Copyright © A. Filippone (1996-2001). All Rights Reserved.

Road Vehicles Aerodynamics

In this Chapter

Road vehicles are probably the only class of systems for which aerodynamics is constrained by non techical performances (fashion, environment, regulations, etc.). An example of input parameters on the aerodynamic design of a passenger vehicle is shown in the figure below.

Figure 1: Typical design inputs

Most often cars must look good. Since designers are more important than aerodynamicists (on a ratio 15:1, or more), we will focus on a few general topics.

From an aerodynamic point of view, road vehicles are three- dimensional bluff bodies in ground proximity. Any attempt to recall the aerodynamics of streamlined bodies will not do.

Base Separation and Wakes

Base separation is one (almost inevitable) cause of large drag. Important experimental studies have been performed on the effect of the scant angle (see Fig.) on the base drag, with remarkable results. There appears to be a critical value of the scant angle. Computational studies are fairly complex.

Figure 2: Slant angle effects

Bluff Bodies in Tandem

Bodies in tandem are two (or more) bodies on each other’s wake. This situation creates an interference that may be beneficial or detrimental for one of them. Typical examples are tractor-trailer, truck-motorcycle, racing cars and cyclists following behind the lead. In automobile racing the effect is called drafting.

Drafting is effective for the trailing body when its drag is reduced. The effect is strongly dependent on the separationx between cars. The CD of the lead and trailing cars increase up to 1 body length then they stay almost constant; the downforce -CL is also affected, with the CL of the trailing car having the most dramatic change within 1 body length from the lead.

Experiments are also available on tandem cars with lateral separation (case occurring during overtaking.)

Experiments on tandem cylinders of various diameter ratios and longitudinal separation show that there is an optimal configuration, with CD as low as 0.02 (while in the worst case the tandem CD is of the order of 0.75 !)

Ground Effects

Ground proximity has enormous consequences on the aerodynamics of all systems. With reference to drag, this is generally higher. One reason is that, even on a symmetric body, the pressure field is altered to maintain the flow parallel to the ground. Separation is sometimes enhanced by this situation. However, the most striking effect is on the lift (positive or negative).

Another effect is the venturi: channels on the under body accelerate the flow, thus creating a loss of pressure. This effect has been used whenever possible on racing cars.

Passenger Vehicles

Passenger vehicles fall into special classification related to their size. Although there are some aerodynamic implications, the reasons for doing this are tax and environmental issues.

Another classification is due to its shape, as it is shown in the figure below. These types (shortly called F, H, N) have different aerodynamic response to cross winds.

Car Types

Figure 3: Car classes

Body work (e.g. aerodynamic add-ons, such as chin spoilers, rear wings, dams, etc.) has very limited use, and seldom comes as a standard from the manifacturer.

Trucks and Buses

Trucks and buses come in a wide variety of sizes, but they are more bluff than passenger vehicles, and streamlining is more difficult. However, there is a number of aerodynamic solutions (especially for trucks) which are quite important:

The first class of systems is used for streamlining and eliminating areas of dead air. The fairings are shown in the figure below.

Truck with NO fairings Truck with fairings

Figure 4: Flow separation

The tangential slots are used for reducing the effects of lateral separation on the vortex/pressure drag; the separation between tractor and trailer is a problem of tandem aerodynamics (see above).

The drag still depends on ground effect and bluff body separation at the rear, as sketched in the figure below.

bluff body separation

Figure 5: Bluff body separation


High-Speed Trains

The resistance of a high-speed train is mostly due to aerodynamics. For example, two streamlined head/tail locomotives pulling 8 carriages (length: 250 metres; mass: 520 metric tons) at speed 300 km/h would have a total resistance of about 64,000 N (or 6,300 Kgf), of which 70-80 % is aerodynamic drag and the rest is rolling resistance and mechanical resistance due to pantograph and cooling flows. (This is only valid on condition of straight plain track with no atmospheric winds).

Aerodynamics is particularly important in these cases:

  • Trains traveling in opposite directions (open air)
  • Trains traveling in opposite directions (tunnels)
  • Trains entering a tunnel at high speed
  • Trains entering a railway station at high speed

A train encountering another train travelling in the opposite direction on a close track can create a pressure wave so strong as to derail both trains. Hence the interest in shaping appropriately the nose in order to minimize this wave. The problem is particularly critical in a tunnel. Details available in Schetz (2001). However, there is a number of other problems, including the ground interference.

Wind Tunnels

Wind tunnel testing remains the most important means of development, although engineering is now placing efforts in the new computational methods. (further material on CD-ROM)

Related Material

(on CD-ROM)
  • Tables of Vehicles CD
  • Racing Aerodynamics

Selected References

  • Katz J. Race Car Aerodynamics, Robert Bentley Publ. Cambridge, MA, 1995.

  • Milliken WF, Milliken DL, Race Car Vehicle Dynamics, SAE International, Warrendale, PA, 1995.

  • Sovran G, Morel T, Mason WT (editors). Aerodynamic Drag Mechanisms of Bluff Bodies and Road Vehicles, Plenum Press, New York, 1978.

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Copyright © A. Filippone (1996-2001). All Rights Reserved.