Reducing a vehicle’s wind resistance to aid performance and efficiency has never been more important. We look at the basic theories and real-world applications.
“A SUPERCAR” is a popular answer to the question of which type of vehicle has the most aerodynamic shape. It’s wrong, though; while a wedge-like shape may look aerodynamic, in reality it is quite inefficient when it comes to cheating the wind at airflow speeds of slower than the speed of sound. So, what’s the ideal slippery shape? Let’s start with the basics.
Coefficient of drag (Cd)
Drag coefficient as applied to the automotive field is a “unit-less” quantity that denotes the efficiency of a shape, regardless of the size, as it passes through the air. The figures above (in Cd) show how general shapes move through the air.
Reducing the Cd of a shape is called streamlining because breakaway airflow (or vortexes) around the edges or rear of a shape results in a larger, low-pressure area behind the shape, increasing its drag. At the bottom of this page, we use the example of a two-dimensional rectangular block.
Drag force
At speed on a level road, the main forces opposing vehicle motion are aerodynamic drag force (Fdrag) and rolling resistance. The formula of the drag force, measured in Newton, is:
Fdrag = ½ ⅹ ᵨ ⅹ V2 ⅹ A ⅹ Cd
Where
ᵨ = air density [kg/m3]
V = vehicle speed [m/s]
A = frontal area [m2]
Cd = coefficient of drag
If we calculate the drag force of the VW Golf 7 GTI (frontal area 2,59 m2, Cd = 0,27, ᵨ = 1,2 kg/m3 ) for vehicle speed up to 250 km/h, the results are shown in the graph below.
It’s clear from the equation that the drag force increases with the square of vehicle speed, proven by the shape of the drag force line versus vehicle speed on the graph. At 120 km/h, the drag force is 466 N, and at 250 km/h it has increased to 2 023 N. The respective power requirements to overcome drag is 16 kW and 141 kW at the given speed points. This also explains why fuel consumption notably increases when you up cruising speed.
Lowering drag force
Manufacturers would love to produce cars that look like the XL1 from VW, but customers’ practical requirements dictate otherwise. Therefore, carmakers tweak shapes by adding small vents or lips to decrease aerodynamic drag. This is mostly done with the help of computer simulations employing computational fluid dynamics programmes. The proposed changes are then tested in a wind tunnel, sometimes with a rolling road, to validate the changes. As an example, the new BMW X5 has air splitters fore of the wheels to guide airflow around them and vents behind the wheels to reduce the high-pressure region that forms behind each wheel. Lips are added to the rear lights as well as air blades on the sides of the rear windows to control the flow separation from the body. On the X5, these tweaks represent a Cd figure of 0,31.
Lately, manufacturers have started concentrating more on the design of the underbody areas in order to create flat surfacing; generally, the drivetrain and exhaust pipes are neatly tucked away in modern vehicles.
Another area in which gains are made is the air inlets (grille) to the engine compartment in a front-engined vehicle. It is more efficient to guide airflow around the engine compartment than through it, although some flow is needed for cooling purposes. On vehicles designed with optimal fuel economy in mind, such as Volkswagen’s BlueMotion products, the grille is covered to allow through only the air required for cooling, thereby reducing drag. High-end vehicles come with electronic grille shutters that can open and close according to cooling needs.
An area that requires further development is that of wheel coverings. The vortexes generated by the wheels upset the airflow past the sides of the vehicle body and increase drag. However, this is an unpopular tweak because it harms the aesthetic appeal of the vehicle. In future, we expect side mirrors to be replaced by cameras for additional efficiency gains.
What’s next?
The spiralling cost of fuel (or energy) will eventually force people to accept overtly aerodynamic vehicle shapes, while technologies like automated road trains may allow close-proximity slipstreaming on motorways to reduce drag. In the meantime, the only way for your vehicle to cut through the air cleaner is by removing roof bars, bike racks and bullbars, and slowing down.
Laminar vs. turbulent flow
Laminar flow occurs when air flows in parallel layers, with no disruption between the layers. This is only possible at very low airflow speeds and over extremely smooth surfaces. In the real world, most airflow is turbulent, including airflow over vehicle bodies. The advantage of turbulent flow is that it clings to the body longer before separating, creating a smaller area of low pressure behind the moving body and thereby reducing drag. This is the reason golf balls have dimples – the indented surface area encourages turbulent flow over the ball and reduces drag that leads to longer driving distances. The study of laminar, transition and turbulent flow uses the Reynolds number as an indication at what speed the transition will occur.
Speed of sound
Aerodynamic studies show that the optimal nose shape of a body travelling through the air at less than the speed of sound is round. This is why commercial airliners have rounded noses. The airflow dynamics, however, change drastically at the speed of sound, with the shockwave attached to the nose being the biggest contributor to drag. A sharp nose design, as found on fighter jets that can exceed the speed of sound (Mach 1), manages to decrease the inclusive angle of the shockwave spreading from the nose and, in doing so, lowers drag. This explains the sharp-nose design of the Bloodhound SSC that plans to break the land-speed record at Hakskeen Pan in 2015. The target speed is 1 000 mph, or 1 609 km/h, which is Mach 1,3 at sea level. What makes the aerodynamics of the Bloodhound SSC substantially more complex than the aircraft equivalent is the reflection of the shockwaves from the surface of the desert.
Downforce vs. drag
To create downforce, aerodynamics engineers who work on race cars often fit wings or splitters at the front or rear of the vehicle, or a diffuser at the latter.
The wings are basically inverted aircraft wings that produce downforce (as opposed to lift) by working the airflow. The diffuser at the rear of the vehicle accelerates the airflow underneath the vehicle, creating a low-pressure region and “sucking” the vehicle to the ground. Unfortunately, downforce has an aerodynamic-drag penalty. That’s the reason why a Bugatti Veyron can achieve its stratospheric top speed of 400-plus km/h only when the rear spoiler is tucked away. F1 cars are designed for maximum downforce and the Cd figure of the cars on a high-downforce circuit can be as great as 1,1.
