AN EXPERIMENTAL INVESTIGATION OF INTERACTING WING-TIP VORTEX PAIRS

The interaction of turbulent trailing vortices shed from the tips of two rectangular wings have been studies through helium-bubble flow visualizations and extensive hot-wire velocity measurements made between 10 and 30 chordlengths downstream. The wings were placed tip to tip at equal and opposite angles of attack to generate pairs of co- and counter-rotating vortices. Meaningful hot- wire measurements could be made because the vortices were found to be insensitive to probe interference and subject only to very small wandering motions. The co-rotating vortices were observed to roll around each other and merge. Upstream of the merger location the vortices have approximate elliptical cores. These are surrounded by the two wing wakes which joint together at their ends. Flow in the vicinity of the cores appears fully developed. During the merging process the cores rotate rapidly about each other winding the wing wakes into a fine spiral structure. Merger roughly doubles the core size and appears to produce turbulence over a broad range of frequencies. The counter-rotating vortices move sideways under their mutual induction and slightly apart, their flow structure changing little with streamwise distance. These cores remain fairly circular and do not become fully developed within the 30 chordlengths of the measurements.