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Vortex formation occurs along the edge of a free axisymmetric jet. This is caused by Kelvin-Helmholtz instability from the shear force between the high velocity air jet and the surrounding stagnant air. Small vortices are created close to the nozzle and grow in size as they are carried along the flow until they are big enough to fully interrupt the jet. The vortices then collapse creating a fully turbulent flow. This visualization has been done by injecting smoke into the fluid (air). The air is forced through a 100mm diameter nozzle. A laser sheet makes it possible to visualize the flow in (almost) 2D. This is done for a Reynolds number of approx. 1e4.
