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Vehicle aerodynamics – effects of side winds

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Mechanical Engineering

Publié dans : Ingénierie

Vehicle aerodynamics – effects of side winds

  1. 1. Vehicle Aerodynamics – Effects of side winds, Vehicle exposed to cross wind
  2. 2. Vehicle Body Aerodynamics?  Is a branch of physics and is concerned when air flows around a body  Deals with a solid body moving through atmosphere and the interaction which takes place  Depends on varying wind speeds and wind direction  The most important factor is reducing fuel economy in aerodynamics
  3. 3. Importance of aerodynamic study? • Drag force is reduced. So maximum speed and acceleration are obtained for the same power output. • Fuel consumption of the vehicle can be reduced to the maximum (about 35% of fuel cost). • Gives better appearance and styling. • By reducing the various forces and moments, good stability and safety can be achieved. • Helps to provide proper ventilation system. • Aerodynamic noise could be reduced which results in quite running of the vehicle.
  4. 4. Features incorporated to aerodynamics? • Large corner radii on the front section • Low level front skirts • Fairings above and behind the driver’s cab • Aerodynamically shaped mirrors • Enclosed steps
  5. 5. Aerodynamic Drag Aerodynamic drag is usually insignificant at low vehicle speed but the magnitude of air resistance becomes considerable with rising speed.
  6. 6. Aerodynamic Drag Drag force depends on the following factors: • The size and shape of the vehicle (area of nose end, vehicle super structures) • Travel speed • Air density • Wind direction and strength
  7. 7. FL = ½ Cd Aρv2 FL : Drag Force Cd : Vehicle’s drag co-efficient A : Area of vehicle’s nose end ρ : Density of air v : Velocity of the vehicle PL = FL v = ½ Cd Aρv3 PL = Power to overcome drag
  8. 8. Types of Drag • Pressure drag (or) Form drag (or) Profile drag - 57% • Induced drag - 8% • Friction drag - 10% • Inference drag - 15% • Cooling and ventilation system drag – 10%
  9. 9. Pressure Drag
  10. 10. Typical static pressure coefficient distribution
  11. 11. Induced Drag (or) Lift Drag • This lift force depends on the contour of the body • Normal Speed - Not a serious problem • High Speed – Serious problem • Lift affects stability and braking performance • To reduce the accelerated flow upper side • To reduce the deceleration flow under side will reduce the aerodynamics lift
  12. 12. Friction Drag (or) surface drag (or) Skin friction • This is caused by friction force between the boundary layer and the body surface • Shear stress generated in the boundary layer • Laminar boundary layer should be maintained • Well – Polished surface is not only attractive but also makes the vehicle more economical • Body smoothness is of the order of 0.5 to 1.0 microns.
  13. 13. Interference drag • The flow over many exterior components interact with the flow over basic body shape and this leads to drag • Exterior components includes door handles, mirrors, aerials and badges which project out from normal surface • Mechanical components Engine parts, suspension system, exhaust system, frame rail • Exterior ornaments must be placed where the velocity is minimum • Door closer must be placed in a close proximity and longitudinally in line with each other
  14. 14. Internal Drag (or) Cooling and ventilation system Drag • Arising from cooling of the engine • Brakes, cabin ventilation flows • Contributes 10% of the overall drag
  15. 15. Drag Co-efficient The aerodynamic drag coefficient (Cd) is a measure of the effectiveness of a streamline aerodynamic body shape in reducing the air resistance to the forward motion of the vehicle.
  16. 16. Aerodynamic forces • Force of air drag in the direction of motion with wind angle along longitudinal axis (Px) • Cross wind force (Py) • Aerodynamic lift (Pz)
  17. 17. Longitudinal air drag (Px)  The longitudinal component of the resultant of pressure distribution  Magnitude is represented by Px = (Cx p A V2 ) / 2
  18. 18. Cross wind air drag (Py)  Its formed by asymmetric flow of air around the vehicle body when the wind angle is not equal to zero  Magnitude is represented by Py= (Cy p A V2 ) / 2
  19. 19. Aerodynamic lift (Pz)  It’s the vertical component of the resultant of the pressure distribution over the vehicle body due to flow of air around it  Magnitude is represented by Pz = (Cz p A V2 ) / 2
  20. 20. Effect of cross wind
  21. 21. Aerodynamic moments • Rolling moment - Mx • Pitching moment – My • Yawing moment - Mz
  22. 22. Rolling moment - Mx  This moment caused by the cross wind force Py about the longitudinal axis  Magnitude is given by Mx = Py a = Cmx p A L V2 / 2  This effect is dangerous for tall van, where side force acts much above the C.G  The only solution is increase the wheel track
  23. 23. Pitching moment - My  This moment caused about y-axis by cross wind force Py or the longitudinal force Px  Magnitude is given by My = Pz b = Cmy p A L V2 / 2
  24. 24. Yawing moment - Mz  This moment caused about z axis by cross wind force Py  Magnitude is given by Mz = Py c = Cmz p A L V2 / 2
  25. 25. DRIVER’S SEAT  Important factors in relaxed, stress free driving is ergonomically optimized seating for the driver and co- driver  Seating position and convenient adjustment  Fundamental importance too that all the control elements easy to reach
  26. 26. Geometrical relations to driver’s seat  Seat position in relation to the steering wheel, foot controls and other secondary controls
  27. 27. Geometrical relations to driver’s seat
  28. 28. Geometrical relations to driver’s seat  The driver’s seat should be adjustable 45mm horizontally and 30mm vertically  Avoid slanted body position will cause abnormal tension in the pelvis muscle leading to tiredness  The back rest must support the trunk in a vertical position and extend in vertical up to spine . The improper seating leads to breathing problems  A good back rest relieves the neck and shoulders  The angle between seat and back rest less than the passenger seat
  29. 29. Geometrical relations to driver’s seat  avoid The seat being tilted too much Insufficient room for the legs An unsuitable angle between the seat and back rest  The seat cushion should be fairly rigid and must having sufficient shock absorbing capacity