Numa Bertola 1 School of Civil Engineering, The University of Queensland, Brisbane QLD 4072, Australia Hang Wang 2 School of Civil Engineering, The University of Queensland, Brisbane QLD 4072, Australia e-mail: hang.wang@uqconnect.edu.au Hubert Chanson School of Civil Engineering, The University of Queensland, Brisbane QLD 4072, Australia Air Bubble Entrainment, Breakup, and Interplay in Vertical Plunging Jets The entrainment, breakup, and interplay of air bubbles were observed in a vertical, two- dimensional supported jet at low impact velocities. Ultra-high-speed movies were ana- lyzed both qualitatively and quantitatively. The onset velocity of bubble entrainment was between 0.9 and 1.1 m/s. Most bubbles were entrained as detached bubbles from elon- gated air cavities at the impingement point. Explosion, stretching, and dejection mecha- nisms were observed for individual bubble breakup, and the bubble interaction behaviors encompassed bubble rebound, “kiss-and-go,” coalescence and breakup induced by approaching bubble(s). The effects of jet impact velocity on the bubble behaviors were investigated for impact velocities from 1.0 to 1.36 m/s, in the presence of a shear flow environment. [DOI: 10.1115/1.4039715] 1 Introduction A plunging jet is a rapid liquid jet plunging into a relatively slow body of the same or a different fluid. The near-field flow region downstream of the impingement point is a turbulent shear flow, with transfer of momentum from the impinging flow to the surrounding bath. The plunging jet flow pattern varies for differ- ent jet impact velocities [1]. For a water jet with free surface open to atmosphere, air entrainment takes place at the impingement point when the jet impact velocity exceeds a critical onset velocity V e [2–4]. The entrained air bubbles affect significantly the charac- teristics of the downstream shear flow by air–water mixing and bubble–turbulence interplay. For example, the plunging pool is highly aerated at large impact velocities, leading to unsteady flow bulking, bubble grouping during advection, and modification of turbulence field [5]. On the other hand, when the impact velocity is relatively small and slightly greater than V e , particle interplay is primarily limited among a small number of neighboring bubbles, in the absence of large-scale turbulent structures in surrounding water [3]. The bubble behavior, including their entrainment, breakup and interplay/coalescence, is directly related to the inter- facial area in mass and heat transfer in the two-phase flow, thus is important in many industrial applications [6,7]. The presence of large eddy structures and turbulent shear forces in a plunging jet can further complicate the bubble motions, deformation, and dynamics. Air entrainment in plunging jets is a process sensitive to both impact velocity and turbulence level in the impinging jet [4,8]. For given fluid properties (e.g., viscosity, surface tension), the occurrence of bubble breakup in a homogeneous turbulent flow is related to the interfacial oscillations induced by the flow velocity fluctuations and the response of surface tension [9–11]. The effects of gravity, shear stress, inhomogeneous turbulence, and any form of flow instabilities may add to nonzero average bubble deformation [12]. A relevant parameter to the bubble coalescence is the relative bubble approach velocity [13]. In applications like plunging jet, approach velocity observations implied an instant coalescence regime with typical coalescence time smaller than 10 2 s[14]. There have been vast amount of analytical, physical, and numerical studies of bubble breakup and coalescence, most of which focused on artificially generated bubbles in well-controlled turbulent environment [15–18]. Fewer studies were dedicated to self-aerated flows like circular and planar plunging jets with in- depth description of the air-phase behavior. One of the bottlenecks of the research is the inadequate measuring techniques, and the lack of physical information hampers the progress in numerical simulation investigation when the coalescence and breakup mod- els need experimental guidance and verification [6]. The aim of this work is to present a statistical description of bubble behavior in self-aerated plunging jets at relatively low impact velocities. Observation of self-entrained individual bub- bles or bubble clusters was performed using high-speed camera visualization at flow conditions close to the onset of air entrain- ment, and the images were analyzed qualitatively and quantitatively. 2 Experimental Setup A two-dimensional planar water jet was issued from a 0.269 m wide rectangular nozzle with a 0.012 m nozzle opening. The pla- nar jet was supported by a full-width polyvinyl chloride sheet extending from the nozzle edge into the receiving pool. The jet support was set at 88.5 deg to the horizontal to prevent flow detachment. It was built with lateral transparent window to facili- tate flow visualization. The receiving tank was a 2.5 m long, 1 m wide, 1.5 m deep, in which the bath water level was controlled with a sharp-crested weir. The deep pool setup ensured that the bottom had no effect on the air entrainment and diffusion process in upper part of the pool. Brisbane tap water was supplied from a constant head tank and the water discharge was measured with an orifice meter installed in the supply pipelines and calibrated on-site. Observations of bubble behavior were carried out using a Phantom Ultra-high- speed digital camera (v2011) equipped with a Carl Zeiss Planar T*85 mm f/1.4 lens, producing images with an absolutely negligi- ble degree (0.1%) of barrel distortion. The camera system was able to record up to 22,000 monochrome frames per second in high definition (1280  800 pixels, pixel size 28 lm) or 1,000,000 frames per second in low definition (128  16 pixels). Herein, the recording was set between 600 fps and 10,000 fps in high defini- tion, and the total number of recorded frames was 33,285 frames, independently of the frame rate. The video movies were analyzed manually to guarantee maximum reliability of the data. The cam- era was positioned beside the plunge pool. The observation win- dow was 10 cm wide and 20 cm long, while the depth of field was less than 20 mm. The observations were two-dimensional, and 1 Present address: ETH-Z€ urich, Future Cities Laboratory, Singapore 138602. 2 Corresponding author. Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received October 2, 2017; final manuscript received March 16, 2018; published online May 2, 2018. Assoc. Editor: Matevz Dular. Journal of Fluids Engineering SEPTEMBER 2018, Vol. 140 / 091301-1 Copyright V C 2018 by ASME