THE STRUCTURE AND DEVELOPMENT OF A COUNTER-ROTATING WING-TIP VORTEX PAIR

Experiments have been performed to examine the turbulence structure an d development of a pair of counter-rotating wing-tip vortices. The vor tices were generated by two rectangular NACA 0012 half wings placed ti p to tip, separated by 0.25 chordlengths. Preliminary studies showed t he vortices to be insensitive to the introduction of a probe and subje ct only to small wandering motions. Meaningful measurements could ther efore be made using hot-wire probes. Three-component velocity measurem ents were made 10 and 30 chordlengths downstream of the wing leading e dges for a chord Reynolds number of 260 000. At 10 chordlengths the Vo rtex cores are laminar. True turbulence levels within them are low and vary little with radius. The turbulence that surrounds the cores is f ormed 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 the vortex cores have become turbulent. True turbu lence levels within them are larger and increase rapidly with radius. The turbulent region surrounding the cores has doubled in size and tur bulence levels have not diminished, apparently being sustained by outw ard diffusion from the core regions. The distribution of the turbulenc e has also changed, the wake spirals having been replaced by a much mo re core-centred turbulence field. This change in flow structure contra sts sharply with what is seen in the equivalent isolated tip vortex, p roduced when one of the wings is removed. Here the vortex core remains laminar and the turbulence surrounding it decays rapidly with downstr eam distance. This implies that the transition to turbulence in the co res of the vortex pair is stimulated by interaction between the vortic es. Spectral measurements at 10 chordlengths suggest that short-wave i nstability may be the cause.