A wind tunnel investigation of a wing-tip trailing vortex
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 uniform. 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.