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204 CHAPTER EIGHT

There are practical problems in building a venturi wind tunnel. You
cannot just contract and expand the tube arbitrarily. This is because
the air must constrict smoothly to reduce the effects of the walls. To
contract too quickly will make the walls act as a block to the airflow,
as illustrated in Figure 8.4. The pressure buildup limits the
effectiveness of the fan. To expand too quickly after the venturi also
causes problem. The air will not be able to follow the walls, causing
the flow to separate from the walls. This causes a buildup of pressure,
which also reduces the effectiveness of the tunnel. Figure 8.5
A rule of thumb for wind tunnels
illustrates how a venturi wind tunnel should work.
is that the walls should not
The wind tunnel just described, where the air passes once
slope more than 7 percent after
though the test section and is then lost, is called an open-
the test section.
circuit wind tunnel. A practical problem with such wind
tunnels is that all the energy put into air is lost and cannot be
recycled. This makes the open-circuit wind tunnels inefficient.
Therefore, you generally do not see large venturi-type tunnels. There
is one exception, the 80 120-ft wind tunnel at NASA Ames Research
Center, discussed a little later.

Closes too fast

Opens too fast
Fig. 8.4. A venturi that contracts and expands too quickly.

Correct curvature

Fig. 8.5. How a venturi wind tunnel should work.