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Modifying the Aerodynamics of Your Road Car: Step-by-step instructions to improve the aerodynamics of road cars
To understand what is occurring as air passes over (and around and under) your car, you need to first understand some theory. I could start by discussing the physical characteristics of air, but I think it makes the subject more accessible if instead we begin by looking at a real-world aspect of car aerodynamics – drag. Through looking at how drag occurs, we can begin to understand airflow and the forces it develops. Then, when we have done that, we can build on those ideas to look at more complex aspects.
Drag is the force imposed by the air that retards the car’s forward movement. Higher drag increases fuel consumption and slows acceleration. So what types of drag are cars subjected to, and why does it occur?
Much on-road aerodynamic testing can be done at quite low speeds. For example, the wool-tuft testing shown in this chapter was completed at speeds of 60-80km/h (35-50mph). This is fast enough that the global airflow pattern becomes visible, and is also fast enough that minor crosswinds will have little impact on the patterns that you can see. Clay testing is done more quickly at slightly higher speeds, but they are not necessary to get good results. For some on-road aerodynamic testing (eg coast-down testing and downforce/lift measurements), it can be an advantage to do testing at much higher speeds, but for flow visualisation, those high speeds are not needed.
The most fascinating view of the Tesla is the rear three-quarters. The airflow down the rear glass is superbly attached, and with the slight kick-up of the rear deck, increased pressure will occur over this area, reducing drag and lift. (The SAE paper states that a small separation bubble exists at the base of the rear glass, however this is not visible in the tuft testing.) Also interesting is the behaviour of the air over the C-pillar. The tech paper states that much effort was put into reducing trailing vortices from these pillars, and while the paper does not say this, it looks to my eyes as if this has been achieved by adding a sharp edge to this pillar. This potentially alters the wrap-around of air from the side of the car onto the hatch, reducing lift. Significantly, this sharp line does not appear to be present on the original Model S concept car.
A unique handbook that assumes no starting knowledge of vehicle aerodynamics.
Unlike changes in total car drag, lift and downforce can be measured accurately on the road over a short testing period, and without it costing a lot. You will still need to do some careful testing (ie keeping constant the atmospheric conditions, speed and car loads) and you will need to install some specific sensors and do a little bit of simple electronics work – but it is straightforward to do.
Before we look at how to do it, let’s look at some fundamental concepts. As I’ve described earlier, the aerodynamic forces that are applied to the car are through the mechanism of pressures felt on the car body. They might be pressures pushing down, they might be pressures pushing up.
As described in Chapter 1, the drag area of a car is found by multiplying the CD by the projected frontal area. You would therefore assume that if you wished to reduce drag, an easy way of doing so would be to reduce frontal area. However, this assumes that the CD does not increase at the same time!
Let’s explore that idea in more detail. Prevalent in older modified cars was the use of a low front spoiler (sometimes called an air dam or chin spoiler). This prevented air passing under the car – with very few exceptions, the underside of the car was rough and so caused great drag. Decrease the amount of air hitting these obstructions, and the overall drag was reduced. But hold on – adding a low front spoiler also increases the frontal area of the car!
As described in Chapter 1, most of the attached flow on a car comprises a turbulent boundary layer. This turbulence excites panels – it causes them to vibrate. The second cause of aerodynamic noise occurs where the flow separates and then reattaches, with these pressure fluctuations creating noise. This type of noise can be associated with external rear vision mirrors, and the A-pillars. (In addition, noise-generating vortices are often formed at the A and D pillars.) Lastly, cavities formed by body openings (eg at the doors) can both generate noise and allow it to be transmitted to the interior.
Once I was lucky enough to sit in a car positioned in a wind tunnel, as the huge fan was brought up to speed.
Publisher : Veloce Publishing (January 8, 2019)
Language : English
Paperback : 248 pages
ISBN-10 : 1787112837
ISBN-13 : 978-1787112834
Item Weight : 1.16 pounds
Dimensions : 8.25 x 0.75 x 10 inches
