Interactions between turbulent waters and the atmosphere may lead to some air-water mixing, often called \u27white water\u27. This study reviews the basic entrainment processes for a wide range of flow situations and presents new evidence leading to a better understanding of the basic multiphase flow dynamics. The focus is on flow situations characterised by very-strong air-water interactions : e.g., vertical plunging jet, stepped chute flow, plunging breaker. The two basic entrainment mechanisms are local and interfacial aerations. In a local aeration process, air is entrapped at the singularity/discontinuity between the impinging jet and the receiving pool of water. At low jet velocities, the bubbles are entrained individually while, at high jet impact velocities, an elongated air cavity is set into motion between the entrained fluid and the jet flow. At impingement, air is entrapped by a Couette flow mechanism. Interfacial aeration occurs when turbulent velocity fluctuations acting next to the free-surface become large enough to overcome surface tension and buoyancy. The sizes of the bubbles and droplets extend over several orders of magnitude. Void fraction distributions may be modelled by advective diffusion models. Turbulence intensity measurements suggest high levels of turbulence across the entire air-water flow, of one to two orders of magnitude greater than in monophase flows. In coastal engineering, air entrainment is characterised by unsteadiness and high levels of aeration. The results demonstrate that air entrainment in coastal and oceanic zones is an important process and cannot be ignored.
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