Bubbles In Acoustic Field Research Articles

Coupled oscillation and shape instability of bubbles in acoustic field

Based on the Lagrange's equation, the dynamic equations and shape mode equations of two bubbles with nonspherical distortion are obtained. The radial oscillations and shape instabilities of two bubbles with nonspherical distortion in an acoustic field are numerically investigated. The numerical results show that there are two coupled modes between two nonspherical bubbles: shape coupled mode and radial coupled mode. The coupled modes between two nonspherical bubbles depend on the shape modes of two bubbles. When the shape mode of the first bubble is equal to that of the second bubble (n=m), the shape coupled mode and radial coupled mode both exist. The interaction force between bubbles is caused by these two coupled modes. If the two bubbles have different shape mode orders (n m), there is a radially coupled mode between two bubbles. The interaction force between two bubbles is caused by radially coupled mode. The interaction caused by the radial coupling and shape coupling has an influence on the instability of gas bubble. The influencing factors depend on the shape mode, the equilibrium radius of neighboring bubble, and the driving acoustic field. The results demonstrate that the shape coupling can change the severity of the collapse of a gas bubble, and increase the ability of a gas bubble to resist distortion under a certain condition. The nonspherical disturbance of a real bubble in an acoustic field is not a single shape mode, but the coupling of different shape modes, so the shape coupling has an obvious influence on the shape instability of a real bubble. These may be the reason why bubbles can form some stable structures and keep stable oscillations in an acoustic field.

Stability mechanisms of oscillating vapor bubbles in acoustic fields

Vapor bubble instability could enhance the sonochemical activities and accelerate the reaction rate. In the present paper, vapor bubble instability in acoustic fields is investigated through combining both the spherical and stiffness stabilities within a wide range of parameter zone (consisting of bubble radius, acoustic frequency and pressure amplitude) in order to determine the stability states of vapor bubbles. The status of bubble oscillations are divided into four zones in terms of their stability characteristics. Influences of several paramount parameters on the bubble stability are demonstrated in detail. Different orders of spherical instability are quantitatively given together with cases in high-frequency and low-frequency limits. The practical applications of the present work are twofold: identification of the parameter zones with rapid sonochemical reactions; validity of the spherical bubble assumption for simplification of the numerical studies.

Dynamics of cavitation bubbles in acoustic field near the rigid wall

Dynamics of multiple cavitation bubbles near the rigid wall subjected to the incident acoustic wave with different arrangements is researched in this paper. Wave equation is used to approximate the prophase and anaphase motion of cavitation bubble to consider the compressibility of fluid. The modified Bernoulli equation is deduced as the boundary condition for cavitation bubble in acoustic field. Influences of compressibility, acoustic wave frequency and amplitude, incident angle, and the bubble arrangements on the dynamics of cavitation bubbles are presented. Numerical results show that the compressibility of fluid will impact the intensity of motion. The dynamics of bubbles is affected by the incident amplitude and frequency, and limited by the rigid wall. The different incident angles and arrangements of bubbles will change the acting pressure phase and interaction between bubbles and rigid wall, which will lead to the motion advancement or delay. The radius difference will increase with the large phase difference and the strong rejection. The jet will be oblique to the incident direction, as well as the rigid wall and bubbles around.

Direct numerical simulation of three-dimensional dynamics of compressible bubbles in acoustic field using boundary element method

In the present work the dynamics of bubbles containing compressible gas is studied in the presence of an acoustic field at low Reynolds numbers. The numerical approach is based on the boundary element method (BEM), which is effective for three-dimensional simulation. The application of the standard BEM to the compressible bubble dynamics faces the problem of the degeneracy of the algebraic system. To solve this problem, additional relationships based on the Lorentz reciprocity principle are used. Test calculations of the dynamics of one and several bubbles in an acoustic field are presented.

Waves of acoustically induced transparency in bubbly liquids: Theory and experiment

The theory of self-organization of bubbles in acoustic fields predicts formation and propagation of waves of self-induced acoustic transparency. This is a strongly nonlinear effect, which is a result of a two-way coupling of the sound field with the bubble distribution. We are challenging the theory with an experiment. Here, a homogeneous distribution of gas bubbles is first generated and then an ultrasonic field is switched on. The pressure waves are below the cavitation threshold and in a frequency range from 50 kHz to 200 kHz, mostly above the linear resonance frequency of the bubbles. The ultrasound leads to a rapidly propagating bubble wave away from the transducer. The dynamics is observed with a high-speed camera and analyzed. Interestingly, this transparent region is propagating through the bubbly liquid at velocities substantially higher than the bubble rise velocity due to the gravity. A simplified theoretical model of this acoustically induced transparency is developed. Both, analytical and numerical solutions are obtained. A comparison of the experimental data with the model is presented and the underlying physics of the problem is discussed.

Simulations of self-organization of bubbles in acoustic fields in three dimensions

Self-organization of bubbles (structure formation) and related self-action of the acoustic field is a strongly non-linear phenomenon, which is observed, e.g., in acoustic cavitation and sonochemical reactors. The phenomenon is a manifestation of a two-way field-particle interaction, when the bubbles change their position and sizes due to the Bjerknes and other forces and rectified diffusion, and affect the acoustic properties of the medium, which leads to restructuring of the acoustic field. The models of this phenomenon available in literature are revised, and a model based on the spatio-temporal averaging, which includes the above mentioned effects, is derived. Based on this model, a 3-D pseudospectral code for simulation of bubble self-organization is developed and tested. Simulations show interesting spatio-temporal behavior of the bubbles and the acoustic field, which is initialized as a standing wave but may drastically change its structure due to bubble dynamics (especially near the resonance). Bubble pattern formation is sensitive to many parameters including parameters of the acoustic field, bubble initial size, number density, spatial distribution, and ambient conditions. The effects found in the result of simulations are discussed. [This research is supported by the Grant of the Ministry of Education and Science of the Russian Federation (G34.31.0040).]

Viscous effects on the interaction force between two small gas bubbles in a weak acoustic field.

The relative motion of two gas bubbles in an acoustic field is investigated by calculating the time-averaged interaction force known as the secondary Bjerknes force. The surrounding medium is assumed to be an incompressible viscous liquid and the separation distance between the bubbles much larger than their radii. A refined formula for the interaction force is derived, which allows for translational oscillations of the bubbles, the vorticity of the linear scattered field, and acoustic streaming. The boundary condition of slippage on the gas-liquid interface is assumed. It is shown that viscous effects can cause small bubbles, driven well below resonance, to repel each other within a relatively wide parameter range. This result discloses a significant boundedness of the classical Bjerknes theory according to which bubbles of this sort are capable of mutual attraction alone.

Plural Sonoluminescing Bubbles in Acoustic Fields of Higher Mode

Plural Sonoluminescing Bubbles in Acoustic Fields of Higher Mode

Nonlinear oscillations of non-spherical cavitation bubbles in acoustic fields

The nonlinear stability of gas bubbles in acoustic fields is studied using a multiple-scale type of expansion. In particular the development of a subharmonic or a synchronous perturbation to the flow is investigated. It is shown when an equilibrium non-spherical shape oscillation of a bubble is stable. If the amplitude of the sound field is ε then it is shown that subharmonic perturbations of order ε½ can exist and be stable. Furthermore synchronous perturbations of order ε can exist and be stable. It is shown that synchronous perturbations, unlike the subharmonic case where the bifurcation is symmetric, bifurcate transcritically when the driving frequency is varied and also undergo secondary bifurcations. It is further shown that, in certain cases, the latter properties of the synchronous modes cause the flow to exhibit a hysteresis phenomenon when the driving frequency is varied.