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)1 = (V * A)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.
Activities:
Guided Tours
-
Wind Tunnels:
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