Previous investigations of wing-wake roll-up have assumed the wake to be a vortex sheet of zero thickness. This immediately leads to the conclusion that, as soon as the process starts, a spiral of near axisymmetric form, with an infinite number of turns, forms at the edge, as predicted by the work of Karen, which must apply to the early stages of roll-up for any sheet of zero thickness. In addition, most investigators, starting with Westwater, have replaced the continuous vortex sheet by discrete vortex lines.
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