Aerodynamics: an introduction
by Chris Woodford. Last updated: December 30, 2016.
Have you ever ridden in an open-top car and felt the wind pushing past your face? It's exhilarating and you feel really alive, but it's also surprising, because we don't normally feel the air at all. Although we're surrounded by this mysterious gas, and life is impossible without it, we hardly ever give it a moment's thought. Understanding how air behaves when we slice through it at speed is incredibly important: without the science of aerodynamics, as it's known, we'd never be able to design planes or spacecraft, land-speed record cars, or bridges that can survive hurricanes. So what exactly is aerodynamics? Let's take a closer look!
Photo: If you can't see airflow, how can you hope to measure it or study it? Scientists have developed all sorts of cunning ways of making airflow visible to help them develop faster and more energy-efficient vehicles, including Schlieren photography, a way of revealing a moving airstream in patterns of lines and color.
What is aerodynamics?
One of the most obvious differences between solids, liquids, and gases is their density: how many atoms of "stuff" there are in a given space. Solids and liquids are much more dense than gases—and you'll know this is if you've ever tried walking through a swimming pool. Compared to walking through the air, it's incredibly hard work to propel your body through water. You literally have to push the water that's in front of you out of the way; as you move forward, the water sloshes around you into the space you've just left behind. It's much faster to swim through water than to walk through it because you can make your body into a long, thin shape that creates less resistance: you glide through the water more smoothly, disturbing it less, and because there's less resistance, you can move faster. (Find out more in our article about the science of swimming.)
Photo: You can swim faster and for longer by ensuring your body disturbs the water as little as possible as it moves along. You can't see air, but exactly the same principle applies. Notice how the "bow waves" this swimmer creates stretch back from his body in a similar way to the shock waves created by the airplane in the photo above. Like all waves, these carry energy away from whatever creates them. To swim or fly efficiently, it pays to create as little wave disturbance as possible. Photo by Joseph M. Clark courtesy of US Navy.
Moving through air is much the same. Like water, air is a fluid (the name we give to liquids and gases that can easily move, or flow) and, generally speaking, most fluids behave the same way. If you want to speed quickly through the air, you're better off in a long, thin vehicle—something like a plane or a train—that creates as little disturbance as possible: planes and trains are tube-shaped for exactly the same reason that we swim horizontally with our bodies laid out long and thin.
Thinking about how to move through a fluid quickly and effectively is really what aerodynamics is all about. If we want a more formal, scientific definition, we can say that aerodynamics is the science of how things move through air (or how air moves around things).
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.
Photo: 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.
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.
Photo: 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.
Photo: The faster you go, the harder you have to work against the air. Air resistance doesn't really matter when you walk, because you're not traveling fast enough. But if you cycle, you need to think about aerodynamics much more and adopt a streamlined posture that disturbs the airflow as little as possible. Like this rider, you might want to get a tear-shaped cycling helmet and wear tight-fitting clothes?
Air resistance—drag, as it's usually known—follows on from the distinction between laminar and turbulent flow. When a sports car speeds through the air, the flow remains relatively laminar; when a truck plows through it, there's much more turbulence. Drag is the force that a moving body feels when the flow of air around it starts to become turbulent. If you ride a bike or you've ever run a sprint race, it'll be very obvious to you that drag increases with speed. But a very important point is that it doesn't increase linearly as your speed increases but according to the square of your speed. In other words, if you double your speed, roughly speaking you quadruple the drag. Fast-moving vehicles use most of their energy overcoming drag; once you reach about 300km/h (180mph), you're using virtually all your energy trying to push the air out of the way. This doesn't just apply to land-speed record cars but to ordinary drivers as well: for stop-start city driving, you waste most of your energy in braking; when you speed along on the highway, most of your energy is being lost pushing aside the air. (To see the simple math behind this, take a look at David MacKay's discussion in his book Sustainable Energy Without Hot Air.)
Photo: Top: Friction drag: The aerodynamic shape of this car allows the airflow around it to remain reasonably laminar. There is drag, but mainly caused by friction between the layers of air moving past one another at different speeds. Notice how the air becomes more turbulent behind the car and vortices start to occur in the wake. Bottom: Form drag: A box-shaped vehicle (such as a large truck) makes no attempt to divert air around itself. As soon as the air smacks into it, it starts to become turbulent. Photo by Eric James courtesy of NASA Ames Laboratory.
Why does drag happen? There are two types called friction drag and form drag and they have different causes. Imagine a car sitting still as the wind speeds past it. If the car is smoothly shaped, the air next to its paintwork isn't moving at all. The layer just beyond that is moving a little bit, and the layer beyond that is moving a little bit more. All these layers of air are sliding past one another in exactly the same way that your foot might slide across the floor: they have to overcome the mutual attraction between one another's molecules, which causes friction. Friction drag happens because it takes energy to make layers of air slide past one another.
The rougher or more obstructive the object, the more turbulent the air flow becomes, the greater the friction between the layers, and the greater the drag. At low speeds, the air flows splits when it meets an object and, providing the object is reasonably aerodynamic, flows right around it, closely following its outline. But the faster the air flow and the less aerodynamic the object, the more the air flow breaks away and becomes turbulent. That's what we mean by form drag.