Air-water Flow Measurements Research Articles

Energy Dissipation down a Stepped Spillway with Nonuniform Step Heights

Although most stepped spillway–design guidelines were developed for uniform step heights, a nonuniform stepped design might be a practical alternative in some cases. A physical study was conducted in a moderate slope-stepped chute (1V:2H) and five stepped configurations were tested for 0.7<dc/h<1.9. Detailed air-water flow measurements were performed for each configuration and the results were compared in terms of flow patterns, energy dissipation, and flow resistance. The basic findings showed minor differences between all configurations and indicated that the rate of energy dissipation was about the same for uniform and nonuniform stepped configurations. But the observations suggested that the nonuniform stepped configurations might induce some flow instabilities for smaller flow rates.

Air–water flow properties in step cavity down a stepped chute

For the last three decades, the research into skimming flows down stepped chutes was driven by needs for better design guidelines. The skimming flow is characterised by some momentum transfer from the main stream to the recirculation zones in the shear layer developing downstream of each step edge. In the present study some physical modelling was conducted in a relatively large facility and detailed air–water flow measurements were conducted at several locations along a triangular cavity. The data implied some self-similarity of the main flow properties in the upper flow region, at step edges as well as at all locations along the step cavity. In the developing shear layer and cavity region (i.e. y/ h < 0.3), the air–water flow properties presented some specific features highlighting the development of the mixing layer downstream of the step edge and the strong interactions between cavity recirculation and mainstream skimming flows. Both void fraction and bubble count rate data showed a local maximum in the developing shear layer, although the local maximum void fraction was always located below the local maximum bubble count rate. The velocity profiles had the same shape as the classical mono-phase flow data. The air–water flow properties highlighted some intense turbulence in the mixing layer that would be associated with large shear stresses and bubble–turbulence interactions.

Turbulent length–time scales distributions in hydraulic jumps

Air–water flow measurements were performed in hydraulic jump flows for a range of inflow Froude numbers. The experiments were conducted in a large-sized facility using phase-detection intrusive probes. The void fraction measurements showed the presence of an advective diffusion shear layer where the air concentration vertical distributions were successfully compared with an analytical solution of the advective diffusion equation for air bubbles. In the air–water shear layer, a new empirical relationship between the maximum air concentration decay as a function of both the distance from the jump toe and the inflow Froude number was derived. Air–water turbulent time and length scales were deduced from auto- and cross-correlation analyses based on the method of Chanson (2007). The result provided some characteristic transverse time and length scales of the eddy structures advecting the air bubbles in the developing shear layer. The turbulence time scale data showed an increase with the relative elevation above the bed, as well as some decrease with increasing distance from the toe. The dimensionless integral turbulent length scale Lxz/d1was closely related to the inflow depth.

Energy dissipation, flow resistance and gas-liquid interfacial area in skimming flows on moderate-slope stepped spillways

With the re-evaluation and revision of a number of design floods, several embankment overtopping protection systems have been developed and a common technique is the construction of a stepped spillway on the downstream slope. For such moderate slope stepped channels, detailed air–water flow measurements were performed in a large facility with a focus on the rate of energy dissipation, flow resistance, air–water interfacial areas and re-aeration rates. Past and present experimental results showed a significant aeration of the flow. The median dimensionless residual head was about 3 × dc for the 21.8° sloping chute and smaller than that for flatter slopes (θ = 3.4° and 15.9°). The flow resistance results yielded an equivalent Darcy friction factor of about 0.25 implying a larger flow resistance for the 21.8° slope angle than for smaller slope angles. The re-aeration rate was deduced from the integration of the mass transfer equation using measured air–water interfacial areas and air–water flow velocities. The results suggested an increasing re-aeration rate with increasing rate of energy dissipation. The stepped invert contributed to intense turbulence production, free-surface aeration and large interfacial areas. The experimental data showed however some distinctive seesaw pattern in the longitudinal distribution of air–water flow properties with a wave length of about two step cavities. While these may be caused by the interactions between successive adjacent step cavities and their interference with the free-surface, the existence of such “instabilities” implies that the traditional concept of normal flow might not exist in skimming flows above moderate-slope stepped spillways.

Turbulence characteristics in skimming flows on stepped spillways

The stepped spillway design is characterized by an increase in the rate of energy dissipation on the chute associated with a reduction of the size of the downstream energy dissipation system. This study presents a thorough investigation of the air–water flow properties in skimming flows with a focus on the turbulent characteristics. New measurements were conducted in a large-size facility (θ = 22°; step height, h = 0.1 m) with several phase-detection intrusive probes. Correlation analyses were applied to estimate the integral turbulent length and time scales. The skimming flow properties presented some basic characteristics that were qualitatively and quantitatively in agreement with previous air–water flow measurements in skimming flows. Present measurements showed some relatively good correlation between turbulence intensities Tuand turbulent length and time scales. These measurements also illustrated large turbulence levels and large turbulent time and length scales in the intermediate region between the spray and bubbly flow regions.

Turbulence measurements in the bubbly flow region of hydraulic jumps

A hydraulic jump is characterized by a highly turbulent flow with macro-scale vortices, some kinetic energy dissipation and a bubbly two-phase flow structure. New air–water flow measurements were performed in a large-size facility using two types of phase-detection intrusive probes: i.e. single-tip and double-tip conductivity probes. These were complemented by some measurements of free-surface fluctuations using ultrasonic displacement meters. The void fraction measurements showed the presence of an advective diffusion shear layer in which the void fractions profiles matched closely an analytical solution of the advective diffusion equation for air bubbles. The free-surface fluctuations measurements showed large turbulent fluctuations that reflected the dynamic, unsteady structure of the hydraulic jumps. The measurements of interfacial velocity and turbulence level distributions provided new information on the turbulent velocity field in the highly-aerated shear region. The velocity profiles tended to follow a wall jet flow pattern. The air–water turbulent integral time and length scales were deduced from some auto- and cross-correlation analyses based upon the method of Chanson [H. Chanson, Bubbly flow structure in hydraulic jump, Eur. J. Mech. B/Fluids 26 (3) (2007) 367–384], providing the turbulent scales of the eddy structures advecting the air bubbles in the developing shear layer. The length scale L xz is an integral air–water turbulence length scale which characterized the transverse size of the large vortical structures advecting the air bubbles. The experimental data showed that the dimensionless integral turbulent length scale L xz / d 1 was closely related to the inflow depth: i.e. L xz / d 1 = 0.2–0.8, with L xz increasing towards the free-surface.

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, highly three-dimensional and strongly aerated.

Advanced post-processing and correlation analyses in high-velocity air–water flows

The interest in air-water flows has not diminished in recent years, but it is accompanied by frequent citations of early, sometimes outdated articles. A basic issue is the inadequate, incomplete interpretation of air-water flow instrumentation by hydraulic engineers and researchers. This article comments on high-velocity air-water flow measurements by means of intrusive phase detection probes. This article focus on the bubbly flow structure of high-velocity air-water flow based upon measurements by means of intrusive phase detection probes. It is shown that some advanced post-processing techniques may yields expanded information on the air-water turbulent flow properties and bubbly flow structures. The outcomes demonstrate simple techniques in high-velocity air-water flow analysis.

Air entrainment processes in a full-scale rectangular dropshaft at large flows

Rectangular dropshafts, commonly used in sewers and storm water systems, are characterized by significant flow aeration. New detailed air–water flow measurements were conducted in a near-full-scale dropshaft at large discharges. In the shaft pool and outflow channel, the results demonstrated the complexity of different competitive air entrainment mechanisms. Bubble size measurements showed a broad range of entrained bubble sizes. Analysis of streamwise distributions of bubbles suggested further some clustering process in the bubbly flow although, in the outflow channel, bubble chords were in average smaller than in the shaft pool. A robust hydrophone was tested to measure bubble acoustic spectra and to assess its field application potential. The acoustic results characterized accurately the order of magnitude of entrained bubble sizes, but the transformation from acoustic frequencies to bubble radii did not predict correctly the probability distribution functions of bubble sizes

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.

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 turbu- lence. 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 pro- totypes. 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 al- though 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.

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.

Unsteady air?water flow measurements in sudden open channel flows

Measurements of air-water flow properties are reasonably simple in steady flows, but not so in unsteady flows. Some studies investigated periodic flows in which instantaneous data were averaged over several cycles. During the present work, new unsteady air-water flow measurements were performed in sudden open channel flow surges. Unsteady air-water flow measurements were performed in the wave front with an array of resistivity probes. The results demonstrated quantitatively strong aeration of the leading edge in terms of void fractions, bubble count rates and specific interface areas. Experi- mental results highlighted that this strongly aerated region was relatively short: i.e. typically 0.3 to 0.5 m long. Mea- surements of air and water chord sizes highlighted a wide range of bubble and droplet sizes. Time-variations of air- water flow structure were observed. List of symbols a Air-water specific area (1/m) defined as the air-water interface area per unit volume of air and water amean Depth averaged specific interface area de- fined in terms of Y90: -

Understanding air–water mass transfer in rectangular dropshafts

A dropshaft is a vertical structure connecting two channels with different invert elevations. Four configurations of rectangular dropshafts were investigated systematically to study the effects of outflow direction and pool depth on particle residence times and flow aeration. The best hydraulic design was that with 180° outflow direction and deep pool shaft. For that design, a full-scale study was conducted, the scaling ratio between prototype and model being 3.1:1. Although similar trends were seen in both model and prototype, scale effects were observed in terms of particle residence times and bubble swarm depths. In the prototype, detailed air–water flow measurements were performed in the shaft pool and the mass transfer equation was integrated using measured interfacial areas and particle residence times. The results demonstrate that the air–water mass transfer is the largest at low flow rates (regime R1) because of large residence times and significant interfacial area. Overall the present study provides new understanding of the basic mechanisms of air–water mass transfer in rectangular dropshafts. Key words: dropshaft, mass transfer, aeration, hydraulics.

Drag reduction in skimming flow on stepped spillways by aeration / Réduction du frottement des écoulements écumant sur les seuils en marches d'escalier par aération

In skimming flows on stepped spillway, flow resistance is predominantly a form drag process. Cavity recirculation and air entrainment are intense. The interactions between entrained air bubbles and mixing layers may cause some reduction in flow resistance. The paper presents new evidence showing a reduction in friction factor in skimming flow with increasing free-surface aeration. The experimental data are based upon detailed air-water flow measurements in large size models and may be extended to prototypes. The results might suggest some drag reduction, although comparable clear-water flow data are rare.

Drag reduction in skimming flow on stepped spillways by aeration

In skimming flows on stepped spillway, flow resistance is predominantly a form drag process. Cavity recirculation and air entrainment are intense. The interactions between entrained air bubbles and mixing layers may cause some reduction in flow resistance. The paper presents new evidence showing a reduction in friction factor in skimming flow with increasing free-surface aeration. The experimental data are based upon detailed air-water flow measurements in large size models and may be extended to prototypes. The results might suggest some drag reduction, although comparable clear-water flow data are rare.

Energy Dissipation and Air Entrainment in Stepped Storm Waterway: 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 (θ=3.4°) 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 (θ<5°).

Air-Water Flow Measurements with Intrusive, Phase-Detection Probes: Can We Improve Their Interpretation?

Interest in air-water flows has not diminished in recent years, as evident by the number of associated papers published in the Journal of Hydraulic Engineering and other journals, such as the Journal of Hydraulic Research, the International Journal of Multiphase Flow and the Journal of Fluids Engineering. The writer believes that a particularly important issue is the often inadequate or incomplete interpretation of air-water flow instrumentation by hydraulic engineers and researchers. The present Forum article briefly comments of the several common techniques for measuring air-water flows by means of intrusive phase detection probes, and it describes a basic data processing method that readily yields expanded information on air-water flow properties.

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 (α ~ 50°). This paper presents an experimental study conducted in a large-size stepped chute (α = 22°, 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 sidewalls.Key words: stepped spillway, air entrainment, two-phase flow properties, skimming flow, transition flow.

Caractéristiques diphasiques des écoulements sur les coursiers en marches d'escalier

Stepped cascades and spillways have been used for more than 3,500 years. The recent regain of interest for the stepped chute design is associated with the introduction of new construction techniques, the development of new design techniques and newer applications. Stepped chute flows are characterised by significant free-surface aeration that cannot be neglected. Two-phase flow measurements were conducted in a large-size model (h =0.10 m, α = 22o). Experimental observations demonstrate the existence of a transition flow regime for a relatively wide range of flow rates. Detailed air-water flow measurements were conducted for both skimming flows and transition flows. Skimming flows exhibit gradual variations of the air-water flow properties, whereas transition flows are characterised by rapid flow redistributions between adjacent steps.