Bubble Field Research Articles

A two-level semi-hybrid-mixed model for Stokes–Brinkman flows with divergence-compatible velocity–pressure elements

A two-level version for a recent semi-hybrid-mixed finite element approach for modeling Stokes and Brinkman flows is proposed. In the context of a domain decomposition of the flow region Ω, composite divergence-compatible finite elements pairs in H(div,Ω)×L2(Ω) are utilized for discretizing velocity and pressure fields, using the same approach previously adopted for two-level mixed Darcy and stress mixed elasticity models. The two-level finite element pairs of spaces in the subregions may have richer internal resolution than the boundary normal trace. Hybridization occurs by the introduction of an unknown (traction) defined over element boundaries, playing the role of a Lagrange multiplier to weakly enforce tangential velocity continuity and Dirichlet boundary condition. The well-posedness of the method requires a proper choice of the finite element space for the traction multiplier, which can be achieved after a proper velocity FE space enrichment with higher order bubble fields. The method is strongly locally conservative, yielding exact divergence-free velocity fields, demonstrating pressure robustness, and facilitating parallel implementations by limiting the communication of local common data to at most two elements. Easier coupling strategies of finite elements regarding different polynomial degree or mesh widths are permitted, provided that mild mesh and normal trace consistency properties are satisfied. Significant improvement in computational performance is achieved by the application of static condensation, where the global system is solved for coarse primary variables. The coarse primary variables are a piecewise constant pressure variable over the subregions, velocity normal trace and tangential traction over subdomain interfaces, as well as a real number used as a multiplier ensuring global zero-mean pressure. Refined details of the solutions are represented by secondary variables, which are post-processed by local solvers. Numerical results are presented for the verification of convergence histories of the method.

Levitation and dynamics of bodies in supersaturated fluids

A body immersed in a supersaturated fluid like carbonated water can accumulate a dynamic field of bubbles upon its surface. If the body is mobile, the attached bubbles can lift it upward against gravity, but a fluid-air interface can clean the surface of these lifting agents and the body may plummet. The process then begins anew, and continues for as long as the concentration of gas in the fluid supports it. In this work, experiments using fixed and free immersed bodies reveal fundamental features of force development and gas escape. A continuum model which incorporates the dynamics of a surface buoyancy field is used to predict the ranges of body mass and size, and fluid properties, for which the system is most dynamic, and those for which body excursions are suppressed. Simulations are then used to probe systems which are dominated by a small number of large bubbles. Body rotations at the surface are critical for driving periodic vertical motions of large bodies, which in turn can produce body wobbling, rolling, and damped surface ’bouncing’ dynamics.

A Review of the Dynamics Progress of Bubble Collapse within Droplet and Droplet Splash

The dynamics of a cavitation bubble within a droplet is one of the hot research topics at present. The present paper summarizes the research progress of bubble collapse within droplets and associated droplet splash. Firstly, three typical structures of bubble collapse are introduced, together with the collapsing shock waves and the distribution of collapsing forces. Secondly, several typical forms of droplet splash are shown in terms of splash speed, splash direction, and dynamic mechanisms. Finally, the unsolved problems in the field of cavitation bubbles within droplets are proposed with perspectives.

Theoretical Study of Supercavitation Bubble Formation Based on Gillespie’s Algorithm

Understanding the creation and development of a supercavitation bubble is essential for the design of supercavitational underwater vehicles and applications. The pressure field of the supercavitation bubble is one of the most significant factors in these processes, and it should be taken into account in the analysis. The underwater vessel is surrounded by a supercavitation bubble which is, in fact, an inhomogeneous fluid containing cavities (also described as microbubbles). The distribution of the cavities in the supercavitation volume dictates the pressure field and thus determines the stresses and forces that act on the vessel and affect its motion and stability. In this research, we suggest a new approach to studying the bubbles’ formation and learning about the cavities’ distribution in the low-pressure volume that envelops the underwater vehicle. We used Logvinovich’s principle to describe a two-dimensional ring of fluid that is created at the front edge of the supercavitation body and moves downstream along the vessel. To describe the distribution of the cavities we used Gillespie’s algorithm, which is usually used to describe biological and chemical systems. The algorithm succeeded in describing the random movement of the cavities in the cross-section under various conditions and also in describing their distribution and effects on the macroscopic system. A few factors of the physical characteristics of the fluid and the flow conditions were examined (the initial bubble supply, and the rate coefficients of creation and collapse). The results led to the conclusion that with an examination of those factors and using Gillespie’s algorithm, predictions of the distribution and thus the development of supercavitation could be achieved. The main finding of the analysis was that asymmetric development of the bubbles took place, in spite of the symmetry of the physical problem, as observed in high-resolution experiments.

Accuracy Assessment of the Eulerian Two-phase Model for the CFD Simulation of Gas Bubbles Dynamics in Alkaline Electrolyzers

To date, the most industrially developed technology to produce green hydrogen is represented by alkaline water electrolysis (AWE). To improve on design and efficiency of these devices, however, multiphysics simulations based on Computational Fluid Dynamics (CFD) are needed, able to account for electrophysical phenomena and multiphase flows. Focusing on internal flow optimization, the requirements for CFD simulations are anyhow extremely challenging, since solving the gas bubbles’ motion implies the solution of a two-phase flow characterized by very low Reynolds numbers and a high fraction of dispersed gas. Despite some interesting studies have been presented in the literature so far, validation of CFD results with detailed experimental measurements is quite rare and, therefore, the reliability of the adopted modelling approaches is not assessed yet. This study presents the results of a multivariate CFD analysis of an electrochemical cell and its validation through a literature test case. Bubbles generation is introduced as a source term, thus overlooking for the moment the electrochemistry to focus on fluid-dynamics. In particular, attention is given to the Eulerian multiphase modelling, investigating the influence of both the inter-phase interaction sub-models’ settings (e.g., lift and drag forces, virtual-mass force) and the general settings of the simulation. The mean velocity field of the PIV-measured bubbles is considered to assess the accuracy of numerical predictions, while the available high-definition flow pictures allow a qualitative assessment of the bubbles size and location. CFD results are shown to be in decent agreement with experimental data and able to reproduce the key flow features such as the spreading of the bubble curtains and the gas shifting towards the inner part of the cell. The effect of the bubbles’ diameter and of source layer thickness is also discussed.

Bubbles Templated Interconnected Porous Metallic Materials: Synthesis, Surface Modification, and their Electrocatalytic Applications for Water Splitting and Alcohols Oxidation

AbstractPorous metallic foams play a vital role in a diverse range of applications including electrocatalysis, electrochemical sensing, energy conversion and storage systems. Recently, template‐free electrochemical deposition or dynamic bubbles templating techniques have been very useful to precisely control the nanostructured metallic architecture without using any additional organic or inorganic template. Dynamic hydrogen/oxygen bubbles template electrodeposited metallic foams give micro/nano porosity with controlled pores size, pores channels and surface decorated nanoparticles. In this review, we describe the uniqueness of bubble‐templating electrochemical deposition techniques, their detailed implementation towards the synthesis of novel nanostructured metallic foam, and the progress made in the field of hydrogen bubbles, oxygen bubbles, and their derived templating approaches for the development of advanced nanometallic foam and their future prospective. Some modifications of porous metallic foams by galvanic replacement, chemical/electrochemical dealloying process, annealing and decoration of metallic and non‐metallic composites on the porous metallic foam are also summarized. Moreover, the role of porous metallic foam in electrochemical water splitting and alcohol oxidation reaction by offering a short diffusion pathway and concluding remarks about the recent development and future perspective of porous foam in the frontiers of surface structure are also critically reviewed.

Pressure field characteristics of underwater detonation gas jet below and above free water surface

An underwater detonation tube (DT) experiment is carried out to investigate the pressure field characteristics of the detonation gas jet propagating under and above a free surface. In the experiment, a 0.78 L DT filled with 0.12 MPa methane-oxygen mixture is detonated. The evolution of jet shape and pressure field are recorded using high-speed photography, pressure sensors, hydrophones and microphones. There are mainly three bubble pulsation periods determined from the bubble evolution and field pressure evolution, and the bubble shape tends to be stable after the pulsation ends. The fast Fourier Transform and Wavelet Transform method are adopted to analyze the frequency characteristics and energy distribution of the pressure signals. The energy of the bubble pulsation and the energy of the shock wave have different contributions in different media. Specifically, the high-frequency energy formed by reverberation contributes more in water, while the low-frequency energy formed by bubble pulsation is the main part of the energy in the air. The shock wave formed by the decoupling of the detonation wave attenuates sharply during its propagation. The distribution characteristics of the detonation acoustic signal in the air are analyzed using the band-pass filter pair, and the transmission coefficient is defined to study the ability of the detonation acoustic signal to transmit through the water–air interface. The result indicates that the low-frequency signals, such as bubble pulsations, can transmit out of the water–air interface at a larger transmission angle more easily than the high-frequency signals. The signal of bubble pulsation has a stronger transmission ability through the water–air interface compared to the shock wave. Combining the bubble evolution and pressure evolution is helpful to analyze the different signal components in the water and air, which plays an important role in the analysis of the pressure field characteristics.

Finding the bubbles in therapeutic ultrasound—For better or for worse

Cavitation has played a variety of roles in therapeutic ultrasound since the field began. Bubble activity has been seen as both a harmful side effect and the means to the therapeutic end. Throughout this range, the monitoring of bubble activity has been an important goal for numerous reasons including safety considerations, progression of treatment, and determining therapeutic efficacy. Methods of cavitation monitoring have been summarized in various contexts including the ASA effort led by Wesley Nyborg, PhD that generated the American National Standards Institute (ANSI) S1.24 TR-2002 (R2007) entitled ANSI Technical Report—Bubble Detection and Cavitation Monitoring. More recently a six-part virtual series, The Detecting, Mapping, and Quantifying Bubble Activity in Therapeutic Ultrasound workshop (https://www.fusfoundation.org/posts/bubble-activity-in-therapeutic-ultrasound-workshop-series/) was held in 2021, sponsored by The Focused Ultrasound Foundation, in partnership with the American Institute of Ultrasound in Medicine’s Future Fund. A summary was released as a white paper. This presentation will provide an introduction to the field of bubble and cavitation detection and monitoring and a summary of this most recent workshop and associated follow-on efforts.

Electric field and temperature analysis of high-voltage XLPE cable insulation bubbles based on COMSOL simulation

Compared with ordinary PVC cables, crosslinked polyethylene insulated cables have been used widely because of their high load capacity, strong overload capacity and long service life.However, in cable production process, small bubbles may appear in XLPE layer, and the bubble defects can cause distortion to the electric field distribution and temperature distribution of the cable.This article simulates electric field and temperature distribution characteristics of bubbles in different sizes and locations in 110kV XLPE cables by COMSOL.The simulation results show that when the bubble appears in the XLPE layer, the electric field intensity near the interface between the bubble and XLPE decreases sharply, and in comparison with the condition that there is no bubble, the electric field intensity is greater.Meanwhile, the closer bubble get to the outside of the XLPE layer, the smaller the maximum field intensity will be.Also,the internal temperature of the bubble in the XLPE layer shows a linear ting trend.

Analysis on the escape phenomenon of oil mist from turbine lower guide bearing based on VOF model

To analyze the cause and mechanism of oil mist escaping from the lower guide bearing during the operation of a turbine, the oil-gas mixture in the lower guide bearing was numerically simulated by using the VOF two-phase flow model and the SST turbulence model. The influences of different sealing clearances and speeds on the flow field and the oil-gas distribution in the oil tank were studied, and the escaping characteristics of oil mist were analyzed. The results show that increasing the clearance of the labyrinth seal will reduce the pressure difference between the inside and outside of the oil tank and reduce the driving force for the escape of the oil mist. However, increasing the clearance will increase the turbulence of the flow field, the number and volume of bubbles in the lubricating oil, which will lead to the uneven distribution of oil and gas and reduce the motion stability of the lower guide bearing. The change of speed will affect the normal use of the shaft-collar pump. High speed will aggravate the generation of bubbles in the oil tank and increase the possibility of cavitation in the lower guide bearing structure. In engineering practices, it is necessary to comprehensively consider factors such as oil mist escape phenomenon, economic benefits, process manufacturing and assembly, and adopt the lower guide bearing structure with a suitable labyrinth seal clearance.

The Relationship Between Flipped Learning And Pupils’ Performance: A Reflection On Alternative Philosophical Underpinnings

This research paper investigates the significance of the relationship between flipped learning and pupils’ academic performance in secondary schools in the Kurdistan Region of Iraq especially with a focus on alternative philosophical underpinnings. Undoubtedly, the significance of this study in the field of educational research bubbles over with controversies. Professional researchers/teachers’ disagreements about what happens in schools emanate from the difference in philosophical lenses employed in understanding phenomena, divergent visions regarding the purpose of schooling, and what constitutes an ideal society and cultural differences. This research attempts to understand teachers’ perception of reality in classrooms and fathoms the relationship between flipped learning and pupil’s academic performance, engagement and achievement. The research methodology employed has focused on mixed methods that allow the data analysis to adopt an interpretive approach and attempted to address the research questions by developing a structured observation and a questionnaire to facilitate the data collection procedure. The findings show that alternative philosophical underpinnings bear profound effects on pupils’ learning: teachers and the epistemologies and ontologies employed to understand reality in classrooms significantly impact flipped learning and pupils’ academic performance. The results suggest that these implications share common critical statements found in the related literature reviews that also indicate reflections on alternative philosophical underpinnings.

Nonlinear oscillation and acoustic scattering of bubbles

The scattered acoustic pressure and scattered cross section of bubbles is studied using the scattered theory of bubbles. The nonlinear oscillations of bubbles and the scattering acoustic fields of a spherical bubble cluster are numerically simulated based on the bubble dynamic and fluid dynamic. The influences of the interaction between bubbles on scattering acoustic field of bubbles are researched. The results of numerical simulation show that the oscillation phases of bubbles are delayed to a certain extent at different positions in the bubble cluster, but the radii of bubbles during oscillation do not differ too much at different positions. Furthermore, directivity of the acoustic scattering of bubbles is obvious. The scattered acoustic pressures of bubbles are different at the different positions inside and outside of the bubble cluster. The scattering acoustic fields of a spherical bubble cluster depend on the driving pressure amplitude, driving frequency, the equilibrium radii of bubbles, bubble number and the radius of the spherical bubble cluster. These theoretical predictions provide a further understanding of physics behind ultrasonic technique and should be useful for guiding ultrasonic application.

Three-Dimensional Reconstruction of Dilute Bubbly Flow Field With Light-Field Images Based on Deep Learning Method

The three-dimensional reconstruction of bubble flow field is of great significance to study the motion of gas-liquid two phase flow. Combined with the abundant information of bubbles in the light field images, a fast and accurate 3D reconstruction method for dilute bubbly flow based on the deep learning algorithm DIF-LeNet (Double Information Fusion with LeNet5 net) is proposed in this work. The calibration method and bubble segmentation algorithm of light field image for data processing is detailed. DIF-LeNet realizes the fusion of different dimension data and regression prediction. By DIF-LeNet, bubble depth could be predicted by fusing the refocused bubble image with the focal distance of the refocused image. Then, the three-dimensional bubble field could be reconstructed with the bubble depth, bubble center and bubble diameter. Comparing with the reconstruction method based on statistics and LeNet5, the reconstruction accuracy and speed of the proposed method based on DIF-LeNet are improved. Especially for the bubble without focused image in the refocused image sequence, the effect of 3D reconstruction based on DIF-LeNet is much prominent.

Effect of Strain Rate on the Deformation Characteristic of AlN Ceramics under Scratching.

To clarify the influence mechanism of strain rate effect on deformation characteristics of aluminum nitride (AlN) ceramics, some varied-velocity nanoscratching tests were carried out using a Berkovich indenter in this paper. The deformation characteristics of the scratched grooves were observed using the scanning electron microscope. The experimental results showed higher scratch speed would lead to shallower penetration depth, fewer cracks, and indenter fewer slipping, which was more conducive to the plastic deformation of AlN ceramics. Considering the strain rate effect and the elastic recovery of material, a model for predicting the Berkovich indenter penetration depth under edge-forward mode was established. The prediction results were consistent with the experimental data, and the error was less than 5%, indicating that the model is effective. Based on the Boussinesq field, the Cerruti field, and the Sliding bubble field, a strain rate dependent scratch stress field model was established. The stress field revealed higher scratch speed may significantly reduce the maximum principal stress in the stress field under the indenter, which is the fundamental reason for reducing the crack damage and promoting the plastic deformation. The above study can provide theoretical guidance for reducing the processing damage of AlN ceramics.

Bubble dynamic evolution, material strengthening and chemical effect induced by laser cavitation peening

The mechanism of laser cavitation peening (LCP) including laser shock wave, bubble collapse shock wave, and water-jet was investigated at various stand-off distances (γ) combined with experiment and simulation. The dynamic characteristics, pressure field, and temperature field of cavitation bubble were investigated. The Q235 steel was impacted by LCP and the strengthening mechanism was analyzed, and the chemical effect in LCP was discussed. The results found that the pressure intensity of shock wave and water-jet decreases with increasing the γ. At γ=0, the laser shock wave, bubble collapse shock wave, and water-jet are 989 Mpa, 763 Mpa, and 369 Mpa respectively. The pressure and temperature of the bubble decrease obviously in the second and third pulsations. The impact of LCP causes plastic deformation on the Q235 steel surface and refines the grains on the surface layer within a depth of 20–30 μm. The enhancement of microhardness and the residual stress increases with the increase of γ, and the optimal value for LCPwc is 0.4. The degradation rate of MB solution in the infinite domain, LCPwc, and LCP is 26.4%, 41.7%, and 34.5%.

The application of flow to an ultrasonic horn system: Phenol degradation and sonoluminescence

Sonochemical (SC) processes can be increased with the application of fluid flow due to changes in bubble characteristics. In this work, a novel flow through set-up was applied to an ultrasonic horn system to investigate the effects of flow on the degradation of phenol. KI dosimetry and sonochemiluminescence (SCL) were also analysed, under the same conditions, to provide comparison of degradation to other SC processes. Further, sonoluminescence (SL) in water and different concentrations of potassium iodide (KI) and phenol solutions was studied to determine the effect of flow on processes inside the bubble that result in SL. The degradation of 0.1 mM phenol solutions, KI dosimetry and SL from phenol (0.1, 20 and 60 mM) and KI (0.1, 1 and 2 M) solutions were analysed under flow rates of 0, 24, 228 and 626 mL/min. For an ultrasonic horn system, all flow rates could augment phenol degradation beyond that of the systems without flow. At the lowest applied power, the amount of degradation was significantly increased with flow, becoming greater than degradation observed at the highest power. A strong correlation between phenol degradation and SC processes indicated that degradation followed an oxidative process. SL intensity from water, KI, and phenol solutions could also be increased with flow beyond the no flow system. For water this occurred most readily at higher powers, then for the solutes there was varied behaviour dependent upon the solute concentration. It was theorised that flow may increase the transfer of radical species to solution to enhance SC processes. An increase in SL, with flow, indicates that flow is acting to change the properties of the bubbles and/or the bubble field such that the active bubbles present collapse with greater total intensity.

Primordial black hole formation by vacuum bubbles. Part II

The discoveries of LIGO/Virgo black holes in recent years have revitalized the study of primordial black holes. In this work we investigate a mechanism where primordial black holes are formed by vacuum bubbles that randomly nucleate during inflation through quantum tunneling. After inflation, these bubbles typically run into the ambient radiation fluid with a large Lorentz factor. In our previous work, we assumed the bubble fields are strongly coupled to the standard model particles so that the bubble wall is impermeable. Here we complete this picture by considering bubbles interacting with the fluid only through gravity. By studying the scenario in several limits, we found that black holes could form in either subcritical or supercritical regime. Depending on the model parameters, the resulting mass spectrum of the black holes could be wide or narrow, and may develop two peaks separated by a large mass range. With different spectra, these black holes may account for the LIGO/Virgo black holes, supermassive black holes, and may play an important role in dark matter.

Experimental investigation of overlapping bubbles based on laser scanning

Image analysis technique is a common method to measure the void fraction and bubble size distribution in the bubbly flow. However, the image recognition algorithm will become invalid finally when the the projection overlaps of bubble become more heavily. In this paper, a new approach on three-dimensional (3D) reconstruction and parameters measurement of overlapping bubbles is developed based on laser scanning. The measurement principle of laser scanning is represented firstly, and appropriate scanning parameters are analyzed for 3D reconstruction at certain temporal and spatial resolution. Several image-preprocessing procedures are performed to extract the features effectively from the original slices of bubbly flow. A compensation method for slices offset is investigated to remove the unaligned feature noise of bubbles. An optimization algorithm based on image convolution of second-order differential averaging operator is proposed to remove the image noise caused by multiple exposures. In experiment, the 3D structure of bubbly flow field is reconstructed, and the void fraction and bubble size are measured with the analysis of distribution. The experimental results show that the method is valid for non-intrusive measurement and reconstruction of bubbly flow without the limitation of projection overlap. The optimization algorithms can effectively reduce the reconstruction deformation and improve the reliability, and the measurement error of void fraction is less than 13%, which indicates the relatively high characterization precision for the bubbly flow.

Large‐Eddy Simulation Model for the Effect of Gas Bubble Dissolution on the Dynamics of Hydrocarbon Plume from Deep‐Water Blowout

AbstractDuring a deep‐water wellhead blowout incident, the dynamics of the released hydrocarbon plume is strongly affected by the gas dissolution process that weakens the bubble‐induced buoyancy for driving the plume. In this study, a new modeling strategy is developed to efficiently incorporate the gas dissolution effect into a fast Eulerian‐Eulerian large‐eddy simulation (LES) model. By simultaneously simulating the evolutions of the bubble mass concentration and number density functions, the average bubble size in each LES computational cell can be calculated locally. Based on the cell‐averaged bubble diameter, the local gas dissolution rate and bubble rise velocity are parameterized, which are then used in the gas transport equations to model the evolution of the gas bubble field due to turbulent transport and gas dissolution. This LES model is applied to simulate several blowout scenarios with different initial bubble sizes. The results show that the plumes that have smaller initial bubble size exhibit faster relative bubble dissolution rate compared to the plumes with larger initial bubble size. As a result, the plumes with smaller bubbles also have lower peel and trap heights than those with larger bubbles. For comparison, a second set of LES runs with identical conditions but without including the gas dissolution effect are also performed. The data analysis shows that the omission of gas dissolution can cause overestimations of the plume peel and trap heights for up to 151% and 127%, respectively, suggesting the importance of including the gas dissolution effect for accurately modeling the near‐field hydrocarbon plumes.

Laser cavitation peening of gray cast iron: Effect of coverage layer on the surface integrity

Copper coverage layer was employed to the laser cavitation peening (LCP) of HT200 gray cast iron. The process of bubble evolution, velocity field of bubble, impact of water-jet, deformation and stress of material were simulated. The effects of coverage layer on the surface integrity (surface morphology and roughness, residual stress, micro-hardness) of LCP treated HT200 under various laser energies and defocusing amounts were systemically investigated. The results indicate that the maximum velocity of water-jet is 380 m/s and the diameter is 0.1 mm. The surface roughness of HT200 increases after the treatment of LCP. The presence of Copper coverage layer makes the HT200 surface more flat and the surface roughness decreases by half. At the case of no coverage layer, the parameters of 200 mJ laser energy and 1 mm defocusing amount are optimal for roughness, compressive residual stress and micro-hardness. Higher laser energy produces higher magnitude of micro-hardness and compressive residual stress in the case of coverage layer, and the optimal defocusing amount is 0 mm.