Vehicle dynamics - Chapter 2 (Road Loads)


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Vehicle dynamics - Chapter 2 (Road Loads)

  2. 2. 2.1 VEHICLE AERODYNAMICS  The study of the aerodynamics of road vehicles.  Its main goals are reducing drag and wind noise, minimizing noise emission, and preventing undesired lift forces.  Its also important to produce down-force to improve traction and thus cornering abilities.  An aerodynamic automobile will integrate the wheel arcs and lights into the overall shape to reduce drag.
  3. 3. 2.2 MECHANICS OF AIR FLOW AROUND A VEHICLE  In fluid mechanical terms, road vehicles are bluff bodies in very close proximity to the ground.  Internal and recessed cavities which communicate freely with the external flow and rotating wheels add to their geometrical and fluid mechanical complexity.  The objectives of improvement of flow past vehicle bodies are to reduce of fuel consumption and improve the driving characteristics.
  4. 4.  Vehicle aerodynamics includes three interacting flow fields which are: a. flow past vehicle body b. flow past vehicle components (wheels, heat exchanger, brakes or windshield) c. flow in passenger compartment.
  5. 5. 2.3 PRESSURE DISTRIBUTION ON A VEHICLE  The pressure distribution over the body surface exerts normal forces which, summed and projected into the freestream direction.  Combination of shock wave effects, vortex system generation effects and wake viscous mechanisms  When the viscosity effect over the pressure distribution is considered separately, the remaining drag force is called pressure (or form) drag.  In the absence of viscosity, the pressure forces on the vehicle cancel each other and, hence, the drag is zero.
  6. 6. 2.4 AERODYNAMIC FORCES  The force exerted on a body whenever there is a relative velocity between the body and the air.  There are only two basic sources of aerodynamic force: a. the pressure distribution b. the frictional shear stress distribution exerted by the airflow on the body surface.  The distribution of pressure and shear stress represent a distributed load over the surface.
  7. 7. 2.5 DRAG COMPONENTS  Drag refers to forces acting opposite to the relative motion of any substance moving in a fluid.  Lift acts perpendicular to the motion.  Drag sources - the skin friction between the molecules of the air and the solid surface of the aircraft.  The skin friction is an interaction between a solid and a gas, so the magnitude of the skin friction depends on properties of both solid and gas.
  8. 8.  The additional drag components caused by the generation of lift is induced drag.  Occurs because the flow near the wing tips is distorted span wise as a result of the pressure difference from the top to the bottom of the wing.  Called induced drag because it has been "induced" by the action of the tip vortices.  It is also called "drag due to lift" because it only occurs on finite, lifting wings.
  10. 10. 2.6.1 BUMPER SPOILER  A motor vehicle bumper which comprises a shield and a spoiler hinged between two positions of stable equilibrium  Namely a high position in which at least a part of the spoiler projects from the shield and,  a low position in which the spoiler extends the shield downwards.  The spoiler is suitable for adopting a third position of stable equilibrium, in which it is completely retracted behind the shield.
  11. 11. 2.6.2 AIR DAMS  A front spoiler (air dam) is positioned under or integrated with the front bumper.  In racing, this spoiler is used to control the dynamics of handling related to the air in front of the vehicle.  To improve the drag coefficient of the body of the vehicle at speed, or to generate down-force.  In passenger vehicles, the focus shifts more to directing the airflow into the engine bay for cooling purposes.  Air dam will keep the nose steady and pointed at the ground in high speed driving.
  12. 12. 2.6.3 DECK LID SPOILERS  The trunk lid or boot lid is the cover allows access to the main storage or luggage compartment.  A spoiler’s function is to 'spoil' unfavorable air movement across a body of a vehicle in motion.  Spoilers are used primarily on sedan-type race cars.  They act like barriers to air flow, in order to build up higher air pressure in front of the spoiler.
  13. 13. 2.6.4 WINDOW AND PILLAR TREATMENTS  Pillars are the vertical or near vertical supports of an automobile's window area or greenhouse-designated.  Which is A, B, C or D-pillar moving in profile view from the front to rear.  Pillars are implied, whether they exist or not; where a design's greenhouse features a break between windows or doors without vertical support at that position  The non-existent pillar is "skipped" when naming the other pillars.
  14. 14. 2.6.5 OPTIMIZATION  A mass reduction enable fuel economy improvement.  Various global optimization methods use to solve drag reduction problems in the automotive industry.  The genetic algorithms (GA) has the major advantage to seek for a global minimum.  But this method is very time consuming because of the large number of cost function evaluations that are needed.  All the hybrid optimization methods have greatly reducing the time cost by coupling a GA.
  16. 16. 2.7.1 AIR DENSITY  In simple terms, density is the mass of anything including air - divided by the volume it occupies.  Lift and drag vary directly with the density of the air-as air density increases, lift and drag increase.  As air density decreases, lift and drag decrease.  Air density is affected by pressure, temperature and humidity.
  17. 17. 2.7.2 DRAG COEFFICIENT  A common measure in automotive design as it pertains to aerodynamics.  The drag coefficient of an automobile impact the way the automobile passes through the surrounding air.  Aerodynamic drag increases with the square of speed; therefore it becomes critically important at higher speeds.  Reduce drag coefficient to improves the performance of the vehicle as it pertains to speed and fuel efficiency.
  18. 18. 2.8 SIDE FORCE  Cornering force is the lateral force produced by a vehicle tire during cornering.  Its generated by tire slip and is proportional to slip angle at low slip angles.  Slip angle describes the deformation of the tire contact patch.  This deflection of the contact patch deforms the tire in a fashion akin to a spring.  The deformation of the tire contact patch generates a reaction force in the tire; the cornering force.
  19. 19. 2.9 LIFT FORCE  A fluid flowing past the surface of a body exerts a force on it. Lift is the component of this force.  Lift is the force generated by propellers and wings to propel aircraft and keep them in the air.  Lift can be in any direction since it is defined to the direction of flow rather than to the direction of gravity.  When an aircraft is climbing, descending, or banking in a turn the lift is tilted with respect to the vertical.
  20. 20. 2.10 PITCHING MOMENT  The moment (or torque) produced by the aerodynamic force on the airfoil.  The pitching moment on the wing of an airplane is part of the total moment that must be balanced.  The lift on an airfoil is a distributed force that can be said to act at a point called the center of pressure.  If the moment is divided by the dynamic pressure, to compute a pitching moment coefficient, this coefficient changes only a little.
  21. 21. 2.11 YAWING MOMENT  A yaw rotation is a movement around the yaw axis of a vehicle that changes the direction the vehicle is facing.  The yaw rate or yaw velocity of a car or other rigid body is the angular velocity of this rotation.  Yawing moment is the projection of a given torque over the yaw axis.  It is important in road vehicles because pitch and roll moments are limited by the floor reaction.
  22. 22. 2.12 ROLLING MOMENT  In a vehicle suspension, roll moment is the moment of inertia of the vehicle's sprung mass.  Product of the sprung mass and the square of the distance between the vehicle's roll center and its center of mass.  In aeronautics, the roll moment is the aerodynamic force applied at a distance from an aircraft's center of mass.  A roll moment can be the result of wind gusts, control surfaces such as ailerons, or simply by flying at an angle of sideslip.
  23. 23. 2.13 CROSSWIND SENSITIVITY  The disturbances such as crosswinds should be minimized.  The increasing of this disturbances will make the driver have difficulties in compensating it.  The sensitivity of a vehicle to cross-wind depends on many factors involving the design of the suspensions and the aerodynamics of the body.  Streamlined bodies with smooth transitions and paralleled underbodies lead to low drag coefficients.
  24. 24.  However, these measures cause the flow velocity around the vehicle to increase.  This also increases the sensitivity of the vehicle’s oncentre handling under non-idealized flow conditions.  A method for estimating the aerodynamic loads on vehicle due to crosswind on a road section is also presented.  The aim to find a relationship between steering feel and crosswind sensitivity.  Aerodynamic loads under real conditions were estimated and the data were thereafter used in a study.
  25. 25. 2.14 ROLLING RESISTANCE  A term used to describe the energy generated by the friction of a tyre rolling over a road surface  Tyre inflation pressure, tread compound, tread design and temperature can affect the rolling resistance  The tyres are usually manufactured with a higher degree of silica built into the compound  Silica allows the tyres grip performance to remain high, especially in wet conditions, whilst rolling resistance is improved.
  27. 27. A. TYRE TEMPERATURE  The temperature grades representing the tire's resistance to heat generation and its ability to dissipate heat.  Tested under controlled conditions on a specified indoor laboratory test wheel.  Sustained high temperature can cause the material of the tire to degenerate and reduce tire life  While excessive temperature can lead to sudden tire failure
  28. 28. B. TYRE INFLATION PRESSURE/LOAD  The level of air in the tire that provides it with loadcarrying capacity.  Affects the overall performance of the vehicle.  A number that indicates the amount of air pressure measured in pounds per square inch (psi).  Manufacturers of passenger vehicles and light trucks determine this number based on the vehicle's design load limit.
  29. 29. C. VELOCITY  Rolling without slipping is a combination of translation and rotation where the point of contact is instantaneously at rest  When an object experiences pure translational motion, all of its points move with the same velocity as the center of mass  The object will also move in a straight line in the absence of a net external force.
  30. 30. D. TYRE MATERIAL AND DESIGN  Tires provide a gripping surface for traction and serve as a cushion for the wheels of a moving vehicle  Rubber is the main raw material used in manufacturing tires, and both natural and synthetic rubber is used.  The other primary ingredient in tire rubber is carbon black.  In the tire design, the main features of a passenger car tire are the tread, the body with sidewalls, and the beads.
  31. 31. E. TYRE SLIP  Slip is the relative motion between a tire and the road surface it is moving on.  Can be generated either by the tire's rotational speed or by the tire's plane of rotation being at an angle to its direction of motion.  In rail vehicle dynamics, this overall slip of the wheel relative to the rail is called creepage.  Its distinguished from the local sliding velocity of surface particles of wheel and rail, which is called micro-slip.
  32. 32. 2.14.2 TYPICAL COEFFICIENTS  Tire Rolling Resistance Coefficient is calculated by dividing the measured rolling resistance force  Comparing Rolling Resistance Coefficients only allows comparing tires within a single size.  Larger tires generate higher Rolling Resistance Forces than smaller tires.  Larger tires will often have a lower Rolling Resistance Coefficient than smaller tires.
  34. 34. 2.15.1 FUEL ECONOMY EFFECTS  Vehicle weight, aerodynamic drag, driving style and rolling resistance can effect the amount of fuel.  Tyres can affect up to 1/3 of the vehicle's total fuel consumption. Each tyre creates drag.  A vehicle's aerodynamics and its travelling speed have an extremely large effect on how much fuel is consumed.  An environmental factors are impossible to control but have a direct effect on fuel consumption.
  35. 35. THANK YOU !