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 co-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, 15, 22 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. Helium bubble flow visualizations were performed to reveal the core trajectories (figure 2).

 The vortices spiral around each other and merge some 20 chordlengths downstream of the wings.  As merger is approached the vortices lose their axisymmetry - their cores develop lopsided tangential velocity fields and the mean vorticity field is convected into filaments. The cores also become part of a single turbulence structure dominated by a braid of high turbulence levels that links them together. The braid, which quite closely resembles the structure formed between adjacent spanwise eddies of transitional mixing layers, grows in intensity with downstream distance and extends into the vortex cores. Unlike a single tip vortex, the unmerged cores appear turbulent.

 The merging of the vortices wraps the cores and the flow structure that surrounds them into a large turbulent region with an intricate double spiral structure. This structure then relaxes to closely axisymmetric state. The merged core appears stable and develops a structure similar to the laminar core of the vortex shed from a single wing. However, the turbulent region formed around the vortex core during the merger process is much larger and more axisymmetric than that found around a single wing tip vortex.

 Detailed description of these results and their implications is given in the paper, W J Devenport, C M Vogel and J S Zsoldos, "Flow Structure Produced By the Interaction and Merger of a Pair of Co-Rotating Wing-Tip Vortices", Journal of Fluid Mechanics, vol. 394, pp. 357-377, 1999.