Bubble Count Rate Research Articles

Enhancing flow aeration on an embankment sloped stepped spillway using a labyrinth weir

This study presents for the first time a detailed laboratory investigation on the influence of a labyrinth weir as an aerator and corresponding effects on the air–water flow properties in a moderate-sloping (3H:1V) stepped chute. For comparison, an identical study was conducted with a broad-crested weir. All experiments included two near-prototype scale step heights (0.1 m and 0.2 m) and were conducted for a wide range of flow conditions (0.6 ≤ dc/h ≤ 3.1) with conductivity probe measurements at half and quarter distances from the side walls. It was observed that the labyrinth significantly modified the flow entering the chute, generated non-uniform flow patterns and enhanced air entrainment immediately downstream of the weir; no clearwater region was present. Air–water flow properties downstream of the labyrinth weir exhibited larger transverse variability that diminished in the streamwise direction. The labyrinth weir produced higher mean air concentrations, increased bubble count rates with smaller bubble sizes, and caused additional bubble property differences, thereby increasing the interfacial area and enhancing air–water mass transfer or re-aeration efficiency relative to the broad-crested weir case.

Self aeration and energy dissipation on a steep stepped chute: how does physical modelling compare to prototype observations?

For the last five decades, a number of overflow stepped chutes were built because the staircase shape is conducive to reduced construction costs and increased rate of energy dissipation. The stepped chute operations are characterised by air‐water flows that are highly turbulent flows with a large rate of energy dissipation, in comparison to smooth chutes. Herein, physical measurements were performed in a large‐size 1 V: 0.80H stepped chute model, with a steep slope typical of modern concrete gravity dams. The results are compared to visual observations of prototype spillway operation under Froude similar conditions. The detailed two‐phase flow measurements were conducted to characterise finely the self‐aeration and air diffusion process downstream of the inception region of free‐surface aeration. The bubble count rate profiles scaled with the instantaneous void fraction variance, and the relationship was biased close to the stepped invert under the influence of large‐scale vortical structures. The rate of energy dissipation was carefully estimated based upon the two‐phase flow measurements and the results are compared to earlier results on similar steep invert slopes and prototype data estimates. At the downstream end of the stepped chute, the rate of energy dissipation ranged from 43 to 46%, i.e. more than twice that on a smooth-invert chute for a similar chute length and discharge range.Graphical abstractCharacteristics of self-aerated stepped chute flows for dc/h = 1.3—(I) Prototype flow at Hinze Dam (Re = 4.0 × 107); (II) Air-water flow properties in the large-size laboratory model (1:15) stepped spillway (Re = 6.1 × 105)

Effect of probe size on phase-detection probe measurements of air-water flow properties in hydraulic jumps

Effect of probe size on phase-detection probe measurements of air-water flow properties in hydraulic jumps

Experiments on two-phase flow in hydraulic jump on pebbled rough bed: Part 1–Turbulence properties and particle chord time and length

Experiments on two-phase flow in hydraulic jump on pebbled rough bed: Part 1–Turbulence properties and particle chord time and length

Air entrainment scale effects in chute aerator flows

Physical study of air–water flow in a spillway aerator is usually conducted using a downscaled Froude-similar model. The Reynolds number much smaller than that in a prototype structure may adversely affect the understanding and correct interpretation of many physical processes, including air entrainment and turbulence development. In this paper, the similarity and scale effects in the aeration process of flows in chute aerators were systematically investigated by using physical models of three different geometric scales. Air–water flow properties related to the air entrainment, turbulence and bubble characteristics were measured for inflow Froude numbers from 4.9 to 12 and Reynolds numbers from 8.26 × 104 to 9.23 × 105. The experimental results showed that the parameters of air concentration and bubble count rate were scale-sensitive, while the velocity and turbulence intensity were almost free of scale effects. The Reynolds number seemed to have a significant effect on bubble size which was observed to increase with increasing Reynolds number.

Air–water properties of unsteady breaking bore part 2: Void fraction and bubble statistics

Air–water properties of unsteady breaking bore part 2: Void fraction and bubble statistics

Transverse Nonuniformity of Air–Water Flow and Lateral Wall Effects in Quasi-Two-Dimensional Hydraulic Jump

Flow field characterization in high-aerated flows often involves flow imaging techniques through transparent sidewalls, and the results may be subject to sidewall effects and thus differ from the central flow behaviors. This paper contributes an experimental investigation of the sidewall effects on the air-water flow distributions in quasi-two-dimensional hydraulic jumps in a rectangular flume. The tested hydraulic jumps are characterized by preaerated approach flows and large inflow Froude numbers from 10.6 to 15.1. The air-water flow properties are measured intrusively along the centerline, sidewall, and transverse cross sections to provide a three-dimensional view of the aerated flow structure. Substantial redistribution of the bubbly flow is observed due to the presence of lateral aeration boundary layers in the high-speed jet-shear region, compared to the less affected free-surface roller. The magnitude difference between the central and lateral air-water flow properties is more distinct in terms of bubble count rate and less evident for void fraction and interfacial velocity, while the affected area is broader for void fraction distributions. The results suggest a concentration of high-speed jet flow in the central bottom flow column, where the jet layer is thicker, the recirculation roller is smaller, and the proportion of small-size bubbles is higher. The width of the central flow region free of the sidewall effects is approximately 60% to 80% of the flume width in the present facility, narrower on the bottom, and broader at the free surface. The findings demonstrate that the transverse variation of air-water flow and the difference between the central and lateral flow fields should not be ignored for the high-speed flow regions of hydraulic jump. A correct assessment of the sidewall effects is essential for interpreting imaging-based measurement of highly-aerated flow.

Air–water flow properties in hydraulic jumps on rough pebbled bed

ABSTRACT In open channel flows, a hydraulic jump is a rapid shift from a supercritical flow to a subcritical flow with a sharp rise in the elevation of the free surface, which is associated with strong energy dissipation, air entrainment, and spray. The present research aims to investigate the air–water flow properties in a hydraulic jump on rough pebbled bed. A range of discharges from 0.06 to 0.1 m3/s, corresponding to inflow Froude numbers Fr 1 between 1.7 and 2.84 and Reynolds numbers Re 1 from 140,000 to 220,000 were considered. The basic parameters of two-phase flow such as the void fraction and bubble count rate are presented. Close to the jump toe, the mean void fraction on rough bed was larger than on smooth bed, while farther downstream larger magnitudes were observed on smooth bed. No significant difference was observed between rough and smooth beds in terms of the maximum void fraction. In the turbulent shear layer, the bubble count rate was comparatively larger on rough bed than on smooth bed, while the longitudinal distribution decayed for both bed configurations.

Hydraulic jumps with low inflow Froude numbers: air–water surface patterns and transverse distributions of two-phase flow properties

Hydraulic jumps are commonly employed as energy dissipators to guarantee long-term operation of hydraulic structures. A comprehensive and in-depth understanding of their main features is therefore fundamental. In this context, the current study focused on hydraulic jumps with low Froude numbers, i.e. Fr1 = 2.1 and 2.4, at relatively high Reynolds number: Re ~2 × 105. Experimental tests employed a combination of dual-tip phase-detection probes and ultra-high-speed video camera to provide a comprehensive characterisation of the main air-water flow properties of the hydraulic jump, including surface flow features, void fraction, bubble count rate and interfacial velocities. The current research also focused on the transverse distributions of air-water flow properties, i.e. across the channel width, with the results revealing lower values of void fraction and bubble count rate next to the sidewalls compared to the channel centreline data. Such a spatial variability in the transverse direction questions whether data near the side walls may be truly representative of the behaviour in the bulk of the flow, raising the issue of sidewall effects in image-based techniques. Overall, these findings provide new information to both researchers and practitioners for a better understanding of the physical processes inside the hydraulic jump with low Froude numbers, leading to an optimised design of hydraulic structures.Article HighlightsExperimental investigation of air-water flow properties in hydraulic jumps with low Froude numbersDetailed description of the main air-water surface features on the breaking rollerTransversal distribution of the air-water flow properties across the channel width and comparison between centreline and sidewall.

High-Velocity Air–Water Flow Measurements in a Prototype Tunnel Chute: Scaling of Void Fraction and Interfacial Velocity

Aeration occurs in many natural and human-made flows and must be considered in engineering design. In water infrastructure, air–water flows can be violent and of very high velocity. To date, most fundamental research and engineering design guidelines involving air–water flows have been based upon laboratory scale measurements with limited validation at prototype scale with larger Reynolds numbers. Herein, unique measurements were conducted in high-velocity air–water flows in the tunnel chute of the 225-m-high Luzzone arch Dam in Switzerland. For each of the two test series, an array of 16 double-tip conductivity probes was installed in the circular tunnel chute of 3 m diameter and slope of ≈37° measuring void fraction, bubble count rate, interfacial velocity, and droplet sizes for four different discharges of up to 15.9 m3/s corresponding to Reynolds numbers of up to 2.4 × 107 and mean flow velocities of up to 38 m/s. Void fraction and interfacial velocity distributions, as well as design parameters such as depth-averaged void fractions and flow resistance, compared well with previous laboratory studies and empirical equations. The droplet chord sizes exhibited scale effects, and care must be taken if air–water mass transfer and droplet momentum exchange processes are assessed at the laboratory scale.

Air-Water Flow Properties in Hydraulic Jumps With Fully and Partially Developed Inflow Conditions

Abstract Novel air–water flow measurements were conducted in fully aerated hydraulic jumps with partially and fully developed supercritical inflow conditions. Irrespective of the inflow conditions, the hydraulic jumps resembled typical flow patterns with strong aeration and instabilities, albeit hydraulic jumps with fully developed inflow conditions had a more upward directed roller motion and a larger clear water core in the second half of the roller. Hydraulic jumps with fully developed inflow conditions had comparatively larger void fractions in the first half of the jump roller and larger bubble count rates throughout, while a comparatively larger number of smaller bubble sizes suggested a stronger break-up of bubbles. This was consistent with slightly larger interfacial velocities and turbulence intensities in the first half of the jump roller with fully developed inflow conditions. An assessment of the required sampling duration for air–water flow properties indicated the requirement to sample for at least five times longer than applied in previous studies. These results highlighted the need to carefully consider the inflow conditions and sampling parameters for aerated hydraulic jumps.

Bubble characteristics affecting air-water exchange in open-channel flow with a jet forming over a sudden bottom drop

Entrainment of air bubbles into flowing water increases significantly the interfacial area between the two phases hence enhancing the air-water mass transfer. The air bubble characteristics are thus of direct relevance to the waterbody aeration and oxygenation, which should be understood in the context of bubbly-flow turbulence development. In this paper, we studied experimentally a canonical aerated flow featured by a two-dimensional jet over a sudden bottom drop followed by a sloping open-channel flow. This is a representative scenario of open-channel flow with simultaneous free-surface air-water exchange and bottom aeration into the deep water. In addition to the air concentration and air flux evolutions upstream and downstream of the jet impact on the channel bed as well as from bottom to the free-surface, the local and accumulative bubble count rate distributions were depicted. The air-water flow structures were revealed at bubble scales with the aid of a further characterisation of the bubble chord length spectrum and mean bubble size distributions, indicating intense bubble breakups following the jet impact and next to the bottom boundary layer. Although the jet impact on the dry bottom was responsible for a substantial air loss, an increasing number of smaller bubbles were produced and prevented drastic drop in the air-water interfacial area. The mean bubble size and specific interfacial area were predicted under some rough assumption for the deep-water mass transfer. An insight into the bubble-turbulence interplay was presented based on a statistic bubble clustering analysis.

Estimation of air-flow parameters and turbulent intensity in hydraulic jump on rough bed using Bayesian model averaging

Abstract A hydraulic jump is an abrupt transition between subcritical and supercritical flows which is associated with energy dissipation, air entrainment, spray, splashing, and surface waves. Both physical and numerical modeling were largely applied to study hydrodynamics, turbulence and air-entrainment in the hydraulic jump, while the literature about the application of classifier models is quite limited. Determining air-flow parameters and turbulent intensity has been merely performed by costly and time-consuming experimental methods, while this study is the first attempt to estimate the mentioned parameters using a computer-based methodology with desired precision. In the present study, air-flow parameters including void fraction (C) and bubble count rate (F), as well as turbulent intensity (Tu) on rough bed were estimated using Bayesian model averaging (BMA) and three multilayer perceptron (MLP), support vector regression (SVR) and generalized regression neural network (GRNN) as classifier models. To develop the stated models, the experimental data from Felder and Chanson (2016) were divided into four classes based on longitudinal distance from the jump toe. Results highlighted that the MLP and GRNN models have more accurate results compared to the SVR model. For F and Tu, the GRNN model and for C, the MLP model showed better performance than other models in four classes. The average acceptance rate between 15 and 30% of the BMA model performance for all classes proved the accuracy and efficiency of the proposed methodology. The average RMSE value of BMA results and the bests classifier models were 0.41 and 0.42, respectively, for the estimation of all three parameters. Results revealed that the BMA model by weighting individual classifier models could be able to estimate parameters with better accuracy than the best classifier model in each class. The significant outcome of this study is that the proposed model is able to render accurate results in a complex system such as hydraulic jump.

Singular air entrapment at vertical and horizontal supported jets: plunging jets versus hydraulic jumps

In plunging jets and at hydraulic jumps, large amounts of air bubbles are entrained at the impingement of the liquid jet into the receiving body. Air is entrapped and advected into a turbulent shear layer with strong interactions between the air bubble advection process and momentum shear flow. In this new physical study, air-water flow measurements were systematically repeated with identical inflow length, inflow depth and inflow velocity in a vertical supported jet (PJ) and a horizontal hydraulic jump (HJ). Detailed measurements were conducted with the same instrumentation. Both similarities and differences were observed between the two multiphase gas-liquid shear flows. Visual observations showed a key difference in the outer region, with a buoyancy-driven flow in the plunging jet with negligible void fraction, versus a strong recirculation motion with uncontrolled interfacial aeration in the hydraulic jump. Differences were also observed at the impingement perimeter, in terms of fluctuation frequencies and amplitudes, for identical inflow conditions. Both flow conditions yielded intense local singular air entrainment and close results were observed in terms of void fraction, bubble count rate, bubble chord sizes and interfacial area in the shear layer, in both the plunging jet and hydraulic jump. The transfer of momentum between impinging jet and receiving water, as well as the effect of buoyancy, were however affected by the flow geometry.

Flow Resistance and Energy Dissipation in Supercritical Air‐Water Flows Down Vegetated Chutes

AbstractVegetation covers on dikes and embankment dams have proven as sustainable and cost‐effective surface protection against external erosion caused by hydraulic, mechanical, or climatic impacts. Determination of the hydraulic loads that act upon these covers requires the knowledge of the flow resistance. While the high‐velocity flows on vegetated slopes are often aerated, the flow aeration has rarely been considered, and no direct measurements of the air‐water flow properties have been conducted to date. The air‐water flow properties are needed for a direct estimation of important design parameters such as friction factors and residual head at the downstream end. Herein, unique air‐water flow measurements were conducted in high‐velocity air‐water flows down a vegetated chute with a 1:3 slope. Several vegetation covers were tested for a range of flow rates. The experiments revealed strong flow aeration within three‐dimensional, fragmented flows associated with complex interactions of vegetation and high‐velocity flows. The air‐water flow properties were measured with phase‐detection intrusive probes providing novel insights into aerated flows on vegetated chutes including distributions of void fraction, bubble count rate, and interfacial velocity as well as direct estimates of energy dissipation and flow resistance. The results highlighted strong flow aeration and energy dissipation for all vegetated configurations. The median equivalent Darcy‐Weisbach friction factors for all vegetations were within 0.19 to 0.45, comparable to aerated flows on stepped spillways. The present results highlighted the significant flow resistance of vegetated covers and the need to consider air‐water flow properties in the design of vegetated chutes.

Nappe flows on a stepped chute with prototype-scale steps height: Observations of flow patterns, air-water flow properties, energy dissipation and dissolved oxygen

Abstract Air-water flows occur commonly in stepped spillways including the nappe flow regime for low flow rates. The present laboratory experiments researched the nappe flow regime in a stepped chute with prototype-scale steps height providing unique insights into the evolution of nappe flows along a stepped chute. Detailed visual observations highlighted the varying flow features along the stepped chute including the evolution of flow aeration, of jet properties and of instationarities in form of jump waves and cavity fluctuations with typical frequencies of around 1 Hz. These instationarities were caused by complex flow interactions at the impingement of the jet on the horizontal step face. Detailed air-water flow measurements revealed the complexity of the flows highlighting both S-shape and jet-like void fraction distributions and jet-like interfacial velocity distributions downstream of the jet impact and at step edges. This resulted in a downwards shift of the bubble count rate distributions closer to the step face. The nappe flows showed strong energy dissipation and reaeration performances along the stepped chute. The present study provided a robust and extensive characterisation of nappe flows and due to the large scale of the experiments, the results should provide confidence for the design of stepped chutes with embankment dam slope.

Three-Dimensional Aerators: Characteristics of the Air Bubbles

Three-dimensional aerators are often used in hydraulic structures to prevent cavitation damage via enhanced air entrainment. However, the mechanisms of aeration and bubble dispersion along the developing shear flow region on such aerators remain unclear. A double-tip conductivity probe is employed in present experimental study to investigate the air concentration, bubble count rate, and bubble size downstream of a three-dimensional aerator involving various approach-flow features and geometric parameters. The results show that the cross-sectional distribution of the air bubble frequency is in accordance with the Gaussian distribution, and the relationship between the air concentration and bubble frequency obeys a quasi-parabolic law. The air bubble frequency reaches an apex at an air concentration (C) of approximately 50% and decreases to zero as C = 0% and C = 100%. The relative location of the air-bubble frequency apex is 0.210, 0.326 and 0.283 times the thickness of the layers at the upper, lower and side nappes, respectively. The air bubble chord length decreases gradually from the air water interface to the core area. The air concentration increases exponentially with the bubble chord length. The air bubble frequency distributions can be fit well using a “modified” gamma distribution function.

A physical study of air–water flow in planar plunging water jet with large inflow distance

Abstract A plunging jet is commonly encountered in nature. It is also widely used in industry for its capacity to enhance fluid mixing and entrain gases into the liquid fluid when the impact velocity exceeds a critical value. This paper presents a physical study of vertical supported planar water plunging jets, in a relatively large-size facility. Air-water flow properties were measured in the falling jet and in the plunging pool using an intrusive phase-detection probe, with jet impact velocities between 2.5 m/s and 7.4 m/s and a fixed jet length. The falling jet was characterised by large disturbance and substantial pre-aeration. Intense air-water mixing was observed downstream of the impingement point. The development of air diffusion layer and turbulent shear layer was characterised by the streamwise evolution of void fraction, bubble count rate, bubble chord length and interfacial velocity profiles. The results compared favourably with the literature, albeit some difference was observed associated with different inflow jet turbulence levels as well as instrumentation development and signal processing refinement, including instrumental size, scanning rate and duration. The clustering properties were derived using the near-wake criterion. Results were comparable to those in horizontal hydraulic jumps. The air-entrainment rate was derived, highlighting the significant contribution of jet pre-aeration.

Application of local optical flow methods to high-velocity free-surface flows: Validation and application to stepped chutes

Abstract The entrainment of air on a high-velocity spillway leads to a rapid bulking in flow depth and developments of complex flow patterns downstream of the inception point of aeration. This study examines the feasibility of two local optical flow techniques – the Lucas-Kanade method and the Farneback method – applied to high-velocity air-water skimming flows above a stepped chute. Such methods are not widely known to the multiphase flow community. Experimental studies were undertaken in a relatively large-size stepped spillway model. Validation test cases were performed using a synchronised ultra-high-speed camera and phase-detection probe setup. The optical flow technique detected changes in brightness due to reflectance difference associated with passages of air-water interfaces. The standard deviation of luminance correlates with void fraction and bubble count rate, and may be used as a predictor for uncertainties in optical flow estimation. The streamwise optical flow properties were in close agreement with those determined by the phase-detection probe next to the sidewall, with increasing differences for void fraction greater than 50% (C > 0.5). In the sidewall region, however the bubble count rate and interfacial velocity distributions were underestimated compared to the channel centreline's interfacial properties. The tests demonstrated that the optical flow methods can provide useful qualitative and quantitative information on complex air-water flow patterns.

Time series video analysis of bubble release processes at natural hydrocarbon seeps in the Northern Gulf of Mexico

This research quantifies the rate and volume of oil and gas released from two natural seep sites in the Gulf of Mexico: lease blocks GC600 (1200 m depth) and MC118 (850 m depth). Our objectives were to determine variability in release rates and bubble size at five individual vents and to investigate the effects of tidal fluctuations on bubble release. Observations with autonomous video cameras captured the formation of individual bubbles as they were released through partially exposed deposits of gas hydrate. Image processing techniques determined bubble type (oily, gaseous, and mixed: oily and gaseous), size distribution, release rate, and temporal variations (observation intervals ranged from 3 h to 26 d). A semi-automatic bubble counting algorithm was developed to analyze bubble count and release rates from video data. This method is suitable for discrete vents with small bubble streams commonly seen at seeps and is adaptable to multiple in situ set-ups. Two vents at GC600 (Birthday Candles 1 and Birthday Candles 2) were analyzed. They released oily bubbles with an average diameter of 5.0 mm at a rate of 4.7 bubbles s−1, and 1.3 bubbles s−1, respectively. Approximately 1 km away, within the GC600 seep site, two more vents (Mega Plume 1 and Mega Plume 2) were analyzed. These vents released a mixture of oily and gaseous bubbles with an average diameter of 3.9 mm at a rate of 49 bubbles s−1, and 81 bubbles s−1, respectively. The fifth vent at MC118 (Rudyville) released gaseous bubbles with an average diameter of 3.0 mm at a rate of 127 bubbles s−1. Pressure records at Mega Plume and Rudyville showed a diurnal tidal cycle (24.5 h). Rudyville was the only vent that demonstrated any positive correlation (ρ = 0.60) to the 24.5 h diurnal tidal cycle. However, these observations were not conclusive regarding tidal effects on bubble release.