Bubble Entrainment, Spray and Splashing at Hydraulic Jumps

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 surrounded by air. Visually significant pray and splashing were significant above the jump roller. The present study is the first comprehensive 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

Flow Resistance and Design Guidelines for Embankment Stepped Chutes

Recently the design flows of many dams have been re-evaluated, resulting in spills often larger than the original ones. In many cases, occurrence of revised flows would result in dam overtopping because of insufficient storage and spillway capacity of reservoirs. Embankment dams whilst common are more likely to fail than other dam types when overtopped in an uncontrolled manner because of breaching or complete erosion of the downstream face of the embankment. Despite the catastrophic effects of dam failure, dam overtopping constitutes still most identified collapses. Stepped chutes with moderate slopes represent a convenient embankment overtopping protection because of its large energy dissipation potential and its compatibility with recent construction methods (RCC, rip-rap gabions). New experimental flow resistance results are presented herein to help designers to predict accurately energy dissipation on moderate slope stepped chutes. Some recent advances in design guidelines are also discussed

Turbulence manipulation in air–water flows on a stepped chute: An experimental study

In a stepped channel operating with large flow rates, the flow skims over the pseudo-bottom formed by the step edges as a coherent stream. Intense three-dimensional recirculation is maintained by shear stress transmission from the mainstream to the step cavities, while significant free-surface aeration takes place. The interactions between free-surface aeration and cavity recirculation are investigated herein with seven step cavity configuration. The experiments were conducted in a large stepped channel operating at large Reynolds numbers. For some experiments, triangular vanes, or longitudinal ribs, were placed across the step cavities to manipulate the flow turbulence to enhance the interactions between the mainstream flow and the cavity recirculation region. The results showed a strong influence of the vanes on the air-water flow properties in both free-stream and cavity flows. The findings demonstrate some passive turbulence manipulation in highly turbulent air-water flows

Experimental Study of Turbulence Manipulation in Stepped Spillways: Implications on Flow Resistance in Skimming Flows

Current expertise in air-water turbulent flows on stepped chutes is limited to laboratory experiments at low to moderate Reynolds numbers on flat horizontal steps. In this study, highly turbulent air-water flows skimming down a large-size stepped chute were systematically investigated with a 22 degree slope. Turbulence manipulation was conducted using vanes or longitudinal ribs to enhance interactions between skimming flows and cavity recirculating regions. Systematic experiments were performed with seven configurations. The results demonstrated the strong influence of vanes on the air-water flow. An increase in flow resistance was observed consistently with maximum flow resistance achieved with vanes placed in a zigzag pattern

Flow Characteristics of Skimming Flows in Stepped Channels

Ohtsu et al. must be congratulated for their systematic study of skimming flows on stepped channels with different slopes and step heights. Their experimental data provide a solid database that might lead to better and improved design criteria. Important outcomes include some estimate of friction factors for skimming flows and a design flow chart for stepped canals. Despite these outstanding results, the writers would like to comment constructively on further design criteria for skimming flows, and to provide additional flow resistance results and a discussion on the physical processes. It is hoped that the present contribution will add to safer stepped chute design. The writers would like to point out that the energy dissipation performances of stepped canals with moderate slopes are far from being totally understood and further experimental research is needed

Stepped Spillways for Embankment Dams: Review, Progress and Development in Overflow Hydraulics

In recent years a number of embankments were designed with concrete overtopping protection shaped in a stepped fashion. During large overflows on stepped chutes, there is no skin friction between mainstream and steps, and flow resistance is basically form drag. Alterations of flow recirculation and of fluid exchanges between free-stream and cavity flow are discussed in this study. A comprehensive series of air-water flow measurements on embankment dam stepped chute are presented. The introduction of vanes demonstrates turbulence manipulation and form drag modification that could lead to more efficient designs

Physical Modelling and Scale Effects of Air-Water Flows on Stepped Spillways

During the last three decades, the introduction of new construction materials (e.g. RCC (Roller Compacted Concrete), strengthened gabions) has increased the interest for stepped channels and spillways. However stepped chute hydraulics is not simple, because of different flow regimes and importantly because of very-strong interactions between entrained air and turbulence. In this study, new air-water flow measurements were conducted in two large-size stepped chute facilities with two step heights in each facility to study experimental distortion caused by scale effects and the soundness of result extrapolation to prototypes. Experimental data included distributions of air concentration, air-water flow velocity, bubble frequency, bubble chord length and air-water flow turbulence intensity. For a Froude similitude, the results implied that scale effects were observed in both facilities, although the geometric scaling ratio was only Lr=2 in each case. The selection of the criterion for scale effects is a critical issue. For example, major differences (i.e. scale effects) were observed in terms of bubble chord sizes and turbulence levels although little scale effects were seen in terms of void fraction and velocity distributions. Overall the findings emphasize that physical modelling of stepped chutes based upon a Froude similitude is more sensitive to scale effects than classical smooth-invert chute studies, and this is consistent with basic dimensional analysis developed herein

Turbulence and Cavity Recirculation in Air-Water Skimming Flows on a Stepped Spillway. Reply

The authors acknowledge the discussion. In the present study, 1.6mm thick aluminum triangular vanes were used, although a number of 6mm thick perspex vanes were also tested. The experimental observations showed no effect of vane thickness on the flow patterns and properties. They did not support the Discussers’ argument. The authors however disagree with the Discusser’s comment on the rate of energy dissipation on stepped spillways and a so-called "most efficient solution" for practical applications. In terms of energy dissipation, the current expertise is restricted to rectangular, prismatic stepped chutes with horizontal steps, constant step height and constant bed slope, as shown in most earlier studies. Recently, detailed air-water flow results on stepped chutes showed systematically lower turbulence levels and larger turbulent length and time scales in comparison between 2:1 Froude similar experiments in laboratory flumes, as well as a smaller number of entrained bubbles and comparatively larger bubble sizes (Chanson and Gonzalez 2005, Chanson 2008). These findings have direct implications on the design of stepped spillway because reduced turbulence levels in laboratory flumes must imply a reduced rate of turbulent dissipation, particularly on long channels. Simply small-size models are likely to underestimate the rate of energy dissipation of prototype stepped spillways for similar flow conditions. The current understanding of energy dissipation on stepped spillway is clearly incomplete and the concept of the most efficient design is therefore an utopia

Interactions Between Free-Surface, Free-Stream Turbulence and Cavity Recirculation in Open Channel Flows: Measurements and Turbulence Manipulation

New experiments were conducted in a large stepped channel with a 22 degree slope operating at large Reynolds numbers. Interactions between free-surface and cavity recirculation were systematically investigated in the skimming flow regime. Turbulence manipulation was further conducted and identical experiments were performed with 4 configurations. Basic results demonstrated a strong influence of cavity vanes (longitudinal ribs) on the air-water flow properties in both free-stream and cavity flows. The air-water flow structure (bubble and droplet size distributions, clustering factor) was significantly affected by the presence of vanes, and greater spray production was observed with the vanes. Air-water velocity and turbulence distributions highlighted further a major impact of the vanes on the entire flow

Effects of Turbulence Manipulation in Skimming Flows: An Experimental Study

Current expertise in air-water flow properties in turbulent flows is limited to low to moderate Reynolds numbers and a few types of surfaces (roughness). Highly turbulent air-water flows cascading down a large-size stepped spillway model were systematically investigated with a 22o slope. Several stepped configurations were tested and turbulence manipulation was conducted to enhance interactions between skimming flows and cavity recirculating regions. Systematic experiments were performed with 3 new configurations to complement an initial study [6] with another three configurations. Turbulence modifiers (vanes or longitudinal ribs) were observed to have a strong influence on the air-water flow properties, with wakes or low-speed streaks observed above each vane