Stereographic Backlit Imaging and Bubble Identification From a Plunging Jet With Floor Interactions

Plunging jets have been extensively studied for their relatively simple set-up but complex multiphase interactions. This phenomenon includes gas carry-under and mixing, which occurs when shear effects between the plunging liquid jet and surrounding gas are sufficient to entrain gas at the impact site. Previous investigations typically assume the floor has an infinite depth and neglect compressive effects caused by the jet interacting with the catch tank floor. While this assumption is ideal for breaking waves in the middle of the ocean, many other applications have to contend with floor effects. These include waterfalls, wastewater treatment, dams, fish farms, mineral separation, and molten metal pouring. It is hypothesized that floor interactions will significantly affect the multiphase flow hydrodynamics, especially in places where the uninhibited jet would approach or pass the floor region. Using a large catch tank with an adjustable floor region designed to hold a constant water level, data were collected using high-speed backlit stereographic imaging to capture and compare the effects of three separate tank depths with those found using an infinite pool assumption. To identify bubbles in each stereographic projection, a uniform bubble recognition procedure was developed that was used across all data sets. This allowed for the automated identification of bubble entrainment regions, which could be compared with different flow conditions. Preliminary results are inconclusive as to the effects of the floor region on the bubble plume dynamics; however, the results showed consistent measurements between trials and the two stereographic cameras, implying the time variation of the jet dynamics was the primary source of uncertainty in the results and not the identification procedure. Therefore, the identification methods have provided a method for plume volume and shape estimation, which will be used in future studies using 3D imaging techniques.