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CH08_Anderson 7/25/01 9:01 AM Page 205

Aerodynamic Testing 205

p 1 p 2
MEASURING THE AIRSPEED IN THE TEST
SECTION
The venturi is an application of the Bernoulli
principle. As the air enters the contraction, it
speeds up, and the static pressure drops. Since
Bernoulli’s theorem states that static pressure and
airspeed are related, this relationship can be used
h
to determine the airspeed in the test section.
Without going through the mathematics, the
velocity in the test section is just proportional to
the square root of the difference between the
static pressure in the test section and the static
pressure in the region with the fan. The
proportionality just depends on the density of the
Fluid
air and the difference in cross-sectional areas of
the two sections. A manometer is used to measure Fig. 8.6. The manometer.
the pressure difference. In its simplest form, a
manometer, shown in Figure 8.6, is just a bent glass
tube filled with liquid connecting the two regions. The
difference in pressure is proportional to the difference in
height of the liquid as shown. The wind-tunnel operator only
has to monitor the difference in height of the liquid and the
air density to know the airspeed in the test section. Kerosene
is often used as the liquid because it will not evaporate and is
safer to handle than mercury.

Closed-Circuit Tunnels
The lost power in an open-circuit wind tunnel can easily be
recovered if you close the airflow circuit, as in Figure 8.7. The
In 1709, Brazilian Jesuit
closed-circuit wind tunnel is the most common design for
Bartolmeu Lourenco de Gusmao
larger tunnels. Once the air is accelerated to operating condi-
demonstrated the first hot-air
tions, the fan only needs to add the power that is lost due to
balloon to the King of Portugal.
drag on the model and friction on the walls of the wind tunnel.
Unfortunately, the fire used in
Be aware that this power loss may be quite significant.
the balloon ignited the royal
Consider a production wind tunnel running many hours a day.
draperies, causing considerable
The friction and drag on the model can result in considerable
damage.
heat input to the air and the walls of the wind tunnel. Some