Similarity laws and vortical structures of spray dynamics

Dynamics of the unsteady spray injected from a swirl injector is investigated both experimentally and numerically. An original three-dimensional (3D) simulation code is developed to analyze the two-phase flow consisting of liquid droplets and ambient air, in which the equations of motion of droplets and the dynamical equation for the ambient air flow are solved simultaneously accounting for secondary atomization of droplets. It is confirmed that the tip penetration obtained by our code agrees well with the experimental results conducted by ourselves. The formation of vortex rings and their role in the process of entrainment of the ambient air are investigated numerically in order to reveal the spray development in an engine cylinder. It is found that many vortex rings are created successively in the early period, but only one vortex ring survives in the later period. The tip penetration increases in a power law with time in the early period, while logarithmically in time in the later period. The difference in the time dependence of the tip penetration between the two periods is accounted by the change in the structure of vortex rings created or surviving.