On coherent structures of spatially oscillating planar liquid jet developing in a quiescent atmosphere

The momentum transport and entrainment of a developing plane jet are dominated by coherent structures. Percipience about the formation, evolution, and interaction of these coherent structures for oscillating planar jets remains unclear. In the present study, numerical investigations are reported for analyzing spatially oscillating planar jets by solving Navier-Stokes equations coupled with the volume of fluid method to track the air-water interface. Coherent structures in the flow for Reynolds numbers (Re) 4500 and 500 are identified using Q-criterion. It is demonstrated that for oscillating jets, the spread and entrainment increase with an increase in Re. These jet characteristics are greatly influenced by the head vortices, which transform into a pair of hairpin vortices, which further undergo leapfrogging. This jet front dynamics is found to be dampened by viscous forces. The numerical results also suggest the existence of sideways hairpin vortices that expel the fluid by forming a channel and are significantly subdued at low Re. A peculiar merging of span-wise Kelvin-Helmholtz rollers at low Re is also reported in the present study. Furthermore, the dominant flow structures are identified and analyzed using proper orthogonal decomposition and dynamic mode decomposition. The results demonstrate the dominance of coherent structures at the downstream for higher Re with vortices in the near field at the jet peaks also contributing to the flow field dynamics.