Wind Tunnel

Wind Tunnel, in aeronautics, research apparatus that simulates the conditions encountered by any object moving through the air. An object studied in a wind tunnel remains stationary as air or gas is forced over it. Wind tunnels are used to study the effects of moving air on such objects as aircraft, spacecraft, missiles, cars, buildings, and bridges.

Wind tunnels vary in size from a few inches to the 12-m by 24-m (40-ft by 80-ft) tunnel at the Ames Research Center of the National Aeronautics and Space Administration (NASA), at Moffet Field, California. This massive wind tunnel can accommodate a full-size aircraft with a wingspan of 22 m (72 ft).

The larger the cross-sectional area of the test section, the more difficult it is to develop and sustain high-speed airflows in a wind tunnel. This is especially a problem in supersonic and hypersonic tunnels in which power requirements are so great that the size of the tunnel must be kept much smaller. Although large low-speed wind tunnels can use motor-driven propellers to force the air, higher speeds require air compressors, the release of gas stored under high pressure, or the explosive discharge of gases. Wind tunnels powered by a gas charge can operate only for short periods of time. Very high velocities are obtained in the hypervelocity tunnel of the Ames Research Center, in which small models of aircraft or spacecraft are propelled by an explosive charge into the wind tunnel in one direction, while another explosive charge simultaneously drives gas into the tunnel from the other direction. These conditions can create air velocities of up to 48,000 km/h (about 30,000 mph) for about one second.

During the flight of supersonic spacecraft, friction causes significant amounts of heat (see Heat Transfer). To study these effects, special wind tunnels are used in which a jet of hot gas flows over the model. Instruments measure the movement of gas and the generation of heat.

Smoke is sometimes added in low-speed tunnels so that the movement of the air over aircraft wing sections is visible. Other specially designed tunnels permit the simulation of very high altitudes and the observance of their influence on aircraft performance. Altitudes of up to 145 km (about 90 mi) have been simulated. These high-altitude tests are also very important for predicting the performance of jet engines under all flight conditions (see Jet Propulsion). A special tunnel at NASA's Lewis Flight Propulsion Laboratory in Cleveland, Ohio, can test full-size jet engines at air velocities of up to 3860 km/h (about 2400 mph) and at altitudes of up to 30,500 m (about 100,000 ft). The European Transonic Wind Tunnel, to be used by European aerospace companies and research establishments, will provide similar capacity. See also Aerospace Industry.