Editing Wind tunnel (section)

{{Short description|Machine used for studying the effects of air moving around objects}}
{{Use dmy dates|date=May 2014}}
[[File:8-Foot Transonic Pressure Tunnel (9443013405).jpg|thumb|A sample wind tunnel layout showing some typical features including a test section and control room, a machine for pumping air continuously through ducting, and a nozzle for setting the test airspeed.]]

A '''wind tunnel''' is "an apparatus for producing a controlled stream of air for conducting aerodynamic experiments".<ref name="Biophysical">{{cite book |url=https://archive.org/details/biophysicalaerod0000ward/mode/2up |title=Biophysical Aerodynamics and the Natural Environment, Glossary of Terms|author1=Ward-Smith, Alfred John |publisher=John Wiley & Sons|location=New York |date=1994| isbn=0471904368 |page=[https://archive.org/details/biophysicalaerod0000ward/page/166/mode/2up 166]}}</ref> The experiment is conducted in the test section of the wind tunnel and a complete tunnel configuration includes air ducting to and from the test section and a device for keeping the air in motion, such as a fan. Wind tunnel uses include assessing the effects of air on an [[aircraft]] in flight or a ground vehicle moving on land, and measuring the effect of wind on [[Building|buildings]] and bridges. Wind tunnel test sections range in size from less than a foot across, to over {{convert|100|ft|m}}, and with air speeds from a light breeze to hypersonic.

The earliest wind tunnels were invented towards the end of the 19th century, in the early days of aeronautical research, as part of the effort to develop heavier-than-air flying machines. The wind tunnel reversed the usual situation. Instead of the air standing still and an aircraft moving, an object would be held still and the air moved around it. In this way, a stationary observer could study the flying object in action, and could measure the aerodynamic forces acting on it.

The development of wind tunnels accompanied the development of the airplane. Large wind tunnels were built during [[World War II]], and as supersonic aircraft were developed, supersonic wind tunnels were constructed to test them. Wind tunnel testing was considered of strategic importance during the [[Cold War]] for development of aircraft and missiles.

Advances in [[computational fluid dynamics]] (CFD) have reduced the demand for wind tunnel testing, but have not completely eliminated it. Many real-world problems can still not be modeled accurately enough by CFD to eliminate the need for wind tunnel testing. Moreover, confidence in a numerical simulation tool depends on comparing its results with experimental data, and these can be obtained, for example, from wind tunnel tests.