Adams, G. N. and Gilmore, D. C., " Some Observations of Vortex Core Structure", Canadian Aeronautical Journal, June 1972.

During observation of propellor tip-vortex locations by means of smoke and stroboscopic lighting, a fortuitous atmospheric condition resulted in the formation of visible condensation in the vortex cores. The core structure was seen to be extremely complex, with evidence of concentric shells of axial flow, bounded by families of ring vortices. Photographs were taken , and a selection of these is presented in the paper. A mechanism of generation of concentric ring vortices is presented. It is seen that the core of the vortex differs from the normally accepted solid body rotation. Two concentric cores were visualized surrounded by vortex rings with opposite directions of rotation. This would explain secondary features in measured vortex velocity profiles observed by some researchers.