ABSTRACT
The exchange of air between the atmosphere and flowing water is usually called air entrainment, air bubble entrainment or self-aeration. The continuous exchange between air and water is most important for the biological and chemical equilibrium on our planet. For example, the air-water mass transfer at the surface of the oceans regulates the composition of the atmosphere. The aeration process drives the exchange of nitrogen, oxygen and carbon dioxide between the atmosphere and the sea, in particular the dissolution of carbon dioxide into the oceans and the release of supersaturated oxygen to the atmosphere. Another form of flow aeration is the entrainment of un-dissolved air bubbles at the air-water free-surface. Air bubble entrainment is observed in chemical, coastal, hydraulic, mechanical and nuclear engineering applications. In Nature, air bubble entrainment is observed at waterfalls, in mountain streams and river rapids, and in breaking waves on the ocean surface. The resulting “white waters” provide some spectacular effects (Figs. 7.1 to 7.4). Figure 7.1 illustrates the air bubble entrainment at a 83 m high waterfall with a lot of splashing and spray generated at nappe impact. Figure 7.2 shows some air entrainment in a hydraulic jump downstream of a spillway, and Figure 7.3 presents some air bubble entrainment at a plunging breaking wave. Figure 7.4 highlights the free-surface aeration downstream of the Three Gorges dam that may be seen from space (Fig. 7.4B).
