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Computer graphic of a closed circuit wind tunnel.

Aerodynamicists use wind tunnels to test models of proposed aircraft. In the tunnel, the engineer can carefully control the flow conditions which affect forces on the aircraft. By making careful measurements of the forces on the model, the engineer can predict the forces on the full scale aircraft. And by using special diagnostic techniques, the engineer can better understand and improve the performance of the aircraft.

Wind tunnels are usually designed for a specific purpose and speed range. There are special tunnels for propulsion, icing research, subsonic, supersonic, and hypersonic flight, and even full scale testing. A wind tunnel may be open and draw air from outside the tunnel into the test section and then exhaust back to the outside, or the tunnel may be closed with the air recirculating inside the tunnel. The tunnel in the figure is a closed tunnel which we are viewing from above. The air inside the tunnel is made to move by the fan on the far side of the tunnel. In this figure, air continuously moves counter-clockwise around the circuit, passing over the model that is mounted in the test section.

The model is instrumented to provide the engineer with test data. To obtain meaningful data, the engineer must insure that the flow similarity parameters of Mach number and Reynolds number match the desired flight conditions. Both the Mach number and the Reynolds number depend on the velocity and gas density in the tunnel. For safety reasons, engineers can not be present in the test section during the operation of the tunnel. The engineers operate the tunnel from a control room in an adjoining building. Data from the model is transferred to the control room through bundles of electrical lines.

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Editor: Nancy Hall
NASA Official: Nancy Hall
Last Updated: May 13 2021

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