Local aeration characteristics

Free-surface flow conditions generating air-water mixtures can be categorized broadly into flow configurations causing ambient surface aeration - such as high-speed open channel flows or jets, which have been treated in the previous chapters - and those giving rise to local aeration. The basic difference between the two types of flow is the fact that in local aeration processes the transport capacity of the flow may drastically differ from its local entrainment capacity, whereas these capacities are always balanced in fully developed flows of ambient surface aeration. In local surface aeration, air may be entrained due to a local disturbance of the free surface at a rate completely independent of the transport capacity: the latter is only of importance with regard to the question how far and for how long the air is kept in suspension.