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Introduction

In 1886, Karl Benz built the world’s first three-wheeled automobile powered by an internal-combustion engine. Its main difference from a horse-drawn carriage was the powertrain, while the basic form of the modern four-wheeled car came from Daimler’s ideas. Over the following century and more, car bodies changed continually as people demanded greater mobility and aesthetic tastes evolved. From the Beetle through the ponton body to the wedge shape, designers consistently sought forms that combined lower drag with visual appeal. This article offers a brief look at the relationship between automotive styling and aerodynamics.

Car Design and Aerodynamic Drag

At constant speed on a level road under normal atmospheric pressure, a car's aerodynamic drag consists of pressure drag and skin-friction drag. Depending on their proportions and the part of the vehicle involved, aerodynamic drag can be further divided into form drag, induced drag, roughness and interference drag, and internal-flow drag. About 69% of a car's fuel is used to overcome air resistance, and form drag accounts for 58% of aerodynamic drag. Body shape is therefore crucial to reducing resistance. We can define the drag coefficient as Cw=F_drag/(ρ/2 v_∞² A), where v_∞ is the relative velocity between the car and the uniform airflow far upstream, and A is the vehicle's frontal area. Figure 1-1 gives the aerodynamic drag coefficients of several body shapes moving at the same constant speed through the same horizontal, uniform, ideal airflow. The differences among vehicle types are substantial.

Figure 1-1

We will now look briefly at each of these forms of aerodynamic drag.

Form Drag

Pressure drag accounts for 80–90% of a car's form drag, while air friction contributes only 10–20%. Figure 1-2 shows the pressure drag on several shapes in the same ideal fluid:

Figure 1-2

For a bluff body such as a flat plate placed across the flow or a rectangular block, pressure rises sharply near the object's largest cross-section. The flow separates, forming a wake of vortices behind the body. This makes the pressure distribution across the surface asymmetric and creates pressure drag. Rounding a corner can keep the flow from separating as it passes an edge. Delaying separation in this way reduces pressure drag. The same reasoning applies to pressure drag on a car.

Now consider skin-friction drag:

Figure 1-3

The velocity gradient and molecular viscosity within the boundary layer create wall shear stress τw at every point on the surface. The sum of the components of that stress in the direction of flow is skin-friction drag.

When the flow does not separate, friction can account for a large share of total drag; when separation is extensive, its share is small. This is why the dimples on a golf ball allow it to travel farther. On a streamlined body, including some vehicles, skin-friction drag makes up a much larger proportion of the whole.

All liquids and gases have viscosity. When adjacent parcels of fluid move at different speeds, friction acts between them. Viscosity also causes fluid next to a solid surface to adhere to it. These attached parcels slow the fluid moving past them and form a friction-producing boundary layer. Compared with a turbulent boundary layer, a laminar boundary layer interacts less strongly with the body's surface, reducing frictional drag and energy absorption. This is why friction represents a comparatively large share of the drag on streamlined bodies.

The car's surface therefore needs a smooth finish.

Induced Drag

Induced drag results from the pressure difference between the upper and lower surfaces of the body, though it is not itself lift. That pressure difference superimposes a vertical component on the horizontal freestream. Air moves around the sides of the body to equalize the pressure, creating vortices that travel downstream along the vehicle. The continuous production of vortices absorbs energy and therefore produces induced drag. Induced drag is related to aerodynamic lift, a point we will return to below.

Figure 2-1. Induced drag around a car

Roughness and Interference Drag

Roughness and interference drag include resistance created by surface discontinuities and accessories that protrude through the boundary layer around the car. Chassis and suspension components, wheels, mirrors, auxiliary lights, and windshield wipers all contribute.

Interference drag may be positive or negative; as the names suggest, the two have opposite effects. Positive interference drag arises between nearby or connected objects. Taking the car as the main body, a side mirror disturbs its original flow field, causes the air to separate earlier, and increases aerodynamic drag. Negative interference drag arises because every object in a flow has a slower-moving region behind it. Another object inside that region experiences less resistance than it would in the undisturbed flow. For example, when two discs are placed one behind the other at a suitable separation, their combined drag is lower than the sum of their drag when tested separately (Figure 2-2). The same principle appears in tractor-trailers: an aerodynamically optimized tractor can reduce the overall drag of the combination.

Figure 2-2

Internal-Flow Drag

Internal-flow drag, also called internal drag, is produced when air passes through the vehicle for engine cooling and ventilation. It includes momentum lost where the air exits and pressure lost as it moves through the radiator and engine compartment. These processes consume additional energy. Internal-flow drag accounts for about 5–12% of total air resistance.

Car Design and Aerodynamic Lift

A car's aerodynamic lift comes from its distinctive shape (Figure 3-1).

Figure 3-1

Under ideal conditions, Bernoulli's equation gives:

Combining this with the car's shape, in the ideal case:

F_top ＜ F_bottom

The result is aerodynamic lift.

In reality, the underbody also matters. Figure 3-2 shows simplified two-dimensional inviscid and viscous flows and pressure distributions around a car:

Figure 3-2

A car's rolling resistance is proportional to the normal force on its wheels. Increasing aerodynamic lift reduces that normal force and therefore reduces rolling resistance. At first, this seems to suggest that lift can lower the vehicle's total resistance. Yet greater lift also reduces traction and stability, while producing additional induced drag that may greatly exceed the reduction in rolling resistance. Increasing lift to reduce drag is therefore a poor trade. A large rise in lift weakens high-speed stability and creates a safety hazard. Above 70 km/s, for example, some cars may begin to feel “light” as aerodynamic instability reduces the driver's sense of contact with the road, potentially contributing to rollover or skidding.

This led to the wedge-shaped car. These aerodynamically optimized bodies, many of them sports cars, may also use spoilers to generate downforce. Their aerodynamic lift can even be negative, greatly improving stability at high speed.

(Some of the text and figures in this article draw on Aerodynamics of Road Vehicles.)