Chapter 15 - Air entrainment in high-velocity water jets

Abstract

This chapter describes the air bubble entrainment along the interfaces of water jets discharging into air. Free-surface aeration is very important leading to highly-aerated mixture flow. Two basic geometries are considered: a two-dimensional developing jet and a circular jet. First, the air bubble diffusion is studied. The results are compared with experimental data, then the velocity distributions are discussed. In high-velocity water jets discharging into the atmosphere, free-surface aeration is observed along the jet interfaces. The distribution of air concentration is approximated by a simple advective diffusion theory in the chapter. In the developing flow region, the velocity distribution follows the same trend as non-aerated turbulent jets. Both circular and two-dimensional jets exhibit a similar trend, the air bubble diffusion seems little affected by the presence of the turbulent shear layer. The air diffusion process is enhanced with developed inflow conditions. The length of the clear-water core is shorter as air bubbles diffuse towards the jet centerline and the outer edge of the spray extends outwards. The addition of polymer inhibits the development of surface roughness and reduces water packets ejections from the main jet.