单车空气动力学aerodynamicsofcycling.ppt

Aerodynamics of Cycling,Stephen Kulju,http:/www.mira.co.uk/Services/images/bike.jpg,Outline,Introduction Basic Fluid Mechanics Drag and Friction Bicycle Aerodynamics Position Velocity & Power Output Reducing Drag Drafting Crosswind effects,Introduction,Aerodynamics, 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 hill Due to the fluidity of air. Composed of normal (Pressure) force and tangential (frictional) force. Extremely geometry dependent.,Fluid Mechanics & Dynamics,Fluid a material that deforms continuously and permanently under the application of a shearing stress. Important properties Density () Specific weight () Specific Gravity (SG) Viscosity (),Shear Force,Velocity Gradient,Velocity of fluid is zero along surface due to friction . (No slip condition),Fluid Mechanics & Dynamics,Streamline Lines tangent to the velocity vector throughout the flow field,Figure from Fundamentals of Fluid Mechanics pg. 97,Fluid Mechanics & Dynamics,Stagnation Point Largest pressure obtainable along a streamline Velocity is zero,Figure from Fundamentals of Fluid Mechanics pg. 108,Fluid Mechanics & Dynamics,Air as a fluid When studying aerodynamics air is treated as a fluid. Follows all laws of motion and all laws of fluid mechanics,http:/pico1.e.ft.fontys.nl/aot/newton.jpg,F = m Conservation of Energy Conservation of Mass,Fluid Mechanics & Dynamics,Continuity equation Mass is conserved V1A1=V2A2 Bernoulli equation P1+1/2 V12 +z1 = P2+1/2 V22 +z2 Relationship between Pressure, Velocity, and Elevation Based on conservation of linear momentum (Kinetic Energy),A1,A2,V1,V2,Aerodynamics,Two effective forces Pressure Friction For cyclists, pressure effect is much larger 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 body,Figure from Bicycle Science pg. 174,Aerodynamics,Drag Coefficient CD = drag/(area x dynamic pressure) Dynamic Pressure can be approximated for speeds under 100 mi/h as: Dynamic pressure = V2/2gc gc = 32.174 lbm-ft/lbf-s2 Drag The force in the direction of relative flow. Propulsion power to overcome drag: = drag force x relative vehicle velocity,Aerodynamics,Drag coefficients of various geometries,Figure from Bicycling Science pg. 191,Aerodynamics,Laminar Flow Layers of fluid flow slide smoothly over one another Turbulent Flow Boundary layer is composed of vortices that increase surface friction. Common at rear end of non-streamlined vehicle,Turbulent,Laminar,http:/www.cheng.cam.ac.uk/research/groups/electrochem/JAVA/electrochemistry/ELEC/l2fig/laminar.gif,Bicycle Aerodynamics,Bicycle is responsible for 20-35% of drag. Loose Clothing increases drag by up to 30%.,Bicycle Aerodynamics - Position,Positions Goals: reduce frontal area & reduce drag coefficient,Bicycle Aerodynamics - Position,Drag Coefficients,Figure from Bicycling Science pg. 188,Bicycle Aerodynamics - Position,Rearward vs. Forward position (23.57 22.28 N drag),Forward seat position decreases drag at the expense of comfort and pedaling mechanics. Union Cycliste Internationale limits the fore-aft position of the saddle requiring it be at least 5 cm behind the bottom bracket spindle Injury preventive measure,Image and caption from Road Cycling Handbook,Bicycle Aerodynamics,Fairings Reduce Drag Coefficient up to 50 %,/sf40blu.jpg,Image from Bicycling Science pg. 191,Bicycle Aerodynamics - Drafting,Drafting,/images/AX933548.jpg?size=67&uid=51D3B79C-B5D0-4A72-B318-B002D5C78EBC,Traveling close behind another rider Broken up air vortices propel second rider Offers advantage to both front and rear rider Riders in group expend 40% less energy than solo riders,Bicycle Aerodynamics - Drafting,Drafting,Negative drag propels object forward at close distances( 1 diameter and under),Image from Bicycling Science pg. 199,Bicycle Aerodynamics - Drafting,Drafting No advantage to side by side drafting.,Aerodynamic interference of two side by side struts. Image from Bicycling Science pg. 201,Higher CD occurs at distances less than the of the width of the strut (or rider),Bicycle Aerodynamics -Crosswinds,Aerodynamic drag is usually calculated assuming calm weather Crosswinds create aerodynamic moments and instability. CP (point of action of aerodynamic forces) should be behind the CG for maximum stability.,Fcrosswind,Fcrosswind,CG,CG,CP,CP,stable,instable,References,“Efluids 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 Fl