Air-Water Mass Transfer on a Stepped Waterway

Stepped waterways are commonly used as river training, debris dam structures, storm water systems and aeration cascades. The present study was focused on analysis of basic air-water flow properties on a low gradient stepped chute, combined with dissolved oxygen measurements. The oxygen aeration efficiency was found to be about 30% for 12 steps with a total drop in invert elevation of 1.4 m, nearly independently of the inflow conditions. Detailed air-water flow measurements, including void fraction, velocity, bubble count rate and interface area, were used to integrate the mass transfer equation and to estimate the aeration potential of the waterway. Direct comparisons with dissolved oxygen measurements showed good agreement between the two methods

Experimental Investigations of Air Entrainment in Transition and Skimming Flows down a Stepped Chute: Application to Embankment Overflow Stepped Spillways

Stepped spillways have been used for about 3,500 years. The last few decades have seen the development of new construction materials, design techniques and applications : e.g., embankment overtopping protection systems. Although it is commonly acknowledged that free-surface aeration is significant in stepped chutes, experimental data are scarce, often limited to very steep slopes. The paper presents an experimental study conducted in a large-size stepped chute. Observations demonstrate the existence of a transition flow pattern for intermediate flow rates between nappe and skimming flows. Detailed air-water flow measurements were conducted in both transition and skimming flows, immediately downstream of the inception point of free-surface aeration where uniform equilibrium flow conditions were not achieved. In skimming flows, a complete characterisation is developed for the distributions of void fraction, bubble count rate and velocity, and flow resistance is estimated including drag reduction effects. Transition flows exhibit significantly different air-water flow properties. They are highly aerated, requiring the design of comparatively high chute sidewalls. The design of embankment overflow stepped spillway is discussed in the light of the new results and design recommendations are provided. Major design issues include the step stability at the plunge point for high tailwater levels, the interactions between free-surface and seepage flows which could lead to further drag reduction, and secondary currents at the connection between steps and abutments

Developing Air-Water Shear Layers Of Two-Dimensional Water Jets Discharging Into Air

The paper presents new experiments performed in the developing flow region of two-dimensional water jets discharging into air. The results indicate that the distributions of void fraction follow closely an analytical solution of the diffusion equation, and that the transfer of momentum between the water jet and the surrounding air is negligible for x/do < 20. An analogy with self-aerated open channel flows is further developed. The distributions of air bubble frequency have the same shape for both flow situations suggesting some similarity in the air-water flow structure

Interfacial Aeration and Bubble Count Rate Distributions in a Supercritical Flow Past a Backward-Facing Step

An example of high-velocity open channel flows is a supercritical flow past an abrupt drop. In such a geometry, the basic air-water flow properties were measured, including distributions of void fraction and bubble count rate, and local air and water chord size distributions, at and downstream of the backward-facing step. The bubble count rate distributions were compared with a conceptual model of streamwise distribution of air and water chords which yields a quasi-parabolic relationship between bubble count rate and void fraction. The proposed model was an attempt to explain the experimental relationship between bubble count rate and void fraction, rather a meticulous breakdown of the complex air-water structure

Energy Dissipation and Air Entrainment in a Stepped Storm Waterway: an Experimental Study

For the last three decades, research focused on steep stepped chutes. Few studies considered flat-slope stepped geometries such as stepped storm waterways or culverts. In this study, experiments were conducted in a large, flat stepped chute (~3.4 degrees) based upon a Froude similitude. Three basic flow regimes were observed: nappe flow without hydraulic jump, transition flow, and skimming flow. Detailed air-water flow measurements were conducted. The results allow a complete characterization of the air concentration and bubble count rate distributions, as well as an accurate estimate of the rate of energy dissipation. The flow resistance, expressed in terms of a modified friction slope, was found to be about 2.5 times greater than in smooth-chute flow. A comparison between smooth- and stepped-invert flows shows that greater aeration and larger residence times take place in the latter geometry. The result confirms the air-water mass transfer potential of stepped cascades, even for flat slopes

Experimental Investigations of Air Entrainment in Transition and Skimming Flows down a Stepped Chute

Stepped spillways have been used for about 3500 years. The last few decades have seen the development of new construction materials, design techniques, and applications, for example, embankment overtopping protection systems. Although it is commonly acknowledged that free-surface aeration is significant in stepped chutes, experimental data are scarce, often limited to very steep slopes (alpha - 50 deg). This paper presents an experimental study conducted in a large-size stepped chute (alpha = 22 deg, h = 0.1 m, W = 1 m). Observations demonstrate the existence of a transition flow pattern for intermediate flow rates between nappe and skimming flows. Detailed air-water flow measurements were conducted in both transition and skimming flows, immediately downstream of the inception point of free-surface aeration where uniform equilibrium flow conditions were not achieved. In skimming flows, a complete characterization is developed for the distributions of void fraction, bubble count rate, and velocity, and flow resistance data are compared with other studies. Transition flows exhibit significantly different air-water flow properties. They are highly aerated, requiring the design of comparatively high chute sidewalk. Les deversoirs en escalier ont ete utilises depuis plus de 3500 ans. Les quelques demieres decennies ont vu apparaitre le developpement de nouveaux materiaux de construction, techniques de conception et d'applications: e.g., systemes de protection contre le debordements des barrages en remblai. Bien qu'il soil frequemment reconnu que l'a6ration en surface libre est importante dans les deversoirs en escalier, les donnees experimentales sont rares, se limitant souvent a des pentes raides (alpha - 50 deg). Cet article presente une etude experimentale conduite dans un grand deversoir en escalier (alpha = 22 deg, h = 0.1 m, W = 1 m). Les observations demontrent l'existence d'un comportement d'ecoulement de transition pour les debits moyens entre l'ecoulement en nappe et l'ecoulement extremement turbulent. Des mesures detainees du debit air-eau ont ete prises pour l'ecoulement de transition et l'ecoulement extremement turbulent, directement en aval du point d'aeration en surface libre off des conditions d'ecoulement uniforme a l'equilibre n'ont pas ete achevees. Pour l'ecoulement extremement turbulent, une caracterisation complete est developpee pour les distributions des fractions de vides, le compte des bulles et leur velocite, et les donnees de la resistance du debit sont comparees avec d'autres etudes. Les ecoulements de transition montrent des differences significatives dans les proprietes de debit air-eau. Its sont grandement aeres, ce qui demande une conception des murs de deversoir plus haut

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

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

Current expertise in air-water turbulent flows on stepped chutes is limited mostly to laboratory experiments at low to moderate Reynolds numbers on chutes with flat horizontal steps. In this study, highly turbulent air-water flows skimming down a large-size stepped chute were investigated with a 1V:2.5H slope. For some experiments, the cavity recirculation was controlled using triangular vanes, or longitudinal ribs, to enhance the interactions between the skimming flow and cavity recirculating region. New experiments were performed with seven configurations. The results demonstrated the strong influence of the vanes on the cavity recirculation patterns and on the air-water flow properties. An increase in flow resistance was observed consistently with maximum rate of energy dissipation achieved with vanes placed in a zigzag pattern

Turbulence, dynamic similarity and scale effects in high-velocity free-surface flows above a stepped chute

In high-velocity free-surface flows, air entrainment is common through the interface, and intense interactions take place between turbulent structures and entrained bubbles. Two-phase flow properties were measured herein in high-velocity open channel flows above a stepped chute. Detailed turbulence measurements were conducted in a large-size facility, and a comparative analysis was applied to test the validity of the Froude and Reynolds similarities. The results showed consistently that the Froude similitude was not satisfied using a 2:1 geometric scaling ratio. Lesser number of entrained bubbles and comparatively greater bubble sizes were observed at the smaller Reynolds numbers, as well as lower turbulence levels and larger turbulent length and time scales. The results implied that small-size models did underestimate the rate of energy dissipation and the aeration efficiency of prototype stepped spillways for similar flow conditions. Similarly a Reynolds similitude was tested. The results showed also some significant scale effects. However a number of self-similar relationships remained invariant under changes of scale and confirmed the analysis of Chanson and Carosi (Exp Fluids 42:385-401, 2007). The finding is significant because self-similarity may provide a picture general enough to be used to characterise the air– water flow field in large prototype channels

Hydraulics of skimming flows on stepped chutes: The effects of inflow conditions?

Modern stepped spillways are typically designed for large discharge capacities corresponding to a skimming flow regime for which flow resistance is predominantly form drag. The writer demonstrates that the inflow conditions have some effect on the skimming flow properties. Boundary layer calculations show that the flow properties at inception of free-surface aeration are substantially different with pressurized intake. The re-analysis of experimental results highlights that the equivalent Darcy friction factor is f similar to 0.2 in average on uncontrolled stepped Chute and f similar to 0.1 on stepped chute with pressurized intake. A simple design chart is presented to estimate the residual flow velocity, and the agreement of the calculations with experimental results is deemed satisfactory for preliminary design