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.