Dynamics of tip vortices on plunging wings

The vortex dynamics of the tip vortices on oscillating wings in periodic and transient plunging motions are investigated via three-dimensional velocimetry measurements in a water tunnel. The ratio of the maximum plunging velocity to the freestream velocity V_pmax/U_∞ is varied in the range of 0.1 to 1.5. At lower plunging velocity and frequency, the tip vortex appears quasi steady as a straight vortex filament. With increasing frequency and amplitude of the wing motion, three-dimensional deformations appear as helical waves on the vortex filament while merging with the trailing-edge vortices also takes place. The instability waves appear as the circulation of the tip vortex approaches near its maximum during the cycle. The wavelength normalized by the vortex core radius varies in the range of λ/a = 2 to 4 for the periodic as well as transient plunging motion. We found reasonable agreement with the theoretical predictions for isolated vortices. It is likely that the presence of the wing flow as well as other vortices result in the strain and disturbances that excite the tip vortex core. We propose that the mechanism of generation of instabilities on tip vortices is similar to those of the leading-edge and trailing-edge vortices.