=drag/(area x dynamic pressure)Dynamic Pressure can be approximated for speeds under 100 mi/h as:Dynamic pressure=V2/2gcgc=32.174 lbm-ft/lbf-s2DragThe force in the direction of relative flow.Propulsion power to overcome drag:=drag force x relat
9、ive vehicle velocityAerodynamicsDrag coefficients of various geometriesFigure from Bicycling Science pg.191AerodynamicsLaminar FlowLayers of fluid flow slide smoothly over one anotherTurbulent FlowBoundary layer is composed of vortices that increase surface friction.Common at rear end of non-streaml
10、ined vehicleTurbulentLaminarBicycle AerodynamicsBicycle is responsible for 20-35%of drag.Loose Clothing increases drag by up to 30%.Bicycle Aerodynamics-PositionPositionsGoals:reduce frontal area&reduce drag coefficientCDFrontal AreaCDAPower to Overcome DragTops1.15.55 m2.632 m2345 WHoods1.0.40 m2.4
11、0 m2220 WDrops.88.36 m2.32 m2176 WBicycle Aerodynamics-PositionDrag CoefficientsFigure from Bicycling Science pg.188Bicycle Aerodynamics-PositionRearward vs.Forward position(23.57 22.28 N drag)Forward seat position decreases drag at the expense of comfort and pedaling mechanics.Union Cycliste Intern
12、ationale limits the fore-aft position of the saddle requiring it be at least 5 cm behind the bottom bracket spindleInjury preventive measureImage and caption from Road Cycling HandbookBicycle AerodynamicsFairingsReduce Drag Coefficient up to 50%:/lightningbikes /sf40blu.jpgImage from Bicycling Scien
13、ce pg.191Bicycle Aerodynamics-DraftingDrafting :/pro.corbis /images/AX933548.jpg?size=67&uid=51D3B79C-B5D0-4A72-B318-B002D5C78EBCTraveling close behind another riderBroken up air vortices propel second riderOffers advantage to both front and rear riderRiders in group expend 40%less energy than solo
14、ridersBicycle Aerodynamics-DraftingDraftingNegative drag propels object forward at close distances(1 diameter and under)Image from Bicycling Science pg.199Bicycle Aerodynamics-DraftingDraftingNo advantageto side by sidedrafting.Aerodynamic interference of two side by side struts.Image from Bicycling
15、 Science pg.201Higher CD occurs at distances less than the of the width of the strut(or rider)Bicycle Aerodynamics-CrosswindsAerodynamic drag is usually calculated assuming calm weatherCrosswinds create aerodynamic moments and instability.CP(point of action of aerodynamic forces)should be behind the
16、 CG for maximum stability.FcrosswindFcrosswindCGCGCPCPstableinstableReferencesEfluids bicycle aerodynamics.EFluids.04 Apr.09.Gregor,Robert J.Road Cycling-Handbook of Sports Medicine and Science.Malden:Oxford,2000.Munson,Bruce R.,Donald F.Young,and Theodore H.Okiishi.Fundamentals of Fluid Mechanics.5th ed.Jon Wiley&Sons,2006.Tamai,Goro.The Leading Edge-Aerodynamic Design of Ultra-streamlined Land Vehicles.Cambridge:Robert Bentley Publihsers,1999.Wilson,David G.,and Jim Papadopoulos.Bicycling Science.3rd ed.MIT P,2004.
