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Air-water bubbly flows are encountered in numerous engineering applications. One type of air-water shear flow, the developing flow region of a plunging jet, is discussed in the light of new experimental evidence.Distributions of air concentration and mean air-water velocity, and bubble chord length distributions are presented for inflow velocities ranging from 2 to 8 m/s. The results indicate that the distributions of void fraction follow closely analytical solutions of the diffusion equation, as developed by CHANSON (1995aCHANSON ( ,1997. In airwater shear layers, the velocity distributions have the same shape as in monophase flows but the characteristic parameters of the shear layer differ from monophase flow results, because of the interactions between the entrained air bubbles and the turbulence.Keywords: air bubble entrainment, plunging water jet, two-phase flow, developing shear flow, air bubble diffusion.

Turbulent water jets discharging into the atmosphere are often characterized by a substantial amount of free-surface aeration. The effects can be detrimental or beneficial. In any case, the knowledge of the air entrainment mechanisms is essential for an optimum design. New experimental data are presented in the developing flow region of two-dimensional water jets discharging into air. The results indicate that the air diffusion takes place rapidly downstream of the nozzle and it is nearly independent of the momentum transfer process. Further, the distribution of air bubble frequency may be related to the air content distribution by a parabolic relationship.