Formation and breakup of twisting ligaments in a viscous swirling liquid jet

We analyze the successive steps of the breakup morphology of a swirling liquid jet. Three-dimensional numerical simulations are carried out using the Volume of Fluid method with adaptive mesh refinement for axial Reynolds numbers of 50 and swirl numbers of 0.50 & LE; S & LE; 1.55. We present fundamental flow features of the swirling jet in terms of time-averaged axial and azimuthal velocity profiles for the considered range of swirl numbers. The provision of a swirl induces helical disturbance at the interface of the jet, which exhibits an azimuthal mode number of m = 4. We identified that viscous forces are the most dominant force in the flow, which causes the suppression of Kelvin-Helmholtz instability at the interface. In contrast, we found the existence of centrifugal instability, which destabilizes the helical rim developing at the interface. As a result, centrifugally induced corrugations in the form of tiny protrusions develop along each of the helical rims, which triggers Rayleigh-Taylor instability. Subsequently, these tiny protrusions get stretched in the radially outward direction and transform into twisting ligaments that break into droplets. We have elucidated the mechanism for the twisting of ligaments and its further disintegration into first-generation droplets, which has not been reported in previous studies.