To recognize the effects of interaction between a pair of vortices upon their development, it is first necessary to understand and document the behavior of an isolated vortex. This was done as part of a companion study in which the wake of an unswept rectangular NACA 0012 half-wing was measured. It was found that found that such an isolated vortex generates no turbulence of its own. Flow inside the core is laminar and the only turbulence outside the core is that associated with the unrolled-up part of the wake, which decays rapidly with streamwise distance.
In the present study turbulence measurements made in a pair of counter-rotating trailing vortices produced using the same configuration used to generate the isolated vortex, but with a second half wing added (figure 1). This allowed, to some extent, the isolated vortex study to be used as a control, distinguishing effects associated with interactions between the vortex pair from those associated with individual vortex development.
Experiments were performed in the Stability Wind Tunnel using miniature four-sensor hot-wire probes. The wings were placed at 5 degrees angle of attack and velocity measurements were made in detail in and around both vortex cores at locations 10 and 30 chordlengths downstream of the wings (x/c=10 and 30). The same filtering and wandering correction schemes used in the isolated vortex study were employed here.
At 10 chordlengths the flow structure produced by vortex pair (figures 2 and 3) appears quite similar to the isolated vortex. The vortex cores are laminar. True turbulence levels (figure 4) within them are low and vary little with radius. The turbulence that surrounds the cores is formed by the roll up of and interaction of the wing wakes that spiral around them. This turbulence is stretched and organized but apparently not produced by the circulating mean velocity fields of the vortices.
At 30 chordlengths (figures 5 and 6) the vortex pair has changed dramatically, the vortex cores have become turbulent. True turbulence levels (figure 7) within them are larger and increase rapidly with radius. The turbulent region surrounding the cores has doubled in size but turbulence levels have not diminished, apparently being sustained by outward diffusion from the core regions. The distribution of the turbulence has also changed, the wake spirals having been replaced by a much more core-centered turbulence field.
This change in flow structure contrasts sharply with the rapidly decaying turbulence levels and continuing laminar flow seen in the isolated tip vortex. This implies that the transition to turbulence in the cores of the vortex pair is stimulated by interaction between the vortices. Spectral measurements at 10 chordlengths suggest that short-wave instability may be the cause.
Detailed description of these results and their implications is given in the paper "The Structure and Development of a Pair of Counter-Rotating Wing-Tip Vortices", William J. Devenport, Jeffrey S. Zsoldos and Christine M. Vogel, Journal of Fluid Mechanics, vol. 332, pp. 71-104, 1997.
