Riaz Sanatian, Automotive sector manager, CD-adapco

From the missile shaped design of the modern F1 Grand Prix racing car to the brick-like structure of a heavy truck, aerodynamics plays a crucial role in how well they behave on the road and how efficiently they perform the tasks they are designed for.

For all vehicles, ranging from small passenger vehicles to commercial buses and trucks, reducing air drag is one of the most efficient ways of improving fuel economy. For example, a 5% improvement in drag for a typical diesel engine heavy tuck, which can simply be achieved by improving the design of the wing mirrors, can result in fuel savings of around 500-1000 litres/year for a typical 150,000 km annual highway driving. On the other end of the scale, in motor racing fuel saving might not be the number one priority, but reaching very high speeds certainly is. To propel a typical Class 1 ITC racing car at 300km/h, around 30 kW of additional power is required for a car with drag coefficient of 0.40, compared to one with 0.36. And when you are operating at the limit of your engine, this can make the difference between winning or losing.

Optimum aerodynamic design
But there is a lot more to external aerodynamics design than simply reducing the air drag. It might come as a surprise to most motor racing enthusiasts that a typical modern F1 Grand Prix car has a higher drag coefficient than the average family saloon we go shopping in, or that an ITC racing car has a much higher drag coefficient than the production vehicle it is based on! Nevertheless this should not come as a big surprise, especially when we look at all the aerodynamic components and features that are there to keep racing vehicles stable and drivable at high speeds, effectively preventing them from flying off the ground. Components such as front wings, diffuser shapedunderbody, brake cooling ducts, engine intake and rear wings are there to improve the car stability and down force, which can be of the order of a tonne at maximum speed for an F1 racing car, but at the same time can add to the drag of the vehicle. Therefore the optimum aerodynamic design has to produce the best balance of low drag and high down force that allows the car to be stable and drivable at very high speeds.

Courtesy of Renault F1, CD-adapco is a Renault F1 team supplier
Passenger and commercial vehicles also have other aerodynamic requirements. Here, high levels of down force are not usually required, but undesirable lift, which can result in unstable handling, should also be avoided. For passenger vehicles the visual effect of aerodynamic components, such as rear spoilers can also play a major role in the vehicle’s final design. With quieter power trains, wind noise and aeroacoustics are also becoming an important consideration in the aerodynamic design. With high-sided vehicles, such as large commercial trucks and buses, there are further safety issues relating to wind loading. Effective management of rain, snow and dirt around the vehicle is also greatly influenced by its aerodynamic design.

Furthermore, we should not forget the influence of external aerodynamics on cabin ventilation, underhood thermal management and brake cooling, to name but a few.
To satisfy the many, and sometimes contradictory, vehicle aerodynamics design requirements there is a need for detailed information about the behaviour of air flow around the vehicle, and here numerical simulation techniques have been making a significant contribution to the aerodynamics design of modern vehicles.

Numerical and experimental approaches
At present, wind tunnel testing and CFD simulation are the two major techniques used to obtain aerodynamic data around a vehicle. Nowadays the choice is not to use one technique or the other, but to combine the capabilities of both techniques to obtain the best set of results for the analysis of airflow around the vehicle.