Editing Wind tunnel (section)

==Measurement of aerodynamic forces and moments==
[[File:Kirsten wind tunnel 08A.jpg|thumb|left|Six-element external balance below the Kirsten Wind Tunnel. Six measurements are required, three forces and three moments.]]
Air speed, direction and pressures are measured in several ways in wind tunnels.

Air speed through the test section is determined by [[Bernoulli's principle]]. The direction of airflow around a model is shown by fluttering tufts of yarn attached to the aerodynamic surfaces. The direction of airflow approaching and leaving a surface can be seen by mounting tufts in the airflow in front of and behind the model. Smoke or bubbles of liquid can be introduced into the airflow upstream of the model, and their paths around the model recorded using photography (see [[particle image velocimetry]]).

Aerodynamic forces on the test model are measured with beam balances.<ref>{{cite web| url=https://archive.org/details/windtunneltestin0000alan_m7c1/page/118/mode/2up?q=beam |title=Wind Tunnel Testing |format=Chapter 4, Model force....measurements}}</ref>

The pressure distribution on a test model has historically been measured by drilling small holes on the surface, and connecting them to [[manometer]]s to measure the pressure at each hole. Pressure distributions can be measured more conveniently using [[pressure-sensitive paint]],<ref>{{cite book| title=Low-Speed Wind Tunnel Testing|url=https://www.wiley.com/en-us/Low-Speed+Wind+Tunnel+Testing%2C+3rd+Edition-p-9780471557746| author1=Barlow, Jewel B.  |author2=Rae, William H. |author3=Pope, Alan|edition=3rd|isbn=978-0-471-55774-6|date=1999-02-01 |page=145|publisher=John Wiley & Sons, Inc.|location=New York}}</ref> in which pressure is indicated by the fluorescence of the paint. They can also be measured with very small electronic pressure sensors mounted on a flexible strip which is attached to the model.<ref>{{cite journal |title=Dynamic flight load measurement with MEMS pressure sensors|journal=CEAS Aeronautical Journal|url=https://link.springer.com/article/10.1007/s13272-021-00529-3|date= 27 July 2021|author1=Raab, Christian |author2=Rohde-Brandenburger, Kai|volume=12|pages=737–753}}</ref>

The aerodynamic properties of an object can vary for a scaled model.<ref name="Lissaman">{{cite journal|title=Low-Reynolds-Number Airfoils|journal = Annual Review of Fluid Mechanics|first=P. B. S.|last=Lissaman|date=1 January 1983|volume=15|issue=1|pages=223–239|doi=10.1146/annurev.fl.15.010183.001255|bibcode=1983AnRFM..15..223L|citeseerx = 10.1.1.506.1131| s2cid=123639541 }}</ref> However, by observing certain similarity rules, a very satisfactory correspondence between the aerodynamic properties of a scaled model and a full-size object can be achieved. The choice of similarity parameters depends on the purpose of the test, but the most important conditions to satisfy are usually:

* Geometric similarity: all dimensions of the object must be proportionally scaled.
* [[Mach number]]: the ratio of the airspeed to the speed of sound should be identical for the scaled model and the actual object (having identical [[Mach number]] in a wind tunnel and around the actual object is not equal to having identical airspeeds).
* [[Reynolds number]]: the ratio of inertial forces to viscous forces should be kept. This parameter is difficult to satisfy with a scaled model and has led to development of pressurized and cryogenic wind tunnels in which the viscosity of the working fluid can be greatly changed to compensate for the reduced scale of the model.

In certain particular test cases, other similarity parameters must be satisfied, such as the [[Froude number]].

===Force and moment measurements===
[[File:Lift curve.svg|thumb|A typical [[lift coefficient]] versus [[angle of attack]] curve]]

The model is mounted on a balance which measures forces and moments. Lift, drag, and lateral forces, as well as yaw, roll, and pitching moments are measured over a range of [[angle of attack]]. Common curves such as [[lift coefficient]] versus angle of attack are produced.

The model must be held stationary, and these external supports create drag and potential turbulence that will affect the measurements. The supporting structures are kept as small as possible and aerodynamically shaped to minimize turbulence.