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Our previous study showed that the frictional drag decreases with increasing void fraction at Re > 1300, while it increases at Re u '– v 'plane). Here, we focus on two specific Reynolds numbers (at Re ≈ 900 and ≈ 1410, which are close to the laminar-to-turbulent transition regime) and discuss bubble effects on sweep ( u ' > 0, v ' u ' v ' > 0) events as a function of the Reynolds number. We also illustrate velocity fluctuations in the vicinity of an individual bubble and a swarm of bubbles on the u '– v ' coordinates. The results show that a bubble swarm suppresses the velocity fluctuations at Re ≈ 1410.

To investigate the effect of microbubble injection into a turbulent boundary layer, we measure the velocity fields of gas–liquid two-phase flow in a horizontal water channel with a Particle Tracking Velocimetry (PTV) technique. In this study, we focus on a thin shear layer where the Reynolds shear stress is the highest. The effect of the shallow focal depth-of-field (DOF) in a lens system assists to improve the spatial resolution of the PTV results. Considering the distribution of luminous flux, we first enhance the illumination intensity to label the objects on images. With their assistance together with an image processing filter, we can then improve the in-depth resolution of tracer particles and determine their positions more accurately. We apply this shallow DOF-based PTV technique to measure the streamwise and spanwise velocity components and visualize the motion of bubbles relative to continuous phase in the near-wall region at three downstream positions. The result shows that the averaged velocity right downstream of the bubble injector clearly decreases due to microbubbles. The local instantaneous flow structure around bubbles shows no clear coherence because of highly turbulent background; however, the frequency spectrum analysis finds that the bubbles reduce the kinetic energy in a frequency range higher than the bubble passing frequency.

Microbubble and air film methods are believed to be applicable to skin friction reduction in ships. Small bubbles are dispersed into the turbulent boundary layer in the former case, and wide air layers cover the wall surface in the latter case. Previous studies did not specifically address the intermediate case between the microbubble and air film conditions. This study is concerned with the possibility and mechanism of drag reduction using relatively large air bubbles compared to the boundary layer thickness in a horizontal turbulent channel flow. The relationship between local skin friction and the bubble’s interfacial structure is investigated by synchronizing the measurement of wall-shear stress with the image acquisition of bubbles. The bubble sizes range from 2 to 90 mm approximately. As a result, a negative correlation between the local skin friction and the local void fraction is confirmed by the time-resolved measurement. A new observation is the fact that the local skin friction decreases drastically in the rear part of individual large bubbles, and rapidly increases after the bubble’s rear interface passes. This characteristic underlies the bubble-size dependency of the average skin friction in the intermediate bubble size condition.

An image analysis method for measuring bubble velocity fields at high bubble number density is proposed. It is based on computing the cross correlation of bubble images with multiple spatial resolutions, i.e. a recursive cross correlation technique. Comparing several PTV and PIV schemes, it is confirmed that the recursive cross correlation leads to the best measurement results because of the robustness with respect to optical and dynamic characteristics of bubbles. The method is applied successfully to the measurement of bubble motion in bubble plumes accompanying a strong unsteadiness over a wide frequency range. The results reveal that the high frequency fluctuations of bubble velocities grow in the shear layer and near the top surface, while low frequency fluctuations dominate in the middle part of the tank. The characteristic frequency divides the frequency spectrum of the bubble fluctuation intensity into two regions: where there are turbulent behaviors and where there are macroscopic convection patterns in the bubble plumes.

Skin frictional drag of bubbly two-phase mixture in a horizontal turbulent channel is investigated experimentally using a shear stress sensor and a high-speed video camera. We choose non micro bubbles having a size of 1 to 10 mm intentionally to confirm that the drag reduction still works with relatively large bubbles to the channel height. The measurement results have shown that the drag reduction realized in case of a void fraction higher than a critical value that depends on liquid flow velocity. Also, significant bubble size dependence was found in the region near bubble injector. A synchronized measurement of local shear stress and interfacial structure has newly shown that the instantaneous friction factor decreased in rear part of individual bubble.

Wall-sliding bubbly flow is experimentally investigated using a shallow focus imaging technique. The technique allows to capture the motion of tracer particles inside a thin layer from a wall. Adding digital image processing provides further fine spatial resolution up to 100 ratio of the length of the viewing area to the focused thickness. Bubble-bubble interaction, which is one of dominant phenomena in wall-sliding bubbles, is analyzed with using Particle Tracking Velocimetry applied to both phases. Transition of bubbles' pattern from vertically chained to horizontally clustered arrangement is discussed with the local liquid phase flow structure around individual bubble. The results reveal that the temporally high fluctuation velocity in the liquid phase generates during the transition, and the circulation of bubble-vicinity liquid induces rapid transition of the arrangement.

This paper concerns with flow visualization and image measurement of bubbly flows around various shapes of cylinders. A coaxial confined double rectangular chamber is constructed in order to provide a wide two-dimensional uniform bubble distribution upstream of the obstacle. The experiment shows that a wide two-phase convection is induced around the obstacle, though such an effect is not observed in research on the single-phase flow around objects. The spatial scale of the two-phase convection depends sensitively on the shape of the obstacle. Dense arrangement of cylinders is also investigated to find the interaction among the convection. The measurement results of void fraction, bubble velocity and liquid phase flow, which are obtained by image processing including particle tracking velocimetry (PTV), explore the detailed mechanism of generating the convection.Copyright © 2003 by ASME

Bubble-driven flow around circular cylinders in a narrow channel is experimentally investigated by means of image processing. The cylinders are settled in the channel with three kinds of arrangement pattern to clarify the difference of the internal two-phase flow structure. Thinning algorithm is applied to the detection of all the bubbles in order to measure the bubble motion in locally high void fraction. Size and spatial distribution of the bubbles are also obtained with the image analysis to discuss the bubble's coalescence/fragmentation due to the collision with the cylinders. The results imply that the rising velocity of the bubble is governed not only by bouncing on the cylinders but also by the liquid phase flow pattern that is sensitively dependent on the gas flow rate and the cylinders' arrangement pattern.

Bubble-Bubble interaction in a free-rising bubbly flow is measured with three-dimensional Particle Tracking Velocimetry (3-D PTV). The present report mentions the dependency of the interaction pattern on Reynolds number of rising bubble from 5 to 75. With the measurement data, it has been newly found that high Re number bubbles had repulsive motion each other both in horizontal and vertical direction as the distance of bubbles was shortened. The technique of PTV for extracting accurately the 3-D velocity vectors of bubbles is also discussed.

気泡流中では気液二相が互いに異なる流場を持ち，その相対速度は，気泡の並進運動特性によって支配される．近年では低Re数気泡の運動方程式モデルはほぼ確立されており，原理的に，気泡の速度ベクトルデータから液相流場を逆算することが可能である．本研究では，気泡速度ベクトルから，気泡軌跡近傍の液相速度ベクトルを逆算する手法と，その時空間的補間スキームの組み合わせにより，液相の流場を詳しく推定するアルゴリズムを提案した．このようなアルゴリズムは，トレーサ粒子を大量に散布できない工業機器や自然環境計測において利用が期待される．本論文では，気泡の並進運動に関する周波数解析から気泡運動からの推定の合理性を示したうえで，Taylor-Greenの渦流れを用いて提案したアルゴリズムの解析的評価について論じた．また，実際の気泡画像に対する実験的デモンストレーションにより，その実用性を明らかにした

A method for measuring the instantaneous 3-D bubble distribution in bubbly flows is proposed by making use of stereoscopic image processing. The method applies to the case of higher rather than dilute void fractions, because it is based on the following two statistic-based procedures. The first process is to obtain the 2-D distribution of local void fraction from visualized images, where a correlation between the bubble shadow fraction and the projection void fraction is derived by introducing the new idea of overlapping probability of the bubble-images under various conditions. The second process is to reconstruct the 3-D distribution of the local void fraction. In this step, a logical method is proposed in order to obtain the bubble-existence probability function which simultaneously satisfies the two distributions of the two projection void fractions. The present method is applied for instantaneous measurement of the 3-D structure of a bubble plume. The measured results show its particular structure, such as the hollow distribution of the bubbles, and the local spiral structure, which cannot be captured by point-measurement experiments.

An experiment showed a quite strong effect of inserting a circular cylinder into a unform rising bubbly flow. Size of the dffected zone was more than several times as that of the cylinder. The purpose of present study is to elucidate the mecahnism of generating the two-phase convection using a numerical analysis based on Elulerian-Lagrangian model by which the bubble's bpundary condition on the cylinder can be treated accurately. The numerical results showed a good agreement with the experimental results and revealed the dependency of the flow on the void fraction bubble radius, blockage ratio as well as the bubble's bouncing condition on the cylinder surface.

The turbulence structure in a rising bubbly flow driven by bubble buoyancy has been analyzed using the Eulerian-Lagrangian model. A new method of calculating the two-way interaction between the dispersed bubbles and the liquid phase was proposed to accurately simulate the local fluctuations of liquid flow induced by bubble migration. The present two-dimensional simulation revealed that, (1) in the case of the larger bubble, the turbulence structure has the longer wavelength in the vertical direction, and, (2) the wave spectrum of kinetic energy of the liquid phase is sensitive to the average void fraction and the lift force of the bubbles.

Bubble curtain is expected to be a new type of oil fence because it can generate a strong and wide surface flow over the bubble generation system. However, the most suitable condition for the bubble generation system has not been investigated. In this work, the surface flow induced by the bubble curtain is measured by particle imagining velocimetry, and the parametric dependency on the gas flow rate, void fraction and bubble size have been made clear. Furthermore, the internal two-phase flow structure in the bubble curtain is discussed with the PIV measurement data.

Water was injected vertically downward through a straight circular pipe onto a water bath contained in a cylindrical vessel. Three types of bubble dispersion patterns were observed with respect to the distance from the pipe exit to the undisturbed bath surface. When the distance was short, small bubbles were generated at the bath surface and they dispersed in the whole bath (type 1). On the other hand, when the distance was long, relatively large bubbles were generated and the bubble dispersion region was localized beneath the pipe exit (type 3). For an intermediate distance, the bubble dispersion patterns of the two types appeared simultaneously (type 2). We focused on types 1 and 3 and carried out LDV measurements to investigate the liquid motion in the bubble dispersion region. In the case of type 1, the mean velocity and turbulence characteristics were not affected by bubbles and agreed well with those for single-phase free jets, while the two characteristics for type 3 were influenced significantly by bubbles. The differences in the two characteristics between types 1 and 3 were found to be associated with turbulence production in the wake of bubbles.

Detailed information on the fluid flow around a bubble is required for better understanding of metallurgical reactions in the steelmaking processes agitated by gas injection. In the current research a model has been used for a cold simulation experiment of water-air two-phase flow. Previous studies have focused mainly on the time-averaged flow structure around bubbles. However, it is well known that the movement of a bubble is transient and three-dimensional, and hence, the flow around the bubble is also transient and three-dimensional. This study was carried out to elucidate the flow around a single bubble rising in a cylindrical water vessel.3-D PTV (Particle Tracking Velocimetry) is used to analyze the movement of gas-bubbles in the water bath. The water flow around a single bubble is investigated by the technique of PIV based on visualization and image processing. An experimental apparatus is set up to generate a bubble of known volume rising from bottom. Velocities of the bubble and the surrounding fluid are obtained simultaneously by a three consecutive time picture cross-correlation 3-D PTV. A new technique that can measure the fluctuating velocity field and ensemble averaged kinetic energy from the time series of particle imaging is developed in data processing.

This paper investigates the behavior of a bubble which bridges the gap in a cross section of a horizontal narrow divergent passage under Earth's gravity conditions ( 1 G) analytically. In a narrow passage, inertia forces are known to be small compared with viscous forces. Also, gravity force is not dominant for bubble behavior in a horizontal narrow passage. In this sense, the bubble behavior in the passage is similar to that under microgravity conditions. It is important to understand the bubble behavior in relation to separating gas from a gas-liquid two-phase flow and controlling a gas-liquid interface under microgravity conditions. A one-dimensional momentum equation for the bubble behavior is derived. The equation is arranged as an ordinary differential equation with respect to the upstream interfacial location and is solved. Analytical results are compared with experimental ones. As a result, the effects of gap size, bubble projected area, and divergent angle on bubble behavior are explained qualitatively.

This study investigates a bubble which is placed such that it bridges a cross section of a horizontal narrow divergent passage under the Earth gravity condition (1-G). In a narrow passage, inertia forces are known to be small compared with viscous forces. Also, gravity force is not dominant for bubble behavior in a horizontal narrow passage under the 1-G condition. In this sense, the bubble behavior in the passage is similar to that under a low-gravity condition. It is important to understand the bubble behavior in relation to separating gas from a gas-liquid two-phase flow and controlling a gas-liquid interface under a low-gravity condition. A single bubble was placed such that it bridged the cross section of a horizontal narrow passage under the 1-G condition. The bubble geometry and its behavior were studied experimentally for gap sizes ranging from 0.5 mm to 2 mm, divergent angles of 1 to 5 degrees, and using ethyl alcohol as a working fluid. The following results were obtained : (1) bubble was found to move to greater cross sectional area, (2) the gas-liquid interface geometry from the top view could be expressed as a "contact circle model" which takes the maximum radius in the passage, and (3) effects of the gap size and the bubble projected area on the bubble behavior were clarified.

Gas injection into a cylindrical bath through a centric bottom nozzle causes a swirl motion like rotary sloshing. Conditions indicating the initiation and cessation of the swirl motion have been made clear by many researchers. So far, the effect of the swirl motion on transport phenomena in the bath is not clear yet. The present study was made to clarify the bubble characteristics (void fraction, bubble frequency) and liquid flow characteristics (mean velocity, turbulence intensity, Reynolds shear stress) during swirl motion of bubbling jet. These two characteristics were investigated using an electro-resistivity probe and a two-dimensional LDV, respectively.

The authors already developed three-dimensional particle tracking velocimetry (3D-PTV) based on a binary correlation method for high speed particle identification. The technique of the 3D-PTV is applied to the measurement of the flows of water and air bubbles in a cylindrical bath with a bottom blowing air bubbling jet.The performance of the stereo matching and reconstruction for threedimensional particle positions and the measurement accuracy of velocity vectors of water flow are discussed. Besides, the trajectories of air bubbles are investigated and discussed.

The technique of the quantitative flow visualization is applied to measure the behaviors of bubbles in a cylindrical bath which is agitated by a bottom blown bubbling jet. Rising velocities of bubbles, distributions of bubbles, flow directions around a bubble and distributions of gas-holdups etc. are measured. Without the technique, measurements of those quantities would be rather difficult, or some are impossible.

In bubbly two‐phase flow, the gas phase and liquid phase have different flow fields. The mathematical expression for the motion of a small bubble at low Re number has been already established, the liquid velocity along the trajectory of the bubble is calculated inversely by using the motion equation. Whole field liquid flow structure has also estimated using a spatial and/or temporal interpolation method. This paper proposes an algorithm for estimating the liquid phase flow field based on measurement data on bubble data on bubble motion. The applicability of the algorithm is examined with Taylor‐Green vortex flow as an analytical test case. Meanwhile, The result, based on the theory, has been applied to reconstruct liquid phase velocity field by using the data of bubble velocity in an aeration tank.

Aeration plays an important role in the treatment of activated sludge. The aeration performance is deeply affected by the movement law of bubble liquid flow in aeration tank. In order to discuss the influence of gas‐phase, liquid‐phase motions in an aeration tank on the sewage disposal, three kinds of boundaries and initial conditions were applied in the experiment to study the law of two‐phase flow. A developed two‐equation turbulence model was employed in this paper. Results obtained by experiment are compared with those from the mathematical model. It is found that the results of measurement and simulation agree with each other.

Bubble-bubble interaction is a quite fundamental issue to understand multiphase flow dynamics and to improve mathematical models of dispersed multiphase flow for higher volume fraction of dispersion. In this study, the bubble-bubble interaction is measured using Particle Tracking Velocimetry (PTV) in various environments. First, bubbles sliding on a vertical wall are measured using 2-D PTV. Second, the free rising bubbles in an unbounded space are measured applying 3-D PTV. Third, the simultaneous measurement for gas and liquid phases in the layer of wall-sliding bubbles is carried out. The measurement data have shown that the average bubble-bubble interaction patterns in the wall-sliding bubbles and in the free rising bubbles were attractive in the vertical direction and repulsive in the horizontal direction. The relation between the carrier phase flow structure and the bubbles’ motion is detected to explain the mechanism of the bubble-bubble interaction.Copyright © 2003 by ASME

Water electrolysis is one of the methods for hydrogen gas production. The objective of this study is to investigate the effect of the electrode surface roughness on the bubble generation from a viewpoint of the multiphase-flow engineering. We vertically set two electrodes into water tank and capture the images of hydrogen bubbles generated from cathode surface using the high-speed video camera. As a result, we found that bubbles are generated and released from fixed positions of the electrode surface. The size and velocity of released bubbles were measured.

Skin frictional drag of the gas-liquid two-phase flow is investigated using silicon oil in a horizontal channel. The silicon oil has three characteristics, i.e. stable interfacial property, high viscosity, and low surface tension in comparison to water. The present study is planned to grasp the skin friction characteristics modified with bubbles in the oil. The measurement and the visulization results show increment of the frictional drag by bubbles because of promotion of the laminar to turbulent flow transition. Bubble shapes changing with the transition is also discussed.

This paper concerns with experimental measurement of bubble-bubble interaction in a free-rising bubbly flow using three-dimensional Particle Tracking Velocimetry (3-D PTV). An accurate labeling method is employed to measure correctly the center of gravity for individual bubble so that the relative velocity vector in a swarm of bubbles is captured with high statistic reliability. The bubble-bubble interaction is discussed for the range of 5

As a basic study of skin friction reduction using bubbles, the relationship between the skin friction and the bulk void fraction in a horizontal rectangular channel is investigated experimentally for Re=9700. The skin friction is reduced for the void fraction larger than 17% in our test facility, in which the bubble size ranges from 2 to 30mm. The negative correlation between the local skin friction and the local void fraction is confirmed by a time-resolved synchronized measurement, and we also found that the local skin friction decreased in the rear region of large bubbles.

Bubbly flow shows complicated behavior due to a strong dynamic interaction between bubble and vortex motions. A technique for generating a strong surface flow by using a bubble curtain has been proposed by the authors. In the future, bubble curtain is expected to be a new type of oil fence because it can generate a strong and wide surface flow over the bubble generation system. The present paper is concerned with the mechanism of generating the surface flow. Furthermore, the internal two-phase flow structure of the surface flow generation induced by the bubble curtain is discussed using PIV (particle imaging velocimetry) measurement data after separating bubbles and particles from the original images.

Bubble-injection methods are considered to be a prospective approach to reduce skin-frictional drag for ships. To apply this technique for unsteady flows, we attempt to introduce a feedback loop of air injection in future applications. Hence, we wish to establish reduced-order models of unsteady bubbly flows from the information at the bottom of the ship, but this must be achieved with a limited number of sensors in practice. This motivates us to develop a reconstruction technique that recovers instantaneous phase information of gas–liquid flows. This study introduces the so-called stochastic estimation and reconstructs bubble distribution based on electrical impedance. We simulate the flow at the bottom of a ship using a horizontal channel and install a 16-channel electrode array on the upper wall to acquire time histories of electrical impedance. The relation between the bubble distribution and the electrical impedance is determined by simultaneous image measurement using a high-speed video camera. Once the coefficients of the linear relation between the electrical impedance and the local void fraction are calculated, two-dimensional bubble distribution can be estimated from only voltages at the 16 electrodes. The proposed technique is examined over a range of void fractions and flow velocities, and the capabilities of the stochastic estimation applied to bubbly flow reconstruction are demonstrated by comparing the reconstructed/predicted images with the original ones.

The conventional PIV or PTV systems cannot work well in the measurement of bubbly flow. Neither the gray level cross-correlation method and binary image cross-correlation method nor fourframe PTV can track bubble properly because whole image of each bubble cannot be always recorded due to illumination problem. High velocity and random motion of bubble are also the causes for the problem. The behavior of bubble in image sometimes causes an uncertainty to a degree and therefore cannot be measured well by the previous PIV methods. Fuzzy logic has proven to be a simple and robust method for process control. In this paper, the fuzzy logic has been applied in the measurement of bubbles in a gas-liquid two-phase flow. Two frames of digitally recorded, single exposure particle images are used. The purpose of current research is to establish a novel PTV method based on fuzzy logic to measure the bubble behavior from bubble images with uncertainty.