Bubble Compression Research Articles

Simulation of air gun bubble motion in the presence of air gun body based on the finite volume method

Air guns are widely used as artificial seismic sources in the exploration of seabed resources. The pressure wave emitted from an air gun reflects from different seabed media, and the reflected wave carries basic information about the seabed structure, which is useful for determining the location and burial depth of resources. This paper presents a compressible air gun bubble model implemented in OpenFOAM and considers the influence of the gun body structure. The bubble pulsation of air guns under different boundary conditions is investigated using the finite volume method (FVM) combined with the volume of fluid method (VOF). We analyze how the opening height, opening position, and air gun length affect fluid pressure. The results show that, when the opening position is below the middle of the air gun, the primary pulse decreases due to a decrease in the mass-transfer rate, and the weakened boundary effect reduces the period and bubble pulse of the pressure wave. Increasing the opening height of the air gun increases the main pulse and bubble pulse, which facilitates the gas motion from the chamber of the air gun to the bubble. When the maximum radius Rm of the bubble is shorter than the length L of the gun body, the gun body has negligible influence on the pressure wave, but in the opposite case, the breakup of the bubble consumes part of the energy, which slightly decreases the bubble pulse.

Cavitation bubble dynamics and sonochemiluminescence activity inside sonicated submerged flow tubes

Bubble dynamics and luminol emissions of cavitation in sub-millimeter-sized perfluoroalkoxy alkane (PFA) flow tubes, submerged in an ultrasonic bath reactor, are studied at 27.2 kHz driving frequency. Nucleation of cavitation inside the tubes only takes place via a free interface, realized here in form of an alternating water-air slug flow. High-speed recordings show that cavitation bubbles in the water slugs often develop localized structures in form of clusters or bubble “plugs”, and that such structures can be seeded via a single pinch-off from the free interface. Within the structures, bubbles strongly interact and frequently undergo merging or splitting events. Due to the mutual interaction and resulting motion, bubbles often collapse with a fast displacement, suggesting jetting dynamics. Bubble compression ratios are estimated on basis of observed individual bubble dynamics and numerical fitting by a single bubble model that takes the tube wall into account. The resulting peak temperatures around 3500 K allow for dissociation of water vapor. This is in accordance with observed sonochemiluminescence from luminol, originating from active bubble zones in the tubes.

Effect of the spatial variation of permeability on air bubble creation and compression

Tow bundles inside a quasi-unidirectional non-crimp fabric are maintained by sewing threads that induce variation in the bundles’ shape. Indeed, the sewing threads apply a light clamping force that gives a periodical and sinusoidal shape to the tow’s cross-sectional area. This tow cross-sectional area heterogeneity, as a function of position in the fabric, induces a variation of the permeability values. Consequently, while injecting liquid into the fibrous bed, preform's impregnation is influenced as well as the fabric’s void content. The aim of this paper is to consider the effect of tow cross-sectional area heterogeneity on the quality of the manufactured composite part. It leads to reveal the influence of the fibrous reinforcement’s microstructure on the air bubble creation and compression phenomena, especially in terms of process time and the micro and macro air bubble distribution.

Oxidative Deterioration Effect of Cavitation Heat Generation on Hydraulic Oil

The thermal effects of cavitation on hydraulic oil is very important. From a micro perspective, the evolution of bubbles, will release amount of energy, especially its collapse in high-pressure area. The energy is converted into heat, which will be absorbed by hydraulic oil, which is indirectly converted into vibration and noise, and other forms. In order to study the effect of cavitation on hydraulic oil from an energy perspective, we investigated the oxidative deterioration of surrounding hydraulic oil during the compression of cavitation bubbles by using theoretical analysis and experimental comparison methods. Firstly, the cause of cavitation heat generation in hydraulic system was analyzed. Secondly, two kinds of hydraulic oils were tested for corrosion under the environment of cavitation heat generation, and the chemical properties changes of these hydraulic oils in the simulated cavitation heat production environment were analyzed. Finally, the thermogravimetric analysis and comparison of the two kinds of hydraulic oils in a simulated cavitation heat-generating environment were performed, and obtained the reason why hydraulic oil was oxidized to form solid carbon particles. The results showed that cavitation heat generation will cause oxidative deterioration of hydraulic oil, which enhanced the corrosion of hydraulic oil, and subsequently generated solid particles in hydraulic oil. The inner surface of hydraulic components will be scratched when solid carbon particles move with hydraulic oil flow in hydraulic system.

‘Uncertainty’ principle in two fluid–mechanics

Hamilton’s principle (or principle of stationary action) is one of the basic modelling tools in finite-degree-of-freedom mechanics. It states that the reversible motion of mechanical systems is completely determined by the corresponding Lagrangian which is the difference between kinetic and potential energy of our system. The governing equations are the Euler-Lagrange equations for Hamil- ton’s action. Hamilton’s principle can be naturally extended to both one-velocity and multi-velocity continuum mechanics (infinite-degree-of-freedom systems). In particular, the motion of multi–velocity continuum is described by a coupled system of ‘Newton’s laws’ (Euler-Lagrange equations) for each component. The introduction of dissipative terms compatible with the second law of thermodynamics and a natural restriction on the behaviour of potential energy (convexity) allows us to derive physically reasonable and mathematically well posed governing equations. I will consider a simplest example of two-velocity fluids where one of the phases is incompressible (for example, flow of dusty air, or flow of compressible bubbles in an incompressible fluid). A very surprising fact is that one can obtain different governing equations from the same Lagrangian. Different types of the governing equations are due to the choice of independent variables and the corresponding virtual motions. Even if the total momentum and total energy equations are the same, the equations for individual components differ from each other by the presence or absence of gyroscopic forces (also called ‘lift’ forces). These forces have no influence on the hyperbolicity of the governing equations, but can drastically change the distribution of density and velocity of components. To the best of my knowledge, such an uncertainty in obtaining the governing equations of multi- phase flows has never been the subject of discussion in a ‘multi-fluid’ community.

Investigation of cavitation and air entrainment during pilot injection in real-size multi-hole diesel nozzles

This paper investigates the complex multiphase flow developing inside the micro-orifices of diesel injector nozzles during pilot injection. High speed micro-visualisations of a transparent serial production nozzle tip replica are used to record the multiphase flow inside the flow orifices as well as near-nozzle spray development. The physical processes taking place are explained with the aid of a three-phase (liquid, vapour and air) homogeneous mixture model utilised in the context of Large Eddy Simulations. Phase-change due to cavitation is considered with a model based on the Rayleigh-Plesset equation, while compressibility of all the phases is accounted for. Numerical simulations shed light on the interaction between the vortex flow and cavitation formation that take place simultaneously with air entrainment from the surrounding environment into the injector’s sac volume during the injection and the dwell time between successive injections. The experimentally observed flow phenomena are well captured by the simulation model. In particular the compression of pre-existing air bubbles inside the injector’s sac volume during the injector opening, cavitation vapour condensation and air suction after the needle closure are well reproduced.

The effect of bubbles on primary breakup of diesel spray

The spray atomization and related nozzle internal flow dictate the subsequent combustion of air/fuel mixture and emissions. The dynamics of internal flow and spray breakup under various injection pressures and initial conditions were studied by employing the combination of high-speed microscope imaging technique and CFD numerical simulation with VOF and RNG K-ε models. Four main types of spray fragmentation were observed and classified. The energy stored in the compressed bubbles was released to accelerate and atomize the fuel film in the front, which is more effective under higher injection pressure. Besides, it is surprising to discover the ‘C’ shaped fuel in the nozzle which divided the bubble into three parts rather than puncturing it directly. The probability of this ‘C’ shaped fuel decreased with increasing injection pressure due to the strong pressure shock. This study is believed to be essential for further revealing the spray breakup mechanisms.

Quantum shell effects in the hydrodynamic equations of a quantum fluid

Numerical analysis of the hydrostatic equilibrium of a compressed gas bubble was carried out by the DFT method. Based on this analysis, we modified the hydrodynamic equations of a quantum fluid taking into account quantum shell effects. An external force acting on the electronic subsystem is added to the modified hydrodynamic equations. A numerical algorithm for solving of the system of modified hydrodynamic equations is discussed.

Energy dissipation through entrained air compression in plunging jets

ABSTRACT The energy dissipation in plunge pools is often related to air entrainment in the plunging jet. Recent experimental work showed that the void fractions below the impinging jet are significantly smaller than outside the jet impact. This was attributed to the compression of the entrained air bubbles. In this paper, the thermodynamics of air bubble compression is introduced as a novel energy dissipation mechanism in civil engineering hydraulics. For the analysis of reported experimental data, isothermal conditions were applied as a lower bound approximation. It was found that the energy dissipation rates range from 2.7–9.7% for a jet velocity of 10 m s−2 and void fractions of 0.11 and 0.31, to 15.9–36.0% for a jet velocity of 24.5 m s−1 and void fractions of 0.35–0.55. The results may explain the difference between energy dissipation rates in plunge pools at model and full scale.

Numerical analysis of the interaction of 3D compressible bubble clusters

Based on the bubble dynamic theory and the compressible two-phase flow solver of the open source software OpenFOAM, a numerical simulation study is carried out on the interactions of bubble clusters in a closed volume. The bubble dynamics and interactions of a single bubble, two bubbles, and four bubbles are investigated under the working conditions without and with the presence of a free surface. Through a parametric study, the qualitative patterns of the variations of the bubble collapse period, the volume compressibility, the bubble pressure peak value, and the breakdown, fusion, and separation phenomena with the parameters such as the bubble pressure, the radius size, the bubble spacing, and the distance from the free surface are obtained. The main factors affecting the bubble morphology and the dynamic characteristics are summarized from numerous parameter experiments. It is shown that, in the absence of a free surface, the main factors are the relative size of the bubbles, the pressure of the liquid, and the pressure differences among the bubbles, while in the presence of a free surface, the main factor is the pressure of the liquid between the upper surface of the bubble and the free surface.

Quantum shell effects in compressed mesoscopic system

This article demonstrates the nontrivial manifestation of quantum shell effects in a compressed mesoscopic system. It is shown that there are two spatial scales in the distribution of degenerate electrons in a spherical well. The first scale is the Fermi length ∼h/pF. By the quantum shell effect, the authors mean the existence of the new spatial scale, which is the order of the system size and much larger than the first scale. The theoretical analysis for the large amount of free electrons (N ≲ 109) in an infinite spherical well demonstrates what causes the appearance of the spatial nonuniformity and gives the analytical expression for the electron distribution function. These results are confirmed by a numerical summation of exact solutions for the electron wave functions in an infinite potential well. It is shown that an analogous effect for the spatial distribution of electrons exists in a compressed hydrogen gas bubble of submicron size (<0.1 μm). The numerical simulation of the electron distribution was carried out by the Density Functional Theory method. The consequence of this effect is the nontrivial dynamics of the compressible cold gas bubble. This system can be realized in the thermonuclear experiments. The limiting factors of the analyzed effect are considered: symmetry of the system, electron temperature, and curvature of the system boundary.

Effects of solid loading on pore structure and properties of porous FeAl intermetallics by gel casting

Solid loading as a key parameter in gel casting influence both rheological behavior and stability of slurry. It also has strong impacts on microstructure and final properties of porous FeAl intermetallics. In this study, a designed non-aqueous gel casting has been developed for fabrication of porous FeAl intermetallics. The SEM analysis, bubble point method and compression test were used to investigate pore structure and properties of porous FeAl intermetallics. Results reveal that higher solid loading will improve the uniformity of pore structure and lead a decrease in maximum pore size. Gas permeability decreases but compressive strength increases with increasing solid loading. The quantitative relationship between maximum pore size (dm) and solid loading (φ) is dm = 10.52 − 0.15φ (35 ≤ φ ≤ 50); the relationship between gas permeability (K) and solid loading (φ) is K = 13.99 − 0.22φ (35 ≤ φ ≤ 50). Further, the distinctive phase transformation and pore formation mechanism are the main reasons for tiny volume expansion and highly open porosity of final products, respectively. Altogether, this work provides a strategy to fabricate porous FeAl intermetallics with adjustable pore structure and properties by controlling the solid loading, suggesting that the non-aqueous gel casting is a promising candidate for the preparation of porous FeAl intermetallics.

Measurements and Modeling of Phase Behavior, Density, and Viscosity of Oil + Carbon Dioxide: Squalane + CO2 at Temperatures (313 to 363) K and Pressures up to 77 MPa

New measurements of compressed liquid density, viscosity, and bubble pressures of mixtures of carbon dioxide and squalane are reported for CO2 mole fractions x = (0.199, 0.299, and 0.519) at temper...

Dynamics of the Localized Pulse in Bubbly Liquid

In nature, the movements of multiphase systems with physico-chemical transformations are widespread. The features of such flows are determined by the interaction of hydrodynamic, thermophysical, and also physicochemical phenomena. The most pronounced multiphase effects are manifested in the propagation of pressure waves in such media. In a liquid with bubbles, the rheological properties of a weakly compressible fluid, which is the carrier phase, change drastically with the addition of gas (in the form of bubbles), which is a dispersed phase. The peculiarity of bubble liquids is due to their high static compressibility while maintaining a high mass density close to the density of the liquid, which in turn leads to a low equilibrium speed of sound. An interesting feature of bubble liquid in dynamic processes is the manifestation of fluid inertia when the volume of a mixture changes due to compression or expansion of bubbles. Anomalously strong compressibility and dissipation of wave energy makes it possible to use bubble curtains of liquid to extinguish shock waves in liquids. To date, one-dimensional waves in a bubble liquid have been studied in sufficient detail, the dynamics of two-dimensional and detonation waves in bubble liquids is being actively studied at the moment. This paper presents the results of studies of the dynamics of a pressure pulse localized in a transverse coordinate in a bubble liquid.

Simulation of a gas bubble compression in water near a wall using the SPH-ALE method

This paper is motivated by the cavitation phenomenon, which occurs when bubbles collapse near a hydraulic machine surface. The bubble compression close to the wall has been addressed as the fundamental mechanism producing cavitation damage, whose general behavior is characterized by the emission of pressure waves and the formation of a micro jet. In order to simulate the collapse of a gas bubble in water, it is proposed a multiphase and compressible model developed in SPH-ALE. This model does not diffuse the interface and guarantees the continuity of normal velocity and pressure at the interface between both fluids, allowing it to deal with interfaces of simple contact. The model solves the mass, momentum and energy conservation equations of Euler system using an equation of state for each phase without phase change. The compressible model was validated through monodimensional configurations, such as shock tube test cases for monophase and multiphase flows. Bubble collapse simulations in 2D are presented highlighting the principle features, i.e. pressure waves and micro jets. Also, it is analyzed the effect of the initial distance between the bubble and the wall (H0). Limitations and perspectives of SPH-ALE method on this particular subject are also discussed.

Phase-resolving simulation of dense bubble clusters under periodic shear

The paper studies the response of a cluster of bubbles to osmotic pressure, and steady and oscillatory shearing by detailed numerical simulations. In contrast to other investigations, the movement of the interstitial fluid is fully resolved. To that end, an immersed-boundary method is employed, yielding the trajectory of each bubble and the flow and pressure field of the fluid. Additionally, a physically motivated collision model ensures realistic bubble interactions. Furthermore, a suitable numerical configuration is proposed which allows imposing osmotic pressure and shear in a way that integrates well into the simulation without generating artefacts. This method allows for the realistic investigation of the compression of bubbles across the jamming limit, demonstrating the influence of the inertia of the interstitial fluid. Applying oscillatory shearing with varying osmotic pressure, shear stress and frequency, the occurrence of shear bands is demonstrated and the influence on rheometric measurements is discussed.

Visual experimental investigations of string cavitation and residual bubbles in the diesel nozzle and effects on initial spray structures

In this article, optical experiments on string cavitation and residual bubbles inside the real-size transparent tapered diesel nozzle and near-nozzle spray structures were performed based on a high-pressure common rail fuel injection system with a high-speed camera. The tapered nozzle which has high flow efficiency with weak or even no geometric cavitation has been widely used in commercial injectors, while there still exists string cavitation which may also influence the in-nozzle flow and subsequent spray. This article put focus on the tapered nozzle and the result indicated that the in-nozzle string cavitation provided a reasonable explanation for the two bumps of spray cone angles during the opening and closing stages of needle of real diesel engine injection processes. The suction and compression of air bubbles at the start and end stages of injection processes, and the various shot-to-shot near-nozzle spray patterns were captured and analyzed. These different near-nozzle spray patterns were attributed to the distribution of residual bubbles inside the nozzle orifice. The residual bubbles were survived from the last injection or sucked into the nozzle during needle opening stages. Stagnant bubbles were compressed and then accelerated the residual fuel which was close to the injector tip, leading to the formation of mushrooms. This study confirmed that the initial mushroom and the tail were generated by the interactions between the residual/sucked bubbles and the residual/initial fuel, and the leading mushroom was incurred by the combination of the transverse expansion of the jet and the laminar layer theory. This work pointed out and analyzed the new sources of the cycle-to-cycle variation of air/fuel mixture and spray.

Strong compression of bubbles in water, acetone and benzol

Strong compression of the medium in cavitation bubbles in water, acetone and benzol realizing in the final stage of their collapse in the initially motionless liquid at a temperature of 293 K is considered. At the beginning of the collapse, the radius of the bubbles is 500 μm, the vapor in the bubbles is in its saturation state at the temperature of the surrounding liquid. The pressure of water and benzol is varied in the range 1 ≤ p0 ≤ 100 bar to determine its values at which the degree of compression of the medium inside the cavitation bubbles in these liquids is close to that achieved inside the cavitation bubble in acetone at the pressure p0 = 15 bar. It is shown that in the case of the bubble in benzol such a degree of the vapor compression can be achieved at p0 = 4 bar, whereas in the case of the bubble in water, even p0 = 100 bar is not enough.

Numerical simulation of bubble dynamics in central region of streamer

A mathematical model and a numerical technique for studying strong expansion and collapse of cavitation bubbles located in the central region of a streamer where the bubbles are almost motionless are developed. They are essentially efficient combinations of the models and techniques previously created by the authors for calculating the dynamics of interacting weakly-non-spherical bubbles in a streamer and the dynamics of a single axisymmetric bubble. The first model and technique are applied at the low-speed stage of expansion and compression of bubbles where their hydrodynamic interaction is significant. The second ones are used at the final high-speed stage of their collapse where the interaction is inessential. The simplest case of the streamer comprising three bubbles is considered as an example to illustrate the features of the developed model and numerical technique. It is shown that under the strong expansion and collapse of an initially spherical cavitation bubble, the presence of neighboring bubbles can substantially deflect the bubble cavity vapor dynamics from what is realized inside a similar but single bubble.

Cavitation bubbles dynamics and cavitation erosion in water jet

Abstract A water jet test rig is used to the cavitation test of the brass samples. The single crater and the pits-group, are found on the specimen surfaces after 15 h test. A water jet cavitation model with the sound interaction among bubbles, is developed by applying a water pressure of water jet flow fields into the Keller-Miksis equation, so as to obtain the dynamic characteristics of the bubbles. The theorical results reveal that, the volume concentration of the bubbles has an slight influence in the excursion time of bubble in water jet test; while a higher water jet velocity or a higher pressure difference between nozzle entrance and exit can lengthen the bubble expansion and compression time. The results also show that, the cavitation erosion pits can form on the specimen surface at stage II, while cannot form at stage I and stage III, which are well verified by the present theoretical model as well as the available experiment results.