Vorticity-dynamics analysis of liquid-jet atomization in crossflow dominated by Kelvin–Helmholtz instability

The liquid jet in crossflow (LJIC) refers to the atomization process resulting from injection of a liquid stream into a gaseous crossflow. This process is widely encountered in engineering applications, including fuel injection in internal combustion engines, gas turbines, ramjets, and scramjets. Instabilities along the liquid jet column induce ligament formation and subsequent breakup, processes strongly influenced by the viscosity and density of the surrounding gas. The Kelvin–Helmholtz (KH) instability plays a dominant role, particularly at low liquid-to-gas density ratios. Previous studies have demonstrated that such instabilities contribute to intensification of vortex cores, which are closely associated with interfacial deformation; however, experimental characterization of these coupled vortex–interface dynamics in LJIC remains challenging due to highly transient, extreme flow conditions and limitations in diagnostic techniques. Consequently, high-fidelity numerical simulations provide an alternative for investigating atomization mechanisms and resolving flow–interface interactions. In this study, a numerical framework examines the influence of gas viscosity on breakup mechanisms in LJIC regimes dominated by KH instability, with attention to the dynamics of vortical structures and their coupling with liquid–gas interface deformation. The results reveal that increasing gaseous viscosity does not suppress KH instability; rather, it enhances its development under certain conditions. This behavior underscores the critical role of viscosity contrast across the interface as a governing factor in instability growth. Furthermore, vertically oriented vortices along the liquid column are identified as a key mechanism driving surface breakup. Various surface breakup scenarios are elucidated through vortex-dynamics analysis, highlighting its role in understanding atomization.