Automotive aerodynamics keep changing.

Recently I read a paper on the development of some late model Audis. Much of the rationale behind the aero development was as you’d expect – minimising flow separation, keeping the area of the wake low, using a smooth underfloor – stuff like that.

However, a significant amount of effort was directed at reducing the size of the vortices being shed from the angled vertical surfaces – the C pillars in sedans and the D pillars in hatchback (or wagon) body styles. After all, if the wake size has been minimised, any further reduction in pressure drag comes from controlling these vortices – the whirling ribbons of air being dragged along behind the car.

Interestingly, much of the technique in reducing these vortices occurred under the car – the shape of the rear diffuser influencing the size of the vortices that were being shed. In fact, the differences between the body shapes (eg sedan and hatch) was such that the underfloor attachments needed to be tailored to suit.

And you can see another, more visible change, in aero occurring as well.

For a long time, the trailing edge of the upper car surfaces has been sharply cut off to promote better flow separation. So think of the roof extension spoilers used on the trailing edges of hatchbacks, for example. Or boot lid extension spoilers on sedans.

However, now the focus has clearly moved to additionally promoting clean separation on the side panels of cars. Take a look at the rear three-quarters of newly-released cars and you’ll often see a vertical crease on the corner of the car. This crease causes flow separation to then occur cleanly at this point, rather than the airflow wrapping around the rear edges. In fact, one Honda Civic I saw had a small vertical spoiler mounted at this location. I thought this factory attachment looked pretty trick – and doubly so when you think of its function.

If you do a lot of driving in slow-moving traffic, looking at the different styling approaches that manufacturers are using to achieve this clean side separation can keep you entertained for hours!

And finally, the way in which front and rear coefficients of lift are regarded is changing. The traditional wisdom has been that a low coefficient of lift (eg at the rear) promotes a stable car, and that a car with a higher coefficient of lift will consequently be less stable.

However, Mazda research indicates that this may not be as valid as first thought.

The trouble is that coefficients of lift tend to be averages – rather than taking into account fast changes in lift values that may occur due to transient changes in local airspeeds. What sort of transient changes, then? Well, the wind does not blow constantly as a steady stream: it contains gusts and other fast speed changes caused by roadside obstacles (and other vehicles) creating turbulence.

It is suggested that the reason that some cars are more aerodynamically stable than others – despite both cars having ostensibly the same lift figures – is due to differing behaviour under these rapidly changing conditions.

Car aero is a fascinating subject….