Mechanisms for Oscillations in Volume of Single Spherical Bubble Due to Sound Excitation in Water

Mechanisms for the evolution of a single spherical bubble subjected to sound excitation in water are studied from the viewpoint of nonlinear dynamics.First,the shooting method is combined with a Poincaré map to obtain the fixed point for the case of forced oscillation in volume.Then,the stabilities are judged by Floquet theory and the bifurcation theorem.Moreover,the transitions of bubble oscillation in volume due to sound excitation in water are explained from the viewpoint of nonlinear dynamics in detail.The results show that with an increase in sound frequency,the period-1 oscillation becomes unstable,and oscillation behaves in a double-periodic manner,then a quasi-periodic manner,and finally chaotically.Additionally,with an increase of the amplitude of the sound pressure,the bubble eventually oscillates with chaos via a series of period-doubling bifurcations.

An Investigation of Bubble Motion in the Fluidic Oscillator

Enhanced gas-liquid mass transfer is significant for the desulfurization and denitration of ship exhaust gases.As a fluid device,the special structure of the fluidic oscillator generates self-excited oscillations that can effectively enhance the mass transfer process of gas-liquid.But there are few studies on the internal gas-liquid flow.The transportation of individual bubbles in the fluidic oscillator was investigated by a high-speed camera and digital image analysis.The results show that the bubble experienced a significant deceleration process in the chamber region of the fluidic oscillator.In addition,the maximum bubble offset increased with the diameter of the initial bubble.The trajectory of the bubble showed zigzag movement due to the deflecting oscillation of the fluidic oscillator.At the same time,the deformation of the bubble was intensified by the deflecting oscillation.The deformation ratio of the bubble increased with the increase of Reynolds number.By studying the transport process of a single bubble in the fluid oscillator,it is considered that the fluid oscillator has the potential to be a new bubble generator.

Numerical Investigations on Effects of Physical Parameters on Single Bubble Generation from Capillary Needles in Newtonian Liquids

The interphase mass,heat transfer efficiency,and flow resistance are strongly dependent on bubble size in gas-liquid two-phase systems,so it is very important for engineering applications to effectively control bubble size.In this paper,the formation,growth,and detachment of single bubbles in Newtonian liquids based on capillary needles were studied in detail using a volume of fluid method.The authors investigated the effects of gas injection velocity,gravitational level,surface tension coefficient,needle radius,and liquid-phase properties(liquid viscosity and density)on the process of bubble generation,and the effects of the above factors on bubble shape,detachment diameter,and time were analyzed.The results show that an increase in gas injection rate,liquid-phase viscosity,needle radius,and surface tension coefficient can lead to an increase in bubble detachment diameter;however,an increase in liquid-phase density and gravitational level can lead to a decrease in bubble detachment diameter.It is found that the effect of the liquid-phase viscosity on bubble detachment diameter and time is slight,but the effect of gravitational level on detachment diameter and time is significant.Among all the forces,buoyancy,surface tension,and pressure are the most important ones that control the generation of bubbles.

The effect of multi-orifice plate configuration on bubble detachment volume

The multi-orifice plate gas sparger, mainly composed of a multi-orifice plate and a gas chamber, is one of the most common sparger facilities.The aeration performance of multi-orifice plate has a close relation with the multiorifice plate configuration.In addition, the weeping phenomenon has a considerable influence on the gas chamber condition which affects the bubble detachment volume directly.This paper conducts a set of visual experiments to study the influence of multi-orifice configuration and gas chamber condition on the aeration performance of gas sparger.For multi-orifice plate, an improved theoretical model is proposed which considers the wave effect of the previous bubbles generated from adjacent orifices and the variance of the number of active bubbling orifice.A parameter is proposed to evaluate the aeration performance in order to overcome the difficulty caused by the randomness of bubble formation process.The experimental results suggest that the gas chamber filled with water is in favor of large bubble formation.The influence of the pitch of orifice on aeration performance can only be observed in high-restricted case.According to the theoretical model and experimental results, the influences of gas flow rate and the number of open orifices on the aeration performance are analyzed and a design criterion for the number of open orifice is proposed.

Pilot Study on the Oscillation and Migration of a Spherical Bubble Escaping from the Lateral Propulsion Hole

In an atrocious ocean environment,the lateral propulsion hole could potentially be partly out of water and capture an air cavity.Bubbles would form when the captured air cavity escapes underwater and they may affect the performance of the sonar.The common commercial computational fluid dynamics software CFX was adopted to calculate the ambient flow field around the lateral propulsion hole generated by a moving vessel.The oscillation of the spherical bubble was based on the Rayleigh-Plesset equation and its migration was modeled using the momentum equation.The radiated noise of the oscillating bubble was also studied.The aim is that the results from this paper would provide some insight into corresponding fluid and acoustic study.

Stability Mechanisms of Gas Bubbles Subject to Ultrasonic Field during the Rectified Diffusion in Liquids

The spherical oscillation of a gas bubble in liquids is important to growth in liquids during rectified diffusions( e. g., the onset of the sonoluminescence and the enhancement of sonochemical reactions). The present paper numerically shows stability maps( divided into four zones),in which gas bubbles maintain the linearly spherical oscillation without nonlinear disturbance of rectified diffusions within a large range of bubble radius. The critical pressures of spherical and diffusional oscillations are two decisive indexes determining the stability status. Specifically,the stability boundaries and influential factors( including acoustic parameters and gas concentration in liquids) were discussed and analyzed. The results show that the variations of gas concentration and acoustic parameters dramatically changed the stable status of the gas bubbles. The gas bubble maintained stable status when external parameters and gas concentration were set between the two critical values properly. The cases of high-frequency and low-frequency limits were also introduced at the end of the whole paper.

Parameters Relevant to Bubble Detachment when Gas-injecting into Polymer Melt Flow Field

The bubble deformation processes were reported when gas was injected into polyme r melt flow field in another paper, the experiments showed that the deformation was severely affected by the volume of the bubble, and in turn, for the different bubbles, several different deformation processes were presented d uring their movement along the flow channel. In addition, we could find that the magnitude of the bubble volume was dependent upon the pressure difference of th e gas injection pressure and the melt pressure. In this paper, more experimental conditions were changed to investigate the parameters relevant to the detachmen t of bubbles from the injection nozzle. The experimental results show that the p ressure difference, the melt flow velocity as well as the melt pressure were all critical for the parameters, such as the bubble detachment time, the maximum bu bble diameters and the magnitude of the bubble volume. The morphology changes of bubble were very large when the flow field was abruptly changed, and the situat ions were more complicated.

Detachment Size Measurement of Two Interacting Bubble Plumes Formed at Neighboring Needles Using an Acoustic Method

Detachment size determination with an acoustic method has been carried out for two interacting bubble plumes formed at neighboring needles in quiescent water. Two sets of needle pairs, one with 1.5mm and 0.8mm inner diameters and the other with the equal 1.5mm inner diameters, were separately used as the bubble pair injectors in the experiments. Consequently, four typical patterns of bubble plumes interaction could be observed in the two cases of needle pair matches. Through measuring the pressure pulses radiated by the bubble pairs immediately after their 'pinching-off ' and by making use of a sophisticated relation between oscillation frequency of volume mode and radius of gas bubble, the detachment size of the bubble plumes have been determined from the amplitude/frequency spectrum of the sound pressure pulses. The experimental results demonstrate that the acoustical method is valid in both of the interacting and non-interacting circumstances in bubble field and the bubble size measurements by this acoustical method agree well with the measurements from photographic analysis. Finally, a comparison has been made on the strong and weak points of the acoustical method with the other size determination methods.

Effect of Translatory Motion on Dynamics Heat and Mass Transfer of Vapor Bubble Radially Oscillating in a Liquid

The phase transitions and the effect on this process of two factors: relative motion and the external influence of the carrier flow (pressure perturbation) are investigated. A mathematical model describing all the stages of the phenomenon is constructed. The model includes mass, momentum and energy balance equations (both for the vapor and for the liquid) or their first integrals.

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.

Particle Simulation Research on the Detailed Process of Bubble Generation

The detailed process of bubble generation in laser-plasma interaction has been studied by particle simulation. During the interaction between laser and plasma, a strong electrostatic field caused by charge separation is observed, whose structure looks like a potential well. Electrons are accelerated to 10 MeV within some tens of femtoseconds when the laser intensity is 5×10^(18) W/cm^(2). After many electrons escape from the potentiaLwell-like structure, electron cavitons are generated. Because of laser filamentation, the intensity of the laser becomes very asymmetric, the caviton structure is squeezed and deformed, and then electron bubbles are generated, whichfinally results in ion bubble formation. It is the electron caviton and laser beam filamentation that cause bubble generation. The i0 MeV superthermal electron generation accompanied by the electron caviton is attributed to EV . E heating, which is a locally oscillating electron heating mechanism.

离心模拟水下爆炸试验技术

随着国际形式日趋紧张,重要基础设施的抗爆安全愈发受到关注。对于水下爆炸研究,超重力场可以实现水下爆炸冲击波和气泡脉动的同时相似,离心模型试验是爆炸毁伤效应的重要研究手段。结合中国水科院十余年来离心模拟水下爆炸的实践经验,系统介绍了相关设备的系统组成,给出了设备满足的技术指标。通过2组离心模型试验,测试了离心机主轴、吊耳部位及模型箱的振动响应,结果表明:①离心机主轴所受的爆炸冲击响应较小,与正常运转条件下的振动速度基本一致,可以忽略爆炸冲击对离心机整体稳定的影响;②吊耳部位所受的冲击加速度及振动速度较大,可以通过在模型箱布置高弹橡胶减轻吊篮的冲击响应;③试验模型箱的变形基本处于弹性范围,模型箱长边可在跨中布置支撑减小跨中的变形,模型箱壁支座可采用螺栓增加支座的稳定。

不同浓度离子溶液中气泡运动特性的实验研究

气泡运动特性可以分析流型形成和演化,是探究两相流模型构建的基础。为了进一步研究气泡在多种溶液内的运动特性,搭建了气泡运动特性实验平台。在3种浓度的FeCl_(3)、CaCl_(2)、NH_(4)Cl离子溶液及去离子水内,对单个气泡运动特性进行了实验研究,利用高速相机拍摄图片并导入MATLAB软件进行了计算。据此,分析了离子溶液内单个气泡的上升时间,脱离时间,初始当量直径,以及纵横比。结果表明:离子溶液,尤其是CaCl_(2)溶液在一定程度上可以促进气泡在喷嘴处的脱离,使得脱离时间减少,当喷嘴为1.5 mm时,促进作用最为明显;三种离子溶液和去离子水比较,气泡上升时间都有所缩短,但是相差不大;三种离子溶液中,FeCl_(3)溶液中气泡纵横比变化最大;NH_(4)Cl溶液对气泡的当量直径促进作用最强,CaCl_(2)溶液会抑制气泡当量直径的发展,因此在特殊流体机械内部,应适当增加钙离子以降低冲击带来的腐蚀与振动;在去离子水和三种离子溶液中气泡的纵横比δ都会有一个“L”型分布,且最大值均出现在气泡脱离喷嘴的瞬间。

黏性对内空泡诱导柱状液滴界面不稳定性的影响

Rayleigh-Taylor不稳定性存在于爆炸、液滴形成和液体喷雾等工程应用过程中,是流体力学关注的经典问题之一.内空泡振荡诱导液滴界面演化问题是其研究中基本模型之一,空泡振荡作用下液滴界面发生扰动并发展,其特征形态主要表现为破碎、通气和稳定.液体黏性是影响界面不稳定性发展的重要因素,文章通过建立高精度的数值模拟方法,开展液体黏性对内空泡诱导柱状液滴界面不稳定性的影响研究.在数值模拟中,基于开源OpenFOAM框架的多相可压缩求解器直接求解Navier-Stokes方程,采用isoAdvector的几何流体体积法捕捉界面演化特征.结果表明,液体黏性的增加会减缓空泡的收缩,进而减缓液滴界面扰动的发展,该影响下通气工况液滴通气发生时间增加,而稳定工况最大扰动幅值减小.最大扰动幅值的减小直接影响了液滴的特征形态,基于一系列数值模拟结果归纳得到液滴不稳定性相图.在文章讨论的参数范围内,随着黏性增加,小液滴(R_(d0)<2 mm)的形态从破碎转变为通气进而变成稳定;中液滴(2 mm <R_(d0)<3 mm)的形态从通气转变为稳定,不出现破碎形态;而大液滴(R_(d0)> 3 mm)的形态不随液体黏性改变,液滴形态会随着液体黏性增加趋于稳定.

浮选气泡尺寸对粗颗粒-气泡涡旋湍流脱附机制的影响

气泡是浮选过程中不可或缺的重要角色,其尺寸对颗粒-气泡间相互作用有着显著影响,然而气泡尺寸对粗颗粒-气泡涡旋湍流脱附机制的影响尚未清晰。为此,采用自制的颗粒-气泡受限湍流脱附测试平台探索了涡旋湍流中不同气泡尺寸的气絮体上颗粒的脱附行为,并运用Image-ProPlus图像处理软件对颗粒脱附过程的动力学参数进行了测量分析。结果表明:在确保气泡浮力的前提下,减小气泡尺寸能够显著提升粗颗粒-气泡的矿化气絮体在涡旋湍流中的稳定性;涡旋湍流中气絮体上颗粒的脱附主要表现为流体剪切、气泡振荡和颗粒离心三种脱附形式,方腔内涡旋结构的改变对气絮体上颗粒的脱附方式至关重要;区别于传统离心脱附理论,涡旋湍流驱使下颗粒并非单独在气泡表面做高速离心运动,而是与气泡一起做离心运动,且小气泡与颗粒在方腔内共同旋转时,位于气泡外缘的颗粒与气泡运动速度接近,相应气絮体稳定性高,颗粒不易发生离心脱附,而大气泡形成的气絮体中,气泡外缘的颗粒与气泡运动速度差异明显增大,颗粒相对于气泡旋转更快,从而使得颗粒所受离心力显著增强,增加了离心脱附的可能性。研究结果有望对涡旋湍流场中粗颗粒的脱附机制提供一个基本认识。

同轴式微通道壁面润湿性对微细气泡生成特性的影响规律

微细气泡的生成时间和大小对污水处理的效率具有重要影响。为探究微通道壁面润湿性对微细气泡生成特性的影响,采用两相流水平集的方法模拟研究不同接触角下微细气泡的生成过程。以接触角为主变量,液体流量、气体压强和气体类型为次变量,探究次变量对微细气泡生成时间和脱离体积的影响。设计并制造了同轴式微流控芯片,开展微细气泡生成特性的试验。结果表明接触角在0~180°递增时,微细气泡的生成时间总体呈下降趋势,脱离体积先增加后减小,其中90°接触角为生成时间的分界点和脱离体积的峰值点。此外,3个次变量中液体流量的变化对微细气泡生成特性的影响尤为显著,液体流量越大,其生成时间和脱离体积越小。实测值与仿真值的偏差在正负百分之十以内,验证了微通道壁面润湿性对微细气泡生成特性的影响规律。

黏弹性介质包裹的液体腔内球状泡群耦合振动模型

高强度超声可激发生物组织空化,软组织常被当作黏弹性介质,因此,黏弹性介质中气泡动力学行为研究可为超声生物治疗提供理论支持.为探索组织液内多气泡动力学影响,构建了球形液体腔内的球状泡团模型,考虑了液体腔外黏弹性介质的动力学效应,得到了球状泡群内气泡耦合振动方程,并基于此分析了气泡的振动行为.结果表明,腔体以及泡群约束虽抑制了气泡振动,但在一定程度上还可以增强气泡的非线性振动特性.约束环境下气泡的非球形振动稳定性主要受驱动声波压力幅值和频率、气泡初始半径以及气泡数密度影响,而腔体半径的影响随驱动压力的增加而增强;存在最小不稳定驱动声压阈值,不同初始半径的气泡不稳定振动阈值压力不同且不稳定分布区主要分布在小于4μm的范围内;驱动频率增大,气泡振动不稳定区的声压阈值也随之增大,且不稳定区域有减小的趋势;随着气泡数密度的增加,气泡不稳定区域逐渐向无规则的斑图状分布,极易受到扰动发生不稳定振动而崩溃.高频声波激励下平衡半径大于4μm范围内气泡的惯性空化阈值受频率和气泡数密度的影响显著.

基于Anton Ziolkowski模型气枪震源子波模拟

气枪震源被广泛用于海洋地震勘探中,它是一种重要的人工地震震源。文章简单介绍了自由气泡震荡的理论基础及其相关公式,并根据理论计算,得到了相应的曲线,分析了影响气枪震源的各项参数,实现了基于Anton Ziolkowski模型气枪震源的子波模拟。

Experimental research of the cavitation bubble dynamics during the second oscillation period near a spherical particle

In this paper,the dynamic behaviors of the cavitation bubble near a fixed spherical particle during the second oscillation period are analyzed based on the high-speed photographic system.The deformation and motion of the bubble during the second period are investigated by changing the distance between the particle and the bubble and the maximum radius of the bubble.Meanwhile,the variation of the equivalent radius and the centroid motions are analyzed,and the dynamic behaviors of the bubble are categorized according to the bubble morphological characteristics during the second period.Through this research,it is found that(1)The dynamic behaviors of the bubble during the second oscillation period could be divided into three typical cases:For case 1,a bulge would exist on the bubble interface away from the particle,and for case 2,a bulge would appear on the bubble interface and evolve towards the particle,while for case 3,the bubble would be divided into two parts.(2)The larger the dimensionless distance between the particle and the bubble,the smaller the maximum bubble equivalent radius in the second period,and the shorter the second oscillation period.(3)When the bubble is close to the particle,a counter-jet appears at the bubble interface away from the particle during the rebound stage.

表面疏水性对气-液界面颗粒运动行为的影响

利用改进的L-B水槽,借助高速动态显微试验装置,基于颗粒间相互作用力计算、界面聚团微观结构分析、聚团重组过程解析以及颗粒所受平衡作用力计算,系统研究了气泡与气-液界面单颗粒层兼并后不同疏水性颗粒的震荡运动行为。结果表明,在界面颗粒吸附密度相同的情况下,低疏水性颗粒(接触角43°)更容易形成密实的聚团,而高疏水性颗粒(接触角75°)形成的聚团则较为松散。气泡与单颗粒层兼并可诱导界面产生震荡,在此影响下,位于界面上的聚团受表面压力梯度和震荡波的影响不断发生重组,并最终使界面颗粒的分散变得更加均匀,其原理和起泡剂分子稳定液膜的Gibbs-Marangoni效应非常类似。在界面聚团重组的过程中,水平曳力和惯性力以及颗粒间的相互作用力起着关键作用。相同试验条件下,低疏水性颗粒由于额外受到垂直向上毛细力的作用,其瞬时速度和均方位移(MSD)均明显大于高疏水性颗粒,这意味着低疏水性颗粒的震荡运动强度更加剧烈,振幅更大,脱附概率也更高。