Air entrainment in rectangular free falling jets

© 2020 IAHR International Symposium on Hydraulic Structures, ISHS 2020.All right reserved.The overflow is a significant concern for all dam safety engineers and regulators. Due to the increasing magnitude of design floods, the current discharge capacity of many large dams seems to be insufficient. This situation may generate overtopping scenarios with new hydrodynamic actions. However, the study of air entrainment in rectangular free falling jets is scarce to date. This paper presents a laboratory analysis of the air-water flow in nappe jets. The experimental device consists in a sharp-crested weir of 1.05 m width with a vertical falling distance up to 2.20 m. Two different specific flows were considered. The air-water properties were measured in different cross-sections of the nappe with a phase detection probe. The output signal was sampled at 20 kHz for 45 s. To characterize the flow properties, several hydrodynamic variables were investigated such as the void fraction distribution, and the air-water phase change frequency. The variation of the jet thickness and the reduction of the solid inner jet core were also analyzed during the fall. The new measurements allow to extend the existing range of previous researches.