Chaotic Shape and Translational Dynamics of 2D Incompressible Bubbles under Forced Vibration in Microgravity

Nonlinear shape oscillations of 2D incompressible bubbles in an inviscid fluid, subject to a forced vibration in microgravity, have been studied numerically. Forced vibration induces an oscillatory translational motion as well as shape oscillations. It is shown that for large enough oscillation amplitudes, the coupling between the shape oscillation and the translational motion of a bubble results in a chaotic behaviour. For two-bubble systems, the bubbles may attract each other. The attraction force is stronger at higher Bond numbers. Higher Bond numbers also yield larger bubble deformation.     

On hydrodynamical properties of ellipsoidal bubbles

A major part of this paper is taken up by the calculation of the first axisymmetric surface oscillation frequency and the (2, 0) mode of an ellipsoidal bubble, in order to compare with experimentally obtained values of bubbles rising in water. First, an energy method is used for an ellipsoid oscillating in stagnant water. Interestingly, with help of a paper by Bjerknes [12] at 1873!. The results compare poorly with experiments. Agreement improves when the oscillation of the rise velocity is taken into account. The remaining difference between the results of theory and experimental values is ascribed to deviation of the bubble shape from an ellipsoid. Finally, volume oscillations of ellipsoidal bubbles are calculated with the energy method and the results compare well with those of an earlier work, based on an electrical analogon.     

Vibration of a single microcapsule with a hard plastic shell in an acoustic standing wave field

Observation techniques for measuring the small vibration of a single microcapsule of tens of nanometers in an acoustic standing wave field are discussed. First, simultaneous optical observation of a microbubble vibration by two methods is investigated, using a high-speed video camera, which permits two-dimensional observation of the bubble vibration, and a laser Doppler vibrometer (LDV), which can observe small bubble vibration amplitudes at high frequency. Bubbles of tens of micrometers size were trapped at the antinode of an acoustic standing wave generated in an observational cell. Bubble vibration at 27 kHz could be observed and the experimental results for the two methods showed good agreement. The radial vibration of microcapsules with a hard plastic shell was observed using the LDV and the measurement of the capsule vibration with radial oscillation amplitude of tens of nanometers was successful. The acoustic radiation force acting on microcapsules in the acoustic standing wave was measured from the trapped position of the standing wave and the radial oscillation amplitude of the capsules was estimated from the theoretical equation of the acoustic radiation force, giving results in good agreement with the LDV measurements. The radial oscillation amplitude of a capsule was found to be proportional to the amplitude of the driving sound pressure. A larger expansion ratio was observed for capsules closer to the resonance condition under the same driving sound pressure and frequency.     

Bubble Dynamics in Nucleate Pool Boiling on Thin Wires in Microgravity

A temperature-controlled pool boiling (TCPB) device has been developed to study the bubble behavior and heat transfer in pool boiling phenomenon both in normal gravity and in microgravity. A thin platinum wire of 60 μm in diameter and 30 mm in length is simultaneously used as heater and thermometer. The fluid is R113 at 0.1 MPa and subcooled by 26°C nominally for all cases. Three modes of heat transfer, namely single-phase natural convection, nucleate boiling, and two-mode transition boiling, are observed in the experiment both in microgravity aboard the 22nd Chinese recoverable satellite and in normal gravity on the ground before and after the space flight. Dynamic behaviors of vapor bubbles observed in these experiments are reported and analyzed in the present paper. In the regime of fully developed nucleate boiling, the interface oscillation due to coalescence of adjacent tiny bubbles is the primary reason of the departure of bubbles in microgravity. On the contrary, in the discrete bubble regime, it’s observed that there exist three critical bubble diameters in microgravity, dividing the whole range of the observed bubbles into four regimes. Firstly, tiny bubbles are continually forming and growing on the heating surface before departing slowly from the wire when their sizes exceed some value of the order of 10⁻¹ mm. The bigger bubbles with about several millimeters in diameter stay on the wire, oscillate along the wire, and coalesce with adjacent bubbles. The biggest bubble with diameter of the order of 10 mm, which was formed immediately after the onset of boiling, stays continuously on the wire and swallows continually up adjacent small bubbles until its size exceeds another critical value. The same behavior of tiny bubbles can also be observed in normal gravity, while the others are observed only in microgravity. Considering the Marangoni effect, a mechanistic model about bubble departure is presented to reveal the mechanism underlying this phenomenon. The predictions are qualitatively consistent with the experimental observations.     

Transient bubble domain configuration in garnet materials observed using high speed photography

The dynamic domain configurations of bubbles in garnet materials have been studied using a sampling optical microscope capable of single exposure photographs with a 10 nsec exposure time. The microscope is an integral part of a sampling system so that the transient shape of the bubble is recorded at various times after a field pulse or, for bubbles in field access devices, during a clock cycle. A triggerable flowing nitrogen gas laser pumping a low Q Rodamine 6G Dye laser is used as an illumination source giving light pulses of ∼1.5 KW for 10 nsec. This light is sufficient to expose Kodak 4 × 16 mm movie film. Standard pulse generators (HP 214A) are used to make free bubble radial velocity measurements. A modified generator allows free bubble collapse measurement to be made. For bubbles propagating at operating frequency within field access devices, a standard bubble exerciser is used, synchronized to the sampling system. In this case, special samples with an internal mirror and epi-mode illumination are used. Illustrative results of bubble domain shapes in a chevron propagating structure and a 90° chevron expander detector are included.     

Investigation on flow pattern evolution and restraint of vibration energy to bubbles in vibrated dense medium fluidized bed

Vibrated dense medium fluidized bed (VDMFB) provides a feasible and low-cost method for fine coal separation. In this study, empirical mode decomposition (EMD) was used to study the evolution of the dominant flow pattern and the restraint effect of vibration energy on the bubble in VDMFB. The internal pressure fluctuation signal of VDMFB was analyzed by EMD, and the intrinsic mode functions (IMFs) of the signal were extracted, and the corresponding relationship between the IMFs and dominant fluidization behavior was verified. The intermediate-frequency signal components (IMF-3 ~ IMF-4) and low-frequency signal components (IMF-5 ~ IMF-8) corresponded to the particle oscillation of emulsion phase and bubble behavior in the particle bed, respectively. Additionally, the energy variation of IMFs in different frequency bands was closely related to the evolution of dominant fluidization behavior. Further, the intermediate-frequency energy decreased and the low-frequency energy increased gradually along the bed height; additionally, the intersection point Tr existed between the two curves, indicating that the dominant energy controlling the flow pattern of the bed changed from vibration to bubbles. The constraint range of vibration energy on bubble behavior was defined, and the quasi-dispersed micro-bubble fluidization environment suitable for fine coal separation was identified in VDMFB, in which the steady vibration energy effectively controlled the unsteady bubble.     

Nonlinear radial oscillations of micro-bubbles in a compressible UCM medium

The nonlinear oscillations of acoustically forced spherical gas bubbles in an upper-convected Maxwell (UCM) compressible fluid are investigated. The nonlinear viscoelastic model used is suitable for large-amplitude excitation of bubbles that cannot be captured by linear models. The effects of acoustic excitation are studied for compressible nonlinear viscoelastic media, which increases the complexity and nonlinearity of the behavior. The Keller–Miksis equation is used to model the dynamics of a single bubble. The constitutive equations of compressible UCM are used for viscoelastic media. These governing equations are non-dimensionalized and coupled to determine the bubble dynamic behavior. The set of derived non-dimensionalized integro-differential equations developed are numerically solved simultaneously using the fourth-order Runge–Kutta method featured by the automatic variable time step-size module. The combined effects of compressibility and viscoelasticity of the fluid on bubble radius are investigated. The results show that the combination of compressibility and nonlinear viscoelasticity for bubble radial oscillations makes forced bubble dynamics more applicable for human needs, especially for large deformations in highly non-Newtonian fluids like industrial polymers or even tissue-like media. It can be seen that compressibility controls the oscillations at higher forcing amplitudes. The relevance and importance of these bubble dynamics to biomedical ultrasound applications and light emissions by sonoluminescence and other industries are evident.     

A numerical study of periodic slug flow at zero gravity and normal gravity

This paper reports numerical simulations of slug flow at zero and normal gravity. The particular experimental results chosen for validation were obtained at microgravity under conditions which resulted in evenly-spaced and evenly-sized Taylor bubbles facilitating a simulation with periodic boundary conditions. The numerical technique was a free-surface method which explicitly tracked the motion of the gas-liquid interface using a volume-of-fluid specification and a finite volume discretisation of the solution domain. The large scale features of the bubble such as the classic bullet-shaped nose were well predicted by the model. Unsteady features of the bubble shape such as waves in the film and fluctuations of the bottom surface were also predicted but are harder to compare quantitatively to the experiments. The velocity field predictions reveal several interesting features of the flow. When viewed by an observer moving with the bubbles, the liquid slug is dominated by a large recirculating region with the flow travelling from the leading to the trailing bubble along the tube centreline. In this frame of reference, the near-wall region features a jet of fluid issuing from the film of the leading bubble which entrains fluid in the slug. As the film of the trailing bubble begins to form, the entrained fluid must be ejected since the flowrate in the film of each bubble must be the same. It appears to be this process that drives the main recirculation.     

一种考虑时延的双气泡耦合振荡模型

基于单气泡Keller-Miksis振荡方程,在考虑时延的情况下,建立了一种双气泡耦合振荡计算模型。该模型将气泡振荡的周期分成若干份,初始扰动引起第一个气泡的半径在极短时间内变化而产生振荡并辐射声压,声压在传播一定时间后作用到第二个气泡,第二个气泡同样在短时间内做耦合振荡并反馈到第一个气泡,然后重复此过程。利用数值仿真在此模型的基础上分别研究了气泡振幅、半径、间距等参数对耦合振荡的影响。结果表明:初始扰动越大、两个气泡半径越接近,气泡耦合效应越明显;初始半径和平衡半径较大的气泡对耦合振荡有显著影响,振荡的频率向低频移动;气泡间距越大,耦合效应越弱;在某个特定距离处,气泡耦合效应的阻尼会异常减小或者增大。     

面向轨道车辆抗蛇行振动的磁流变脂阻尼器设计与台架测试

针对磁流变液阻尼器存在磁场利用率不高和磁流变液沉降导致控制特性劣化的问题,提出一种基于多级径向流动模式的磁流变脂阻尼器方案,将磁流变脂的多级径向流动分解为源流与汇流的对称组合,建立了磁流变脂径向流动的分析模型。利用磁流变脂微单元平衡得出了磁流变脂尊静态径向流动微分方程,采用磁流变脂双粘度本构模型和无滑动边界条件,导出了磁流变脂径向流动速度分布函数和径向压力梯度分布函数。绘制了磁流变脂在不同半径处流动速度分布图,得到了磁流变脂阻尼器的阻尼力计算方法。按照轨道车辆抗蛇行减振器的技术要求,设计制作了基于多级径向流动模式的磁流变脂阻尼器样机,利用J95-I型油压减振器试验台对其示功特性进行了测试,结果表明在不同激励电流下的磁流变脂阻尼器理论示功特性与实验示功特性能较好吻合。     

Jets in bubbles

Different types of jet formation in collapsing cavitation and gas bubbles near a rigid boundary are explored by using an advanced boundary-integral technique which incorporates the transition from simply connected to multiply connected bubbles (i.e. toroidal bubbles). Physical interpretation and understanding is facilitated by the calculation of the evolving bubble shape, fluid velocities and pressures, the partitioning of kinetic, potential and gravitational energies, the circulation around the bubble and the Kelvin impulse associated with both the complete bubble and the high-speed liquid jet. In the most vigorous jet formation examples considered it is found that upto 31% of the total energy and upto 53% of the Kelvin impulse is associated with the jet. Practical implications of this study beyond the usual damage mechanisms imply that the level of bubble compression will be signiffcantly lessened leading to lower bubble gas temperatures and thence the corresponding change in the chemical reactivity of its contents or the emission of light. Calculations also suggest interesting phenomena around a standoff distance of 1?2 maximum bubble radii where the circulation around the bubble and the kinetic energy of the jet appear to have maximum values. The practical implications and experimental confirmation of this are yet to be explored.     

Optical observations of acoustical radiation force effects on individual air bubbles

Previous studies dealing with contrast agent microbubbles have demonstrated that ultrasound (US) can significantly influence the movement of microbubbles. In this paper, we investigated the influence of the acoustic radiation force on individual air bubbles using high-speed photography. We emphasize the effects of the US parameters (pulse length, acoustic pressure) on different bubble patterns and their consequences on the translational motion of the bubbles. A stream of uniform air bubbles with diameter ranging from 35 microm to 79 microm was generated and insonified with a single US pulse emitted at a frequency of 130 kHz. The bubble sizes have been chosen to be above, below, and at resonance. The peak acoustic pressures used in these experiments ranged from 40 kPa to 120 kPa. The axial displacements of the bubbles produced by the action of the US pulse were optically recorded using a high-speed camera at 1 kHz frame rate. The experimental results were compared to a simplified force balance theoretical model, including the action of the primary radiation force and the fluid drag force. Although the model is quite simple and does not take into account phenomena like bubble shape oscillations and added mass, the experimental findings agree with the predictions. The measured axial displacement increases quasilinearly with the burst length and the transmitted acoustic pressure. The axial displacement varies with the size and the density of the air bubbles, reaching a maximum at the resonance size of 48 microm. The predicted displacement values differ by 15% from the measured data, except for resonant bubbles for which the displacement was overestimated by about 40%. This study demonstrates that even a single US pulse produces radiation forces that are strong enough to affect the bubble position.     

Oscillatory marangoni convection around bubbles and drops in heterogenous solutions of surfactants

Experimental investigation was performed to study the concentration convection around stationary gas bubbles and insoluble drops in a thin liquid layer placed in a vertical Hele-Shaw cell. The bubbles or drops, squeezed between the two parallel cell walls, took the shape of short cylinders with free lateral surfaces. The cell was filled in with an aqueous solution of a surface-tension active fluid (surfactant) with vertically stratified concentration. A special wire frame prevented bubbles from rising up under the buoyancy force, thus modelling the microgravity conditions. A convective motion in the mixture develops at the bubble or drop interface, due to the solutocapillary Marangoni forces. Owing to a small thickness of the liquid layer (∼1mm), the arising flows and surfactant concentration distributions are nearly two-dimensional so that it is possible to investigate their structure and evolution by interferometric technique. The experiments revealed the development of oscillatory convection around the drop interface, which was similar to that observed in bubble tests. The period and duration of oscillations were determined in relation to time, surfactant concentration gradient and concentration Marangoni number. The analysis of bubble and drop behavior showed that the existence of self-oscillatory modes is related to the specific interaction between the solutocapillary and soluto-gravitational mechanisms of motion.     

Lateral motion and interaction of gas bubbles growing over spherical and plate heaters

This work investigates the motion of CO₂ bubbles emerging in n-heptane when a heat pulse given to a submerged heater creates local supersaturation. The ensuing slow diffusion-induced bubble expansion makes bubble motion easy to observe. The low gravity environment of a parabolic flight allows bubbles to reach large sizes without departing from the heater while retaining their spherical shape. A fast lateral displacement of single bubbles has often been noticed on two type of heaters. In cases where many bubbles grow adjacent to each other, they soon start to interact. Phenomena such as bubbles clustering, coalescence and lift-off from the heater of a large bubble induced by neighboring small ones, have been repeatedly observed. An interesting thermocapillary attraction has also been noticed between bubbles adhering to the heater and others free-floating in the nearby liquid.     

水下爆炸气泡对内加筋圆柱壳结构毁伤机理分析

以内加筋圆柱壳为研究对象,对近距离非接触爆炸作用下气泡的脉动载荷以及气泡溃破高速射流对内加筋圆柱壳结构的毁伤机理进行计算分析,并且探讨结构参数、药包位置等相关物理量对结构变形特征与毁伤模式的影响。研究结果表明,水下爆炸气泡膨胀产生的膨胀脉动冲击造成结构的整体变形,另一方面气泡溃破所造成的高速射流(速度 > 100 m/s)则会对局部区域造成严重破坏,弹塑性边界以及自由液面效应会对气泡的形状产生显著的影响。     

均流式多管导流型气泡泵提升性能实验研究

本文为改善多管导流型气泡泵工作过程中气泡分配不均,提高Einstern制冷循环的性能,在大气压下以饱和水为工质,通过改变提升管数量,对比研究了普通多管导流型气泡泵及均流式多管导流型气泡泵的提升性能。结果表明:在气泡泵提升过程中,低功率下气泡泵液体总提升量增加幅度在较大和平缓现象之间不停的交替出现,但在高功率下气泡泵液体总提升量增加幅度较缓的过程所持续的时间越来越短。如管数量为3根,管径为10 mm,沉浸比为0.5,加热功率为450 W时,运行75.2 min时出水管开始有水流出,在75.2～100.1 min时,液体总提升量增加幅度在较大和平缓现象之间不停的交替出现。在添加均流器工况下,随着管数量倍增,气泡分配不均问题得以改善,如管径为10 mm,沉浸比为0.4,管数量为1根时,普通多管导流型气泡泵最大提升速率为15.00 g/s;管数量为3根时,最大提升速率为26.50 g/s;有均流器下,管数量为3根时,最大提升速率为36.50 g/s。因此,均流器网孔的孔径和安装位置对气泡泵的提升性能有一定影响。     

Theoretical Investigation of Capture,Collection, and Targeted Transfer of Underwater Gas Bubbles Using Janus-Faced Carbon Cloth

Recently, a straightforward method for the preparation of a Janus carbon cloth (Janus-CC) is reported. The method relies on polydimethylsiloxane (PDMS) coating, followed by flame exposure of one face of the PDMS-coated CC. The nonexposed to the flame face of the Janus-CC is superhydrophobic, while the exposed to the flame face is superhydrophilic. Underwater, the superhydrophobic face shows superaerophilicity, while the supherhydophilic face shows superaerophobicity. A tethered gas bubble onto the superhydrophobic face has a paraboloid of revolution shape due to the cohesion force between the bubble and the gaseous film. Herein, the geometrical parameters of underwater tethered gas bubbles onto the Janus-CC are obtained, and the tendency of Janus-CC for capturing of bubbles is interpreted based on the derived Gibbs free-energy change equation. It is experimentally observed that a small sited bubble moves toward the bigger one through the formed gaseous film, but this observation cannot be interpreted based on the classical Young–Laplace equation because here the smaller bubble has bigger curvature radius. Herein, a modified Young–Laplace equation is derived and applied for interpretation of the movement of a small bubble toward the bigger one. Also, the buoyancy force for a sited bubble on the gaseous film is discussed.     

A modified Rayleigh–Plesset model for a non-spherically symmetric oscillating bubble with applications to boundary integral methods

In this article the analytical solution to the Rayleigh–Plesset equation for a spherically symmetric oscillating bubble is extended to apply to the much more general (non-spherically symmetric) bubble configuration. An equivalent bubble radius and an equivalent bubble wall velocity are introduced in order to do so. The influence of gravity, surface tension, nearby solid walls, vapor bubbles, bubbles filled with adiabatic or isothermal gas have been considered in the model. An interesting outcome is that the equivalent bubble wall velocity is no longer the time derivative of the equivalent bubble radius. This observation can possibly explain why in various numerical and experimental observations the oscillation time of a bubble changes when compared to that of a standalone bubble; near a solid surface it increases while it decreases when the bubble is placed near a free surface. The current developed theory can be further employed to ascertain the accuracy of a numerical scheme simulating bubble dynamics in an incompressible surrounding flow approximation. An often used numerical technique to simulate such bubble dynamics is the boundary integral method (BIM).     

Disruption of an Aligned Dendritic Network by Bubbles During Re-melting in a Microgravity Environment

The Pore Formation and Mobility Investigation (PFMI) utilized quartz tubes containing succinonitrile and 0.24 wt% water “alloys” for directional solidification (DS) experiments which were conducted in the microgravity environment aboard the International Space Station (ISS; 2002–2006). The sample mixture was initially melted back under controlled conditions in order to establish an equilibrium solid-liquid interface. During this procedure thermocapillary convection initiated when the directional melting exposed a previously trapped bubble. The induced fluid flow was capable of detaching and redistributing large arrays of aligned dendrite branches. In other cases, rapidly translating bubbles originating in the mushy zone dislodged dendrite fragments from the interface. The detrimental consequence of randomly oriented dendrite arms at the equilibrium interface upon reinitiating controlled solidification is discussed.     

浸没排气过程声信号与流场振荡规律研究

采用流体体积(Volume of Fluid,VOF)模型和威廉姆斯-霍金斯(Ffowcs Williams and Hawkings,FW-H)模型对浸没排气过程声信号进行数值模拟,研究竖直向下浸没排气过程声信号产生机制及理论依据。将模拟结果与实验结果进行对比,验证模型的适用性和准确性。研究结果表明,声信号与流场流动状态相关,气泡间分离和气体再注入过程引起的气泡体积剧烈振动是影响气泡流声信号产生的关键因素。在同一深度处,在径向上声压随位置的变化服从指数衰减,在周向上声信号呈现球源信号传播特点。