Vehicle's Aerodynamic


Vehicle Aerodynamic Factors

  • Aerodynamic Forces
  • Laminar Separation
  • Tripping of Boundary Layer
  • Pressure Distribution
  • Wake
  • Tires
  • Glass and Trim
  • General Improvements
  • Unconventional Features



Aerodynamic Forces

  • Lift force
  • Drag force
  • Side force
  • Effects of aerodynamic forces are profound
  • Force coefficients
  • Example
    • What is a vehicle’s drag force, with a frontal area of 1.5 m2, CD of 0.4, and traveling at 30 m/s
  • Aerodynamic down force
    • Opposite of lift in direction
    • Uses an inverted airfoil
    • Increases load on tires without increasing the vehicle’s weight (up to 10% of vehicle’s weight)
    • Improves cornering performance with no weight penalty
    • First discovered in 1960s!
  • Example of down force
  • Example
    • Rear Spoiler (Mazda RX-7 R-2)
    • CD = 0.31 (0.29 without spoiler)
    • CL front = 0.10 (0.16 without spoiler)
    • CL rear = 0.08 (0.08 without spoiler)
  • Underbody improvements
    • Aerodynamic properties
    • Reduce drag
    • Increase down force



Laminar Separation

  • Laminar Separation
    • Flow separation inside the boundary layer
  • Laminar Bubble
    • Streamlines enclosed within the laminar separation
  • Laminar bubble area is sensitive and can easily separate, resulting in excess drag
  • Can appear in low Re range (104-105), and disappear as speed increases, causing severe discrepancies in flow visualization and analysis
  • The rear end shape is the most critical factor in lowering the drag coefficient
  • Flow separation above the rear window can cause annoying dirt deposits on the glass



Tripping of Boundary Layer

  • Introduction of aerodynamic disturbances
    • Fins
    • Vortex generators
    • Strips of coarse sand paper
  • Forcing laminar to turbulent flow
  • Drag reduction due to delay in the onset of flow separation



Pressure Distribution

  • Helps the placement of inlets and outlets
    • Lower pressure at the outlet
    • Higher pressure at the inlet
  • Favorable pressure distribution
    • Prevents flow separation
  • Unfavorable pressure distribution
    • Promotes flow separation
    • Promotes turbulent flow within boundary layer
  • Example of inlet
  • Radiator inlet configurations




  • The disturbed air flow left behind the vehicle
  • Usually in the form of a vortex
  • Caused by merging air flows at different velocities near sharp edges
  • Increases drag
  • Presents danger to the following vehicles
  • Can be controlled with small fins or smooth edges




  • Tires influence a vehicle’s aerodynamic properties
    • Cross sectional area
    • Frontal area
    • Rotation of tires
  • Effects of tire rotation
  • Effect of all-wheel-steering



Glass and Trim

  • Drag can be reduced by making glass and trim as flush with the body as possible
  • Elimination of rain gutter improves the vehicle aerodynamics



General Improvements


1- Front spoiler

2- Ducted engine cooling

3- Shrouded windshield wiper arms

4- Aerodynamic mirrors

5- Smooth windshield transitions

6- Smooth side window transitions

7- Smooth rear window transition

8- Optimized trunk corner radii

9- Optimized lower rear panel

10 - Smooth fuel tank and underbody

11- Optimized rocker panels

12- Flush wheel covers

13- Elimination of the rain gutter



Unconventional Features

  • Large rear fins promote lateral stability in the 1966 Peugeot CD
  • 1969 Chaparral 2J used auxiliary fans to create suction under the car
  • Ford’s rear mounted transverse engine
  • Fans improve aerodynamic properties and reduce drag