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

Streaklines over a model car

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. - 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

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.

Drag Coefficient and Drag Area

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] Drag area figures that do not reflect drag coefficient and frontal area figures from independent aerodynamic testing (e.g. drag areas based on manufacturer-reported figures or educated speculation) are indicated with an asterisk (*).

Drag area of production cars Drag area (Cd x Ft2) Calendar Year Automobile 3.1 2012 Volkswagen XL1[5] 4.2* 1996 GM EV1[5] 6.0* 1999 Honda Insight[5] 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 CR-X 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 GT-R 6.13* 1991 Acura NSX 6.15* 1989 Suzuki Swift GT 6.17* 1995 Lamborghini Diablo 6.19* 1969 Porsche 914 6.2 2010 Toyota Prius[5] 6.2 2012 Tesla Model S[5] 6.24* 2004 Toyota Prius 6.27* 1986 Porsche 911 Carrera 6.27* 1992 Chevrolet Corvette 6.35* 1999 Lotus Elise 6.7 2010 Chevrolet Volt[5] 6.77* 1995 BMW M3 6.79* 1993 Corolla DX 6.81* 1989 Subaru Legacy 6.96* 1988 Porsche 944 S 7.0 2013 Mercedes-Benz CLA250[5] 7.02* 1992 BMW 325I 7.10* 1978 Saab 900 7.13* 2007 SSC Ultimate Aero 7.31* 2015 Mazda3 7.48* 1993 Chevrolet Camaro Z28 7.57* 1992 Toyota Camry 7.8 2012 Nissan Leaf SL[5] 8.70* 1990 Volvo 740 Turbo 8.71* 1991 Buick LeSabre Limited 9.54* 1992 Chevrolet 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 Main article: 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 Drafting Drag reduction system Downforce Flight dynamics Fluid dynamics Ground effect in cars Slipstream Wing References ↑ 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 ↑ Proceedings: Institution of Mechanical Engineers (Great Britain). Automobile Division: Institution of Mechanical Engineers, Great Britain (1957) ↑ 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 ↑ Larry Mayfield (c. 2013), Index to Coefficient of Drag for Many Vehicles Plus Index to Horsepower vs Speed Curves 1 2 3 4 5 6 7 8 Sherman, Don. "Drag Queens: Aerodynamics Compared" (PDF). Car and Driver (June 2014). Hearst Communications. p. https://www.caranddriver.com/features/drag-queens-aerodynamics-compared-comparison-test. Retrieved 29 December 2017. ↑ "Background Research." Automobile Aerodynamics. 18 May 2008. DHS. 18 May 2009 <http://web-aerodynamics.webs.com/backgroundresearch.htm> Archived September 2, 2011, at the Wayback Machine.. External links Wikimedia Commons has media related to Aerodynamics in road vehicles. One of the first cars to generate downforce - The Prevost analysed in CFD