Numerical Investigation Of Cavity Formation And Bubble Entrapment Mechanism For A Micron-Waterdrop Impact On A Deep Pool

As one of the most fundamental and iconic fluid motions,droplet impact exists widely in scientific technologies and natural environment,and the phenomenon has been studied both for fundamental mechanism and for industrial applications ranging from aerospace engineering,inkjet printing,agricultural irrigation and hydraulic structure erosion.Therefore,it is of great significance to study such basic movements for understanding the interfacial deformation of gas and liquid flow and improving the application of droplet impact movement in engineering.Droplet impacting on a deep liquid pool has been exhaustively investigated for droplets with millimeter diameter.In this article,focusing on the cavity formation mechanism during a Micron-sized waterdrop impact on a deep pool,we perform systematic numerical simulations with adaptive mesh refinement technique and VOF method to study the impact of a 290/im water droplet on a deep water pool at velocities in the range 2.5～6.5m/s.The free surface motion,geometry variation of the cavity,local pressure field and vorticity field at selected times are presented to identify the pool-drop water mixing,capillary wave propagation,cavity formation,vortex ring generation and bubble entrapment phenomenon,and the dynamic mechanism of cavity motion is further explored.It is found that under the premise of neglecting the surface tension effects on the cavity and the cavity depth is in the range of h∈(d,hmax),the cavity growth time to reach its maximum depth still scales as t och5/2,but in the end,the formation of the bottom of the cavity is driven by capillary waves.There are two types of the initial cavity shape,one of them is U-shape and the other is hemispherical shape,the former one generally changes to V-shape,and in the latter occasion,the bottom of the cavity will gradually transform into cylindrical,resulting in a thin jet and possible bubble entrapment.Cavity collapse is closely related to capillary wave propagation.When the impact velocity is low,the low-pressure zone is initially generated at the junction between the cavity sidewall and the bottom,a large vortex ring is then generated near the free surface and the bottom of the cavity,respectively.Under high impact velocities,the thin jet is observed,the generation of the vortex ring is suppressed.The low-pressure zone is first generated at the junction between the wave bottom and the cavity sidewall,after the cavity becomes cylindrical,the cavity collapses before the capillary wave arrives at the bottom,causing a bubble entrapment.The shape of an oscillating droplet at impact under different surrounding medium and initial condition is a key factor on the subsequent cavity formation and bubble entrapment.In this study,five kinds of deformed water droplets with different aspect ratios are selected to investigate the influence of drop deformation and impact velocity on the bubble entrapment,capillary wave propagation,and vortex ring evolution.The results show that at low impact velocities,the shape of water droplet does not cause significant changes in the cavity formation and bubble entrapment However,under higher impact velocity,deformed droplet with an aspect ratio of 1.33 coalescences with the pool,and large bubble entrainment occurred.The large bubble entrapment is mainly affected by the vortex ring generated under the free surface at the neck between the droplet and the pool.The vortex ring moves penetrated deeper before it pulls the free surface to generate a rolling jet at the upper interface of the cavity.The rolling jets then contacted at the center of the cavity and collapsed to entrain a large bubble.At the end of the bubble entrapment phenomenon,the cyclone inside the cavity pushed the sidewall of the cavity continuously,and effectively increased the lateral volume of the bubble,which played a vital role in the bubble entrainment process.In the initial stage of the impact,the flatter the shape of the droplet,the greater the curvature of the j et generated on the neck between the droplet and the pool,the greater the strength of the vortex ring generated under the free surface.