Image-based measurements of air-water flow properties in plunging air-water jets

A high-speed water jet plunging into a slower receiving waterbody results in an air entrainment process. In hydraulic engineering, a plunging jet is commonly used as an energy dissipater downstream of a hydraulic structure, such as stepped chute flow and water treatment plants. The energy dissipation is related to the bubble-turbulence interplay associated with advection of turbulent structures, which needs to be better understood. In addition, the penetration depth of the aerated region in a plunging pool represents a key parameter for practical engineers. The present study aims at an in-depth flow characterisation in two-dimensional vertical impinging water jets using image-based techniques. An ultra-high-speed video camera was used to record the video data, in full HD resolution with a sampling rate of up to 10,000 fps. The air-water flow regions were detected using a constant threshold technique, and the interfacial velocities were obtained using an optical flow technique. Based on the penetration depth and visual observation from various flow conditions, the jet flows were classified into different cases. The optical flow results were validated using the phase-detection probe data from a previous study. The results are expected to provide a solid database for numerical modeling, while delivering some useful information for the design of optimised hydraulic structures.