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Aerodynamic Testing 219

over time. This type of wind tunnel requires very little power but
requires quite a long time between tests. The NASA Hypersonic
Tunnel Facility at Plum Brook can generate speed up to Mach 7. This
blowdown facility can accommodate a 5-minute test every 24 hours.
The Twenty-Inch Supersonic Wind Tunnel at the Langley Research
Center can generate flows with Mach numbers from 1.4 to 5 for 1.5 to
5 minutes.
Another option, which is more common, is the vacuum supersonic
wind tunnel shown schematically in Figure 8.20. Rather than pump a
chamber to a high pressure, which is dangerous, the chamber is
evacuated and the airflow is in the other direction through the test
section. Thus, the upstream reservoir of air is just the atmosphere and
the air is being drawn through the throat and test section into a
vacuum.
In all supersonic venturis, the air expands on the high-speed side
and thus cools. For continuous supersonic wind tunnels this is not a

Test section

High-pressure gas

Fig. 8.19. Blowdown supersonic wind tunnel.

Test section

Vacuum

Fig. 8.20. Vacuum supersonic wind tunnel.