Aerodynamicists use
wind tunnels
to test
models
of proposed aircraft and engine components.
During a test, the model is placed in the
test section
of the tunnel and air is made to flow past the model.
In some wind tunnel tests, the
aerodynamic forces on the model are measured.
In some wind tunnel tests, the model is instrumented to provide
diagnostic information
about the flow of air around the model.
In some wind tunnel tests,
flow visualization
techniques are used to
provide diagnostic information about the flow around the model.
The figure shows an F-16XL wind tunnel model in the
NASA Langley Basic Aerodynamic Research Tunnel (BART).
The flow over the top of the aircraft has been illuminated by three laser sheets.
In the laser sheet technique, a fine mist of seed material is introduced upstream in
the tunnel. The assumption is made that the smoke or seed material
moves exactly with the flow. A thin sheet of light is then introduced perpendicular to the
flow, illuminating the seed material. Laser light is ideal for the light sheet because of the
fineness of the beam and the monochromatic nature of the light. In the photo, we see that the
laser sheets have captured the vortex shed by the leading edge extension of the F-16 wing at
angle of attack. The vortex core moves away from the body as it flows downstream to the right.
In a modern development, one can use laser sheets to not only visualize the flow but also to
determine the local velocity in the flow. Digital Particle Image Velocimetry (DPIV) uses multiple
laser sheet images and high speed computers to track the motion of the seed particles and,
from the particle motion, derive the local velocity. DPIV is an attractive technique because
it is non-invasive (the technique does not alter or disturb the flow) and it quickly produces large
amounts of data which would be tedious to acquire using older flow mapping techniques like traversing
probes. One disadvantage of the laser sheet technique is the flow region of interest must be
visible to the source of the laser light. In our photo, we obtain no visualization of the lower
part of the aircraft because the laser sheet cannot pass through the body from above.
You can investigate some wind tunnel visualization techniques by using our
interactive tunnel testing applet.
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