page_title: Wind tunnel
section_title: Qualitative methods
hierarchical_section_title: Wind tunnel / Flow visualization / Qualitative methods
caption_attribution_description: English: Fog (water particle) wind tunnel visualization of a NACA 4412 airfoil at a low speed flow (Re=20.000). The image is released to the public domain courtesy of Smart Blade GmbH (www.smart-blade.com)
context_page_description: Wind tunnels are large tubes with air blowing through them. The tunnels are used to replicate the actions of an object flying through the air or moving along the ground. Researchers use wind tunnels to learn more about how an aircraft will fly. NASA uses wind tunnels to test scale models of aircraft and spacecraft. Some wind tunnels are large enough to contain full-size versions of vehicles. The wind tunnel moves air around an object, making it seem as if the object is really flying.
  Most of the time, large powerful fans blow air through the tube. The object being tested is held securely inside the tunnel so that it remains stationary and does not move. The object can be a small model of a vehicle, or it can be just any part of a vehicle. It can be a full-size aircraft or spacecraft. It can even be a common object like a tennis ball. The air moving around the stationary object shows what would happen if the object was moving through the air. The motion of the air can be studied in different ways; smoke or dye can be placed in the air and can be seen as it moves around the object. Coloured threads can also be attached to the object to show how the air moves around it.
context_section_description: 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.
  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, tempera paint 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 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 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.
caption: Fog (water particle) wind tunnel visualization of a NACA 4412 airfoil at a low-speed flow (Re=20.000)