How Wind Tunnels Work

In hopes of taking humans to the heavens, early flight engineers tried to follow the example of birds. Leonardo da Vinci, for instance, sketched a so-called "ornithopter" in 1485. Yet our winged friends proved less than helpful when it came to revealing the secrets of flight. Numerous inventors fabricated bird-inspired machines, only to watch them flop around helplessly in the dirt.

It became clear that in order for humans to fly, they needed a better understanding of the interplay between wings and winds. So, these fledgling fanciers of flight went in search of hilltops, valleys and caves with powerful, somewhat predictable winds. But natural winds didn't provide the steady flow that could offer helpful design feedback -- artificial winds were necessary.

Enter the whirling arms. In 1746, Benjamin Robins, an English mathematician and scientist, attached a horizontal arm to a vertical pole, which he rotated, sending the arm spinning in a circle. At the end of the arm, he affixed a variety of objects and subjected them to the forces of his homemade centrifuge. His tests immediately confirmed that the shape of things had a tremendous effect on air resistance (also known as drag, an element of aerodynamic force).

Other experimenters, such as Sir George Cayley, soon built whirling arms. Cayley, in particular, tested airfoil shapes, which looked a lot like a cross-section of an airplane wing, to investigate principles of drag and lift. Lift is an element of force that moves perpendicular to the direction of an object's motion.

The rotating arm had a serious side effect, however, in that it chopped up the air as it spun, basically creating hellacious turbulence that greatly impacted all results and observations. But the arm did result in one monumental breakthrough: Engineers began to realize that by quickly propelling an object through the air, they could develop lift. That meant it wasn't necessary to build flapping wings in order to fly. Instead, humans needed enough power and the right kind of wing construction. Scientists needed better investigative tools to work out those important questions. Wind tunnels were the answer.

On the next page, you'll find out how spinning arms evolved into wind tunnels -- and you'll see how those tunnels were instrumental to one of the biggest technological achievements in the history of humankind.

Because whirling arms chopped the air and created wake that invalidated many experiments, scientists needed calmer, artificial winds. Frank H. Wenham, an Englishman active with the Aeronautical Society of Great Britain, convinced the organization to help finance the construction of the first wind tunnel, which debuted in 1871.

Wenham's tunnel was 12 feet (3.7 meters) long and 18 inches (45.7 centimeters) square. It produced 40 mile-per-hour (64 kilometer-per-hour) winds, thanks to a steam-powered fan at the end of the tunnel. In his tunnel, Wenham tested the effects of lift and drag on airfoils of different shapes. As he moved the front edge (called the leading edge) of the airfoil up and down, changing what's called the angle of attack, he found that certain shapes resulted in better lift than anticipated. Man-powered flight suddenly seemed more possible than ever before.

Yet the tunnel's rough design created winds that were too unsteady for consistent test results. Better tunnels were needed for systematic testing and reliable results. In 1894, Englishman Horatio Philips substituted a steam injection system for fans, resulting in steadier, less turbulent air flow.

Across the Atlantic Ocean, in Ohio, the Wright brothers, Orville and Wilbur, were following developments in aerodynamics studies and conjuring ideas for glider designs. But real-world testing of their models was proving to be too time-consuming; it also didn't provide them with enough data to improve their plans.

They knew they needed a wind tunnel. So, after a bit of tinkering, they constructed a tunnel with a 16-inch (40.6-centimeter) test section. They experimented with around 200 different types of wing shapes by attaching airfoils to two balances -- one for drag, and one for lift. The balances converted airfoil performance into measurable mechanical action that the brothers used to complete their calculations.

Slowly, they worked to find the right combination of drag and lift. They began to realize that narrow, long wings resulted in much more lift than short, thick wings, and in 1903, their meticulous wind tunnel testing paid off. The Wright brothers flew the first manned, powered airplane in Kill Devil Hills, N.C. A new age of technological innovation had begun, in large part thanks to wind tunnels.

Next, you'll see exactly how wind tunnels work their invisible magic and help blow humankind into a new technological era.

The first wind tunnels were just ducts with fans at one end. These tunnels made choppy, uneven air, so engineers steadily worked to improve airflow by tweaking tunnel layouts. Modern tunnels provide much smoother airflow thanks to a fundamental design that incorporates five basic sections: the settling chamber, contraction cone, test section, diffuser and drive section.

Air is a swirling, chaotic mess as it enters the tunnel. The settling chamber does exactly what its name implies: It helps to settle and straighten the air, often through the use of panels with honeycomb-shaped holes or even a mesh screen. The air is then immediately forced through the contraction cone, a constricted space that greatly increases airflow velocity.

Engineers place their scaled models in the test section, which is where sensors record data and scientists make visual observations. The air subsequently flows into the diffuser, which has a conical shape that widens, and thus, smoothly slows the air's velocity without causing turbulence in the test section.

The drive section houses the axial fan that creates high-speed airflow. This fan is always placed downstream of the test section, at the end of the tunnel, rather than at the entrance. This setup allows the fan to pull air into a smooth stream instead of pushing it, which would result in much choppier airflow.

Most wind tunnels are just long, straight boxes, or open-circuit (open-return) tunnels. However, some are built in closed circuits (or closed return), which are basically ovals that send the air around and around the same path, like a racetrack, using vanes and honeycomb panels to precisely guide and direct the flow.

The walls of the tunnel are exceedingly smooth because any imperfections could act as speed bumps and cause turbulence. Most wind tunnels are also moderately sized and small enough to fit into a university science lab, which means that test objects must be scaled down to fit into the tunnel. These scale models might be entire airplanes in miniature, built (at great expense) with exacting precision. Or they might just be a single part of an airplane wing or other product.

Engineers mount models into the test section using different methods, but usually, the models are kept stationary using wires or metal poles, which are placed behind the model to avoid causing disruptions in the airflow They may attach sensors to the model that record wind velocity, temperature, air pressure and other variables.

Keep reading to learn more about how wind tunnels help scientists piece together more complicated aerodynamics puzzles and how their findings spur technological advances.

Lift and drag are just two elements of aerodynamics forces that come into play inside a wind tunnel. For aircraft testing in particular, there are dozens of variables (like pitch, yaw, roll and many others), that can affect the outcome of experiments.

Other factors also come into play during testing no matter what the test subject might be. For example, the quality of the air in the tunnel is changeable and has a tremendous bearing on test results. In addition to carefully gauging the shape and speed of the object (or the wind blowing past the object) testers must consider the viscosity (or tackiness) and compressibility (bounciness) of the air during their experiments.

You don't normally think of air as a sticky substance, of course, but as air moves over an object, its molecules strike its surface and cling to it, if only for an instant. This creates a boundary layer, a layer of air next to the object that affects airflow, just as the object itself does. Altitude, temperature, and other variables can affect viscosity and compressibility, which in turn changes the boundary layer properties and drag, and the aerodynamics of the test object as a whole.

Figuring out just how all these conditions affect the test object requires a system of sensors and computers for logging sensor data. Pitot tubes are used to measure airflow velocity, but advanced tunnels deploy laser anemometers that detect wind speed by "seeing" airborne particles in the airstream. Pressure probes monitor air pressure and water vapor pressure sensors track humidity.

In addition to sensors, visual observations are also extremely useful, but to make airflow visible, scientists rely on various flow visualization techniques. They may fill the test section with colored smoke or a fine mist of liquid, such as water, to see how air moves over the model. They may apply thick, colored oils to the model to see how the wind pushes the oil along the model's surface.

High-speed video cameras may record the smoke or oils as they move to help scientists detect clues that aren't obvious to the unaided eye. In some cases, lasers are used to illuminate mist or smoke and reveal airflow details.

Wind tunnels offer endless configurations for testing limitless ideas and concepts. Keep reading, and you'll see the wildly imaginative tunnels that engineers build when they find the money to turn a breeze of an idea into a full-scale technological tempest.

Supersonic and hypersonic tunnels don't use fans. To generate these breakneck air velocities, scientists use blasts of compressed air stored in pressurized tanks placed upstream of the test section, which is why they are sometimes called blowdown tunnels. Similarly, hypersonic tunnels are sometimes called shock tubes, a reference to the high-powered but very brief blasts they produce. Both have enormous power requirements, which generally make them best for short or intermittent tests.

Air pressure capabilities also differentiate wind tunnels. Some tunnels have controls for lowering or raising air pressure. For example, in testing space vehicles, NASA could set up a tunnel to mimic the low-pressure atmosphere of Mars.

You can also categorize tunnels by size. Some are relatively small, and thus, are useful only for testing scaled-down models or sections of an object. Others are full-scale and big enough to test full-sized vehicles.

And some wind tunnels are just…well, really big.

NASA's Ames Research Center, near San Jose, Calif. is home to the world's largest wind tunnel. It's about 180 feet (54.8 meters) high, more than 1,400 feet (426.7 meters) long, with one test section that's 80 feet (24 meters) tall and 120 feet (36.5 meters) wide, big enough to accommodate a plane with a 100-foot (30-meter) wingspan. The tunnel uses six, four-story high fans, each driven by six 22,500 horsepower motors that can drive winds up to 115mph (185 kph).

Size isn't the only factor in extraordinary wind tunnels. Keep reading, and you'll find out just how modern some of these tunnels really are.

Engineers often need to test multiple aerodynamic and environmental variables simultaneously. That's why some tunnels offer a broad array of testing possibilities in a single location. The Vienna Large Climatic Wind Tunnel, used mostly for automobile and rail vehicle testing, is one such tunnel. The test section alone is 328 feet (100 meters) long, through which wind speeds of up to 186 mph (299 kph) flow.

Engineers can adjust relative humidity from 10 to 98 percent and push temperatures from as low as -49 degrees to 140 degrees Fahrenheit (-45 to 60 Celsius). True to its name, the Vienna Climatic Tunnel comes complete with rain, snow and ice capabilities, in addition to solar exposure simulators.

Icing capability, in particular, has been a critical component in wind tunnels for decades, because ice buildup on airplane surfaces can be disastrous, causing a plane to crash. Icing tunnels have refrigeration systems that cool the air and then spray fine droplets of water into the airflow, producing a glaze on the test models. Engineers can then tinker with solutions to counter ice buildup, for example, by installing heating systems that warm the surfaces of the plane.

There are a lot of other tunnel types designed for specific purposes. Some designs skip poles or wires for securing the model and instead use powerful magnets that suspend metallic models in the test section. Others provide remote control wires that let scientists actually "fly" a model plane within the test area.

The University of Texas at Arlington's Aerodynamics Research Center has what's called an arc jet tunnel, which generates supersonic streams of very hot gas at temperatures up to 8,540 degrees Fahrenheit (4,727 Celsius). These kinds of temperatures are especially useful for NASA, which subjects its spacecraft to high heat as they re-enter Earth's atmosphere.

Some tunnels omit air entirely and instead use water. Water flows much like air, but it has greater density than air and is more visible, too. Those properties help scientists visualize flow patterns around submarines and ship hulls, or even better see shockwaves created by very fast aircraft and missiles.

So what's the point of blowing all of this hot and cool air around, anyway? It's not just so that scientists can get their geek on -- on the next page, you'll see how wind tunnels help us do a lot more than fly.

Engineers and manufacturing specialists use wind tunnels to improve not just airplanes and spacecraft, but an entire assortment of industrial and consumer products. Automobile makers, in particular, rely heavily on wind tunnels.

General Motors' Aerodynamics Laboratory has the biggest wind tunnel for studying car aerodynamics. Since building the tunnel three decades ago, the company's engineers have reduced the coefficient of drag of their vehicles by around 25 percent. That kind of improvement boosts fuel economy by two to three miles per gallon.

Race-car makers use the tunnels to improve car aerodynamics, particularly speed and efficiency, to help them get a competitive edge. AeroDyn Wind Tunnel, for example, is located in North Carolina and specializes in testing full-size NASCAR stock cars and other racing cars and trucks. Another company, called Windshear, also operates in North Carolina and owns an advanced closed-circuit tunnel with a built-in rolling road, which is basically a huge treadmill for cars.

Electronics engineers use small wind tunnels to see how airflow affects heat buildup in components. Then they can design cooler computer chips and motherboards that last longer. Utilities managers use wind tunnels to test wind turbines used to generate electricity. Wind tunnels help make the turbines and their blades more efficient, effective and durable, so they can withstand constant, powerful gusts. But wind tunnels also help engineers determine wind farm layouts and turbine spacing, so as to maximize efficiency while minimizing power-sucking turbulence.

Wind tunnels and test models aren't cheap to build. That's why more and more organizations are deactivating their wind tunnels and shifting to computer modeling (also called computational fluid dynamics), which is now often used in place of physical models and tunnels. What's more, computers let engineers adjust infinite variables of the model and the test section without time-consuming (and expensive) manual labor. Physical tunnels are sometimes used only to retest the results of computer modeling.

Construction engineers use computer modeling for wind engineering testing to help them design and build skyscrapers, bridges and other structures. They investigate the interplay of building shapes and materials and wind to make them safer and stronger.

For now, though, wind tunnels are still in active use all around the world, helping scientists make safer and more efficient products and vehicles of all types. And even if newer virtual technologies do eventually replace physical wind tunnels, these marvels of engineering will always have a place in the history of humankind's development.