Automotive aerodynamics

A truck with added bodywork on top of the cab to reduce drag.

Automotive aerodynamics is 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 and other causes of aerodynamic instability at high speeds. Air is also considered a fluid in this case. For some classes of racing vehicles, it may also be important to produce downforce to improve traction and thus cornering abilities.

History

The frictional force of aerodynamic drag increases significantly with vehicle speed.^[1] As early as the 1920s engineers began to consider automobile shape in reducing aerodynamic drag at higher speeds. By the 1950s German and British automotive engineers were systematically analyzing the effects of automotive drag for the higher performance vehicles.^[2] By the late 1960s scientists also became aware of the significant increase in sound levels emitted by automobiles at high speed. These effects were understood to increase the intensity of sound levels for adjacent land uses at a non-linear rate.^[3] Soon highway engineers began to design roadways to consider the speed effects of aerodynamic drag produced sound levels, and automobile manufacturers considered the same factors in vehicle design.

Features of Aerodynamic Vehicles

An aerodynamic automobile will integrate the wheel arcs and lights into the overall shape to reduce drag. It will be streamlined; for example, it does not have sharp edges crossing the wind stream above the windshield and will feature a sort of tail called a fastback or Kammback or liftback. Note that the Aptera 2e, the Loremo, and the Volkswagen 1-litre car try to reduce the area of their back. It will have a flat and smooth floor to support the Venturi effect and produce desirable downwards aerodynamic forces. The air that rams into the engine bay, is used for cooling, combustion, and for passengers, then reaccelerated by a nozzle and then ejected under the floor. For mid and rear engines air is decelerated and pressurized in a diffuser, loses some pressure as it passes the engine bay, and fills the slipstream. These cars need a seal between the low pressure region around the wheels and the high pressure around the gearbox. They all have a closed engine bay floor. The suspension is either streamlined (Aptera) or retracted. Door handles, the antenna, and roof rails can have a streamlined shape. The side mirror can only have a round fairing as a nose. Air flow through the wheel-bays is said to increase drag (German source) though race cars need it for brake cooling and many cars emit the air from the radiator into the wheel bay.

Comparison with Aircraft Aerodynamics

Automotive aerodynamics differs from aircraft aerodynamics in several ways. First, the characteristic shape of a road vehicle is much less streamlined compared to an aircraft. Second, the vehicle operates very close to the ground, rather than in free air. Third, the operating speeds are lower (and aerodynamic drag varies as the square of speed). Fourth, a ground vehicle has fewer degrees of freedom than an aircraft, and its motion is less affected by aerodynamic forces. Fifth, passenger and commercial ground vehicles have very specific design constraints such as their intended purpose, high safety standards (requiring, for example, more 'dead' structural space to act as crumple zones), and certain regulations.

Methods of Studying Aerodynamics

One of the side effects of automotive aerodynamics is seed dispersal.

Automotive aerodynamics is studied using both computer modelling and wind tunnel testing. For the most accurate results from a wind tunnel test, the tunnel is sometimes equipped with a rolling road. This is a movable floor for the working section, which moves at the same speed as the air flow. This prevents a boundary layer from forming on the floor of the working section and affecting the results. An example of such a rolling road wind tunnel is Wind Shear's Full Scale, Rolling Road, Automotive Wind Tunnel in Concord, North Carolina and Auto Research Center in Indianapolis, Indiana USA.

Drag coefficient

Drag coefficient (C_d) is a commonly published rating of a car's aerodynamic smoothness, related to the shape of the car. Multiplying C_d by the car's frontal area gives an index of total drag. The result is called drag area, and is listed below for several cars. The width and height of curvy cars lead to gross overestimation of frontal area. These numbers use the manufacturer's frontal area specifications from the Mayfield Company unless noted.^[4]

Some examples:

Drag area ( Cd x Ft^2)	Year	Automobile
3.0 sq ft (0.28 m^2)	2012	Volkswagen XL1[5]
3.95	1996	GM EV1
5.10	1999	Honda Insight
5.40	1989	Opel Calibra
5.54	1980	Ferrari 308 GTB
5.61	1993	Mazda RX-7
5.61	1993	McLaren F1
5.63	1991	Opel Calibra
5.64	1990	Bugatti EB110
5.71	1990	Honda CRX
5.74	2002	Acura NSX
5.76	1968	Toyota 2000GT
5.88	1990	Nissan 240SX
5.86	2001	Audi A2 1.2 TDI 3L
5.91	1986	Citroën AX
5.92	1994	Porsche 911 Speedster
5.95	1994	McLaren F1
6.00	2011	Lamborghini Aventador S
6.00	1992	Subaru SVX
6.06	2003	Opel Astra Coupe Turbo
6.08	2008	Nissan GTR
6.13	1991	Acura NSX
6.15	1989	Suzuki Swift GT
6.17	1995	Lamborghini Diablo
6.19	1969	Porsche 914
6.2	2012	Tesla Model S
6.24	2004	Toyota Prius
6.27	1986	Porsche 911 Carrera
6.27	1992	Chevrolet Corvette
6.35	1999	Lotus Elise
6.77	1995	BMW M3
6.79	1993	Corolla DX
6.81	1989	Subaru Legacy
6.96	1988	Porsche 944 S
7.02	1992	BMW 325I
7.10	1978	Saab 900
7.13	2007	SSC Ultimate Aero
7.31	2015	Mazda 3
7.48	1993	Chevrolet Camaro Z28
7.57	1992	Toyota Camry
8.70	1990	Volvo 740 Turbo
8.71	1991	Buick LeSabre Limited
9.54	1992	Chevy Caprice Wagon
10.7	1992	Chevrolet S-10 Blazer
11.63	1991	Jeep Cherokee
13.10	1990	Range Rover Classic
13.76	1994	Toyota T100 SR5 4x4
14.52	1994	Toyota Land Cruiser
17.43	1992	Land Rover Discovery
18.03	1992	Land Rover Defender 90
18.06	1993	Hummer H1
20.24	1993	Land Rover Defender 110
26.32	2006	Hummer H2

Downforce

Downforce describes the downward pressure created by the aerodynamic characteristics of a car that allows it to travel faster through a corner by holding the car to the track or road surface. Some elements to increase vehicle downforce will also increase drag. It is very important to produce a good downward aerodynamic force because it affects the car’s speed and traction.^[6]

See also

• Aerodynamic stability
• Downforce
• Flight dynamics
• Fluid dynamics
• Ground effect in cars
• Slipstream
• Wing

References

1. Tuncer Cebeci, Jian P. Shao, Fassi Kafyeke, Eric Laurendeau, Computational Fluid Dynamics for Engineers: From Panel to Navier-Stokes, Springer, 2005, ISBN 3-540-24451-4
2. Proceedings: Institution of Mechanical Engineers (Great Britain). Automobile Division: Institution of Mechanical Engineers, Great Britain (1957)
3. C. Michael Hogan & Gary L. Latshaw, The relationship between highway planning and urban noise, Proceedings of the ASCE, Urban Transportation Division specialty conference, May 21/23, 1973, Chicago, Illinois. by American Society of Civil Engineers. Urban Transportation Division
4. Larry Mayfield (c. 2013), Index to Coefficient of Drag for Many Vehicles Plus Index to Horsepower vs Speed Curves
5. "Volkswagen CarScene TV: Volkswagen XL1 - Vision wird Realität (in german)". Youtube.com. 2011-02-03. Retrieved 2013-06-19.
6. "Background Research." Automobile Aerodynamics. 18 May 2008. DHS. 18 May 2009 <http://web-aerodynamics.webs.com/backgroundresearch.htm>.

External links

• One of the first cars to generate downforce - The Prevost analysed in CFD