Chanson / J Zhejiang Univ SCIENCE A 2006 7(8):1396-1405 1396 Bubble entrainment, spray and splashing at hydraulic jumps CHANSON Hubert (Division of Civil Engineering, The University of Queensland, Brisbane QLD 4072, Australia) E-mail: h.chanson@uq.edu.au Received Mar. 16, 2006; revision accepted Apr. 8, 2006 Abstract: The sudden transition from a high-velocity, supercritical open channel flow into a slow-moving sub-critical flow is a hydraulic jump. Such a flow is characterised by a sudden rise of the free-surface, with some strong energy dissipation and air entrainment, waves and spray. New two-phase flow measurements were performed in the developing flow region using a large-size facility operating at large Reynolds numbers. The experimental results demonstrated the complexity of the flow with a developing mixing layer in which entrained bubbles are advected in a high shear stress flow. The relationship between bubble count rates and void fractions was non-unique in the shear zone, supporting earlier observations of some form of double diffusion process between momentum and air bubbles. In the upper region, the flow consisted primarily of water drops and packets sur- rounded by air. Visually significant pray and splashing were significant above the jump roller. The present study is the first com- prehensive study detailing the two-phase flow properties of both the bubbly and spray regions of hydraulic jumps, a first step towards understanding the interactions between bubble entrainment and droplet ejection processes. Key words: Hydraulic jump, Air bubble entrainment, Spray and splashing physical modelling, Particle chord time distributions doi:10.1631/jzus.2006.A1396 Document code: A CLC number: O359 + .1 INTRODUCTION A hydraulic jump is the sudden transition from a high-velocity, supercritical open channel flow into a slow-moving, sub-critical flow (Belanger, 1840; Bakhmeteff, 1932; Henderson, 1966; Chanson, 2004a). It is characterised by a sudden rise of the free-surface, with some strong energy dissipation and mixing, large-scale turbulence, air entrainment, waves and spray (Fig.1). Early experimental studies of air bubble entrainment were conducted in terms of the quantity of entrained air primarily (Wisner, 1965; Rao and Kobus, 1971). Experimental studies of the bubbly flow’s microscopic structure included Rajaratnam (1962) and Resch and Leutheusser (1972), and more recent advanced studies encompassed the works of (Mossa and Tolve, 1998; Chanson and Brattberg, 2000; Murzyn et al., 2005). Despite these pertinent studies, the interactions between air bubble entrain- ment, and spray and splashing are not yet completely understood. In the present study, new air-water flow meas- urements were performed in hydraulic jumps. The experiments were performed in a relatively large-size facility with a fine phase-detection probe. It is the purpose of this study to document the air bubble en- trainment process, as well as the spray and splashing above the roller. Dimensional considerations In the study of air bubble entrainment in hy- draulic jumps, the relevant parameters for a dimen- sional analysis include the fluid properties and physical constants, the channel geometry and inflow conditions, and the air-water flow properties includ- ing the bubble and droplet characteristics. Consider- ing the simple case of a hydraulic jump in a horizontal, rectangular channel, a dimensional analysis yields: C, F, V, d ab , d wd , …=F 1 (x, y, z, d 1 , V 1 , x 1 , δ, W, k s , g, ρ air , ρ w , µ air , µ w , …), (1) where C is the void fraction, F is the bubble count rate, V is the velocity, d ab is a characteristic size of Journal of Zhejiang University SCIENCE A ISSN 1009-3095 (Print); ISSN 1862-1775 (Online) www.zju.edu.cn/jzus; www.springerlink.com E-mail: jzus@zju.edu.cn