to test models of proposed aircraft.
In the tunnel, the engineer can carefully control the
flow conditions which affect the
on the aircraft. By making careful measurements of the forces on
the model, the engineer can predict the forces on the full scale
aircraft. And by using special diagnostic techniques, the engineer
can better understand and improve the performance of the aircraft.
Wind tunnels are designed for a specific purpose and
and there is a wide variety of
wind tunnel types
and model instrumentation.
The tunnel shown in the figure is a low-speed,
closed tunnel which we are viewing from above.
The air inside the tunnel is made to move by the
on the far side of the tunnel. Air continuously circulates through the
wind tunnel, passing over the
that is mounted in the test section.
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 air density rho times the area A
is a constant.
(rho is a Greek letter commonly used to denote the air density.)
rho * V * A = constant
For a low speed tunnel the density remains constant through the
tunnel and we can further simplify the equation. Between any two places in
( V * A)1 = (V * A)2
Decreasing the area increases the velocity in a
subsonic wind tunnel.
We want the highest velocity in the test section
and that is why the test section of the tunnel is
to have the smallest cross-section.
The model is instrumented to provide the engineer with test data. To obtain
meaningful data, the engineer must insure that the flow
match the desired flight conditions. Both the Mach number and the Reynolds number
depend on the velocity and density in the tunnel.
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