Gas-liquid Two-phase Shear Flow Interface Instability And Fragmentation, Direct Numerical Simulation,

A numerical study of temporal and spatial evolving two-dimensional mixing layer of two immiscible fluids separated by the interface with surface tension is performed by solving the Navier-Stokes equations in order to investigate the mechanism of instability and breakup of interface. A VOF method is used to tracking the moving interface.The characteristic of nonlinear growth of disturbances and evolution of the interface is determined mainly by two competing effects, the Kelvin-Helmholtz instability and dispersion due to surface tension. The Weber number We measures the strength of the K-H instability relative to the dispersive stabilization effect associated with surface tension. For small We, the interface simply oscillates, with no apparent development of the new structure. For intermediate We, the interface forms elongating fingers that interpenetrate each fluid into the other, then breakup occurs. For large We, the interface rolls up into folded fingers, then break up into many small droplets. When the surface tension is zero, the model is reduced to corresponding free shear, the vorticity concentration and vortex-pairing can been observed.The effect of the density ratio has also been considered. The curvature of the finger penetrating into the lighter fluid is higher than that of into the heavier fluid, so the spike and the bubble are formed respectively. The viscosity delays the breakup, smoothes the shape of interface, and increases the size of droplets.A numerical study of temporal and spatial evolving two-dimensional coflowing jet is also performed. The sinuous mode and varicose mode are both considered. The evolution is also determined by the two competing effects of the Kelvin-Helmholtz instability of the wake velocity profile and the dispersive stabilization effect due to surface tension. For uniform density jet, when the Weber number is large, the interface rolls up and forms staggered fingers or folded fingers, then the jet breaks up into droplets as a whole; when the Weber number is small enough, the sheet simply oscillates. When the inner fluid is the heavier and the outer coflowing one is the lighter, the large inertia restrains the evolution of instability. By decreasing Weber number, the breakup is delayed. The viscosity also restrains the evolution of shear instability.