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The migration and wake structure of a single bubble in a viscous medium near a vertical wall have been experimentally investigated using a two-camera orthogonal shadowing method combined with a particle image velocimetry system. The effects of bubble size, liquid properties, and migration modes on the motion of a near-wall bubble and its wake structure are discussed. It is shown that, unlike migration in pure water, collisions between bubbles and walls are rare in viscous liquids. As viscosity increases, the mobility of the bubble near the wall changes from 3D to 2D and finally exhibits 1D motion with only one degree of freedom. While 1D motion is dominated by viscous forces, 2D migration is influenced by both wall effects and viscous forces, while 3D migration is primarily affected by bubble size. After analyzing the wakes produced by different factors, it was found that the wakes generated by mobile bubbles in three dimensions exhibit similar shedding patterns to Karman vortex streets, while two-dimensional wakes show stronger continuity. The average topological flow field structure of the bubble wake is obtained through periodic phase averaging, and the effect of different factors on the evolution of near-wall bubble wakes is analyzed. • Phase-averaged analysis of the wake behind a near-wall bubble. • The impact of size and viscosity on the migration of a near-wall bubbles. • Shedding of near-wall bubble wakes. [ABSTRACT FROM AUTHOR]

Bubble migration in a vibrating zero gravity environment is numerically investigated using ANSYS-FLUENT software. A 3D CFD model is developed describing the two-phase flow of a nitrogen bubble immersed in a container full of ethanol. The Volume of Fluid (VOF) method and the geometric reconstruction scheme are used to track the gas–liquid interface. The liquid in the container is vibrated horizontally in the x momentum with different frequencies from 0 to 1 Hz, and amplitudes from 0.005 to 0.1 m/s2. The vibration impact on the bubble arrival time to the top and its ensuing dynamic is analyzed. Different bubble trajectory shapes are observed, other than the conventional vertical translation induced by the temperature difference. Compared to the no vibration case, the bubble motion is slightly either accelerated or decelerated for very low vibration amplitudes. For a fixed frequency f = 1 Hz, the bubble takes more time to reach the top with the vibration amplitude increment relatively to the no vibration case. The vibration effect becomes more intense with the Marangoni number decrease when f = 0.2 Hz and Ab = 0.005 m/s2. Those results are difficult to obtain experimentally, signifying the importance of this numerical study to understand bubble motion and migration in space. [ABSTRACT FROM AUTHOR]

The presence of a rigid wall causes a microjet of the cavitation bubble to collapse to move toward the wall, while a free surface does the opposite. When a rigid wall surface is combined with a free surface, it may affect the direction of the microjet. The motive of this study is to find out the influence of the dynamic behavior of a laser-induced bubble near the rigid wall with a gas-containing hole. Evolutions of the bubble at different distances from the gas-containing hole in the horizontal and vertical directions were recorded by a high-speed camera (2.3×105 fps). When the bubble collapse near the boundary, the bubble will produce two situations: away from or toward the boundary. It focuses on the direction of the bubble, the oscillation period of a bubble, and reflection angle, and quantitative analysis of the results. It was found that the boundary not only changes the morphologic of the bubble and the overall direction of movement but also affects the oscillation period. In addition, it can control the deflection of the bubble. [ABSTRACT FROM AUTHOR]

The migration of deformable bubbles with different sizes near a vertical wall in pure water was experimentally studied by the dual-camera orthogonal shadow method. The key factors affecting the migration are discussed. It was found that the effect of size and initial distance on bubble migration is indirect. For bubbles of the same diameter (deq=3.10 mm), the same mode occurs when the initial distance (0.5≤L* ≤3.5) is changed. When the initial distance (L* =0.5) is constant, the same mode appears for bubbles of different sizes (2.27 mm≤deq≤3.40 mm). Furthermore, different modes are found in the same initial distance (L* =0.5) and size (deq=3.10 mm). In fact, the migration mode is determined by the initial distance, Reynolds number and Weber number. The size and initial distance can influence the frequency of different modes. Considering a variety of factors, four basic migration modes are summarized. New correlations are proposed to predict bubble behavior in different modes near the wall. The kinematic and dynamic parameters of different modes are discussed. It is expected that the results can be helpful to predict the gas-liquid flow near a wall and to improve related heat and mass transfer process. [ABSTRACT FROM AUTHOR]

The lead-cooled fast reactor (LFR) is one of the most promising fast neutron reactors using molten lead or the lead–bismuth eutectic (LBE) alloy as a coolant. Under postulated severe accidents, the fuel rod of LFR may be damaged, which would cause the release of fission gas, and the migration of fission gas bubbles in the reactor molten pool will affect the release and absorption of radioactive substances in the reactor. In this paper, a three-dimensional numerical study on the release and migration behavior of fission gas in the molten LBE pool of LFR is carried out based on the volume of fluid method. The bubbles are continuously released by gas injection, and the research mainly focuses on the detachment time, the rising velocity, and the size of the bubble when it detaches at the orifice. The coalescence of bubbles is observed, and the acceleration effect of the bubble wake is confirmed. The distribution of the bubble terminal rising velocity with diameter has no simple or linear relationship. The effects of the gas injection velocity, the release depth, and the gas injection angle are studied. A lower gas injection velocity will delay the detachment and reduce the size of the bubble. The increase of release depth tends to release smaller bubbles. The bubbles released from a vertical surface will attach to the wall. The simulations and theoretical analysis are comparable and have similar tendencies. The distribution of the bubble terminal rising velocity with equivalent diameter may predict the migration behavior of bubbles in molten LBE.

The main objective of the study is to investigate the bubble migration between foreign exchange and housing markets in Iran using seasonal data of 1966-1396. In this regard, bubble dating is discovered by using of Phillips et al (2015) method; then, bubble migration from the foreign exchange market to the housing market and from the housing market to the foreign exchange market is examined, using the Gomez-Gonzales et al. (2016) method. The results show that there are 7 bubble periods in the foreign exchange market; and 7 bubble periods in the Housing market. four hypotheses were defined for examine the bubble migration, according to the bubble dates (from the foreign exchange to housing market and vice versa). The results of the hypothesis testes showed that the first house bubble (2007Q2-2008Q3) led to the creation of the first bubble of the foreign exchange market (2007Q4-2008Q3). It seems that in the currency restriction conditions, the probability of creating a foreign exchange bubble and its migration to the housing market is rather high and vice versa.

• The bubble migration behavior in flowing liquids is studied with similarity experiments. • Visualization of migration behavior of bubbles in size of 1∼8 mm in downward flow. • The bubble theories in stagnant liquids are applicable for bubbles in downward flow. • The empirical correlations of slip velocity and drag coefficients are established. Under the Steam Generator Tube Rupture (SGTR) accident scenario of Lead-cooled Fast Reactor (LFR), the second loop coolant pressurized water interacts vigorously with the primary loop coolant LBE, resulting in the generation of a large number of bubbles. These bubbles may migrate to the core along with the flowing liquid metal, causing reactive disturbances and even deterioration of core heat transfer. Aiming at the bubble migration issue in the flowing liquid under SGTR accident conditions, several similarity experiments are performed by injecting gas into downward flow by utilizing visualization technology to simulate the bubble migration behavior in flowing Lead-Bismuth Eutectic (LBE). The bubbles with diameter of 2 ∼ 8 mm are injected into downward flows with flow velocities from −350 to −235 mm/s, and their migration process are captured by the high-speed camera. The comparison of the existing bubble rising theories in stagnant liquid and experimental results shows that the basic research methods for bubble terminal velocity and drag coefficient are applicable for the bubble in downward flow with certain errors. An empirical modified correlation is recommended for bubbles with equivalent diameters from 2 mm to 8 mm in downward flow based on experimental data and dimensional analysis. These data can provide a reference on analyzing bubble migration process in flowing liquid under SGTR accidents. [ABSTRACT FROM AUTHOR]

The bubble migration in liquid titanium melt during vertical centrifugal casting process has been investigated by hydraulic experiments. Results show that the gas bubble in the simple cavity ultimately migrates like a line parallel to the wall in the opposite direction to the rotational casting mould. The deviation distance of the bubble in the simple geometry cavity tends to increase with the increment of the mould rotational speed during the migration process. And the gas bubble is much easier to migrate like a line when its initial position is nearer to the casting mould wall which is opposite to the mould rotational direction. The migration trajectories of bubbles located at different position in the complex cavity are more complicated than that in the simple cavity. The casting mould in the complex cavity can hamper both the radial movement and the circular movement of the bubble. And gas bubbles will gather, re-nucleate and form new bigger bubbles beside the casting mould wall. The re-formed gas bubbles in the complex cavity become bigger than which escape from bubble generation chamber.

The horizontal bubbly two-phase flow is preferably used in various industrial applications because it provides high interfacial areas which enhance the heat and mass transfer. In the present research, the phase distribution of controlled air-water flow in a horizontal acrylic round pipe with 60 mm inside diameter (D) has been investigated experimentally and modeled numerically. The modeled differential pressure and the mixture velocity profile at a distance of 33D from the mixing section (fully developed region) are computed numerically and compared with those obtained experimentally from the two-phase flow system established and maintained at the National Institute of Standards (NIS-Egypt). Furthermore, the numerical and the experimental data have been compared with previous correlations and models. Reasonable quantitative agreement between all data is found. An electronic device based on Arduino Uno board was designed and used with careful data manipulation for measuring the slug bubble velocity. The results point out that the air volume fraction has a maximum value at the upper pipe wall as the gas bubbles tend to migrate to the upper wall. A new correlation was obtained for bubble migration length to the upper pipe wall which is very important in chemical industrial processes and other engineering application. [ABSTRACT FROM AUTHOR]

In order to resist bubble loading, anisotropic composite materials are the development direction of the future. The objective of this paper was to experimentally investigate the hydrodynamic performance of anisotropic laminate composite plates, with a focus on the effect of its anisotropic characteristics on single bubble migration. In these experiments, the bubble was generated in a transparent water tank filled with sufficiently degassed water by Joule heating at the connecting point of the electrodes through the discharge of a 6600 μF charge to 800 V, and a high-speed camera system with a recording speed of 40,000 frames per second was used to record the temporal evolution of bubble patterns and the dynamic responses of the laminated composite plates. The results are presented for two anisotropic cantilever composite beams with different ply angles, namely, 0° and 30°. Several variables, such as the shapes of the bubble, the curved trail of motion of the bubble center, bubble collapse time, and bubble initial standoff distances were extracted from the photographic images. The results showed that bubble migration near the 30° plate presents a curved bubble trail with an evident tilted angle during the collapse and rebound stages, which is very different from bubbles that all move vertically above the 0° plate. Furthermore, a characterization method for bubble migration was proposed to quantitatively describe the curved bubble trails and the deformation of the composite beams in temporal and spatial scales. This method shows that the curved bubble trails near the 30° plate are closely related to the dynamic response of composite beams, with a focus on the bending-twisting coupling effect.

• Studied the migration mechanism of gas bubble in the meso coal fissure. • Explained the mechanism of gas bubbles aggregation in the water injection fissure. • Revealed the correlation between fissure structure and bubble influence. Coal seam water injection is a key technology for achieving the co-mining of coal and coalbed methane. In response to the unclear mechanism of bubble migration in meso fissures that currently dominate the displacement of coalbed methane, this paper proposes a numerical simulation method based on CT images for exploring the migration mechanism of gas bubbles in the meso fissures of coal water injection. The meso fissure geometric modeling of coal water injection was reconstructed using CT scanning image analysis technology. Based on level-set theory, a bubble migration model under a two-phase fluid environment was constructed, and the influence characteristics of different fissure structures on bubble migration were explored. Furthermore, the mechanism of gas bubble migration within the meso fissures of coal water injection was revealed. Results show that the shape of the bubble with a radius of 0.03 mm oscillates obviously in the fissure structure with a length–width ratio of 1.3. The smaller the fissure structure diameter is, the larger the structure length–width ratio is, and the easier the bubble is to form gas resistance. Meanwhile, the larger the curvature of the fissure structure, the greater the contact angle between the bubble and the wall, and the easier it is for the bubble to detach from the wall. During the migration process, the size of the bubble constantly changes. With the increasing length–width ratio and width of the fissure structure, the equivalent radius of the bubble initially increases and then decreases. A negative correlation function is observed between bubble size and flow velocity. At low flow velocity, the volume of the bubble can increase by 18.17 %, and as the flow velocity increases, the volume of bubble can decrease by 54.53 %. In the bubbles aggregation process, an exponential function relationship can be observed between the bubble leading edge position and the bubbles spacing with a negative correlation. The influence of fissure structure on the bubbles is less than the interaction among bubbles. These results provide a theoretical basis for the efficient and increased extraction of coalbed methane. [ABSTRACT FROM AUTHOR]

Gas bubbles can be generated naturally or introduced artificially in porous media. Gas bubble migration through porous media governs the rate of gas emission to the atmosphere as well as the hydraulic and mechanical properties of sediments. The migration of air bubbles through water-wet porous media of uniform geometry was studied using a glass micromodel. Experiments were conducted to measure the velocity of bubbles of various lengths rising in a glass micromodel saturated with different test liquids and varying elevation angles. The results showed a linear dependency of the average bubble velocity on the bubble length and the sine of inclination angle of the micromodel. Comparisons were made using experimental data for air bubbles rising in kerosene, Soltrol 170 and dyed white oil. The effective permeability of the micromodel for the gas bubble, Kg, was calculated for different systems at different inclination angles, assuming that the effective length for viscous dissipation is equal to the initial static bubble length. It was found that the calculated permeability of the medium for gas bubbles had an increasing trend with increasing the bubble length. To visualize the periodic nature of the flow of rising bubbles in a porous medium, the motion of the air bubbles in white oil was video recorded by a digital camera, reviewed and analyzed using PowerDVDTM11 software. The bubble shape, exact positions of the bubble front and bubble tail during motion and, hence, the dynamic bubble length were determined through image analysis. Numerical simulation was performed by modifying an existing simulation code for the rise velocity of a gas bubble and the induced pressure field while it migrates through the pore network. The results showed that the rise velocity of a gas bubble is affected by the grid size of the pore network in the direction perpendicular to the bubble migration. The findings of this study can have important implications for studies on the migration of injected gas bubbles in geoenvironmental applications, as well as fundamental studies on bubble transport and behavior in porous media. [ABSTRACT FROM AUTHOR]

Polished high-purity aluminium was irradiated by 500 keV helium ions to a fluence of 3 × 1016 ions/cm2 at 450 °C. Then a micro-tensile sample was prepared by focused ion beam. The evolution of nanobubbles in irradiated aluminium upon stressing was investigated by employing in-situ tensile testing in a transmission electron microscope. In the case of helium-ions irradiation, dense bubbles with similar sizes are formed in a certain depth region from the irradiated surface. Upon tensile stressing, it was found that bubbles disappeared in the crack tip, and bubble coalescence occurred along the crack-propagation direction, which both contributed to the decrease of bubble number, whereas bubble sizes as well as bubble number showed almost no changes in the region far away the crack tip. This was caused by the non-uniform stresses in the stress fields, in which bubble migration occurred under the stress gradients while bubble evolution was hardly caused in a much low stressed region. Moreover, bubble sizes increased on the grain boundary and bubbles close to the grain boundary gradually disappeared upon stressing. This can be explained by the migration of helium bubbles toward the grain boundary under the stress gradients near the grain boundary. • An in-situ TEM tension was performed on a thin helium-bubbled aluminium. • Bubble disappearance near crack tip and coalescence along crack-propagation direction under the stress gradients. • No significant evolution of bubbles in the region far away from the crack tip. • Increased bubble sizes on grain boundary and gradual bubble disappearance near grain boundary under the stress gradients. [ABSTRACT FROM AUTHOR]

Highlights • We find evidence for explosiveness in EMU government bond yield spreads. • This behavior is coincident with the financial crisis and the sovereign debt crisis. • We find compelling evidence for bubbles in the REITs index. • We test against migration effects from the housing bubble to explosive yield spreads. • We find migration effects during the financial crisis, not during the debt crisis. Abstract This article sheds light on the question whether arising sovereign credit risk in the EMU has been triggered by the US subprime crunch. By adapting recent econometric methodologies suggested in the related field of speculative bubbles, we find clear evidence for fast diverging (and even explosive) behavior of EMU government bond yields of peripheral countries relative to Germany during the financial and the European debt crisis. This might be caused by flight-to-quality effects to German government bonds coincident with the collapse of Lehman Brothers and by a loss of confidence in the fiscal stability of Greece, Ireland, Italy, Portugal and Spain during the European debt crisis. First, we find compelling evidence for bubbles in the Dow Jones Equity Real Estate Investment Trusts (REITs) index which serves as a weekly measure of economic activity in the North American real estate sector. Second, in our main analysis, we test whether the collapsing bubble in the housing market triggered the diverging government bond yields during two crisis regimes. Our findings indicate that this was the case in the course of the financial crisis, but not during the EMU sovereign debt crisis. These results suggest that the severe fiscal problems in peripheral countries are homemade rather than imported from the US. [ABSTRACT FROM AUTHOR]

We report evidence of the existence of house price bubbles in several Canadian provinces using a wealth of monthly data for over thirty years. We apply bubble detection tests to both the price-to-rent ratio and a measure of the non-fundamental component of housing prices and find several episodes of housing price exuberance in various Canadian provinces. Furthermore, we show evidence of bubble migration and show that migration intensities are time-varying. In all cases an inverted U-shape is encountered, with a maximum point around 2012. Our results have important implications for the design of housing market policies. [ABSTRACT FROM AUTHOR]

Based on the pool type lead-based reactor, the two-phase flow phenomenon remains a longstanding challenge in a Steam Generator Tube Rupture (SGTR) scenario. In order to investigate the bubble rising velocity in opaque liquid Lead Bismuth Eutectic (LBE), experiments were carried out by injecting argon gas into four transparent liquids (water, glycerol, alcohol and FC-3283). The appropriate drag coefficient of a single bubble was obtained by dimensionless Eötvos number under different liquids. For bubble equivalent diameter from 4 mm to 6 mm, the bubble flow located in the regime of surface tension dominant, where the bubble terminal rising velocity tended to a value almost independent of the bubble diameter. The single bubble rising velocity remained about 0.252 m/s to 0.274 m/s in gas-liquid metal bubbly flow. An appropriate modified correlation was recommended to roughly forecast the single bubble terminal rising velocity in two phase flow with high liquid-to-gas density ratio. These data can provide a reference on analyzing bubble migration process under accident condition. [ABSTRACT FROM AUTHOR]

Recent theoretical models have suggested that bubbles are unlikely to undergo significant migration in a compaction crystal mush by capillary invasion while the system remains partly molten. To test this, experiments of bubble migration during compaction in a crystal-liquid mush were modeled using deformable foam crystals in corn syrup in a volumetric burette, compacted with rods of varying weights. A bubble source was provided by sodium bicarbonate (Alka-Seltzer®). Large bubbles (>several crystal sizes) are pinched by the compacting matrix and become overpressured and deformed as the bubbles experience a load change from hydrostatic to lithostatic. Once they begin to move, they move much faster than the compaction-driven liquid. Bubbles that are about the same size as the crystals but larger than the narrower pore throats move by deformation or breaking into smaller bubbles as they are forced through pore restrictions. Bubbles that are less than the typical pore diameter generally move with the liquid: The liquid + bubble mixture behaves as a single phase with a lower density than the bubble-free liquid, and as a consequence it rises faster than bubble-free liquid and allows for faster compaction. The overpressure required to force a bubble through the matrix (max grain size = 5 mm) is modest, about 5 %, and it is estimated that for a grain size of 1 mm, the required overpressure would be about 25 %. Using apatite distribution in a Stillwater olivine gabbro as an analog for bubble nucleation and growth, it is suggested that relatively large bubbles initially nucleate and grow in liquid-rich channels that develop late in the compaction history. Overpressure from compaction allows bubbles to rise higher into hotter parts of the crystal pile, where they redissolve and increase the volatile content of the liquid over what it would have without the bubble migration, leading to progressively earlier vapor saturation during crystallization of the interstitial liquid. Bubbles can also move rapidly by ‘surfing’ on porosity waves that can develop in a compacting mush. [ABSTRACT FROM AUTHOR]

Based on the phase-field interface-capturing scheme, a novel and common simulation strategy is put forward with the finite volume method to study the thermocapillary flows, which avoids the fourth-order derivative efficaciously and adopts the mean parameter method to preserve the convergence in the large density ratio flows. Through simulating the thermocapillary migration of bubble, the results show that the new strategy can successfully describe the performance of bubble with different density ratios, i.e. 0.1 and 0.01, and dimensionless parameters Re =1.5×10 -2 and 1.5×10 2 . When Re is 1.5×10 2 , the bubble has an apparent deformation, and a larger migration velocity of the bubble is observed with a larger temperature gradient along the surface. [ABSTRACT FROM AUTHOR]

DNA damage alone or DNA replication fork arrest at damaged sites may induce DNA double-strand breaks and initiate homologous recombination. This event can result in a crossover with a homologous chromosome, causing loss of heterozygosity along the chromosome. It is known that Srs2 acts as an antirecombinase at the replication fork: it is recruited by the SUMO (a small ubiquitin-related modifier)-conjugated DNA-polymerase sliding clamp (PCNA) and interferes with Rad51/Rad52-mediated homologous recombination. Here, we report that Srs2 promotes another type of homologous recombination that produces noncrossover products only, in collaboration with PCNA and Rad51. Srs2 proteins lacking the Rad51-binding domain, PCNA-SUMO-binding motifs, or ATP hydrolysis-dependent DNA helicase activity reduce this noncrossover recombination. However, the removal of either the Rad51-binding domain or the PCNA-binding motif strongly increases crossovers. Srs2 gene mutations are epistatic to mutations in the PCNA modification-related genes encoding PCNA, Siz1 (a SUMO ligase) and Rad6 (a ubiquitin-conjugating protein). Knocking out RAD51 blocked this recombination but enhanced nonhomologous end-joining. We hypothesize that, during DNA double-strand break repair, Srs2 mediates collaboration between the Rad51 nucleofilament and PCNA-SUMO and directs the heteroduplex intermediate to DNA synthesis in a moving bubble. This Rad51/Rad52/Srs2/PCNA-mediated noncrossover pathway avoids both interchromosomal crossover and imprecise end-joining, two potential paths leading to loss of heterozygosity, and contributes to genome maintenance and human health. [ABSTRACT FROM AUTHOR]

In order to resist bubble loading, anisotropic composite materials are the development direction of the future. The objective of this paper was to experimentally investigate the hydrodynamic performance of anisotropic laminate composite plates, with a focus on the effect of its anisotropic characteristics on single bubble migration. In these experiments, the bubble was generated in a transparent water tank filled with sufficiently degassed water by Joule heating at the connecting point of the electrodes through the discharge of a 6600 μF charge to 800 V, and a high-speed camera system with a recording speed of 40,000 frames per second was used to record the temporal evolution of bubble patterns and the dynamic responses of the laminated composite plates. The results are presented for two anisotropic cantilever composite beams with different ply angles, namely, 0° and 30°. Several variables, such as the shapes of the bubble, the curved trail of motion of the bubble center, bubble collapse time, and bubble initial standoff distances were extracted from the photographic images. The results showed that bubble migration near the 30° plate presents a curved bubble trail with an evident tilted angle during the collapse and rebound stages, which is very different from bubbles that all move vertically above the 0° plate. Furthermore, a characterization method for bubble migration was proposed to quantitatively describe the curved bubble trails and the deformation of the composite beams in temporal and spatial scales. This method shows that the curved bubble trails near the 30° plate are closely related to the dynamic response of composite beams, with a focus on the bending-twisting coupling effect. [ABSTRACT FROM AUTHOR]

Abstract: The bubble and liquid turbulence characteristics of air–water bubbly flow in a 200mm diameter vertical pipe was experimentally investigated. The bubble characteristics were measured using a dual optical probe, while the liquid-phase turbulence was measured using hot-film anemometry. Measurements were performed at six liquid superficial velocities in the range of 0.2–0.68m/s and gas superficial velocity from 0.005 to 0.18m/s, corresponding to an area average void fraction from 1.2% to 15.4%. At low void fraction flow, the radial void fraction distribution showed a wall peak which changed to a core peak profile as the void fraction was increased. The liquid average velocity and the turbulence intensities were less uniform in the core region of the pipe as the void fraction profile changed from a wall to a core peak. In general, there is an increase in the turbulence intensities when the bubbles are introduced into the flow. However, a turbulence suppression was observed close to the wall at high liquid superficial velocities for low void fractions up to about 1.6%. The net radial interfacial force on the bubbles was estimated from the momentum equations using the measured profiles. The radial migration of the bubbles in the core region of the pipe, which determines the shape of the void profile, was related to the balance between the turbulent dispersion and the lift forces. The ratio between these forces was characterized by a dimensionless group that includes the area averaged Eötvös number, slip ratio, and the ratio between the apparent added kinetic energy to the actual kinetic energy of the liquid. A non-dimensional map based on this dimensionless group and the force ratio is proposed to distinguish the conditions under which a wall or core peak void profile occurs in bubbly flows. [Copyright &y& Elsevier]

Abstract: The wall void peaking distribution observed in an upward turbulent bubbly boundary layer along a flat plate is generated by bubbles that move towards the plate, come into contact with the wall and then slide along it. This transverse ‘migration’ has been studied using flow visualization, high speed video and particle tracking techniques to measure the trajectories of mono-disperse air bubbles at very low void fractions. Investigations have been performed at four Reynolds numbers in the range 280< Re θ <3000, covering both the laminar and turbulent regimes, with mono-disperse bubbles of mean equivalent diameter between 2mm and 6mm. Lagrangian statistics calculated from hundreds of trajectories show that the migration only occurs in the turbulent regime and for bubble diameters below some critical value: 3.5mm< d eqcrit <4mm. Above this size (We >3), the interface deformation is such that bubbles do not remain at the wall, even when they are released at the surface. Also, bubble migration is shown to be non-systematic, to have a non-deterministic character in the sense that trajectories differ significantly, to increase with Reynolds number and to take place on a short time scale. A series of experiments with isolated bubbles demonstrates that these results are not influenced by bubble–bubble interactions and confirm that two-way coupling in the flow is limited. Flow visualizations show that the migration originates with the capture of bubbles inside the large turbulent structures of the boundary layer (‘bulges’). The bubbles begin to move towards the wall as they cross these structures, and the point at which they reach the wall is strongly correlated with the position of the deep ‘valleys’ which separate the turbulent ‘bulges’. The analysis of the mean Lagrangian trajectories of migrating bubbles confirms these observations. Firstly, the average time of migration calculated from these trajectories coincides with the mean transit time of the bubbles across the structures. Secondly, once the trajectories have been scaled by this transit time and the boundary layer thickness δ, they all have the same form in the region y/δ <0.4, independent of the Reynolds number. [Copyright &y& Elsevier]

Abstract: Kinetic Monte-Carlo simulations were performed to investigate the migration of He-bubbles in bcc Fe, using the binding energies of point defects to a He-bubble and the migration energies of an Fe atom on bubble surface, both of which are obtained by molecular dynamics and molecular static calculations. This approach linking multiple simulation methods can remove the degrees of freedom for lattice vibrations effectively and enables us to perform long time scale simulations successfully. When the equilibrium concentration of point defects in a matrix is assumed, a He-bubble can migrate by the surface diffusion mechanism and show the Brownian motion. The diffusion coefficient of larger He-bubbles than about 0.8nm in diameter is proportional to d −4, where d is bubble diameter. It is in good agreement with conventional continuum theory predictions. [Copyright &y& Elsevier]

The dynamics of a gas bubble situated in a sound-irradiated liquid and near a rigid boundary was studied theoretically to get a better understanding of the role of cavitation microjets in the therapeutic applications of ultrasound (US). The boundary integral method was adopted to simulate the temporal development of the bubble shape, jet formation during bubble collapse and bubble migration. It was found that the dynamic behaviour of the jet and the migratory characteristics of the bubble depend not only on the distance between bubble and boundary but, also, on the properties of the acoustic field. For frequencies of sound fields smaller than or equal to the resonance frequency of the bubble, jet formation and bubble migration toward the boundary are the main features of the interaction. No jet formation was observed for frequencies of sound fields larger than the resonance frequency of the bubble, and the bubble kept its initial position from the boundary throughout its motion. The pressure generated by the impact of the jet developed during bubble collapse close to the boundary may result in the fragmentation of brittle objects, such as renal calculi, dental tartar or intraocular lens. (E-mail: eabrujan@yahoo.com) [Copyright &y& Elsevier]

The spontaneous bubble migration and the active manipulation have widely applications in outer space. In this paper, the bubble migration driven by the thermal capillary effect is numerically investigated under variable buoyancy. The numerical model is built up through the transient two-dimensional axisymmetric model with a level set method. It is found that the magnitude and direction of bubble migration velocity is determined by the competition of upward buoyant effect and downward thermal capillary effect. These two effects can be controlled by changing the ratios of density, viscosity, thermal conductivity of bubble over surrounding liquid, as well as Reynolds number, Froude number, Peclet number and Marangoni number. With increasing Froude number or decreasing density ratio, the thermal capillary effect becomes dominant over the buoyant effect, the bubble will migrate from upward to downward. The Marangoni number has negligible effect on the bubble migration at low Reynolds numbers, while at high Reynolds numbers, increasing Marangoni number can reduce the upward migration velocity due to stronger thermal capillary effect. At low Reynolds numbers, with the increasing ratios of viscosity and thermal conductivity, the downward driving force is increased, while at high Reynolds numbers, the ratios have negligible effect on the bubble migration. These findings may provide the guidance for actively manipulating the bubbles under variable buoyancy. Image 1 • Bubble migration can be manipulated actively. • Both thermal capillarity and buoyancy can determine migration velocity. • Ratios of density, viscosity and thermal conductivity affect migration. • The effect of Re , Fr , Pe and Ma on migration is investigated. [ABSTRACT FROM AUTHOR]

Immiscible gas injection recovery is one of the most important methods used for EOR. However, it is very difficult to directly observe the flow of gas bubbles in the reservoir stratum. In our experimental work, we investigated visually the morphological configurations of gas bubbles migration, fragment, coalescence and trapping in different liquid-simulated porous media packs of glass beads. Also, the volume fraction of the trapped gas bubbles in the drainage process were measured. A fast video recording technique was used for the visual observations of the morphological characteristics of the rising gas bubbles in the saturated porous media. The obtained experimental images recorded reveal three types of the rising migration bubbles namely: free migration in the pore throat, attached wall migration on the beads and deformation migration. Additionally, coalescence and fragment have an interdependency for a rising gas bubble. The adsorption and capillary trappings are both two kinds of important trapping mechanisms, which determine the volume fraction of the trapped bubbles. The volume fraction of the trapped bubbles increases with an increment in the gas injection time and rates with the trapped bubbles fluctuating upward and downward due to the more coalescence and fragment after a long gas injection time/period. Lastly, the volume fraction of the trapped bubbles increases with a decrease in the inclination angles and porosities, which influences the areal sweep efficiency for enhanced liquid recovery in a porous media. • The visual experiments of the rising bubbles in porous media are conducted. • The motion characteristics show migration, fragment, coalescence and trapping. • Adsorption and capillary trapping determine the volume fraction of trapped bubbles. • The volume fraction of the trapped bubbles is measured at different factors. [ABSTRACT FROM AUTHOR]