Computer graphic of a closed circuit wind tunnel. From mass
 conservation, density times velocity times area equals a constant.

Aerodynamicists use wind tunnels to test models of proposed aircraft. In the tunnel, the engineer can carefully control the flow conditions which affect aircraft performance. The model is placed in the test section of the tunnel and 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 flight conditions. A model often contains small ports to measure pressures on the aircraft, or the model may be mounted on a balance to directly measure the aircraft lift or drag.

Wind tunnels are usually designed for a specific purpose and speed range. There are special tunnels for propulsion, icing research, supersonic and hypersonic flight, and even full scale testing. A wind tunnel may be open and draw air from the room into the test section, or the tunnel may be closed with the air recirculating around the circuit. The tunnel in the figure is a closed tunnel which we are viewing from above. The test section is the red box at the bottom and the power to move the air in the tunnel is provided by a motor shown in green at the top. The amount of air in the tunnel is a constant, and we can use the conservation of mass to relate local speed in the tunnel to the cross-sectional area. At every point in the tunnel, the velocity (V) times the density (r) times the area (A) is a constant.

r * V * A = constant

For a low speed tunnel the density remains constant through the tunnel and we can further simplify the equation. Between any places in the tunnel:

( V * A) location 1 = (V * A) location 2

Decreasing the area increases the velocity. We usually want the highest velocity in the test section and that is why the test section of the tunnel has the smallest cross-section.

The Learning Technologies Project provides a wind tunnel index page which links to other sites around the country. From this page you can study the history of wind tunnels, or find plans to build your own tunnel.

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byTom Benson
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