Our mechanical engineer delves deeper into automotive aerodynamics in the second of a two-part article.
"Coupes and sports cars are much easier to “aerotune” than saloons and hatchbacks."
Plenty of complications surround the scientific formulations of moving air.
The underlying reason is that air, though gaseous in state, is nonetheless considered a fluid. The physical predictability of wind remains a dark science.
There are few “air scientists” who know exactly how air behaves over a moving object.
One of them is Adrian Newey, Red Bull F1’s chief engineer and possibly the world’s top automotive aerodynamicist. That Newey’s scalp is devoid of any turbulence-inducing hair may not be a mere coincidence!
And like David Copperfield’s magic tricks, we’ll probably never know how Newey performs his wizardry.
A smooth, uninterrupted surface is practically impossible to achieve for a car body because a car needs ventilation apertures, windscreens, wipers, wheels and tyres, windows, door handles, door mirrors, etc – all of which are culprits of drag.
And cars need to look good. Seriously, would you drive an egg on wheels or an Audi R8? That is the perpetual conflict between styling and aerodynamic efficiency.
That’s tough enough, but there is yet another aspect of vehicular aerodynamics that throws a spanner in the works – “aerodynamic instability”.
While a low-drag shape is desirable to cut cleanly through the air, the aerodynamic forces underneath, over and along the sides of the car give rise to lift and directional instability.
Any object on Earth that moves will displace the air along its path. Naturally then, all that air forced to move out of the way will do what fluids do best – flow whichever way possible and that includes any opening.
In the case of cars, air will flow over the body, between undercarriage and road, through the grilles, over the tyres, and if it is a convertible, into the passenger compartment, too.
It is not as simple as it sounds. Everywhere along the airstreams, various changes in the characteristics of the air take place, resulting in forces of different magnitudes and directions, acting at different places on the car. This is what causes instability and drag.
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For instance, air that flows over the bonnet and roof increases in velocity. According to the physics of fluid dynamics, an increase in velocity causes an inverse effect on pressure. Hence, on the upper surface of the car, there will be less force per unit area than under the car, where the air takes a less deviated route and thus experiences little or no drop in velocity. The result is a nett force that acts upwards, causing “lift”.
At the same time, every surface that can be projected perpendicularly to the direction of travel (everything you see headon in two-dimension) induces drag. There is no running away from the fact that the projected area has a direct effect on drag, which means a smaller “face” and skinnier tyres would create less aerodynamic resistance.
Major car manufacturers have spent hundreds of millions on wind tunnel test facilities in order to develop aerodynamically efficient cars.
However, in order to alleviate the undesirable effects of moving air, such as lift, there are compromises that will have to be made to aerodynamic efficiency.
The best wind-cheating car shapes in the business have barely any spoilers, deflectors, fins and skirts which are immediately visible. Not surprisingly, these are the products of the most sophisticated wind tunnels.
Coupes and sports cars are much easier to “aero-tune” than saloons and hatchbacks, because they typically have lower frontal areas, tapering front and rear, and smaller gaps between undercarriage and road surface.
No matter what body shape, aerodynamicists with the help of wind tunnel data know how to make subtle changes to the body contours to optimise efficiency and stability.
Perhaps the most complicated bodystyle is the convertible, because with the roof down, so much wind could rush in from the top and the sides that the turbulence inside the cabin would be impossible to tolerate.
Wind tunnel experts have become rather good at shaping the details to achieve remarkably turbulence-free convertible interiors, where occupants can enjoy roof-down motoring.
Exactly how it’s achieved is never really revealed, but some of the results have been startling.
A recent example is Bentley’s Continental GT Speed Convertible. Cruising at 160km/h in the topless vehicle is surprisingly comfortable. The air-conditioning continues to work perfectly, turbulence is non- existent, and apart from a slight increase in noise (all road noise, mind you), the only reason to put up the roof is rain. Not long ago, open-air motoring was barely tolerable beyond 70km/h.