Sub-Scale Modeling of Aircraft Trailing Vortices

Abstract

Most experimental studies of trailing vortices have been conducted with one of the following three methods: (1) wind-tunnel study, (2) intercepting the vortices shed by one airplane with another airplane, (3) measurements of the vortices shed by a low-flying airplane with ground-based instruments. The wind-tunnel method is suitable for the study of vortex formation, but not the long-time decay process. The second method would have been the most realistic approach, but unfortunately, the difficulty in coordinating the two traveling airplanes plus the various unknown effects of the atmosphere have so far precluded any consistent and useful data. The third kind of study inevitably includes the ground effect in addition to all the unpredictable atmospheric effects. The data obtained, though useful to the air-traffic control near the runways, are nevertheless hard to analyze; therefore it is difficult to separate the various mechanisms responsible for the vortex decay.

This paper describes a pilot study of trailing vortices using a sub-scale model in a more controllable laboratory environment. The essential feature of this laboratory facility is to have a vortex-generating wing moving along a pair of elevated rails so that the vortices shed by the wing remain relatively fixed with respect to ground-based instrumentation and can be observed and measured throughout their entire life-span. The vortices are rendered visible by two methods: by smoke traces and by drifting soap bubbles. A 16-mm movie camera is used to record the visible vortices. The smoke-traced vortices give better qualitative pictures, but the soap-bubble pictures are more suitable for quantitative studies. Hot-wire anemometer probes are also used to study the turbulence nature of the vortices. Ease of operation, repeatability, and versatility are some of the obvious advantages of this experimental setup.

Geometrically similar wings of different sizes running at various speeds and angles of attack have been tested. The vortices generated by these model wings all contain high levels of turbulence. They generally last for a few seconds. A scaling study is also described based on existing theories that predict the decay rate of a turbulent line vortex. None of the theories examined is capable of correlating the results of the sub-scale experiments (chord Reynolds number of the order of 10⁵) and those of full-scale airplanes (chord Reynolds number ranging from 10⁶ to 10⁸). This indicates that a satisfactory theory of turbulent vortex decay is still lacking.