A description of the incipient bending of a round incompressible jet issuing into a weak crossflow is presented. Axial vorticity is shown to appear from the early stages of the jet evolution owing to the distortion and reorientation of the azimuthal vorticity, and it eventually dominates the flow around the jet and determines the shape of its cross-section.
The crossflow is considered weak enough for the distortion of the jet to occur downstream of its development region, where diffusion already influences the whole cross-section and the jet can be modelled as a point source of momentum. Axial pressure gradient and axial diffusion are negligible under these conditions, since the jet is a slender structure.
Sufficiently near the origin, a finite-length entraining wake is identified on the lee side of the jet, which gradually merges with the main core. At the same time, the cross-section begins to acquire a characteristic elongated shape, with the jet concentrating in a thin layer. Farther downstream the axial vorticity of the jet rearranges into a couple of large locally two-dimensional contrarotating vortices standing against the wind under the action of their own induced velocity, and a smaller vortex sheet coinciding with the distorted jet.