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Colored Schlieren photo of model airplane nose in a wind tunnel.

Aerodynamics: an introduction

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by Chris Woodford. Last updated: November 21, 2017.

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.)

Swimmer reaching arm forward as far as possible during front crawl.

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.

As you empty water from a bottle, you're filling it with air.

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.

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.

225kW wind turbine in Staffordshire, England

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.

Drag

Photo of a speed cyclist wearing a helmet.

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.)

Comparing the laminar and turbulent airflow around a car and a boxy truck in a wind tunnel at NASA Ames.

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.

Supersonic!

Jet airplane: sonic boom and visual mist created as a fighter jet breaks the sound barrier

Photo: Supersonic planes create a sonic boom when they overtake their own sound waves. Photo by John Gay courtesy of US Navy.

The faster you go, the harder it is go faster still—that's the tricky science that makes it so difficult to break speed records in cars, boats, and planes. In theory, the laws of fluid dynamics (of which aerodynamics is a part) apply in much the same way, whether you're speeding over salt flats in a rocket-propelled car, skimming over the waves in a hydrofoil boat, or screaming through the air in a military jet. Planes, however, are in a different category to cars and boats because they can go 5–10 times faster. Once they hit a certain speed, the speed of sound, different rules of aerodynamics come into play. Push your jet plane through the sound barrier and huge, cone-shaped shock waves form at the nose and the tail, where they can greatly increase drag. That's why supersonic (faster-than-sound) jet planes have sharp noses and sharp, swept-back wings. Go faster still and the aerodynamic rules change once again. At hypersonic speeds (around five times faster than sound), shorter wings work best and they need to be positioned further back from the nose than on a supersonic plane.

Continuity

It might seem obvious, but if fluid's flowing through or around an object, the amount of fluid you have at the end is the same as the amount you have at the start. Write this in math form and you get what's called the continuity equation. More formally speaking, it says that the volume of fluid flowing in one place is the same as the volume of fluid flowing in another place. It follows from this that the area through which the fluid flows multiplied by the velocity of the fluid is a constant: if a fluid flows into a narrower space, it has to speed up; if it flows into a wider space, it has to slow down. That helps to explain why wind really whistles down alleys between buildings and why, if you pinch the end of a hosepipe, the water squirts out faster. (It's also the reason that water being poured from a bottle, or falling from a faucet/tap, goes from a wide stream at the top to a much narrower one—because it's speeding up due to gravity and the release of pressure. You can see that clearly in the photo of water pouring up above.) We can use the continuity equation to help understand two other very useful bits of fluid dynamics: Bernoulli's principle and the Venturi effect.

Bernoulli's principle

Photo: Bernoulli on your tabletop: fold a sheet of paper into a tunnel shape and blow through it (top) and you'll see the paper squash down (bottom). The air speeds up inside the tunnel, the pressure goes down, and the paper collapses because atmospheric pressure pushes down on it from above.

Simple table-top demonstration of Bernoulli's principle: blowing through a paper tunnel makes it collapse.

Make yourself a rectangular tube of paper, put it on a table, and blow through it. As you do so, the paper will collapse down, then spring back up again when you run out of breath. What's happening? When a fluid flows from one place to another, it has to conserve its energy. In other words, there has to be as much energy at the end as there was at the start. We know this from the fundamental law of physics called the conservation of energy, which explains that you can't create or destroy energy, only change it from one form into another. Think about the air flowing through your homemade tube. The air just outside the tube, just where you're blowing, has three types of energy: potential energy, kinetic energy, and energy because of its pressure. The air in the middle of the tube has the same three types of energy. However, because the air is moving faster there, its kinetic energy must be greater. Since we can't have created energy out of nothing, there must have been a reduction in one of the other two types of energy. You're blowing straight across a table so the air doesn't rise or fall—and doesn't change its potential energy. The only place we can compensate for the extra kinetic energy is in the fluid's pressure. As the air speeds up, its pressure goes down. Since the air inside the tube is at a lower pressure than the air above it, the tube collapses until you stop blowing. Stated simply, Bernoulli's principle (pronounced Bur-noo-ee's) simply reminds us that the total energy in a moving fluid is constant. But you're likely to see it described a different way: if a fluid speeds up, its pressure goes down (and vice-versa).

How wings really work

A lot of science books tell us that Bernoulli's principle is the key to understanding how airfoils (curved wings on airplanes, also known as aerofoils) generate lift. The standard explanation goes like this. As air hits an airfoil, it splits into two streams, one of which shoots over the wing as the other dives underneath. People used to think that a simple difference in the speed of the two air streams caused the lift on the wing, but we now know this is wrong. The argument went like this: the upper surface of an airfoil is curved, while the lower surface is straight. We know from the continuity equation that there's as much air coming out from behind an airfoil wing as there is going into it at the front. So, theoretically, the air going above the stream has to go faster than the air going underneath it, because it has to go further. Bernoulli's principle tells us that fast-moving air is at a lower pressure than slower-moving air, so there's less pressure above the airfoil, and this is what generates the lift (upward force) as it travels through the air.

Unfortunately, this turns out to be false, both experimentally and in theory. With simple experiments, we can show that a plane can fly if its airfoils have identical upper and lower profiles (if they're symmetrical, in other words): a paper airplane with flat wings will fly perfectly well. The theoretical explanation is also easy to understand: we're talking about two continuous streams of air, one above and one below the airfoil, and there's absolutely no reason why two air molecules that separate at the front of an airfoil (one taking the upper route, one the lower) should neatly meet up again at the back, having traveled different distances in the same time; one molecule could easily take longer than the other and meet up with a different air molecule at the back. The real explanation of why airfoils create lift is down to a combination of pressure differences and Newton's third law of motion. An airfoil wing generates lift because it's both curved and tilted back, so the oncoming air is accelerated over the top surface and then forced downward. This creates a region of low pressure directly above the wing, which generates lift. The wing's tilted angle forces the air downward, and that also pushes the plane upward (Newton's third law). Find our more in our article on airplanes.

Air flow around an airfoil wing in a wind tunnel

Photo: When a plane tips its wings at too steep an angle, it "stalls." Instead of smooth laminar flow around the wing, as in normal flight, we get turbulent flow instead. The wing no longer generates lift and the plane can plummet downward. In this photo, note the smooth laminar flow in front of the wing (on the right of the photo), above it, and below it. Compare that with the turbulent flow just behind it where the smooth lines of laminar flow have been churned up and disrupted. Photo courtesy of NASA Langley Research Center.

The Venturi effect

Have you ever been on a canal barge as it sails upstream through calm water next to another, similar boat? As the two boats whizz along, they're very likely to drift and bump together. This is an example of the Venturi effect, which follows on from the continuity equation and Bernoulli's principle. The basic idea is that when a fluid flows into a narrower space, it speeds up and the pressure drops. So the speeding water between the two boats creates a low-pressure zone between them that moves them together. This is one of the reasons why wind farms are sometimes built in valleys between hills or mountains, where the wind speed is higher. You can also see the Venturi effect (and other aerodynamic principles such as Bernoulli's) at work in the Dyson Air Multiplier fan. (Here's a neat video of it on YouTube.)

Why aerodynamics matters

How a fairing mounted on a truck's cab diverts airflow, saving energy and fuel.

Photo: A simple plastic fairing mounted on top of a truck's cab can save a huge amount of fuel. It works by diverting the airflow (yellow arrows) more smoothly above and around the sides of the huge, boxy cargo container behind the cab.

Why should we care about aerodynamics? Why does it matter? Suppose you run a haulage firm and you have 500 trucks driving around the country delivering supplies to supermarkets. Apart from the trucks themselves and the wages of the drivers, the biggest cost your business faces is fuel. If you fit a relatively inexpensive fairing (a sloped piece of plastic) to the top of your trucks so the air is deflected smoothly up and over the cargo container behind, you'll cut the fuel consumption by 10–20 percent and save a huge amount of money. Fitting side-shields to the underside of the cargo container (to stop turbulent airflow underneath them) will save more. The same goes for cars. Driving around with a roof-rack in place when you're carrying nothing on it will increase the fuel you use (and the amount you have to pay in gasoline/petrol) by about five percent. Why? Because the rack drags in the air and slows you down.

For airplanes and space rockets, aerodynamics is even more important. When spacecraft return to Earth, they pass from the virtual vacuum of space into Earth's atmosphere at high speed, which heats them up dangerously; in February 2003, the Space Shuttle Columbia was tragically destroyed, killing all seven astronauts onboard, when it overheated on reentry. Understanding better how air moves over a spacecraft is essential if we want to avoid such things happening in future.

Aerodynamics matters for the rest of us too. If you're a keen cyclist and you want to win a race, you need to use your energy as efficiently possible, losing as little to the air as you can. If you're a motorist who travels reasonably long distances on the freeway (motorway), minimizing air resistance is one of your best ways of saving fuel, saving money, and helping the planet.

A brief history of aerodynamics

Railton Mobil Special

Photo: Record breaker! Whether you're on land, air, or water, you can't break speed records unless you understand aerodynamics. This is the Railton Mobil Special, which became the first car to go faster than 640 km/h (400 mph) in the 1940s. Here the streamlined bodywork has been lifted up high off the chassis so you can inspect the engine inside. See it for yourself at Think Tank, the science museum in Birmingham, England.

Here's a quick tour through some of the major moments and key figures in the history of aerodynamics:

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Woodford, Chris. (2012/2017) Aerodynamics. Retrieved from http://www.explainthatstuff.com/aerodynamics.html. [Accessed (Insert date here)]

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