超声搅拌磁流变抛光液的声场仿真分析

为了探究超声搅拌磁流变抛光液的制备及优化工艺,利用多物理场数值计算方法,建立了超声搅拌磁流变抛光液的声场仿真模型。研究了20 kHz下不同液位深度、超声变幅杆探入深度、不同功率下磁流变抛光液的声场分布。通过测量磁流变抛光液的声场强度对声场仿真进行了验证。结果表明:随着距变幅杆距离的增加,声强逐渐减弱,高声强区域主要分布在换能器轴线附近。声强在距变幅杆输出端20 mm范围内急剧衰减,变幅杆最佳探入深度为10 mm,增大功率有助于空化区域的扩大。声场仿真结果与实验测量结果基本一致,对磁流变抛光液的制备提供了数值计算基础。     

流体控制方程的超声空化泡动力学模拟*

本文基于流体动力学控制方程和VOF模型,在FLUENT 14.5软件环境下对超声空化泡进行数值模拟。首先研究了超声空化泡一个周期内的形态变化,并且利用空化泡形态变化的最大面积、最小面积、膨胀时间、收缩时间等数值结果分析超声参数对空化效果的影响。同时探究了双频超声作用下空化泡运动的变化,计算结果表明:在其他条件相同的情况下,在1~5MPa范围内,超声声压幅值为3MPa时空化效果最好;当超声频率大于20kHz时,空化效果随着超声频率的增大而降低。对于频率相同的双频超声,较声压幅值为其两倍的单频超声有更好的空化效果;对于频率不同的双频超声,空化效果受到频率差的影响。     

超声振动珩磨作用下空化泡动力学及影响参数

为了合理利用超声振动珩磨作用下的空化效应,以磨削区单个空化泡为研究对象,考虑珩磨头合成扰动速度和珩磨压力的作用建立了磨削区空化泡的动力学模型。数值模拟了空化泡初始半径,珩磨压力,液体静压力和超声声压幅值对磨削区空化效应的影响。研究表明考虑超声振动珩磨作用时,空化泡膨胀的幅值会受到抑制,其溃灭时间也会缩短,而且较容易出现稳态空化。珩磨压力和液体静压力对磨削区空化主要起抑制作用,超声波声压幅值在一定范围内能够促进磨削区空化效果的提升。本文的研究为进一步理解超声振动珩磨的空化机理提供了理论支持。     

Cone-like bubble formation in ultrasonic cavitation field

A new phenomenon in ultrasonic cavitation field is reported. Cavitation bubbles are observed to self-arrange in a cone-like macrostructure in the vicinity of transducer radiating surface. The cone-like macrostructure is stable while its branch-like pattern microstructure changes rapidly. The structure is constituted by moving bubbles which undergo attractive and repulsive Bjerknes forces caused by high acoustic pressure gradients and strongly nonlinear oscillations of cavitation bubbles. The cone-like bubble structure is a chemically active formation. Its remarkably high activity is confirmed by chemiluminescence experiments.     

Novel cavitation fluid jet polishing process based on negative pressure effects

Traditional abrasive fluid jet polishing (FJP) is limited by its high-pressure equipment, unstable material removal rate, and applicability to ultra-smooth surfaces because of the evident air turbulence, fluid expansion, and a large polishing spot in high-pressure FJP. This paper presents a novel cavitation fluid jet polishing (CFJP) method and process based on FJP technology. It can implement high-efficiency polishing on small-scale surfaces in a low-pressure environment. CFJP uses the purposely designed polishing equipment with a sealed chamber, which can generate a cavitation effect in negative pressure environment. Moreover, the collapse of cavitation bubbles can spray out a high-energy microjet and shock wave to enhance the material removal. Its feasibility is verified through researching the flow behavior and the cavitation results of the negative pressure cavitation machining of pure water in reversing suction flow. The mechanism is analyzed through a computational fluid dynamics simulation. Thus, its cavitation and surface removal mechanisms in the vertical CFJP and inclined CFJP are studied. A series of polishing experiments on different materials and polishing parameters are conducted to validate its polishing performance compared with FJP. The maximum removal depth increases, and surface roughness gradually decreases with increasing negative outlet pressures. The surface becomes smooth with the increase of polishing time. The experimental results confirm that the CFJP process can realize a high material removal rate and smooth surface with low energy consumption in the low-pressure environment, together with compatible surface roughness to FJP.     

Effect of supercritical water shell on cavitation bubble dynamics

Based on reported experimental data, a new model for single cavitation bubble dynamics is proposed considering a supercritical water(SCW) shell surrounding the bubble. Theoretical investigations show that the SCW shell apparently slows down the oscillation of the bubble and cools the gas temperature inside the collapsing bubble. Furthermore, the model is simplified to a Rayleigh–Plesset-like equation for a thin SCW shell. The dependence of the bubble dynamics on the thickness and density of the SCW shell is studied. The results show the bubble dynamics depends on the thickness but is insensitive to the density of the SCW shell. The thicker the SCW shell is, the smaller are the wall velocity and the gas temperature in the bubble. In the authors' opinion, the SCW shell works as a buffering agent. In collapsing, it is compressed to absorb a good deal of the work transformed into the bubble internal energy during bubble collapse so that it weakens the bubble oscillations.     

声场中水力空化泡的动力学特性

以水为工作介质,考虑了液体黏性、表面张力、可压缩性及湍流作用等情况,对文丘里管反应器中空化泡在声场作用下的动力学行为特性进行了数值研究.分析了超声波频率、声压及喉径比对空化泡运动特性以及空化泡崩溃时所形成泡温以及压力脉冲的影响.结果表明,超声将水力空化泡运动调制成稳态空化,有利于增强空化效果. 关键词： 超声波 水力空化 湍流 气泡动力学     

黏性液体中激光空泡脉动特性的理论和实验研究

通过自行研制的光偏转测试系统对黏性液体中激光空泡脉动特性进行了实验，获得了激光等离子体空泡前两次脉动全过程，从而判定了空泡在脉动过程中对应的最大和最小泡半径；并将不同黏性系数下空泡脉动结果与基于空泡溃灭理论的计算结果进行了对比.结果表明：液体黏性对空泡生存周期、空泡半径，以及泡壁的运动速度等均有影响.受液体黏性影响，空泡膨胀或收缩过程明显变缓，其相应的生存周期也越长.这一研究结果不仅可促进空化研究的深入，也可为合理利用空化现象提供参考. 关键词： 激光泡 脉动特性 黏性 光纤传感器     

Effects of radiator shapes on the bubble diving and dispersion of ultrasonic argon process

In this work, three ultrasonic radiators in different shapes have been designed in order to investigate the effects of radiator shapes on the argon bubble dispersion and diving as well as the degassing efficiency on magnesium melt. The radiator shape has a strong influence on the bubble diving and dispersion by ultrasound. A massive argon bubble slowly flows out from the radiator with the hemispherical cap, due to the covering hemispherical cap. Using a concave radiator can intensively crush the argon bubbles and drive them much deep into the water/melt, depending on the competition between the argon flow and opposite joint shear force from the concave surface. The evolution of wall bubbles involves the ultrasonic cavities carrying dissolved gas, migrating to the vessel wall, and escaping from the liquid. Hydrogen removal can be efficiently achieved using a concave radiator. The hydrogen content can be reduced from 22.3 μg/g down to 8.7 μg/g. Mechanical properties are significantly promoted, due to the structure refinement and efficient hydrogen removal.     

基于Kahan线性化快速计算梯度磁场下磁流变抛光区域抛光粉沉降特性

针对抛光粉沉降特性数值计算这一超大规模非线性问题,基于Kahan线性化解决了超大规模流固耦合计算问题。研究了以羟基铁粉和硅油为主要成分组合而成的抛光粉多相流在梯度磁场下抛光区域的沉降特性。以质量分数70%、粒径5 μm的羟基铁粉和粘度为0.973 Pa·s的硅油组合而成的抛光粉为研究对象,实现了不同的抛光轮转速、不同嵌入深度以及不同羰基铁粉质量分数情况下的沉降规律分析。结果发现:磁流变抛光区域的抛光粉会随着抛光轮转速的增大而增多;当到达出口时,抛光粉的分布趋于稳定状态;抛光粉会随着嵌入深度的增加而增多并存在饱和区;羟基铁粉的质量分数以非线性的方式影响沉降能力。     

Insights into numerical simulation of controlled ultrasonic waveforms driving single cavitation bubble activity

A computational study treating cavitation phenomenon within a single bubble undergoing various controlled ultrasonic waveforms is presented in this paper. Numerical simulations using sinusoidal, square, triangular and sawtooth waves crossing an aqueous media, saturated with oxygen, are conducted upon various operational conditions of frequency and amplitude. Bubble radius, temperature and pressure were estimated over time for 64 combined cases. The obtained results show that at relatively low acoustic pressure, i.e. 1.5 and 2 atm, the square wave is proved to generate the highest temperature and pressure inside the bubble, while triangular and sawtooth ones remain the less interesting waveforms for sonochemical application within the same operational conditions. At higher amplitudes above 2.5 atm, this trend is changed, especially at low frequencies, i.e. 200 and 300 kHz, where square wave showed some limitations in attaining the optimal values of the strong collapse within one acoustic cycle.     

超声空化对丙酮激光喇曼光谱的影响

本研究设计了一种可吸入超声的喇曼光谱样品池,对丙酮的激光喇光曼谱的检测表明,超声空化降低了喇曼光谱的强度,我们认为,超声空化减少了激光光路上参与散射的丙酮分子数是光谱强度降低的主要原因。     

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.     

Evolution of an 1 MHz ultrasonic cavitation bubble field in a chopped irradiation mode

In this paper the evolution of an ultrasonic cavitation bubble field is studied by means of the void rate, the cavitation noise power and the electric power that feeds the ultrasonic transducer. The ultrasonic irradiation is performed in chopped mode. It is observed two kinds of evolution that differ respectively by a slow (A regime) and a fast (B regime) void rate increase. They correspond to cavitation regimes that are identified as respectively a stable cavitation and an inertial cavitation field. The beginning of the fast increase of the void rate is delayed from the irradiation start and this delay depends on the chopping frequency of the irradiation mode.     

含气量对黏性液体中空泡脉动特性的影响

采用自行研制的高灵敏度光偏转测试系统,研究黏性液体中激光空泡脉动特性.判定了空泡两次脉动对应的最大和最小泡半径,进而计算了激光空泡在前两次脉动过程中泡内的含气量.研究表明：泡内含气量对泡脉动特性有较大影响.随着脉动次数的增加,空泡泡内含气量增大.空泡最大泡半径随含气量的增加而增大.此外,受液体黏性影响,空泡膨胀和收缩过程明显变缓. 关键词： 含气量 黏性 激光空泡 脉动特性     

Observations of water cavitation intensity under practical ultrasonic cleaning conditions

In the industrial cleaning processes either organic solvents or water solutions are used as the cleaning media. The primary causative factor of ultrasonic cleaning is cavitation. Below are presented results of investigations into the influence of temperature, gas content and the solution level in an ultrasonic cleaner on cavitation intensity in the tap water. Previous investigations have revealed a great deal of information on the influence of the above factors on the cavitation intensity and these are confirmed. It has now been found that the tap water reaches the highest cavitation intensity at temperatures below 20 degrees C but during heating at higher temperatures (20-40 degrees C) a second peak of cavitation intensity may appear-depending on the height of water in the bath and air content.     

Dynamics and measurement of cavitation bubble

Based on the introduction of international progress, our investigations on acoustic cavitation have been reported. Firstly we considered the cavity’s dynamics under the drive of the asymmetrical acoustic pressure. An aspheric dynamical model was proposed and a new stable and aspheric solution was found in numerical simulation of the theoretical framework of the aspheric model. Then, a dual Mie-scattering technique was developed to measure the cavity’s aspheric pulsation. A significant asynchronous pulsation signal between two Mie-scattering channels was caught in the case of large cavity driven by low acoustic pressure. As a direct deduction, we observed an evidence of cavity’s aspheric pulsation. Furthermore, we studied the dependency of the asynchronous pulsation signal on the various parameters, such as the amplitude and frequency of the driving acoustic pressure, and the surface tension, viscosity and gas concentration of the liquid. Finally, we introduced a new numeric imaging technique to measure the shapes of the periodic pulsation cavities. The time-resolution was in the order of 20 ns, one order of magnitude lower than that in the previous work, say, 200 ns.     

内凹面磁流变槽路抛光方法的研究

针对高陡度非球面光学元件的内凹面抛光难题,提出了一种内凹面磁流变槽路抛光方法。设计与待加工内凹面形状匹配的凸模,并在凸模上开出供磁流变液循环通过的槽路,当磁流变液经过设有磁场的区域时发生流变作用形成柔性抛光磨头对内凹面产生材料去除作用。通过工件的旋转和外部磁极的移动完成对整个内凹面的抛光加工。建立了实验平台并开展了相关初步实验和分析。结果表明该方法能够适应内凹面抛光加工的需要,可获得较高表面质量,具有一定的可行性和应用潜力。     

基于梯度聚类离散思想的磁流变抛光自适应轨迹

针对磁流变抛光过程中抛光轨迹会引入迭代误差的问题,设计了步长和行距随光学表面梯度自适应变化的光栅线抛光轨迹。首先根据光学元件的表面误差分布,利用标准五点法获得面形各点的梯度值,再基于聚类离散思想将所有面形点根据梯度值大小进行了归类,从而得到轨迹步长和行距随面形误差变化的自适应轨迹。在自研的磁流变加工机床上进行了实验研究,将一块直径50 mm的微晶玻璃,从峰谷值为65 nm、均方根值为12 nm收敛到峰谷值为21 nm、均方根值为2.5 nm,并且在加工后的表面功率谱密度曲线上没有出现明显的尖峰误差。实验结果表明,这种自适应轨迹能有效抑制中高频误差。     

Translation of cavitation bubble near the different walls

The interaction between spherical cavitation bubble and flat wall is transformed into that between the real bubble and imaging bubble by the method of images. Firstly, we investigate the dynamics of real bubble and matched, inversed or mis-matched imaging bubble driven by a small amplitude ultrasound, revealing the characteristics of the interaction between cavitation bubble and rigid, soft and impedance walls. Then, we emphatically study the dynamics of real bubble and mis-matched imaging bubble driven by a finite amplitude ultrasound, and the interaction characteristics between cavitation bubble and real impedance wall are revealed. The results show that the cavitation bubble is always close to the rigid wall and far away from the soft wall; For the impedance wall, whether the cavitation bubble is far away or close depends on the specific wall parameters. Moreover, the direction and magnitude of bubble's translation velocity can be changed by adjusting the driving parameters. Understanding the interaction between cavitation bubble and impedance wall is of great significance for efficient application of ultrasonic cavitation.