Bubble Count Research Articles

An investigation of anisotropy in the bubbly turbulent flow via direct numerical simulations

We investigated the effects of bubble count, flow direction, and Eötvös number on deformable bubbles in turbulent channel flow. For a given shear Reynolds number Re = 180 and fixed bubble volume fractions (1.263% and 2.525%), we conducted a series of direct numerical simulations using a coupled level-set and volume-of-fluid solver to evaluate their impact on bubble volume fraction distribution, velocity fields, and turbulence characteristics. Each aspect was studied based on the microscopic equations of two-phase flow, and the accuracy of the modeling terms used in current Reynolds-averaged Navier–Stokes equation (RANS) models was assessed. The influence on the anisotropic state was analyzed using the Lumley triangle, and the anisotropy of Reynolds stresses was captured through the exact balance equations. The results indicate that in upward flow, bubbles tend to accumulate near the wall, with smaller Eötvös numbers leading to closer proximity to the wall and greater attenuation of the liquid-phase velocity. This distribution enhances energy dissipation and turbulence isotropy. In downward flow, bubbles cluster in the channel center, generating additional pseudo-turbulence and attenuating energy in the buffer layer. Moreover, the interfacial transfer of turbulent energy, as currently modeled in RANS, is found to be inadequate for upward flows.

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)

Ultrasonic mode conversion for in-line foam structure measurement in highly aerated batters using machine learning

An ultrasonic-based method was developed to enable in-line measurements of foam structure parameters for highly aerated batters by mode conversion. Biscuit batters were foamed to different degrees (density: 364–922 g/L) by varying the mixing head speed and pressure. Density and foam structure changes were detected by efficient offline analytics (nref measurement = 96). Ultrasonic signal data were recorded using two ultrasonic sensors attached to an industry-standard tube. Mode conversion effects in the ultrasonic signals were obtained to predict the rheological parameters of the batters. The frequency range in which surface waves are expected was particularly suitable for detecting rheological changes in highly aerated batters. An ultrasonic-based, online-capable method for process monitoring was implemented and established regarding feature selection in combination with machine learning and 5-fold cross-validation. The developed ultrasonic sensor system shows high accuracy for online density measurement (R2 = 0.98) and offers decent accuracy for measurements of foam structure parameters (Bubble count: R2 = 0.95, Relative span: R2 = 0.93, Sauter diameter: R2 = 0.83). The main benefit of this novel technique is that integrating ultrasonic signal features based on mode conversion leads to a robust foam structure analysis, which has the advantage of being retrofitable into existing processes.

Effect of electropolishing on ultrasonic cavitation in hybrid post-processing of additively manufactured metal surfaces

This investigation centered on fundamental understanding of a hybrid post-processing method that synergistically combines electrochemical polishing and ultrasonic cavitation to improve the surface roughness and mechanical properties of additively manufactured parts efficiently and simultaneously in a single step. High-speed imaging was utilized to understand the effect of the DC bias applied during electropolishing on the cavitation generated at various ultrasonic amplitudes. This was aimed at validating the hypothesis that the hybrid process is characterized by more bubbles than with ultrasonic excitation alone, and, in the presence of the DC bias, the bubbles are transported from the cathode to the anode (workpiece) where their impacts cause surface hardening. High-speed imaging confirmed a higher bubble count in the hybrid process than in the cavitation-only process at different ultrasonic amplitudes. Microhardness measurements confirmed that the hybrid process yielded higher microhardness compared to cavitation peening alone, while microhardness in electrochemical polishing showed only incremental changes. This observation was also corroborated by the grain size measurements, with the hybrid process exhibiting more grain refinement than cavitation-only or electropolishing-only post-processing modes. Surface topography analysis revealed minimal improvement in surface roughness due to cavitation-only, emphasizing the importance of the hybrid process in enhancing both surface roughness and the mechanical properties. Scanning electron microscope (SEM) images showed randomly distributed micro-dimples on the surface produced by the hybrid process at the highest ultrasonic amplitude, attributed to the increased number of cavitation bubbles. This research provides fundamental insights into why the hybrid process excels in simultaneously reducing surface roughness and achieving superior surface microhardness.

The effect of collision parameters on the 3D Eulerian simulation of a thin rectangular bubbling fluidized bed

It is well known that the hydrodynamics of dense gas–particle flows depends heavily on inter-particle and particle–wall collisions; however, the subtleties of these interactions are not always fully recognized or appreciated. In this study, the two-fluid model is used to systematically examine the effects of collision parameters, i.e., particle–particle restitution coefficient, particle–wall restitution coefficient, and wall specularity coefficient, on simulations of the hydrodynamics of a pseudo two-dimensional bubbling fluidized bed.The utilization of high-resolution Eulerian simulations facilitates the representation of meso-scale structures without the use of a sub-grid scale model, while qualitatively reflecting the experimental data. To ensure statistically robust results and increase confidence in the simulations, a careful assessment of the time-averaging interval was first undertaken to smooth out the fluctuations that persist even after the bed reaches steady state.A quantitative analysis was then performed to analyze important facets of the flow, including bubble characteristics, particle velocity fields, and granular temperature. The underlying physical formulation was employed to elucidate how collision parameters impact the hydrodynamics of the flow.The notable findings of this study are threefold. First, the specific details of the collisions between particles are shown to significantly influence the bubble characteristics and overall flow pattern. However, the effect of the particle–particle restitution coefficient turns out to be minimal on the bed circulation time except in the limit of particle elastic collisions. It is also found that the elastic collision of particles highly affects the bubble count and unexpectedly changes the role of the wall from a source to a sink of granular temperature. Second, the effect of the particle–wall restitution coefficient is appreciable on the granular temperature while having a minimal effect on the flow hydrodynamics. Third, the effect of the wall specularity coefficient is felt across the bed such that an increase in this parameter increases the bed circulation time. The results of this systematic study enhance the understanding of how collision parameters impact flow characteristics while also providing insight into the selection of their values in simulations.

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

This experimental study investigated the air-water flow properties and bubble characteristics in hydraulic jumps with Froude numbers Fr1 = 2.4 and 6.3, using four dual-tip phase detection probes with sensor sizes from 0.25 mm to 0.64 mm. The hydraulic jumps were characterized by a fully breaking roller with substantial air entrainment and turbulent two-phase flow patterns. The measurements encompassed distributions of void fraction, bubble count rate, interfacial velocity and bubble clustering properties and the data sets were consistent with previous studies. The comparison of the different probes showed a small impact of the tested sensor sizes on the air-water flow properties, in terms of the trends, magnitude and maximum values. Some differences were observed in terms of the bubble count rate, with the larger probes detecting a lesser number of bubbles. The trend was further confirmed through a comparison with the data set of Chanson and Brattberg (2000) [10] with a smaller probe sensor size (Ø1 = 0.025 mm), in which the maxima of bubble count rates were almost twice that of the present dataset for identical flow conditions. The present results confirm that the traditional signal processing techniques can be used for relatively small probe sizes, although different approaches might be needed for larger probes which cannot detect sub-millimetric bubbles. Overall, the findings should facilitate the development of sturdier phase-detection needle probes and help breaching the gap between laboratory and prototype.

Acoustic sensing for investigating critical heat flux enhancement during pool boiling on electrodeposited copper foams

Nano-engineered surfaces have shown great promise in improving boiling heat flux and heat transfer coefficient for thermal management. Electrodeposited copper foams have attracted lots of scientific and engineering interest owing to their multi-tier structures. In this study, hierarchical copper foams with ultra-wicking properties were fabricated using cathodic deposition at different pH, including both acidic (pH 0) and basic (pH 10 and 12) solutions. Pendant droplet wicking tests and pool boiling experiments were performed to characterize the wicking flux and the critical heat flux (CHF) of the fabricated copper foams, respectively. The CHF enhancement mechanism in hierarchical copper foams was investigated with high-speed optical imaging and acoustic emission (AE) sensing during both the wicking and boiling tests. The results show that the wicking flux of the electrodeposited copper foams increases with the pH due to smaller dendrites and crystallites. Boiling CHF is also higher on the base-deposited copper foams (pH 10 and pH 12) than on the acid-deposited foams (pH 0). Consistent acoustic signatures are observed between wicking and boiling, i.e., smaller AE amplitudes are observed at higher wicking flux and higher CHF, which supports wicking-based CHF enhancement theories. A spike in the AE amplitude is observed at CHF during water boiling on copper foam surfaces due to the rapid growth of the vapor layer blanketing the boiling surface. During the nucleate boiling regime, the AE amplitude decreases with the heat flux in the low heat flux stage but increases with the heat flux in the high heat flux stage. This relationship is found to be due to the interplay of bubble size and bubble count in the stress field that bubbles exert on the heater surface. By analyzing the maximum bubble size, bubble count, and the power and dominant frequency of vapor fraction at varying heat fluxes from high-speed videos, our work shows no obvious effect of the surface structures on the dynamics of the apparent liquid–vapor interface. Combining the optical and acoustic analysis, it is concluded that the key role that surface structures play in CHF enhancement is improving capillary wicking underneath the bubbles rather than affecting the dynamics of the apparent liquid–vapor interfaces. This study not only explores the relationship between electrochemical control and wicking properties but also demonstrates the role of wicking in structure-enhanced boiling CHF through acoustic emission analysis.

Water chemistry role in the stability of CO2 foam for carbon sequestration in water aquifers

Injection of CO2 foam into saline aquifers is a practical method to store CO2; however, generating stable CO2 foam at high temperatures and salinities remains a challenging task. Therefore, this research fully explains the effect of formation water components (NaCl, CaCl2, MgCl2, Na2SO4, and NaHCO3) on the stability of generated foam under harsh conditions. In this regard, a high-temperature and high-pressure (HPHT) foam analyzer was used to investigate the foamability and stability of CO2 foam at 100 °C and 1000 psi. The liquid phase contained 0.5 wt% of Armovis surfactant dissolved into 0.1, 0.5, and 1M concentrations of formation water ions. Moreover, the dynamic surface tension of the selected surfactant solutions was investigated using an optical tensiometer. For the first time, this paper investigates the behavior of foam during the foaming process. During the injection of CO2, the average bubble count for the low salt concentration increased sharply and then dropped until it plateaued. However, the average bubble count for the higher salt concentration gradually increased until the foaming process stopped. These phenomena occurred because of the kinetic adsorption rate of the surfactant micelles at the interface. Additionally, we studied the effect of salt concentrations on bubble count half-life, bubble coarsening, and foam volume decay during the static condition. We observed that the increase in salt concentration resulted in more stable foam due to the reduction of CO2 solubility in water. The foam half-life for 0.1 M of NaCl was 190 min, while it was more than 250 min for 1M of NaCl. A similar trend was observed for CaCl2 and MgCl2. The results of the HPHT foam analyzer concluded that the higher concentration of salt ions presented a more stable foam. Also, the use of Armovis surfactant is promising for the stabilization of CO2 foam at high temperatures and salinities.

Investigation of amine-based surfactants for foamed acid stimulation at high temperature, pressure, and salinity

This research investigated the rheology of foamed acid using different amine-based surfactants including Duomeen TTM (TTM), Ethoduomeen T/13 (T/13), Ethomeen C/12 (C/12), and Armogel O at 150 °C and 1000 psi using a modified (high pressure-high temperature) HPHT foam rheometer. The study was conducted in high-salinity water as a replacement for freshwater to promote sustainability. Foam properties such as foam stability, foamability, foam structure, bubble count, and size were investigated through a novel HPHT foam analyzer under different conditions; (23 °C, 14.7 psi) and (120 °C, 1000 psi). The obtained results showed that Duomeen TTM exhibited the highest foam viscosity as a result of the generation of stable wormlike micelles at 150 °C and 1000 psi. Additionally, both Duomeen and Ethoduomeen T/13 displayed stable foam viscosity at a steady shear rate of 500 1/s for 8 h. The temperature has a major impact on foam viscosity; as the temperature raised from 100 °C to 150 °C, the apparent foam viscosity dropped by 36.9% at 500 1/s and at least by 26.5% at 2000 1/s. The outcomes of the HPHT foam analyzer showed that the surfactants, except for Ethomeen C/12, showed poor foamability under ambient conditions. However, at HPHT, high bubbles count was generated with a uniformly fine texture. For Duomeen TTM, the initial bubble count increased, almost by 76.7%, indicating better foamability at HPHT. Additionally, the foam half-life time of all surfactants except for Duomeen TTM decreased sharply once pressure and temperature increased. The stability decreased by 9.7% for Duomeen TTM, 57.7% for Armogel O, 69.1% for Ethoduomeen T/13, and 85.7% for Ethomeen C/12 as compared to ambient conditions. This paper offers a wide-ranging understanding of amine-based surfactants on stimulating foam viscosity and stability at HPHT.

An Experimental Study of High-Pressure Microscopy and Enhanced Oil Recovery with Nanoparticle-Stabilised Foams in Carbonate Oil Reservoir

Foams have been successfully implemented to overcome the challenges associated with gas-enhanced oil recovery (EOR) over time. Generally, the foam helps to increase the viscosity of the injected gas, which in turn improves the effectiveness of EOR. However, this technology has rarely been applied in the oilfield due to technological and economical limitations. It is widely considered that nanoparticles may be added to foam to enhance its performance in harsh reservoir conditions to overcome some of these limitations. In this study, we employed high-pressure microscopy (HPM) as an advanced technique to examine the stability of N2 and CO2 foams at reservoir conditions, both with and without nanoparticles. The experiments were conducted under vapour and supercritical conditions. Our results indicated that foams produced at 80% quality were more stable than foams produced at 50% quality because the bubble size was significantly smaller and the bubble count was higher. Additionally, foams under supercritical conditions (sc) exhibited greater stability than foams under vapour conditions. This is because at supercritical conditions, the high density of gases helps to strengthen the foam lamella by enhancing the intermolecular contacts between the gas and the hydrophobic part of the liquid phase. Furthermore, core flooding studies were performed to investigate their effect on oil displacement and mobility control in both real and artificial core samples. Rather than focusing on precise quantitative results, our objective was to assess the effect of foams on oil recovery qualitatively. The results indicated that foam injection could significantly increase displacement efficiency, as foam injection raised total displacement efficiency from an initial 48.9% to 89.7% in the artificial core sample. Similarly, in the real core model, CO2 foam injection was implemented as a tertiary recovery method, and a recovery factor of 28.91% was obtained. These findings highlight the potential benefits of foams for EOR purposes and their ability to mitigate early gas breakthrough, which was observed after injecting approximately 0.14 PV during scCO2 injection.

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

This study reported and discussed turbulence characteristics, such as turbulence intensity, correlation time scales, and advective length scales. The characteristic air–water time scale, including the particle chord time and length and their probability density functions (PDFs), was investigated. The results demonstrated that turbulence intensity was relatively greater on a rough bed in the roller length, whereas further downstream, the decay rate was higher. In addition, the relationship between turbulence intensity and dimensionless bubble count rate reflected an increase in turbulence intensity associated with the number of entrained particles. Triple decomposition analysis (TDA) was performed to determine the contributions of slow and fast turbulent components. The TDA results indicated that, regardless of bed type and inflow conditions, the sum of the band-pass (Tu′) and high-pass (Tu″) filtered turbulence intensities was equal to the turbulence intensity of the raw signal data (Tu). Tu″ highlighted a higher turbulence intensity and larger vorticities on the rough bed for an identical inflow Froude number. Additional TDA results were presented in terms of the interfacial velocity, auto- and cross-correlation time scales, and longitudinal advection length scale, with the effects of low- and high-frequency signal components on each highlighted parameter. The analysis of the air chord time indicated an increase in the proportion of small bubbles moving downstream. The second part of this research focused on the basic properties of particle grouping and clustering.

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.

Corrosion inhibitor and chelating agent impact on foam stability for formation stimulation applications

Injection of foam into petroleum reservoirs has attracted special interest in the last years. Utilizing foam in well stimulation is promising as it consumes less water than water-based fracturing fluids. Nevertheless, the success of foam application mainly depends on its stability. Thus, it is essential to understand the role of gas type, water chemistry, and other additives such as acids (e.g., chelating agent) and corrosion inhibitors on the generation and stabilization of foam. Hence, a dynamic foam analyzer (DFA) and optical interfacial tensiometer were used to study foam stability under different conditions. Surfactant screening showed that Duomeen TTM and Armovis are superior foamers at low pH (3.5), and therefore were used in this study. The liquid phase typically contained high salinity water, 15 %wt. Chelating agent, L-glutamic acid-N, N-diacetic acid (GLDA), and 1 %wt. surfactant with and without a corrosion inhibitor (CI). First, we investigated the effect of gas types on the stability of the foam. The results revealed that N2-foam is more stable compared to CO2-foam. Also, the results showed that the corrosion inhibitor reduced the foam stability for both N2-foam and CO2-foam. Three types of water were examined; distilled water (DI), produced water (PW), and seawater (SW) for foam stabilization. The water chemistry had little effect on the stability of TTM compared to its impact on Armovis. Finally, we found that GLDA addition to Armovis increased the stability of foam by four times compared to no GLDA addition. Moreover, the stability GLDA/Armovis system was six times higher with CO2 and more than 20 times higher with N2 as compared to TTM stability. The foamability, foam volume, bubble count, and foam structure were studied to determine the foam behavior at given conditions. This study provides a broad understanding of additives’ impact on stimulating foam stability.

Effect of bubble diameter and bubble count on overall mass transfer coefficient using ImageJ analysis in a down flow jet loop sparged reactor

Effect of bubble diameter and bubble count on overall mass transfer coefficient using ImageJ analysis in a down flow jet loop sparged reactor

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

Continuing from the part 1 (Shi et al., 2022) this paper presents an experimental investigation of transient void fraction and bubble statistics in a highly turbulent breaking bore with Fr1=2.4. The measurements were conducted using a combination of dual-tip phase-detection probes and an ultra-high-speed video camera. The enclosed bubble detection technique (EBDT) used the synchronised probe and camera signals to provide the contour of instantaneous void fraction in the bore roller. The ensemble-averaged void fraction was derived, and compared to analytical solutions of air diffusion models. The bubble statistics were characterised by the bubble clustering properties, pseudo bubble count rate and bubble size spectrum. The clustering data showed the non-random bubble grouping in the shear layer, and the bubble size distributions N(r) followed a commonly adopted bubble break-up model: N(r)∝r−m, where r was the equivalent bubble radius in the present study. The comparison indicated that, in the breaking bore, its air diffusion process was similar to that in a stationary hydraulic jump, and the bubble break-up process was comparable to that in breaking waves.

The Impact of Green Chelating Agent and Corrosion Inhibitor on Foam Rheology and Stability at High Temperature, Pressure, and Salinity

Summary This research fills the gap in understanding the impact of corrosion inhibitors (CIs) and a chelating agent on the rheology and stability of foam under harsh conditions. In this regard, a modified high-pressure, high-temperature (HPHT) foam rheometer and HPHT foam analyzer were used to investigate foam rheology and stability at 1,000 psi and 120 to 150°C with carbon dioxide (CO2) in the gas phase. Surfactant screening showed that Duomeen TTM and Armovis are thermally stable at high temperature and high water salinity and thus were used in this study. The liquid phase generally contained produced water (PW) (total dissolved solids ~ 24,611 ppm), 15 wt% chelating agent [L-glutamic acid-N, N-diacetic acid (GLDA)], and 1 wt% surfactant with and without a CI. First, we screened the viscosity and stability of Duomeen TTM and Armovis; the results showed that Duomeen TTM has a higher viscosity (at least by 82%) at a low shear rate, but both have similar viscosity at a higher shear rate. However, Armovis produced more stable foam. Once the GLDA was added to the Duomeen TTM solution, the viscosity increased significantly by 135% at a high shear rate (1,000–1,500). For the Armovis system, the viscosity improved by 77% and 68% at the low and high shear rates by adding GLDA. Additionally, foam stability was improved remarkably in both systems; half-life time almost doubled. Finally, we reported the effect of CI on the fluid systems, showing it considerably reduced the foam viscosity and stability. It reduced the half-life of the Armovis system by 79.4% and hindered the generation of foam for the Duomeen TTM system. A detailed discussion of foam properties, such as foamability, bubble count, and bubble radius, is provided. This study provides a wide-ranging understanding of additives’ impact on stimulating foam stability at HPHT.

Comparison of vapor bubble kinetics and cleaning efficacy of different root canal irrigation techniques in the apical area beyond the fractured instrument

Background/purposeIntracanal fractured instruments hamper adequate root canal disinfection. The aim of this study was to evaluate vapor bubble kinetics and cleaning efficacy of different irrigation techniques in the apical area beyond the fractured instrument. Materials and methodsSixty curved root canal models, in which a 3-mm fragment of a #20 K-file or a WaveOne Gold Primary (WOG) instrument was intentionally separated at 3 mm from the apical foramen, were irrigated with laser-activated irrigation with photon-induced photoacoustic streaming (LAI-PIPS; 20 mJ/15 Hz), laser-activated irrigation using an Er:YAG laser unit (LAI; 30 mJ/20 Hz), or ultrasonic-activated irrigation (UAI) for 5 s. Vapor bubble velocity and counts were analyzed using high-speed video imaging. To evaluate canal wall cleanliness, 40 canals of extracted human teeth with an intentionally separated 3-mm WOG fragment in the canal at 3 mm from the apical foramen were irrigated with LAI-PIPS, LAI, UAI or conventional syringe irrigation, using 17% EDTA (30 s, two cycles), saline (30 s), and 3% NaOCl (30 s, three cycles) as irrigants. Debris and smear layer on the apical canal wall beyond the fractured instrument were scored using scanning electron microscopy. ResultsLAI-PIPS and LAI demonstrated higher vapor bubble counts than UAI. The WOG fragment permitted higher bubble velocity and count than the K-file fragment. LAI-PIPS and LAI showed better debris and smear removal than the other techniques. ConclusionLAI and LAI-PIPS demonstrated higher vaporized bubble kinetics and better cleaning efficacy in the apical area, even in the presence of a fractured instrument.

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.