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

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

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

Scale Effects in Moderate Slope Stepped Spillways. Experimental Studies in Air-Water Flows

Air-water flow measurements were conducted in two large-size stepped chute facilities (theta = 3.4 & 16 degrees) to study experimental distortion caused by scale effects and result extrapolation to prototypes. The stepped geometries corresponded to moderate slopes typical of embankment dams and storm waterways. Experimental data included distributions of air concentration, air-water flow velocity, bubble frequency, bubble chord length and turbulence intensity. For a Froude similitude, scale effects were observed in both facilities, although the geometric scaling ratio was only Lr = 2 in each case. The criterion selection for scale effects is a critical issue. In the 16 degree chute, 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. The findings emphasise that physical modelling of stepped chutes based upon a Froude similitude is mor

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

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

Minimum Specific Energy and Critical Flow Conditions in Open Channels

In open channels, the relationship between the specific energy and the flow depth exhibits a minimum, and the corresponding flow conditions are called critical flow conditions. Herein they are re-analysed on the basis of the depth-averaged Bernoulli equation. At critical flow, there is only one possible flow depth, and a new analytical expression of that characteristic depth is developed for ideal-fluid flow situations with non-hydrostatic pressure distribution and non-uniform velocity distribution. The results are applied to relevant critical flow conditions : e.g., at the crest of a spillway. The finding may be applied to predict more accurately the discharge on weir and spillway crests

Characteristics of clustered particles in skimming flows on a stepped spillway

Air–water flows at hydraulic structures are commonly observed and called white waters. The free-surface aeration is characterised by some intense exchanges of air and water leading to complex air–water structures including some clustering. The number and properties of clusters may provide some measure of the level of particle-turbulence and particle–particle interactions in the high-velocity air–water flows. Herein a re-analysis of air–water clusters was applied to a highly aerated free-surface flow data set (Chanson and Carosi, Exp Fluids 42:385–401, 2007). A two-dimensional cluster analysis was introduced combining a longitudinal clustering criterion based on near-wake effect and a side-by-side particle detection method. The results highlighted a significant number of clustered particles in the high-velocity free-surface flows. The number of bubble/droplet clusters per second and the percentage of clustered particles were significantly larger using the two-dimensional cluster analysis than those derived from earlier longitudinal detection techniques only. A number of large cluster structures were further detected. The results illustrated the complex interactions between entrained air and turbulent structures in skimming flow on a stepped spillway, and the cluster detection method may apply to other highly aerated free-surface flows

Flow patterns in nappe flow regime down low gradient stepped chutes

Although modern gravity dam spillways include often steep chutes operating in skimming flow regime, succession of free-falling nappes (i.e. nappe flow regime) are more common on low gradient chutes and cascades, and this flow situation received little attention to date. New experiments were conducted in nappe flows without hydraulic jump in two large-size facilities with flat slopes. The flow on the stepped cascade displayed complex, three-dimensional patterns. Detailed air-water flow measurements were performed in the jet, at nappe impact and in the downstream flow region. Key results demonstrated that the flow on each step was rapidly varied (RVF), highly three-dimensional and strongly aerated