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 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.
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 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 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.
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 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.
Base Separation and Wakes
Figure 2: Slant angle effects
Bluff Bodies in Tandem
Ground Effects
Passenger Vehicles
Trucks and Buses
|
|
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.
Figure 5: Bluff body separation
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:
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 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)
High-Speed Trains
Wind Tunnels
Related Material
(on CD-ROM)
Selected References