Editing Wind tunnel (section)

==Flow visualization==
Because air is transparent, it is difficult to directly observe the air movement itself. Instead, multiple methods of both quantitative and qualitative flow visualization methods have been developed for testing in a wind tunnel.

===Qualitative methods===
[[File:Cessna 182 model-wingtip-vortex.jpg|thumb|A scale model light aircraft showing helium-filled bubbles which follow [[Streamlines, streaklines, and pathlines|pathlines]] of the [[wingtip vortices]]]]
* Smoke
* Carbon dioxide injection
* Tufts, mini-tufts, or flow cones can be applied to a model and remain attached during testing. Tufts can be used to gauge air flow patterns and flow separation.  Tufts are sometimes made of fluorescent material and are illuminated under black light to aid in visualization. Tufts are also used on aircraft in flight. During flight-testing of the experimental [[rocket plane]] [[SpaceShipOne]] tufts were attached to the surface of the wings. No wind tunnel testing was done on the aircraft.
*Evaporating suspensions are simply a mixture of some sort or fine powder, talc, or clay mixed into a liquid with a low latent heat of evaporation. When the wind is turned on the liquid quickly evaporates, leaving behind the clay in a pattern characteristic of the air flow.
* Oil: When oil is applied to the model surface it can clearly show the transition from laminar to turbulent flow as well as flow separation.
* Tempera paint: Similar to oil, it can be applied to the surface of the model by initially applying the paint in spaced out dots.  After running the wind tunnel, the flow direction and separation can be identified.  An additional strategy in the use of tempera paint is to use blacklights to create a luminous flow pattern with the tempera paint.
* Fog (usually from water particles) is created with an [[ultrasonics|ultrasonic]] [[piezoelectric]] [[nebulizer]]. The fog is transported inside the wind tunnel (preferably of the closed circuit and closed test section type). An electrically heated grid is inserted before the test section, which evaporates the water particles at its vicinity, thus forming fog sheets. The fog sheets function as streamlines over the test model when illuminated by a light sheet.
* Sublimation: If the air movement in the tunnel is sufficiently non-turbulent, a particle stream released into the airflow will not break up as the air moves along, but stay together as a sharp thin line. Multiple particle streams released from a grid of many nozzles can provide a dynamic three-dimensional shape of the airflow around a body. As with the force balance, these injection pipes and nozzles need to be shaped in a manner that minimizes the introduction of turbulent airflow into the airstream.
* Sublimation (alternate definition): A flow visualization technique is to coat the model in a sublimatable material where once the wind is turned on in regions where the airflow is laminar, the material will remain attached to the model, while conversely in turbulent areas the material will evaporate off of the model.  This technique is primarily employed to verify that trip dots placed at the leading edge in order to force a transition are successfully achieving the intended goal.

High-speed turbulence and vortices can be difficult to see directly, but [[strobe lights]] and film cameras or high-speed digital cameras can help to capture events that are a blur to the naked eye.

High-speed cameras are also required when the subject of the test is itself moving at high speed, such as an airplane propeller. The camera can capture [[strobe lights|stop-motion]] images of how the blade cuts through the particulate streams and how vortices are generated along the trailing edges of the moving blade.

===Quantitative methods===
* [[Pressure-sensitive paint]] (PSP): the air pressure on a surface is measured with paint coatings which reacts to variations in pressure by changing color.
* [[Particle image velocimetry|Particle Image Velocimetry]] (PIV) and [[laser Doppler velocimetry]]: air velocity is measured with lasers.
* Model Deformation Measurement (MDM): measures how much a model bends and twists.<ref>{{cite web | url=https://www.onera.fr/en/windtunnel/model-deformation-measurements | title=Model Deformation Measurements }}</ref>