General Motors owns the world's biggest wind tunnel dedicated to auto testing. The fan has a 43-foot (13-meter) diameter.

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Wicked Wind Tunnels

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.