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Seeing Airflow: You Can Look, But You Can't Touch
One of the challenges of wind tunnel testing is "seeing" the air. Researchers need to know details about how air is flowing around a model, but putting an instrument into the flow would interfere with the processes being studied.

Over the years, NASA researchers have used and developed many measurement techniques that have little or no effect on experiments. These are called minimally intrusive or nonintrusive techniques. One of the oldest of such techniques is the use of "tufts," or strips of yarn, mounted on the floor or walls of the wind tunnel or on the model itself. As the air flows across the surface, it causes the tufts to "go with the flow" of the air. This technique is particularly helpful for showing the direction of airflow. Another traditional technique uses pressure-sensitive paint dots to show how airflows impinging on a surface are distributed. The brightness of the paint dots changes as the air pressure changes.

Although ordinary light sources and cameras cannot view the complex airflows of aircraft engine systems, several cutting-edge methods help researchers to see these complex processes in action without interfering with the results.

Photograph: Optics for the Rayleigh-scattering flow diagnostic being adjusted Image right: Optics for the Rayleigh-scattering flow diagnostic being adjusted in Glenn's 9- by 15-Foot Low-Speed Wind Tunnel to test the hot gas ingestion of the ASTOVL model. Credit: NASA

Lasers have played an important role in wind tunnel testing. Two sheets of lasers have been used to illuminate the flow field at different heights. Water mist was often added to make the airflow visible. In the 1990s, Glenn began using fiber optics to transmit the laser light beam in wind tunnels. Fiber-optic technology brought an added measure of safety to the test procedure, and it minimized loss in the transmitted laser light beam.

With another tool, a thermovision system, researchers can view the hot parts of an aircraft, a simulated runway surface, and the gas plume of an engine exhaust. Carbon dioxide is used to image heated surfaces, and an infrared camera shows the various temperature levels in different colors.

The data gathered are processed by computers that visualize the test results. In turn, programming the computers to predict different conditions reduces the number of tests required and helps designers to solve problems faster and with greater confidence.

Researchers at the NASA Glenn Research Center and other NASA Centers are developing minimally intrusive optical measurement systems, advanced optical instrumentation for aerospace propulsion testing facilities, new optical components for aerospace flight, advanced electro-optic circuits for space exploration, and other technologies. Advanced data processing techniques are being used and developed to decrease processing time. Data from these systems will help designers to understand the basic physics of new systems and to validate computer and life models. They will lead to improved designs, increased safety and security, and reduced design times for many technologies developed at Glenn.

Photograph: Front view of ASTOVL model Image left: Front view of ASTOVL model installed in Glenn's 9- by 15-Foot Low-Speed Wind Tunnel. Credit: NASA

Many of theses techniques were used to evaluate NASA's advanced short takeoff and vertical landing (ASTOVL) aircraft in Glenn's 9- by 15-Foot Low-Speed Wind Tunnel. Testing enabled designers to study the problem of hot gas ingestion. This occurs when exhaust from an engine's nozzles, instead of the cooler surrounding air, enters the engine's inlet. Hot gas ingestion can stall an engine or impair performance in critical phases of flight.

Tests conducted at Glenn provide data that aircraft designers need to understand problems such as hot gas ingestion. This knowledge is being applied to make new aircraft safer and more efficient. In addition, the instrumentation and visualization techniques pioneered in Glenn's wind tunnels are now being used in other research areas, manufacturing, and medical applications.

David DeFelice