Vehicle Aerodynamics

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

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Effects of aerodynamic forces are profound

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Force coefficients

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Example

What is a vehicle’s drag force, with a frontal area of 1.5 m², C_D of 0.4, and traveling at 30 m/s

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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

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Example

Rear Spoiler (Mazda RX-7 R-2)
C_D = 0.31 (0.29 without spoiler)
C_{L front} = 0.10 (0.16 without spoiler)
C_{L rear} = 0.08 (0.08 without spoiler)

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Underbody improvements

Aerodynamic properties
Reduce drag
Increase down force

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Laminar Separation

Flow separation inside the boundary layer

Laminar Bubble

Streamlines enclosed within the laminar separation

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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

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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

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Radiator inlet configurations

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Wake

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

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Increases drag

Presents danger to the following vehicles

Can be controlled with small fins or smooth edges

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Tires

Tires influence a vehicle’s aerodynamic properties

Cross sectional area
Frontal area
Rotation of tires

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Effects of tire rotation

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Effect of all-wheel-steering

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Glass and Trim

Drag can be reduced by making glass and trim as flush with the body as possible

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Elimination of rain gutter improves the vehicle aerodynamics

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General Improvements

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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

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1969 Chaparral 2J used auxiliary fans to create suction under the car

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Ford’s rear mounted transverse engine

Fans improve aerodynamic properties and reduce drag

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TEC452 Home

Aero Index

Introduction

Aerodynamic Theory

Wind Tunnel

Computational Fluid Dynamics

Vehicle Aerodynamics