Near field of the tip vortex behind an oscillating rectangular wing

Data on the evolution of the unsteady three-dimensional tip vortex in the near wake (0-3 chordlengths behind the wing) of a rectangular NACA 0015 wing oscillating sinusoidally in pitch about its quarter-chord axis are presented. These data were obtained using three-component laser Doppler velocimetry (LDV). The experiments were performed in a low-speed wind tunnel at a Reynolds number of 1.8 x 10(5). The wing had a semiaspect ratio of 2. It was oscillated at a frequency of 1 Hz and an amplitude of 5 deg around a mean incidence of 10 deg. The instantaneous LDV data were used to obtain information on the distribution of phase-locked velocity and vorticity components across the vortex, as well as the phase-locked circulation associated with the evolving vortex. The data indicate that the average trajectory of the oscillating vortex in the near field is very nearly the same as for a stationary wing at the mean incidence. The length and circulation scales, as well as the maximum circulation carried by the vortex flow under the conditions studied, are modulated in a significantly nonquasisteady manner. Nevertheless, beyond about 0.7 chordlength, the normalized circulation distribution across most of the inner part of the vortex exhibits, at all times, the same universal behavior as the vortex behind a stationary wing.