A 3 'x2' subsonic wind tunnel has been constructed and calibrated. The flow in the
wind tunnel accelerates 6.1 % along the test section due to boundary layer growth. The
potential core flow at both the entrance and exit locations is strongly unifonn. The mean
velocity varies .4% across the entrance and exit planes. The turbulence level at the
entrance and exit locations is approximately .2%.
The tunnel was used to study a trailing vortex during its development at the wingtip
and at 10 chordlengths downstream. The vortex was found to be well developed by 10
chordlengths. Helium bubble flow visualization was used to study the development of the
vortex in the near-tip regions. Hot-wire anemometry was used to study the vortex at 10
chordlengths.
Two vortices were found in the tip-flow studies, a primary vortex core formed on the
suction side surface of the wingtip and a weaker secondary vortex formed on the endcap.
The primary vortex core lifts from the surface of the wingtip as it proceeds along the
chord. The vertical displacement is a strong function of angle of attack. The primary
vortex is displaced from the endcap towards the wing root at the trailing edge. The
spanwise displacement is a strong function of chord Reynolds number.
The primary and secondary vortices co-rotate after leaving the wing, and the primary
vortex core follows a steady helical path. The radius of the helical path quickly diminishes
with downstream distance.
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