Automotive aerodynamics
From Wikipedia, the free encyclopedia
Automotive aerodynamics is the study of the aerodynamics of road vehicles. The main concerns of automotive aerodynamics are reducing drag, reducing wind noise, and preventing undesired lift forces at high speeds. For some classes of racing vehicles, it may also be important to produce desirable downwards aerodynamic forces to improve traction and thus cornering abilities.
An aerodynamic automobile will integrate the wheel and lights in its shape to have a small surface. 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. It will have a flat and smooth floor to support the Venturi effect and produce desirable downwards aerodynamic forces. The air rams into the engine bay, is used (cooling, combustion, and for passengers), reaccelerated by a nozzle and then ejected under the floor.
Automotive aerodynamics differs from aircraft aerodynamics in several ways. First, the characteristic shape of a road vehicle is bluff , compared to an aircraft. Second, the vehicle operates very close to the ground, rather than in free air. Third, the operating speeds are lower. Fourth, the ground vehicle has fewer degrees of freedom than the aircraft, and its motion is less affected by aerodynamic forces.
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 forming on the floor of the working section and affecting the results.
Drag coefficient ( Cd) is a commonly published rating of a car's aerodynamic smoothness, related to the shape of the car. Multiplying Cd 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 overestimations of frontal area. These numbers use the manufacturer's frontal area specs from <http://www.mayfco.com/tbls.htm>
Some examples:
Drag area ( Cd x Ft2)
- 5.76 - 1968 Toyota 2000GT
- 5.92 - 1994 Porsche 911 Speedster
- 5.88 - 1990 Nissan 240SX
- 6.24 - 2004 Toyota Prius
- 6.27 - 1986 Porsche 911 Carrera
- 6.57 - 1985 Chevrolet Corvette
- 6.77 - 1995 BMW M3
- 6.79 - 1993 Corolla DX
- 6.96 - 1988 Porsche 944 S
- 7.02 - 1992 BMW 325I
- 7.10 - Saab 900
- 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
- 26.3 - 2006 Hummer H2
