Bubble Behavior Research Articles

Cavitating bubbly flow of non-Newtonian second grade fluid through nozzles: Application of reduction of cavitation damage and noise

This research is focused on studying the behavior of bubbles in one-dimensional nozzle flows. The study delves into the complex flow of cavitating bubbles within geometries of varying cross-sectional areas, using a second-grade fluid. The cavitation phenomenon poses a potential threat, as it can harm surfaces upon bubble collapse or interaction with adjacent boundaries. Thus, the proposed model unveils the intricate behavior of cavitating bubbles in response to nozzle shape and fluid properties. The governing equations are first modeled using the Rayleigh–Plesset equation along with continuity and momentum equations for bubbly liquids. They are then simplified and nondimensionalized to solve by means of the RK method. The influence of several nondimensional parameters, including the Reynolds number, fluid parameter, cavitation number and the number of bubbles, on the flow behavior of second-grade fluid through various channels is illustrated using graphs. To validate the results, the Newtonian formulation was applied in the current setting, and it was found that the obtained results matched the available literature. Also, critical values of void fractions for the radius of the bubble are presented. The non-Newtonian properties of the fluid are observed a significant outcome for the reduction of cavitation damage and noise.

Performance Evaluation of UOWC Systems from an Empirical Channel Model Approach for Air Bubble-Induced Scattering.

Underwater optical wireless communication (UOWC) systems provide the potential to establish secure high-data-rate communication links in underwater environments. The uniqueness of oceanic impairments, such as absorption, scattering, oceanic turbulence, and air bubbles demands accurate statistical channel models based on empirical measurements for the development of UOWC systems adapted to different types of water and link conditions. Recently, generalized Gamma and a mixture of two generalized Gamma probability density functions (PDF) were proposed to describe the statistical behavior of small and large air bubbles, respectively, when considering several levels of particle-induced scattering. In this paper, we derive novel closed-form analytic expressions to compute the bit error rate (BER) and outage performance using both proposed PDFs for various scattering conditions. Furthermore, simple asymptotic expressions are obtained to determine the diversity order of each scenario. Monte Carlo simulation results verify the obtained theoretical expressions. Our results also reveal that UOWC systems present lower BER and outage performance under more turbid water cases with respect to the tap water case due to the higher diversity order and despite the significant increases in pathloss at short link distances. Particle-induced scattering provides an inherent mechanism of turbid waters to mitigate air bubble-induced fluctuations and light blockages.

One Man’s Bubble Is Another Man’s Rational Behavior: Comparing Alternative Macroeconomic Hypotheses for the US Housing Market

Competing macroeconomic hypotheses have been developed to explain the US housing market and possible bubble behavior. We employ both seasonally adjusted (SA) and non-seasonally adjusted (NSA) monthly data for about 30 independent variables to examine alternative macro hypotheses for home prices. Using a neural network model as an atheoretical non-linear approach to capture the relative importance of alternative macro variables, we show that these hypotheses generate different macro relevance. As an alternative to testing housing time series, we focus on bubble identification being hypothesis dependent. Model forecast errors (residuals) identify the potential presence of bubbles through standardized residual CUSUM tests for structural breaks. By testing for housing bubbles from these unstructured models, we generate conclusions on the presence of bubbles prior to the Great Financial Crisis and the post-pandemic periods. Competing macro hypotheses or narratives will generate different conclusions on the presence of bubbles and create bubble identification issues.

Bridging the Gap: Unraveling the Mechanisms of Nanobubble Detachment from Electrode Surfaces

Gas evolution reactions play a fundamental role in the functionality of various electrochemical devices. Recent experimental studies have provided valuable insights into the behavior of gas bubble formation on nanoelectrodes; however, the experiments have limited spatial resolution to fully understand the underlying mechanisms governing the formation of nanobubbles and their stability on the surface as well as the effects of nanoscopic surface defects on their behavior. A comprehensive understanding of how bubbles behave on electrode surfaces and in the electrolyte is essential for optimizing and designing electrochemical processes. In this work, we employ all-atom molecular simulations to explore the effects of surface chemistry and defect geometry on detachment behavior of hydrogen gas bubbles on nickel electrodes at molecular level. Analyzing the dynamics of bubble detachment on the surfaces with different chemistries reveals that the rate of bubble detachment is higher on the strongly hydrophilic surface compared to the surface exhibiting weak hydrophilicity. In addition, our findings indicate that the defect geometry has significant effects on the detachment of bubbles from the surface. Specifically, the width and depth of canonical-shaped defects are demonstrated to be critical parameters affecting the behavior of the hydrogen bubbles on the nickel surface. Overall, our results show there is a complex balance between the degree of surface wettability and surface defects, determining the stability of nanobubble formation on the electrode surfaces. The results of this study could enhance our understanding of bubble detachment dynamics on the electrode surfaces, providing valuable insights for the future design of practical applications.This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC5207NA27344. Release: LLNL-ABS-858292

Dynamics of a single cavitation bubble near a cylindrical blind hole

Cavitation bubble dynamics, such as microjets and moving shock waves, are strongly influenced by their surroundings. We investigated the complex bubble behavior during the growth and collapse stages near a blind hole. The simultaneous study, which is a combination of experiment and numerical methods, was performed. The single bubble was induced by a laser focus beam near a blind hole in the cuvette and measured by a high-speed camera with an image processing method. Numerical predictions were conducted for a detailed further investigation of the pressure and velocity fields near and inside the bubble. A fully compressible two-phase Navier-Stokes solver based on the homogeneous flow assumption was used. Both experimental and numerical results showed good agreement with each other. Strong deformation of the bottom bubble surface evolution was observed at low standoffs above the hole entrance. An extremely high-velocity microjet with a mushroom cap-shaped bubble was predicted under high standoffs below the hole entrance. Secondary cavitation in the blind hole was observed experimentally, and we discussed the low-pressure region caused by the interaction among the shock wave, bubble, and blind hole. Consequently, the tendencies of maximum microjet velocity and time-dependent surface pressure in the collapse phase are suggested.

Investigation of Taylor bubble behavior in upward and downward vertical and inclined pipe flows

The Taylor bubble behavior in two-phase flow is critical for various engineering applications, including momentum, heat, and mass transfer efficiencies. Therefore, understanding the dynamics of Taylor bubbles is crucial for the optimal design, operation, and safety of reactors and pipelines. In slug flow in pipes, the successive translation of liquid slugs and Taylor bubbles is characterized by the Taylor bubble velocity, which is a function of the flow distribution coefficient (C0). This study aims to compile a large database of Taylor bubble velocity and flow distribution coefficient in upward, horizontal, and downward flows, evaluate existing models, and propose a unified C0 model. As a result, it is found that the physics governing the flow distribution coefficient in downward flow is drastically different from that in upward flow, and it goes through two flow transitions, which are governed by the pipe inclination angle. This behavior is modeled by the proposed model, which predicts the Taylor bubble flow distribution coefficient through both transitions and captures its physical behavior in the downward flow. A validation study of the proposed model revealed that the proposed model outperforms the existing models as it has an average absolute percent error and standard deviation of 4.2% and 6.2%, respectively.

Experimental research of bubble pulsation and jet under a double plate with circular hole

The pulsation and jet of underwater explosion bubble heavily affected by the boundary conditions. To explore the pulsation behavior and jet load of underwater explosion bubble near a damaged double structure, some double plates with circular hole were designed. The electric spark-generated bubble was employed to act as the underwater explosion bubble, and the experiment research on bubble pulsation and jet below the double plate with circular hole was carried out. The bubble pulsation and jet process were recorded by a high-speed camera, and the bubble jet load was measured by a wall pressure sensor. The results show that with the growth of the standoff distance, the bubble jet changes from the type I “gas-liquid two-phase jet” to the type II “gas-liquid two-phase jet”, and finally into the water jet. The load of the type I “gas-liquid two-phase jet” are characterized with high peak pressure with short duration in the initial pressure stage, and low pressure with long duration in the standing pressure stage. Whereas the type II “gas-liquid two-phase jet” feature high peak pressure with short duration in the initial pressure stage without standing pressure stage.

Mechanical and physicochemical characteristics of sub-millimeter and macrobubbles: Insights from AFM analysis, zeta potential measurements and rise velocity

This study presents the change in mechanical and physicochemical behaviour of macrobubbles and sub-millimeter bubbles with respect to pH. While macrobubbles conventionally range from 100 μm to 2 mm in diameter, this study focuses on the comparatively less-explored sub-millimeter bubble range, centered around a median diameter of 500 μm. The difference in sub-millimeter bubbles (500 μm) compared to their larger counterparts (1100 μm) within macrobubble size range were evidenced through atomic force microscopy (AFM) analysis, zeta potential measurements, and rise velocity experiments. The adhesion force and energy between bubbles and the silicon nitride cantilever tip showed a decreasing trend with increasing pH, notably diminishing by about 50% beyond pH 9. Although no significant differences were statistically revealed between the two bubble sizes at lower pH, the adhesion force and energy were consistently smaller for macrobubble compared to sub-millimeter bubble at higher pH beyond pH 9. This finding suggests that the electrical double layer (EDL) interactions are influenced by the specific surface area, particularly in the alkaline region. Zeta potential measurements exhibited a cubic trend, with an isoelectric point at pH 3.5 and a minimum zeta potential at pH 11. The study also depicted a similar trend in the rise velocity of bubbles, suggesting a direct relationship between bubble stiffness and its rise velocity. Therefore, changes in pH affected the bubbles' properties, while on the other hand, certain physicochemical properties are also influenced by bubble sizes.

Muzzle bubble dynamics characterization of underwater launching

To comprehensively understand the dynamic behavior of muzzle bubbles during underwater launching, an emptying process aligned with the muzzle flow characteristics is established and an evaporative condensation mechanism is modeled according to the high temperature and pressure properties of the propellant gas. Utilizing the spherical bubble theory, which comprises the inflation process and evaporative condensation effects, the dynamics of muzzle bubbles and their corresponding pressure waves are investigated. The numerical simulation results well agree with the experimental observations in terms of bubble radius and near-field pressure waves. Furthermore, the influence of two key factors on the bubble dynamics is examined: underwater launching depth and initial muzzle pressures. The results illustrate that the inflation process needs to be accurately described for precise pressure wave predictions. Using the evaporation condensation model, the bubble radius and frequency can be accurately characterized. Moreover, the launching depth influences the free expansion radius and oscillation frequency mostly due to the increase in hydrostatic pressure, which decreases by 33% and increases by 150% in the 1–20 m range, respectively. The initial muzzle pressure affects the initial expansion velocity and initial shock wave mainly due to the increase in the mass flow rate, which increase by 56% and 82% in the 35–65 MPa range, respectively.

Influence of cavity geometry on the bubble dynamics of nucleate pool boiling

Nucleate pool boiling is known for its exceptional heat transfer coefficients, with the use of cavities further improving bubble nucleation and heat transfer rate. To promote this heat transfer enhancement technique, a thorough understanding of the influence of cavity geometry on single bubble dynamics is required. The influence of depth and radius of cylindrical and conical cavities on the bubble dynamics of nucleate pool boiling of R1234yf were numerically investigated. The cavity radius was varied between 50 and 400 μm and the cavity depth between 100 and 1000 μm at a fixed heat flux of 28 kW/m2. It was found that the maximum equivalent diameter prior to departure was constant for cavities with radii smaller than 120 μm, while it increased linearly when increasing the cavity radius further. Cylindrical cavities exhibited high stability regardless of cavity radius or depth whereas conical cavities showed a decrease in vapor retention with increasing cavity angle. During the necking phase, the bubble interface became pinned at the cavity edge, depending on conical cavity angle, implying that smaller radii allowed for enhanced surface rewetting. Conical cavities could be considered as cylindrical cavities when the cavity angle was less than a quarter of the interface contact angle. When translating the single cavity findings to cavity array design, cylindrical cavities were recommended as they allowed for stable bubble behavior. For increased nucleation zones and rewetting, a sub-critical radius was recommended. Wider cavities were recommended for high superheat conditions as larger bubbles could enhance bubble growth.

Bubble behavior of single-pulse discharge in EDM

The removal of machining debris in the narrow discharge gap primarily relies on high pressure bubble expansion and fluid agitation induced by the bubble pulsation in electrical discharge machining (EDM). Observing bubble behavior and elucidating the mechanism of bubble behavior is an increasing concern in EDM. The purpose of this study is to investigate the bubble behavior of single pulse discharge in EDM. Using a high-speed camera, this study observed the bubble evolution and measured the discharge duration dependent bubble characteristics. The results show that the bubble experiences rapid expansion, contraction, primary pulsation, intense pulsation in the post-discharge process, and subsequent bubble energy dissipation behavior. Furthermore, the arc plasma persists for a duration even after the discharge ends in positive polarity machining. The findings suggest that bubble behavior in single-pulse discharges is primarily dominated by the discharge energy, with the bubble behavior during the discharge significantly influenced by the jumping and oscillation of the arc plasma. Our research may introduce new ideas for improving the depth-to-diameter ratio of deep micro-holes in EDM and provide fresh perspectives on the mechanism analysis of how machining parameters affect machining performance. For instance, by synchronizing the pulse intervals with the bubble stabilization time, bubble coalescence can be reduced, thus increasing bubble removal in the small machining gap. The stability of deep micro-hole EDM may depend on bubble behavior, which is determined by the electrical parameters.

Numerical simulation study on opening blood–brain barrier by ultrasonic cavitation

Experimental studies have shown that ultrasonic cavitation can reversibly open the blood–brain barrier (BBB) to assist drug delivery. Nevertheless, the majority of the present study focused on experimental aspects of BBB opening. In this study, we developed a three-bubble-liquid-solid model to investigate the dynamic behavior of multiple bubbles within the blood vessels, and elucidate the physical mechanism of drug molecules through endothelial cells under ultrasonic cavitation excitation. The results showed that the large bubbles have a significant inhibitory effect on the movement of small bubbles, and the vibration morphology of intravascular microbubbles was affected by the acoustic parameters, microbubble size, and the distance between the microbubbles. The ultrasonic cavitation can significantly enhance the unidirectional flux of drug molecules, and the unidirectional flux growth rate of the wall can reach more than 5 %. Microjets and shock waves emitted from microbubbles generate different stress distribution patterns on the vascular wall, which in turn affects the pore size of the vessel wall and the permeability of drug molecules. The vibration morphology of microbubbles is related to the concentration, arrangement and scale of microbubbles, and the drug permeation impact can be enhanced by optimizing bubble size and acoustic parameters. The results offer an extensive depiction of the factors influencing the blood–brain barrier opening through ultrasonic cavitation, and the model may provide a potential technique to actively regulate the penetration capacity of drugs through endothelial layer of the neurovascular system by regulating BBB opening.

Heat transfer characteristics during bottom-blown bubble coalescence

Bubble swarm evolution and behaviors are the key to study the heat transfer between a bubble swarm and its surrounding flow field, and are of significant and systematic theoretical guidance for exploring the process reinforcement technology and processes. In this study targeted at Therminol®66 and R245fa, we analyzed the three factors affecting gas-liquid two-phase flow. The results show that the flow rate of the dispersed phase has a significant effect on the heat transfer coefficient. When the flow rate of the working medium increases from 0.4 kg s-1 to 0.8 kg s-1, the heat transfer coefficient increases by about 1.25 times. The initial temperature difference is positively correlated with the heat transfer coefficient, but the continuous phase height increases the heat transfer coefficient first and then decreases. The heat transfer coefficient model is in good agreement with the results of Kwon, and the deviation is within 20 %, accounting for 92.464 %, which can be further used to describe the heat transfer process of heat exchanger or bottom-blown melting pool. Correlation equations for the Nusselt number (Nu) is developed, with average absolute deviation of 18.49 %.

The comovement of bubbles’ responses to monetary policy shocks

In this paper, we present a two-step methodology designed to elucidate the extent of co-movement in the behavior of asset bubbles across a selected group of developed and emerging economies, with observations extending from the year 2000 onwards. We characterize the behavior of these bubbles, conceptualized as the disparity between fundamental and market values, by examining their responses to monetary policy shocks. Our investigation into their co-movement dynamics post-2000 involves the estimation of a Bayesian Dynamic Factor Model that incorporates the reactions of asset bubbles to monetary policy shocks, both in the short term and the long term. Notably, our findings reveal a convergence in short-term responses among these bubbles prior to the onset of the financial crisis. However, as the crisis unfolded, our results indicate a shift towards divergence. Concerning the long-term response of bubbles to monetary policy, divergence was observed before the crisis, but once the crisis had taken hold, there was a noticeable trend towards convergence. An influential idiosyncratic factor was identified for countries like the United States, South Korea, and Japan, impacting both short-term and long-term behavior. Conversely, the long-term behavior of bubbles in most emerging economies appears to be influenced by idiosyncratic factors. Furthermore, our results largely maintain their robustness even when utilizing an alternative measure of monetary policy stance during the period of the zero lower bound. Nevertheless, this secondary analysis suggests heightened volatility in the factors affecting both emerging and advanced markets.

Study on inclusion removal in two-strand tundish with gas curtain considering bubble-inclusion attachment

ABSTRACT Application of a gas curtain is one of the main means to improve the cleanliness of liquid steel in tundish. In particular, the bubble-inclusion attachment is one of the important ways to remove inclusions, which cannot be ignored. The bubble size and gas flow are the main parameters of the gas curtain. In this paper, the Euler–Lagrange–Lagrange approach is used to study the three-phase interaction behaviour of liquid steel, bubble, and inclusion. The effects of inclusion size, bubble diameter, and gas flow on inclusion removal rate were studied with normal continuous casting conditions. The calculation results show that the cleanliness of molten steel can be improved by replacing the dam with a gas curtain. In the range of gas flow rate from 1.35 to 2.15 Nm3 h−1, the removal rate of bubble-inclusion attachment and total inclusion removal rate increase with the increase of gas flow. In the range of diameter from 4.5 mm to 6.0 mm with the same gas flow, the total removal rate and bubble-inclusion attachment removal rate increase with the bubble size decrease. The results provide a reference for improving the cleanliness of molten steel by properly increasing the gas flow and reducing the bubble size in industrial operations.

In-situ investigation of helium effect on diverse precipitate/matrix interface areas in irradiated 5052 aluminum alloy

Due to its low neutron cross-section and high irradiation swelling resistance, 5052 aluminum alloy is extensively utilized in the core structure and irradiation shielding of research nuclear reactors. Indeed, irradiation-induced defects may accumulate preferentially in the precipitates of aluminum alloys during service, leading to prior impairment of these precipitates, thereby inducing the degradation of mechanical properties. The interaction between irradiation defects and precipitates under irradiation, however, remains inadequately understood. In this work, the microstructure of 5052 aluminum alloy irradiated in-situ with He+ at different temperatures and subsequent annealing was investigated by transmission electron microscopy. After 1.0 dpa irradiation at different temperatures, the evolution of He bubbles within the precipitate was greater than in the matrix, and manifested a pronounced growth threshold between 373 K and 473 K. During post-irradiation annealing, compared to the matrix, also the dynamic evolution of bubbles was observed on the inner side of the precipitate interface, where bubbles coalesced and rapidly dissolved. The dynamic behavior of He bubbles within different precipitates manifested a certain discrepancy, and sink strength of two typical Al-containing precipitates was estimated. The difference in bubble evolution can be attributed to the different sink efficiency and vacancy concentration gradient around the precipitate.

Novel concept for 3D-printed, baffled micro-fluidized beds for the regulation of bubble behavior and the suppression of solid back-mixing: Experiments and simulations

In Micro-Fluidized Beds (MFBs), large bubbles (or slugs) and strong solid back-mixing might hinder good mixing, heat and mass transfer, while reducing conversion rates, and limiting selectivity in certain gas–solid reactions. In this study, a series of novel baffled MFBs were produced using 3D-printing. The influence of the number and position of baffles on the regulation of bubble performance and solid back-mixing in MFBs was investigated through fluidization experiments and Two-Fluid Model (TFM) simulations. The results show that the introduction of baffles effectively suppress slug formation and expand the bubbling regime. Baffles placed at mid-height disrupt fully developed slugs more effectively than those at the bottom. Increasing the number of baffles enhances bubble-breaking efficiency. Numerical simulations suggest that baffles positioned in the lower portion of the bed are more effective for the inhibition of global solid back-mixing. This study shed new light on the design and optimization of baffled MFBs.

Quantitative analysis of the upwelling behavior of methane bubbles in nature and numerical simulations

In recent years, methane gas released from the seafloor has been considered a sub-oceanic resource, and comprehending the dynamics of methane bubbles in the ocean has garnered increasing attention. Therefore, in this study, we conducted a quantitative analysis of the behavior of ascending methane bubbles with and without a hydrate layer. The bubbles were filmed at two natural gas seep sites and reproduced by numerical simulations using two-dimensional motion analysis software. The simulations were performed with gas bubbles and methane hydrate (MH)1 bubbles spouting from a nozzle in a computational domain filled with pure water to assess the validity of image analysis for in-situ data, whereas numerical models and physical properties were utilized for the current two-phase (gas-liquid) simulations. The rising velocity, size, circumference, circularity, and maximum diameter of the methane bubbles were examined to understand the effects of the MH layer on the statistical and stochastic features of ascending methane bubbles. Based on the statistics of the above variables, the gas bubbles had a higher rising velocity and smaller circularity than the MH bubbles when the bubble sizes were identical. In addition, stochastic analysis indicated that the circularity of the MH bubble was uniquely determined by their size, due to the more rigid skin of the MH bubbles compared to that of gas bubbles. Consequently, discrepancies in bubble dynamics between methane gas and MH bubbles were clarified in this study.

Numerical simulation for pressure fluctuations caused by the growth of a single boiling bubble

The pressure fluctuations caused by the rapid volume changes during the initial growth stage for a single bubble are crucial excitations of boiling induced vibration. Therefore, it is necessary to accurately model the dynamic behavior of bubbles during this stage to obtain reasonable excitation forces, which can be used to estimate the vibration response of structures. The present study develops a dynamics model, which is verified and validated to be applicable to the growth of spherical bubbles in infinite superheated liquid and hemispherical bubbles growing on a vertical heated surface. The effects of mass transfer are considered for the vapor-liquid interface motion, and the temperature variations at the liquid side are determined by the finite difference method. The numerical data match well with the theoretical and experimental results, and the characteristics of the excitation forces caused by a single boiling bubble are obtained and analyzed. In addition, the influences of superheat, accommodation coefficient for phase change, and the temperature gradient at the liquid side on excitation forces caused by bubble growth are studied. The results show that the temperature distributions near the heated surface strongly affect the frequency domain characteristics of the excitation forces.

Experimental Study On Bubble Pairs And Induced Flow Fields Using Tomographic Particle Image Velocimetry

Gas-liquid two-phase flows, such as bubbly flows, are widely used across various engineering and military applications due to their distinctive fluid dynamics. This study utilizes advanced imaging techniques, including shadowgraphy and laser-induced fluorescence tomographic particle image velocimetry (LIF-TPIV), to provide a quantitative analysis of bubble motions and the resulting flow fields. We systematically investigate the effects of orifice distances (s) set at 10, 15, 20, and 25 mm to assess the impact of spacing on bubble behavior and interactions. Our findings indicate that at orifice distances of 20 mm or less, the proximity of bubbles facilitates interactions, promoting dynamic bubble behaviors. In contrast, at distances greater than 25 mm, interactions become markedly weaker, leading to increased separation between bubbles during their free oscillation stages. This weak interaction directly influences the characteristics of bubble-induced flow fields. As orifice spacing increases, there is a noticeable decrease in the velocity of the flow field between bubbles, particularly noticeable at spacings of 20 mm and above. At these larger spacings, a low-velocity zone develops between the bubbles at higher heights, which, according to Bernoulli's principle, corresponds to a high-pressure area. This high pressure contributes to a reduction in bubble volume with increasing orifice spacing. Furthermore, the intensity of wake vortices between two bubbles is observed to be highest at the smallest spacing of 10 mm, closely resembling the flow structure characteristic of a single rising bubble. For spacings of 15 and 20 mm, the induced flow fields of the bubbles continue to interact significantly. However, at a spacing of 25 mm, the flow fields appear to function independently, suggesting a threshold beyond which bubble interactions stop to significantly affect their surrounding fluid environments. These findings are helpful for comprehending the two-phase flow dynamics, particularly in multi-orifices bubbly flow.