Air-Entrainment Mechanisms in Plunging Jets and Breaking Waves

Air entrainment in liquids is a complex phenomenon that has important applications in industry and the environment. This article addresses recent research on how air is entrained during the impact of a liquid stream on a pool of the same material in a variety of scenarios. At the fundamental level, these scenarios include the prototype flows of impacting stationary laminar and turbulent steady jets, the transient impact of isolated masses of liquid, the impact of jets with organized disturbances, and translating steady and transient jets. Although significant advances have been made recently, the complexity of this multiphase, three-dimensional, and frequently turbulent flow phenomenon leaves many unanswered questions. To help elucidate the problems still to be addressed in future research, the final section of the article examines air entrainment in the more complex application of plunging breaking waves and points out the many parts of this process that are poorly understood.