超声场中气泡融合的影响因素分析

采用高速摄影机对不同声压和频率作用下接触气泡的融合时间进行了实验研究,运用实验和数值计算相结合的方法分析了次Bjerknes力和最大径向振动速度对气泡融合时间的影响,结果表明在不同的超声声压和频率作用下,气泡的融合时间会随着次Bjerknes力和最大径向振动速度的增大而增大,对两个因素进行对比分析得到次Bjerknes力是影响气泡融合的关键因素。     

Dynamics of a bubble cluster in an acoustic field

A mathematical model describing the dynamics of clustered gas bubbles under the effect of an acoustic field is presented. The proposed model is used as the basis for an analytical study of small bubble oscillations in monodisperse and polydisperse clusters and for a numerical study of nonlinear bubble oscillations under high-amplitude external pressures. The following effects are found to occur in a polydisperse cluster: a synchronization of the collapse phases of bubbles with different radii and a collapse intensification for bubbles of one size in the presence of bubbles of another size. These effects are explained by the interaction between bubbles of different radii in the cluster.     

Study on the bubble transport mechanism in an acoustic standing wave field

The use of bubbles in applications such as surface chemistry, drug delivery, and ultrasonic cleaning etc. has been enormously popular in the past two decades. It has been recognized that acoustically-driven bubbles can be used to disturb the flow field near a boundary in order to accelerate physical or chemical reactions on the surface. The interactions between bubbles and a surface have been studied experimentally and analytically. However, most of the investigations focused on violently oscillating bubbles (also known as cavitation bubble), less attention has been given to understand the interactions between moderately oscillating bubbles and a boundary. Moreover, cavitation bubbles were normally generated in situ by a high intensity laser beam, little experimental work has been carried out to study the translational trajectory of a moderately oscillating bubble in an acoustic field and subsequent interactions with the surface. This paper describes the design of an ultrasonic test cell and explores the mechanism of bubble manipulation within the test cell. The test cell consists of a transducer, a liquid medium and a glass backing plate. The acoustic field within the multi-layered stack was designed in such a way that it was effectively one dimensional. This was then successfully simulated by a one dimensional network model. The model can accurately predict the impedance of the test cell as well as the mode shape (distribution of particle velocity and stress/pressure field) within the whole assembly. The mode shape of the stack was designed so that bubbles can be pushed from their injection point onto a backing glass plate. Bubble radial oscillation was simulated by a modified Keller–Miksis equation and bubble translational motion was derived from an equation obtained by applying Newton’s second law to a bubble in a liquid medium. Results indicated that the bubble trajectory depends on the acoustic pressure amplitude and initial bubble size: an increase of pressure amplitude or a decrease of bubble size forces bubbles larger than their resonant size to arrive at the target plate at lower heights, while the trajectories of smaller bubbles are less influenced by these factors. The test cell is also suitable for testing the effects of drag force on the bubble motion and for studying the bubble behavior near a surface.     

声场中气泡运动的混沌特性

以水为工作介质,考虑了液体的可压缩性,研究了声场中气泡的运动特性,模拟了声波频率、声压幅值、气泡初始半径以及液体的表面张力和黏滞系数的变化对气泡运动状态的影响. 分析了空化处理效果与气泡运动状态之间关系. 结果表明:气泡运动处于混沌状态,是提高声空化降解有机污染物能力的最重要因素. 关键词： 声空化 混沌 相图 功率谱图     

声场与电场作用下空化泡的动力学特性

以液体为工作介质, 利用空化泡的RP控制方程, 模拟分析了无量纲化的电场频率、场强的幅值以及无量纲化的声波频率、 声压幅值的变化对空化泡运动特性的影响. 结果表明: 声场和电场联合作用时, 空化泡运动处于混沌区域范围远高于两者单独作用下空化泡的混沌区域范围. 这不仅对声空化的进一步研究具有重要的理论意义, 而且对于提高和改进空化降解有机污染物的技术也具有指导意义.     

Investigation on bubble dynamics characteristics and power spectral variation in acoustic field

Based on Keller-Miksis model,the influences of multiple control parameters,such as acoustic pressure amplitude,acoustic frequency and bubble radius at rest,on the complicated dynamics characteristics of nonlinear bubble oscillation driven by acoustic wave are discussed by utilizing a variety of numerical analysis methods,and the restrictive relationships among different parameters are analyzed.It is shown that chaotic state can occur only in the condition of all of the parameters in the suitable threshold,as the same time,chaotic state is the result of interaction of multiple control parameters.Furthermore,the power spectral expansion and energy conversion are existed in this nonlinear system.It is certified that the stronger acoustic pressure amplitude,the greater the sub-harmonic energy,besides,the energy attenuation of fundamental harmonic is also much greater.     

Theoretical estimation of sonochemical yield in bubble cluster in acoustic field

In order to learn more about the physical phenomena occurring in cloud cavitation, the nonlinear dynamics of a spherical cluster of cavitation bubbles and cavitation bubbles in cluster in an acoustic field excited by a square pressure wave are numerically investigated by considering viscosity, surface tension, and the weak compressibility of the liquid.The theoretical prediction of the yield of oxidants produced inside bubbles during the strong collapse stage of cavitation bubbles is also investigated. The effects of acoustic frequency, acoustic pressure amplitude, and the number of bubbles in cluster on bubble temperature and the quantity of oxidants produced inside bubbles are analyzed. The results show that the change of acoustic frequency, acoustic pressure amplitude, and the number of bubbles in cluster have an effect not only on temperature and the quantity of oxidants inside the bubble, but also on the degradation types of pollutants, which provides a guidance in improving the sonochemical degradation of organic pollutants.     

声驻波场中空化泡的动力学特性

以水为工作介质, 考虑了液体的可压缩性, 研究了驻波声场中空化泡的运动特性, 模拟了驻波场中各位置处空化泡的运动状态以及相关参数对各位置处空化泡在主Bjerknes力作用下运动方向的影响. 结果表明: 驻波声场中, 空化泡的运动状态分为三个区域, 即在声压波腹附近空化泡做稳态空化, 在偏离波腹处空化泡做瞬态空化, 在声压波节附近, 空化泡在主Bjerknes 力作用下, 一直向声压波节处移动, 显示不发生空化现象; 驻波场中声压幅值增加有利于空化的发生, 但声压幅值增加到一定上限时, 压力波腹区域将排斥空化泡, 并驱赶空化泡向压力波节移动, 不利于空化现象的发生; 当声频率小于初始空化泡的共振频率时, 声频率越高, 由于主Bjerknes 力的作用将有更多的空化泡向声压波节移动, 不利于空化的发生, 尤其是驻波场液面的高度不应是声波波长的1/4; 当声频率一定时, 空化泡初始半径越大越有利于空化现象的发生, 但当空化泡的初始半径超过声频率的共振半径时, 由于主Bjerknes力的作用将有更多的空化泡向声压波节移动, 不利于空化的发生.     

The effect of reflector geometry on the acoustic field and bubble dynamics produced by an electrohydraulic shock wave lithotripter

A theoretical model for the propagation of shock wave from an axisymmetric reflector was developed by modifying the initial conditions for the conventional solution of a nonlinear parabolic wave equation (i.e., the Khokhlov-Zabolotskaya-Kuznestsov equation). The ellipsoidal reflector of an HM-3 lithotripter is modeled equivalently as a self-focusing spherically distributed pressure source. The pressure wave form generated by the spark discharge of the HM-3 electrode was measured by a fiber optic probe hydrophone and used as source conditions in the numerical calculation. The simulated pressure wave forms, accounting for the effects of diffraction, nonlinearity, and thermoviscous absorption in wave propagation and focusing, were compared with the measured results and a reasonably good agreement was found. Furthermore, the primary characteristics in the pressure wave forms produced by different reflector geometries, such as that produced by a reflector insert, can also be predicted by this model. It is interesting to note that when the interpulse delay time calculated by linear geometric model is less than about 1.5 micros, two pulses from the reflector insert and the uncovered bottom of the original HM-3 reflector will merge together. Coupling the simulated pressure wave form with the Gilmore model was carried out to evaluate the effect of reflector geometry on resultant bubble dynamics in a lithotripter field. Altogether, the equivalent reflector model was found to provide a useful tool for the prediction of pressure wave form generated in a lithotripter field. This model may be used to guide the design optimization of reflector geometries for improving the performance and safety of clinical lithotripters.     

Gas bubble fragmentation in an ultrasonic field

A model system consisting of a thin layer of vacuum-deposited metallic aluminium on a glass microscope slide was developed to demonstrate the effectiveness of cavitational activity (occurring within the cooling water supply of a dental ultrasonic descaler operating at 25 kHz) in the removal of particulate matter from solid surfaces. The pattern of particulate matter removal using this model system demonstrated both the mechanism of bubble activity and the erosive nature of microbubbles.Non-resonant bubbles were formed by surface wave activity and adhered to the surface of the slide. There was some removal of the aluminium metal at the periphery of the bubble (probably by a microstreaming mechanism) giving a ‘ghost’ outline. The majority of aluminium removal was caused by numerous microbubbles of non-resonant sizes (typically 1 to 2 μm diameter) formed by surface wave induced fragmentation of the parent bubble.The damaging and erosive effects of transient cavitational activity appear to be the result of sub-resonant sized microbubble formation from larger parent bubbles.     

金属表面下氦泡融合与释放研究

采用分子动力学方法研究了钛金属表面下不同深度处氦泡的行为,分析了氦泡融合与释放的竞争,对比了不同深度处氦泡的释放对金属的影响.结果表明:在接近金属表面处,氦泡很难通过融合无限长大,当达到临界尺寸后,氦泡将会释放而不再与邻近的氦泡发生融合;植入深度对氦泡的融合有一定的影响,深度越大,越有利于形成具有较高氦密度的大氦泡;较深处氦泡的释放会在金属表面形成较大的突起和表面针孔.实验中观察到的不同尺寸的表面孔,其部分原因来自于金属表面下不同深度处氦泡的释放.     

金属表面下氦泡融合与释放研究

采用分子动力学方法研究了钛金属表面下不同深度处氦泡的行为,分析了氦泡融合与释放的竞争,对比了不同深度处氦泡的释放对金属的影响。结果表明：在接近金属表面处,氦泡很难通过融合无限长大,当达到临界尺寸后,氦泡将会释放而不再与邻近的氦泡发生融合;植入深度对氦泡的融合有一定的影响,深度越大,越有利于形成具有较高氦密度的大氦泡;较深处氦泡的释放会在金属表面形成较大的突起和表面针孔。实验中观察到的不同尺寸的表面孔,其部分原因来自于金属表面下不同深度处氦泡的释放。     

Interaction of a bubble and a bubble cluster in an ultrasonic field

Using an appropriate approximation, we have formulated the interacting equation of multi-bubble motion for a system of a single bubble and a spherical bubble cluster. The behavior of the bubbles is observed in coupled and uncoupled states. The oscillation of bubbles inside the cluster is in a coupled state. The numerical simulation demonstrates that the secondary Bjerknes force can be influenced by the number density, initial radius, distance, driving frequency, and amplitude of ultrasound. However, if a bubble approaches a bubble cluster of the same initial radii, coupled oscillation would be induced and a repulsive force is evoked, which may be the reason why the bubble cluster can exist steadily. With the increment of the number density of the bubble cluster, a secondary Bjerknes force acting on the bubbles inside the cluster decreases due to the strong suppression of the coupled bubbles. It is shown that there may be an optimal number density for a bubble cluster which can generate an optimal cavitation effect in liquid for a stable driving ultrasound.     

影响宽带幅度调制信号检测的因素分析

通过推导宽带解调处理的输出信噪比公式,系统地分析了影响幅度调制的宽带信号检测的各种因素。分析表明,宽带调制信号检测性能取决于以下因素:信号的周期调制的深度、前置处理器带通滤波器的带宽、调制噪声在总的噪声中的比重、采样速率,以及计算解调谱时的傅里叶变换的长度、降样倍数、时间窗、数据块重叠程度以及平均次数。     

Numerical simulation of the coalescence of two bubbles in an ultrasound field

The coalescence of two bubbles under ultrasound irradiation is numerically investigated. The results indicate that ultrasound may accelerate the coalescence process, depending on the initial phase. The time-averaged nonzero Bjerknes force promotes bubble coalescence by dragging bubbles to nodes or antinodes, depending on their size. At the beginning of the coalescence process, a film forms between the two bubbles. The film drainage time first increases then decreases as a function of initial distance. This study contributes to an understanding of the effects of ultrasound on bubble coalescence.     

Experimental investigation of conical bubble structure and acoustic flow structure in ultrasonic field

The objective of this paper is to investigate the transient conical bubble structure (CBS) and acoustic flow structure in ultrasonic field. In the experiment, the high-speed video and particle image velocimetry (PIV) techniques are used to measure the acoustic cavitation patterns, as well as the flow velocity and vorticity fields. Results are presented for a high power ultrasound with a frequency of 18 kHz, and the range of the input power is from 50 W to 250 W. The results of the experiment show the input power significantly affects the structures of CBS, with the increase of input power, the cavity region of CBS and the velocity of bubbles increase evidently. For the transient motion of bubbles on radiating surface, two different types could be classified, namely the formation, aggregation and coalescence of cavitation bubbles, and the aggregation, shrink, expansion and collapse of bubble cluster. Furthermore, the thickness of turbulent boundary layer near the sonotrode region is found to be much thicker, and the turbulent intensities are much higher for relatively higher input power. The vorticity distribution is prominently affected by the spatial position and input power.     

Collective bubble dynamics near a surface in a weak acoustic standing wave field

The transport of bubbles to a neighboring surface is very important in surface chemistry, bioengineering, and ultrasonic cleaning, etc. This paper proposes a multi-bubble transport method by using an acoustic standing wave field and establishes a model that explains the multi-bubble translation by expressing the balance between Bjerknes forces and hydrodynamic forces on a bubble in a liquid medium. Results indicated that the influence of primary Bjerknes force, secondary Bjerknes force, and buoyancy force on the bubble translation depends on the position of the target bubble in the acoustic field. Moreover, it was found that increasing the size of a bubble or pressure amplitude can accelerate the bubble motion and enhance the bubble-bubble interaction. The secondary Bjerknes force between two bubbles can switch from an attractive one when they oscillate in phase to a repulsive one when the bubble oscillations are out of phase. These findings provide an insight into the multi-bubble translation near a surface and can be applied to future bubble motion control studies, especially in drug delivery, sonoporation, and ultrasonic cleaning.     

Bubbles in an acoustic field: an overview

Acoustic cavitation is the fundamental process responsible for the initiation of most of the sonochemical reactions in liquids. Acoustic cavitation originates from the interaction between sound waves and bubbles. In an acoustic field, bubbles can undergo growth by rectified diffusion, bubble-bubble coalescence, bubble dissolution or bubble collapse leading to the generation of primary radicals and other secondary chemical reactions. Surface active solutes have been used in association with a number of experimental techniques in order to isolate and understand these activities. A strobe technique has been used for monitoring the growth of a single bubble by rectified diffusion. Multibubble sonoluminescence has been used for monitoring the growth of the bubbles as well as coalescence between bubbles. The extent of bubble coalescence has also been monitored using a newly developed capillary technique. An overview of the various experimental results has been presented in order to highlight the complexities involved in acoustic cavitation processes, which on the other hand arise from a simple, mechanical interaction between sound waves and bubbles.     

Characterization of an acoustic cavitation bubble structure at 230 kHz

A generic bubble structure in a 230 kHz ultrasonic field is observed in a partly developed standing wave field in water. It is characterized by high-speed imaging, sonoluminescence recordings, and surface cleaning tests. The structure has two distinct bubble populations. Bigger bubbles (much larger than linear resonance size) group on rings in planes parallel to the transducer surface, apparently in locations of driving pressure minima. They slowly rise in a jittering, but synchronous way, and they can have smaller satellite bubbles, thus resembling the arrays of bubbles observed by Miller [D. Miller, Stable arrays of resonant bubbles in a 1-MHz standing-wave acoustic field, J. Acoust. Soc. Am. 62 (1977) 12]. Smaller bubbles (below and near linear resonance size) show a fast "streamer" motion perpendicular to and away from the transducer surface. While the bigger bubbles do not emit light, the smaller bubbles in the streamers show sonoluminescence when they pass the planes of high driving pressure. Both bubble populations exhibit cleaning potential with respect to micro-particles attached to a glass substrate. The respective mechanisms of particle removal, though, might be different.