Editing Wind tunnel (section)

==Classification==
There are many different kinds of wind tunnels.  They are typically classified by the range of speeds that are achieved in the test section, as follows:

* Low-speed wind tunnel
* [[Subsonic and transonic wind tunnel]]
* [[Supersonic wind tunnel]]
* [[Hypersonic wind tunnel]]
* High enthalpy wind tunnel

Wind tunnels are also classified by the orientation of air flow in the test section with respect to gravity.  Typically they are oriented horizontally, as happens during [[level flight]].  A different class of wind tunnels are oriented vertically so that gravity can be balanced by drag instead of lift, and these have become a popular form of recreation for simulating [[sky-diving]]:

* [[Vertical wind tunnel]]
Wind tunnels are also classified based on their main use. For those used with land vehicles such as cars and trucks the type of floor aerodynamics is also important. These vary from stationary floors through to full moving floors, with smaller moving floors and some attempt at boundary level control also being important.

===Aeronautical wind tunnels===
The main subcategories in the aeronautical wind tunnels are:

====High Reynolds number tunnels====
[[Reynolds number]] is one of the governing similarity parameters for the simulation of flow in a wind tunnel. For [[mach number]] less than 0.3, it is the primary parameter that governs the flow characteristics. There are three main ways to simulate high Reynolds number, since it is not practical to obtain full scale Reynolds number by use of a full scale vehicle.{{citation needed|date=April 2022}}
* Pressurised tunnels: Test gases are pressurised to increase the Reynolds number.
* Heavy gas tunnels: Heavier gases like [[freon]] and [[R-134a]] are used as test gases. The transonic dynamics tunnel at [[NASA]] Langley is an example of such a tunnel.
* Cryogenic tunnels: Test gas is cooled down to increase the Reynolds number. The [[European transonic wind tunnel]] uses this technique.
* High-altitude tunnels: These are designed to test the effects of shock waves against various aircraft shapes in near vacuum.  In 1952 the University of California constructed the first two high-altitude wind tunnels: one for testing objects at {{convert|50|to|70|mi|km}} above the earth and the second for tests at {{convert|80|to|200|mi|km}} above the earth.<ref>{{Citation|url=https://books.google.com/books?id=8dwDAAAAMBAJ&pg=PA105|title= "Windless Wind Tunnels for High Altitude Tests."|date= February 1952|publisher= Hearst Magazines}}</ref>

====V/STOL tunnels====
[[V/STOL]] tunnels require large cross section area, but only small velocities. Since power varies with the cube of velocity, the power required for the operation is also less. An example of a V/STOL tunnel is the [[NASA]] Langley {{convert|14|by|22|ft|m|adj=on}} tunnel.<ref>{{Citation|url=http://www.aeronautics.nasa.gov/atp/facilities/14x22/index.html|archive-url=https://web.archive.org/web/20090321014629/http://www.aeronautics.nasa.gov/atp/facilities/14x22/index.html|archive-date=2009-03-21|title=14'x22' Subsonic Wind Tunnel}}</ref>

====Tunnels with vertical airflow====
[[File:Vertical wind tunnel at TsAGI.jpg|right|thumb|Vertical wind tunnel T-105 at [[TsAGI|Central Aerohydrodynamic Institute]], Moscow, built in 1941 for aircraft testing]]
[[File:Spin Tunnel Model - GPN-2000-001916.jpg|thumb|Model aircraft in a vertical tunnel showing parachute used to help recovery from a spin.]]
Vertical wind tunnels have a test section with air flowing upwards. Photography is used to record free-flight spin characteristics of aircraft models. Nets are installed above and below the test section to prevent the model from moving too high and to catch it when the air stops flowing.<ref>{{cite book|url=https://archive.org/details/nasa_techdoc_19670012052| title=Wind Tunnels and Their Instrumentation |author1=Gorlin, S. M.|author2=Slezinger, I. I|date=1966-01-01 |page=[https://archive.org/details/nasa_techdoc_19670012052/page/n49/mode/2up 44]}}</ref>

===Automotive tunnels===
Automotive wind tunnels fall into two categories:
* those which are used to determine the aerodynamic coefficients of the vehicle,
* climatic tunnels that evaluate vehicle operability under a wide range of simulated environmental conditions including extreme cold, snow, solar loading and humidity.

Wind tunnel testing of automobiles began in the 1920s,<ref>{{Cite journal |last=Ludvigsen |first=Karl E. |date=1970 |title=The Time Tunnel - An Historical Survey of Automotive Aerodynamics |url=https://www.academia.edu/17643611 |journal=SAE Technical Paper Series |volume=1 |doi=10.4271/700035 |issn=0148-7191}}</ref> on cars such as the [[Rumpler Tropfenwagen]], and the [[Chrysler Airflow]]. Initially, scale models were tested, then larger wind tunnels were built to test full-scale cars with the capability to measure aerodynamic drag which enables improvements to be made for reducing fuel consumption. [[Wunibald Kamm]] built the first full-scale wind tunnel for motor vehicles.<ref name="kraftfahrwesen">{{cite web|title=History (1930–1945)|url=http://www.fkfs.de/english/company/history/|publisher=Forschungsinstitut für Kraftfahrwesen und Fahrzeugmotoren Stuttgart|access-date=3 September 2010|archive-url=https://web.archive.org/web/20110719020415/http://www.fkfs.de/english/company/history/|archive-date=19 July 2011|df=dmy-all}}</ref>

===Low speed tunnels===
Wind tunnels have been used to test sporting equipment including golf clubs, golf balls, bobsleds, cyclists, and race car helmets. Helmet aerodynamics are particularly important in open cockpit race cars such as Indycar and Formula One. Aerodynamic forces on the helmet at high speeds can cause considerable neck strain on the driver; and flow separation on the back side of the helmet can cause turbulent buffeting and thus blurred vision for the driver.<ref>{{Cite web|title=Racing Helmet Design|author=James C. Paul, P.E. |publisher=[[Airflow Sciences Corporation]]|url=http://www.airflowsciences.com/sites/default/files/casestudies/Racing_Helmet_Design.pdf |archive-url=https://web.archive.org/web/20180420010409/http://www.airflowsciences.com/sites/default/files/casestudies/Racing_Helmet_Design.pdf |archive-date=2018-04-20 |url-status=live}}</ref>

Other problems are also studied with wind tunnels. The effects of wind on man-made structures need to be studied when buildings became tall enough to be significantly affected by the wind. Very tall buildings present large surfaces to the wind, and the resulting forces have to be resisted by the building's internal structure or else the building will collapse. Determining such forces was required before [[building code]]s could specify the required strength of such buildings and [[Wind engineering|these tests]] continue to be used for large or unusual buildings.

===Aeroacoustic tunnels===
These tunnels are used in the studies of noise generated by flow and its suppression.

===High enthalpy===
A high enthalpy wind tunnel is intended to study flow of air around objects moving at speeds much faster than the local speed of sound ([[hypersonic]] speeds). "[[Enthalpy]]" is the total energy of a gas stream, composed of internal energy due to temperature, the product of pressure and volume, and the velocity of flow. Duplication of the conditions of hypersonic flight requires large volumes of high-pressure, heated air; large pressurized hot reservoirs, and electric arcs, are two techniques used.<ref>Ronald Smelt (ed), ''Review of Aeronautical Wind Tunnel Facilities'' National Academies, 1988 pp. 34–37</ref>

===Aquadynamic flume===
The aerodynamic principles of the wind tunnel work equally on watercraft, except the water is more viscous and so sets greater forces on the object being tested. A looping [[flume]] is typically used for underwater aquadynamic testing. The interaction between two different types of fluids means that pure wind tunnel testing is only partly relevant. However, a similar sort of research is done in a [[towing tank]].

===Low-speed oversize liquid testing===
Air is not always the best test medium for studying small-scale aerodynamic principles, due to the speed of the air flow and airfoil movement. A study of fruit fly wings designed to understand how the wings produce lift was performed using a large tank of mineral oil and wings 100 times larger than actual size, in order to slow down the wing beats and make the [[vortices]] generated by the insect wings easier to see and understand.<ref>{{cite web |url=http://www.carlzimmer.com/articles/2002/articles_2002_Flyorama.html |title=''Popular Science, Dec 2002'' |publisher=Carlzimmer.com |access-date=2011-06-28 |archive-date=8 July 2011 |archive-url=https://web.archive.org/web/20110708122120/http://www.carlzimmer.com/articles/2002/articles_2002_Flyorama.html }}</ref>

===Environmental wind tunnels===
[[File:Схема ПСА.jpg|thumb|355x355px|A drawing of the test section in an environmental wind tunnel. It shows the velocity profile generated by the test section obstructions. The profile is a simulation of the natural wind boundary layer with height 'H' and turbulent velocity profile 'u'.]]
Wind tunnel tests are used to determine wind velocities around buildings and bridges, and the wind forces on them.<ref>{{cite journal |last1=Chanetz |first1=Bruno |title=A century of wind tunnels since Eiffel |journal=Comptes Rendus Mécanique |date=August 2017 |volume=345 |issue=8 |pages=581–94 |doi=10.1016/j.crme.2017.05.012 |url=https://hal.archives-ouvertes.fr/hal-01570740/file/BCA.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://hal.archives-ouvertes.fr/hal-01570740/file/BCA.pdf |archive-date=2022-10-09 |url-status=live |bibcode=2017CRMec.345..581C |doi-access=free }}</ref>

Environmental wind tunnels are used to simulate the boundary layer of the atmosphere in windy conditions near the earth's surface. The wind near the ground is highly turbulent.<ref>{{cite book |title=Micrometeorology; A Study Of Physical Processes In The Lowest Layers Of the Earth's Atmosphere| url=https://archive.org/details/micrometeorology0000sutt/page/n7/mode/2up|author1=Sutton, O. G.(Oliver Graham) |date=1953|publisher=McGraw-Hill |location=New York |page=[https://archive.org/details/micrometeorology0000sutt/page/56/mode/2up 56]}}</ref> Whereas vehicle wind tunnels have features to produce steady, straight-line air approaching the test model environmental tunnels need spires followed by small cubes on the floor to make the air represent the atmosphere boundary layer as it approaches the test object.<ref>{{cite book|url=https://www.academia.edu/35129053/LOW_SPEED_WIND_TUNNEL_TESTING_THIRD_EDITION_A_WILEY_MTERSCIENCE_PUBLICATION|title=Environmental Wind Tunnel Testing|author1=Kumar, Sathis|publisher=Wiley-Mter Science Publication|edition=Third |page=4}}</ref> The forces caused by wind on high-rise buildings and bridges have to be understood so they can be built using a minimum of construction materials while still being safe in very high winds. Another significant application for boundary layer wind tunnel modeling is for understanding exhaust gas dispersion patterns for hospitals, laboratories, and other emitting sources. Other examples of boundary layer wind tunnel applications are assessments of pedestrian comfort and snow drifting. Wind tunnel modeling is accepted as a method for aiding in [[green building]] design. For instance, the use of boundary layer wind tunnel modeling can be used as a credit for [[Leadership in Energy and Environmental Design]] (LEED) certification through the US Green Building Council.