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Aerodynamic Testing 203

Low V High V Low V

High p Low p High p

Fig. 8.1. The venturi.

Fig. 8.2. The “half” venturi.

Fig. 8.3. Wind tunnel.

The change in cross-sectional area of the tunnel from the fan
section to the test section is called the contraction ratio. If you reduce
the cross section of your cardboard wind tunnel to one-fifth, a
contraction ratio of 5, the airspeed in the test section will be five times
the airspeed produced by the fan. So a fan with a 2-ft (61-cm)
diameter moving air at a speed of 15 mi/h (24 km/h) would produce
70 mi/h (112 km/h) in a test section about 11 inches (27 cm) in
diameter. On the other hand, if you want a test section that is
5 ft in diameter, you will need a fan 11 ft (3.3 m) in diameter.
A wind-tunnel model for a major
A more practical solution is to increase the fan’s airspeed and
development program can cost
reduce the contraction ratio to achieve a larger test section at
millions of dollars.
the same speed.