Very-near-field dynamics in the injection of two-dimensional gas jets and thin liquid sheets between two parallel high-speed gas streams.

A numerical investigation of the velocity, pressure and vorticity fields very near the injection of flat and thin two-dimensional gas jets or liquid sheets between two parallel high-speed gas coflows is performed. The motivation of this research is to uncover some basic physical mechanisms underlying twin-fluid atomization. Conservation equations and boundary and initial conditions are presented for both single-phase jets and two-phase liquid sheet/gas-stream systems. Both infinitely thin and thick solid walls are considered. Apart from the gas Strouhal and Reynolds numbers appearing in the dimensionless single-phase flow equations, the liquid Reynolds number, the momentum flux ratio, the gas/liquid velocity ratio and the Weber number enter the two-phase flow dimensionless formulation. The classical numerical techniques for single-phase jets are supplemented with the volume-of-fluid (VOF) method for interface tracking and the continuum surface force (CSF) method to include surface tension in two-phase flow systems. Ad hoc convection algorithms in combination with a developed version of the fractional-step scheme allows a significant reduction of the numerical diffusion, maintaining localized and sharp interfaces. The action of the surface tension is correctly found via the CSF with a smoothed scalar-field approximation.