The Rundown on Aerodynamics

It’s unpleasant to think about, but imagine what would happen if you drove your car into a brick wall at­ 65 miles per hour (104.6 kilometers per hour). Metal would twist and tear. Glass would shatter. Airbags would burst forth to protect you. But even with all the advancements in safety we have on our modern automobiles, this would likely be a tough accident to walk away from. A car simply isn’t designed to go through a brick wall.

But there is another type of “wall” that cars are designed to move through, and have been for a long time — the wall of air that pushes against a vehicle at high speeds.

Most of us don’t think of air or wind as a wall. At low speeds and on days when it’s not very windy outside, it’s hard to notice the way air interacts with our vehicles. But at high speeds, and on exceptionally windy days, air resistance has­ a tremendous effect on the way a car accelerates, handles and achieves fuel mileage.

This is where the science of aerodynamics comes into play. Aerodynamics is the study of forces and the resulting motion of objects through the air. For several decades, cars have been designed with aerodynamics in mind, and carmakers have come up with a variety of innovations that make cutting through that “wall” of air easier and less of an impact on daily driving.

­Essentially, having a car designed with airflow in mind means it has less difficulty accelerating and can achieve better fuel economy numbers because the engine doesn’t have to work nearly as hard to push the car through the wall of air.

Engineers have developed several ways of doing this. For instance, more rounded designs and shapes on the exterior of the vehicle are crafted to channel air in a way so th­at it flows around the car with the least resistance possible. Some high-performance cars even have parts that move air smoothly across the underside of the car (much more on that later). Many also include a spoiler and rear wing to keep the air from lifting the car’s wheels and making it unstable at high speeds. Although, as you’ll read later, most of the spoilers / wings  that you see on cars are simply for decoration more than anything else.

In this article, we’ll look at the physics of aerodynamics and air resistance, the history of how cars have been designed with these factors in mind and how with the trend toward “greener” cars, aerodynamics is now more important than ever.

 

The Science of Aerodynamics

Aerodynamics is part of a branch of physics called fluid dynamics, which is all about studying liquids and gases that are moving. Although it can involve very complex math, the basic principles are relatively easy-to-understand; they include how fluids flow in different ways, what causes drag (fluid resistance), and how fluids conserve their volume and energy as they flow. Another important idea is that when an object moves through a stationary fluid, the science is pretty much the same as if the fluid moved and the object were still. That’s why it’s possible to study the aerodynamic performance of a car or an airplane in a wind tunnel: blasting high-speed air around a still model of a plane or car is the same as flying or driving through the air at the same speed.

Laminar and turbulent flow

When you empty water from a plastic bottle, you’ve probably noticed you can do it in two very different ways. If you tip the bottle at a shallow angle, the water comes out very smoothly; air moves past it, in the opposite direction, filling the bottle with “emptiness.” If you tip the bottle more, or hold it vertically, the water comes out noisily, in jerks; that’s because the air and the water have to fight at the neck of the bottle. Sometimes the water wins and rushes out, sometimes the air wins and rushes in, briefly stopping the water flow. The fight between water exiting and air entering gives you the characteristic “glug-glug” sound as you pour.

Pour water slowly from a bottle and you get smooth, laminar flow. Tip the bottle up more and the flow will become turbulent. Also, can you see how the spout of water dripping down from this bottle gets narrower toward the bottom where the water moves faster (after being accelerated by gravity)? That’s an example of fluid continuity, which is explained below.

What we see here are the two extreme types of fluid flow. In the first case, we have the water and the air sliding very smoothly past one another in layers, which is called laminar flow (or streamline flow because the fluid flows in parallel lines called streamlines). In the second case, the air and water move in a more erratic way, which we called turbulent flow. If we’re trying to design something like a sports car, ideally we want to shape the body so the flow of air around it is as smooth as possible—so it’s laminar rather than turbulent. The more turbulence there is, the more air resistance the car will experience, the more energy it will waste, and the slower it will go.

Boundary layer

The speed at which a fluid flows past an object varies according to how far from the object you are. If you’re sitting in a parked car and a gale-force wind is howling past you at 200km/h (125mph), you might think the difference in speed between the air and the car is 200km/h—and it is! But there’s not a sudden, drastic discontinuity between the stationary car and the fast-moving air. Right next to the car, the air speed is actually zero: the air sticks to the car because there are attractive forces between the molecules of the car’s paintwork and the air molecules that touch them. The further away from the car you get, the higher the wind speed. A certain distance from the car, the air will be traveling at its full speed of 200km/h. The region surrounding the car where the air speed increases from zero to its maximum is known as the boundary layer. We get laminar flow when the fluid can flow efficiently, gently and smoothly increasing in speed across the boundary layer; we get turbulent flown when this doesn’t happen—when the fluid jumbles and mixes up chaotically instead of sliding past itself in smooth layers.

Wind speed increases with distance from the ground. In theory, a wind turbine tower has to be high enough to ensure the rotors are operating outside the boundary layer. In practice, turbine designers have to compromise: very high turbines may be unacceptable for all kinds of environmental and safety reasons.

The idea of the boundary layer leads to all kinds of interesting things. It explains why, for example, your car can be dusty and dirty even though it’s racing through the air at high speed. Although it’s traveling fast, the air right next to the paintwork isn’t moving at all, so particles of dirt aren’t blown away as you might expect them to be. The same applies when you try to blow the dust off a bookshelf. You can blow really hard, but you’ll never blow all the dust away: at best, you just blow the dust (the upper layers of dust particles) off the dust (the lower layers that stay stuck to the shelf)! The boundary layer concept also explains why wind turbines have to be so high. The closer to the ground you are, the lower the wind speed: at ground level, on something like concrete, the wind speed is actually zero. Build a wind turbine that’s way up in the sky and you’re (hopefully) reaching beyond the boundary layer to the place where the air speed is a maximum and the wind has higher kinetic energy to drive the turbine’s rotors.

Even More Science of Aerodynamics