• Wind tunnels are used in testing all types of vehicles, including bicycles.Andy Sacks/Photodisc/Getty Images
  
  Engineers use wind tunnels to test, measure and control air flow conditions on various types of vehicles. Designed for specific purposes, different types and classifications of wind tunnels exist. NASA classifies wind tunnels by tunnel geometry and speed regime, along with other criteria, according to their website.
  
  

Circuit Tunnels

• Types of circuit tunnels, whether open or closed, depend on how air flow is discharged from the test section of the wind tunnel. Open-circuit tunnels discharge air flow out of the test chamber into another area. Air forced through closed-circuit or closed-return tunnels is re-circulated, generally with a small vent or "breather" located somewhere along the circuit. The vent allows for the release of internal pressure created as the air heats. Using directional vanes, the air stream in a closed-circuit tunnel is generally turned four times by 90 degrees each time.
  
  

Suckdown Tunnels

• In a suckdown tunnel air flows from a top entry point down to exhaust louvers at the bottom, with the outdoors as the return path, thus the "suckdown" configuration. Although different sizes of open-circuit suckdown tunnels exist, the most common possess a test section measuring approximately four feet square. As of 2010, NASA's Ames Research Center's open-circuit suckdown tunnel at 80 feet by 120 feet is the world's largest, according to the Wind Tunnel Design website.
  
  

Blowdown Tunnels

• Blowdown wind tunnels deal with upstream and downstream air pressure, according to the NASA website. Located upstream is the high pressure vessel, with the low pressure downstream of the test section. Used primarily for supersonic testing, the blowdown tunnel is classified as a closed-circuit wind tunnel. Using a compressed-air reservoir, air is intermittently discharged, producing a supersonic or transonic flow. Although no longer in operation, the first pressure wind tunnel, NASA's Variable Density Tunnel, was located at the Langley Research Center.