Interaction between free-surface aeration and total pressure on a stepped chute Gangfu Zhang, Hubert Chanson ⇑ The University of Queensland, School Civil Engineering, Brisbane, QLD 4072, Australia article info Article history: Received 21 September 2015 Received in revised form 10 December 2015 Accepted 12 December 2015 Available online 30 December 2015 Keywords: Air bubble entrainment, Total pressure Turbulence Coupling Physical modelling Stepped spillways abstract Stepped chutes have been used as flood release facilities for several centuries. Key features are the intense free-surface aeration of both prototype and laboratory systems and the macro-roughness caused by the stepped cavities. Herein the air bubble entrainment and turbulence were investigated in a stepped spill- way model, to characterise the interplay between air bubble entrainment and turbulence, and the com- plicated interactions between mainstream flow and cavity recirculation motion. New experiments were conducted in a large steep stepped chute (h = 45°, h = 0.10 m, W = 0.985 m). Detailed two-phase flow measurements were conducted for a range of discharges corresponding to Reynolds numbers between 2  10 5 and 9  10 5 . The total pressure, air–water flow and turbulence properties were documented sys- tematically in the mainstream and cavity flows. Energy calculations showed an overall energy dissipation of about 50% regardless of the discharge. Overall the data indicated that the bottom roughness (i.e. stepped profile) was a determining factor on the energy dissipation performance of the stepped structure, as well as on the longitudinal changes in air–water flow properties. Comparative results showed that the cavity aspect ratio, hence the slope, has a marked effect on the residual energy. Ó 2015 Elsevier Inc. All rights reserved. 1. Introduction Stepped spillways have been used as flood release facilities for several centuries [11]. In the past few decades, advances in con- struction materials and techniques led to a regained interest in stepped spillway design [1,20,10,12]. The steps contribute to some dissipation of the turbulent kinetic energy and reduce or eliminate the need for a downstream stilling structure [15]. Stepped spillway flows are characterised by strong turbulence and air entrainment (Fig. 1). Early physical studies were conducted by Horner [29], Sor- ensen [43], and Peyras et al. [36] with a focus on flow patterns and energy dissipation. Many studies focused on steep chute slopes typical of concrete gravity dams ([39,9,34,8]. More recent studies were conducted on physical models with moderate slopes typical of embankment structures [35,30,22,5,6,45,50]. A key feature of stepped chute flows is the intense free-surface aeration observed in both prototype and laboratory (Figs. 1 and 2). A number of laboratory studies investigated systematically the air– water flow properties at step edges [32,17,44,7,4]. A few studies measured the two-phase flow properties inside and above the step cavities [26,23]. The stepped cavities act as macro-roughness, with intense cavity recirculation. To date the findings hinted a strong interplay between air bubble entrainment and turbulence, and complicated interactions between mainstream flow and cavity recirculation motion, although no definite conclusion has been drawn in terms of stepped spillway design. The goal of this contribution is to examine the air bubble entrainment and turbulence in a stepped spillway model. New experiments were conducted in a large steep chute (h = 45°) equipped with 12 flat impervious steps (h = 0.10 m, W = 0.985 m). Detailed two-phase flow measurements were conducted for a range of discharges corresponding to the transition and skimming flow regimes. The total pressure, air–water flow and turbulence properties in the mainstream and cavity flows were documented systematically. It is the aim of this work to quantify the interplay between air bubble entrainment, turbulence and energy dissipation. 2. Experimental facility and instrumentation New experiments were conducted in a large-size stepped spill- way model located at the University of Queensland (Figs. 2 and 3). The facility consisted of a 12.4 m long channel. Three pumps driven by adjustable frequency AC motors delivered a controlled dis- charge to a 5 m wide, 2.7 m wide and 1.7 m deep intake basin http://dx.doi.org/10.1016/j.expthermflusci.2015.12.011 0894-1777/Ó 2015 Elsevier Inc. All rights reserved. ⇑ Corresponding author. Fax: +61 (7) 33 65 45 99. E-mail address: h.chanson@uq.edu.au (H. Chanson). URL: http://www.uq.edu.au/~e2hchans/ (H. Chanson). Experimental Thermal and Fluid Science 74 (2016) 368–381 Contents lists available at ScienceDirect Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs CrossMark