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Onset of orbital motion in a trailing vortex from an oscillating wing

The onset and development of orbital motion of a trailing vortex from a wing undergoing small amplitude heaving motion is investigated using stereo particle image velocimetry in conjunction with three-dimensional reconstruction techniques. The effect of Strouhal number is examined via space–time representations of axial and azimuthal vorticity, axial velocity deficit and swirl ratio. At low Strouhal number, the undulation of the vortex remains unidirectional with no amplification in the streamwise direction. In contrast, at high Strouhal number, the amplitude of vortex undulation can increase by up to a factor of ten in the streamwise direction. These large amplitudes occur during orbital motion of the vortex. Irrespective of the value of either the Strouhal number of excitation or the streamwise location along the undulating vortex, generic physical mechanisms occur. Changes in curvature along the vortex are closely related to changes in the axial velocity deficit, extreme values of axial vorticity and swirl ratio and the onset and attenuation of pronounced azimuthal vorticity.