碎片云超高速撞击声发射信号特征分析

为了掌握带防护屏的航天器结构受空间碎片超高速撞击时的声发射信号特征,利用二级轻气炮发射球形弹丸撞击铝合金双层板结构,获取了碎片云撞击铝合金板舱壁产生的声发射信号,并利用小波包技术和能量熵理论对信号进行了分析。实验结果表明:弹丸初始速度、防护屏厚度及弹丸直径是决定二次碎片云形态及声发射信号特征的重要因素;在本实验工况范围内,小波包能量熵值能够描述声发射信号频率的复杂程度;当弹丸初始速度处于破碎段(3~7km/s)时,随着初始速度的增大,二次碎片云进一步细化,撞击产生的声发射信号幅值趋于减小、频率成分趋于复杂化,其小波包能量熵值逐渐增大;防护屏厚度对声发射信号的小波包能量熵值影响较大,弹丸直径对其影响较小。研究结果有助于实现对碎片云撞击舱壁结构的损伤模式识别。     

气泡线性振动时近海面气泡群的声散射

海洋中的不同成因的气泡群是常见的水下声学目标及声呐混响源,因此对水下气泡群进行声学建模意义重大。利用有效媒质理论描述气泡群内部的相速度及声衰减变化,并考虑到海洋中气泡群往往产生于不同界面附近,进一步利用球面波叠加原理描述海面对气泡群散射声波的再辐射,导出了平海面作用下气泡群声散射截面的一般表达式,建立了其声散射模型,研究了单一尺寸及混合尺寸气泡群的声学特性。数值分析表明,气泡群的谐振频率会随其半径或孔隙率增加而降低;由于海面的存在,气泡群声散射截面会随频率进行周期性变化,且随气泡群远离海面,这一变化逐渐加剧。此外,若气泡的黏滞阻尼项在全部阻尼项中占比较高,气泡群声散射强度会在谐振频率附近存在起伏振荡。该模型可为近海面鱼群、气泡羽流及海底泄漏的甲烷气体的声学建模提供一定的理论基础。     

1991年声学

在50年前,人们发现,当将照相感光乳剂放置在一流体的表面之上,而在这流体中有一中等振幅(约1atm)的声场作用时,这些乳剂就会暴光.促成声发光发现的因素是对可以形成H_2O_2的认识,其形成与出现强烈振动的气泡有关.出现强烈振动气泡的现象叫声空化.在声发光中,空化气泡破碎时发射光量子.因为导致空化/声发光所需要的声场能量密度是十分小的(马赫数为10~(-5),或10~(-11)V/atom),低能量的光子的出现已经暗示放大或会聚了11个数量级.     

微气泡激发的声微流对细胞声孔效应的影响

理论及实验研究了微气泡激发的声微流对声孔效应的影响。实验采用低幅度(0.05~0.3MIPa)连续超声波信号照射MCF-7细胞,PEI:DNA的复合质粒和造影剂气泡的混合溶液,通过扫描电子显微镜测量细胞膜声孔效应。结果表明声孔大小随着激励声压的幅度和照射时间的增加而增大,平均孔径范围为100 nm~1.25μm。基于Marmottant微气泡振动模型的理论计算结果表明微气泡振动所产生的声微流引起的剪切力在低幅度超声引发声致穿孔作用中起着关键作用。      

流体机械微气泡发生器的结构优化研究

本文基于CFD技术对文丘里式微气泡发生器进行了研究，分析了微气泡发生器内流动特性及微气泡在管内扩散段的发展过程，得出气泡在湍流剪切作用下逐渐破碎。同时，分析了喉部、渐扩角对气泡发生器发泡性能的影响规律，通过实验对模拟结果进行了验证。结果表明：随着喉部内径的增大，气泡粒径呈逐渐增大趋势，入口压力也逐渐增大；随着扩张角的增大，气泡粒径呈先减小后增大的趋势；改变喉部结构形式，当采用阶梯喉部结构时，气泡粒径更小。     

方波驱动下双气泡的动力学行为*

探索方波驱动下双气泡的脉动规律,能够促进方波在声空化工程中的实际应用。本文通过数值求解双气泡耦合方程组,研究了方波驱动下双气泡的动力学行为,得到了多种条件下不同时刻两个气泡半径的数值,并以此计算出气泡间的次Bjerknes力。研究表明,增大驱动频率会使得两个气泡膨胀时能达到的最大半径和次Bjerknes力减小。当两个气泡的平衡半径不同时,其中一个气泡的剧烈收缩会使得另一个气泡产生一个振动方向相反的声脉冲。随着两个气泡平衡半径差距的增加,气泡收缩的时间间隔增大。此外,当驱动声压幅值逐渐增大时,气泡脉动规律也会发生很大的变化。     

单颗金刚石切削无氧铜的声发射关联维特征*

声发射技术可以实现无氧铜切削加工特征的监测与评价。采用声发射技术监测单颗金刚石磨粒旋转切削无氧铜,利用G-P算法重构出声发射时域信号相空间,采用自相关函数法计算出相空间时间延迟参数,通过相空间双对数曲线的计算,得到不同切削工况下的关联维数。研究结果表明,进给速度和切削速度对声发射信号影响较不显著,切深与声发射信号振幅呈正效应关系;声发射信号双对数曲线呈现阶段性增加趋势,并逐渐收敛于饱和状态,关联维数随着嵌入维数的增加先快速下降后趋于平稳;金刚石切削无氧铜的声发射信号具有混沌运动变化特性,在较小嵌入维数时,关联维数与切深和切削速度呈现线性负效应关系,与进给速度呈现线性正效应关系。该研究为无氧铜的切削加工提供理论参考。     

含气泡水的强非线性声学特性

本文提出一种描述含气泡水的非线性声场的物理模型。在声波驱动下，气泡壁作受迫振动，遵循Ｒａｙｌｅｉｇｈ－Ｐｌｅｓｓｅｔ方程，当共振时振幅很大，产生强烈的非线性振动。这非线性力学振动成为二次谐波声压的源，从而声场表现为强非线性。理论计算与ＷＵ和Ｚｈｕ的实验结果进行了比较，诸如强二次谐波声压等重要声学特性符合得比较满意。     

单压吸收式制冷系统气泡泵理论模型与验证

应用两相流理论,对单压吸收式制冷系统中气泡泵在绝热块状流工况下的工作特性进行理论分析,建立了气泡泵的理论模型,并对以饱和水为工质的气泡泵稳态工作运行进行了实验研究。分析结果表明,气泡泵提升管内的两相流流型与加热功率和沉浸比有关,随着加热功率或沉浸比的增加,提升管中两相流流型将发生改变,气泡泵提升效率先增大后减小,实验结果与理论分析曲线逐渐偏离。     

Q235钢板局部腐蚀声发射信号特性研究

为解决腐蚀声发射源特征提取和识别的难题,直接从理论上证明了腐蚀声发射监测的有效性并推导出声发射信号特征。气泡破裂声发射信号幅值近似与气泡半径平方和液位高度成正比,频率与气泡半径成反比;钢板及其腐蚀产物开裂声发射信号幅值与开裂位置局部应力强度成正比,频率与裂纹扩展速度成正比,与裂纹扩展距离成反比。用低频和高频两套声发射系统,同时长时间监测Q235钢板在10%FeCl_3·6H_2O、10%FeCl_3·6H_2O加0.01 mol/L HCl混合液、5%CuSO_4·5H_2O溶液中的腐蚀情况,辅以监测钢板及钢板腐蚀产物开裂作为验证实验。通过对声发射信号的参数及谱分析,得出不同声发射源可以通过撞击数及功率谱在频域的分布来有效区分。实验结果与理论分析相吻合,研究结果对腐蚀声发射监测技术具有重要指导意义。     

Experimental studies on perturbed acoustic resonant spectroscopy by a small rock sample in a cylindrical cavity

A measurement system for acoustic resonant spectroscopy (ARS) is established, and the effects of resonant cavity geometry, inner perturbation samples and environmental temperature on the ARS are investigated. The ARSs of the small samples with various sizes and acoustic properties are measured. The results show that at the normal pressure, the resonant frequency decreases gradually with the increase of liquid temperature in the cylindrical cavity, while the resonant amplitude increases. At certain pressure and temperature, both the resonant frequency and the amplitude decrease greatly when there exist air bubbles inside the cavity fluid. The ARS is apparently affected by the sample porosity and the sample location in the resonant cavity. At the middle of the cavity, the resonant frequencies reach their maximum values for all of the measurement samples. The resonant frequencies of the porous rock samples are smaller than those of the compacted samples if other acoustic parameters are the same. As the sample is moved from the top to the middle of the cavity along its axis, the resonant amplitude increases gradually for the compacted rocks while decreases for the unconsolidated rocks. Furthermore, the resonant amplitude increases firstly and then decreases if the porosity of the rock sample is relatively small. In addition, through the comparisons between the experimental and theoretical results, it is found that the effects of the acoustic parameters and sizes of the samples and the size of the cylindrical cavity on the laboratory results agree well with the theoretical ones qualitatively. These results may provide basic reference for the experiment study of rock acoustic properties in a low frequency using ARS.     

含混合气泡液体中声波共振传播的抑制效应

当声波在含气泡的液体中传播时会出现共振传播现象,即在气泡的共振频率附近声衰减和声速会显著地增大,这是声空化领域的一个重要现象.以往的研究一般假设液体中只存在单一种类的气泡,因此忽略了声波共振传播的某些重要信息.本文研究了含混合气泡液体中声波的共振传播,混合气泡是指液体中包含多种静态半径不同的气泡.结果显示:在这种系统中存在声波共振传播的抑制效应,即与含单一种类气泡的系统相比,在含混合气泡的系统中声波的共振衰减和共振声速会明显变小.对于两种气泡混合、多种气泡混合以及气泡满足某种连续分布的系统,研究了抑制效应的本质和主要特征,此外还探究了黏性和空化率等对抑制效应的影响.本文的研究结果是对该领域现有知识的必要补充.     

Dependence of the characteristics of bubbles on types of sonochemical reactors

Computer simulations of bubble oscillations in liquid water irradiated by an ultrasonic wave have revealed that the characteristic of bubbles depends on types of sonochemical reactors: a horn-type reactor and a standing-wave type reactor. When the acoustic amplitude is large at 20 kHz, the bubble content is mostly water vapor even at the end of the bubble collapse and the temperature inside a bubble at the collapse is relatively low. On the other hand, when the acoustic amplitude is relatively low, the bubble content is mostly noncondensable gas at the end of the bubble collapse and the bubble temperature is relatively high. In a horn-type sonochemical reactor, the former type of bubbles are dominant because many bubbles exist near the horn-tip where the acoustic amplitude is large, while in a standing-wave type reactor the latter type of bubbles are dominant because the Bjerknes force gathers bubbles at a region where acoustic amplitude is relatively low.     

Effect of static pressure on acoustic energy radiated by cavitation bubbles in viscous liquids under ultrasound

The effect of static pressure on acoustic emissions including shock-wave emissions from cavitation bubbles in viscous liquids under ultrasound has been studied by numerical simulations in order to investigate the effect of static pressure on dispersion of nano-particles in liquids by ultrasound. The results of the numerical simulations for bubbles of 5 μm in equilibrium radius at 20 kHz have indicated that the optimal static pressure which maximizes the energy of acoustic waves radiated by a bubble per acoustic cycle increases as the acoustic pressure amplitude increases or the viscosity of the solution decreases. It qualitatively agrees with the experimental results by Sauter et al. [Ultrason. Sonochem. 15, 517 (2008)]. In liquids with relatively high viscosity (～200 mPa s), a bubble collapses more violently than in pure water when the acoustic pressure amplitude is relatively large (～20 bar). In a mixture of bubbles of different equilibrium radius (3 and 5 μm), the acoustic energy radiated by a 5 μm bubble is much larger than that by a 3 μm bubble due to the interaction with bubbles of different equilibrium radius. The acoustic energy radiated by a 5 μm bubble is substantially increased by the interaction with 3 μm bubbles.     

Effects of argon sparging rate,ultrasonic power,and frequency on multibubble sonoluminescence spectra and bubble dynamics in NaCl aqueous solutions

The sonoluminescence spectra from acoustic cavitation in aqueous NaCl solutions are systematically studied in a large range of ultrasonic frequencies under variation of electrical power and argon sparging. At the same time, bubble dynamics are analysed by high-speed imaging. Sodium line and continuum emission are evaluated for acoustic driving at 34.5, 90, 150, 365, and 945 kHz in the same reactor vessel. The results show that the ratio of sodium line to continuum emission can be shifted by the experimental parameters: an increase in the argon flow increases the ratio, while an increase in power leads to a decrease. At 945 kHz, the sodium line is drastically reduced, while the continuum stays at elevated level. Bubble observations reveal a remarkable effect of argon in terms of bubble distribution and stability: larger bubbles of non-spherical shapes form and eject small daughter bubbles which in turn populate the whole liquid. As a consequence, the bubble interactions (splitting, merging) appear enhanced which supports a link between non-spherical bubble dynamics and sodium line emission.     

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.     

Enhancement of oscillation amplitude of cavitation bubble due to acoustic wake effect in multibubble environment

Acoustic cavitation occurs in ultrasonic treatment causing various phenomena such as chemical synthesis, chemical decomposition, and emulsification. Nonlinear oscillations of cavitation bubbles are assumed to be responsible for these phenomena, and the neighboring bubbles may interact each other. In the present study, we numerically investigated the dynamic behavior of cavitation bubbles in multi-bubble systems. The results reveal that the oscillation amplitude of a cavitation bubble surrounded by other bubbles in a multi-bubble system becomes larger compared with that in the single-bubble case. It is found that this is caused by an acoustic wake effect, which reduces the pressure near a bubble surrounded by other bubbles and increases the time delay between the bubble contraction/expansion cycles and sound pressure oscillations. A new parameter, called “cover ratio” is introduced to quantitatively evaluate the variation in the bubble oscillation amplitude, the time delay, and the maximum bubble radius.     

气泡体积分数对沙质沉积物低频声学特性的影响

由于有机物质分解等原因,实际的海底沉积物中存在气泡,气泡的存在会显著影响沉积物低频段的声学特性,因此研究气泡对沉积物低频段声速的影响机理具有重要意义.考虑到外场环境的不可控性,在室内水池中搭建了大尺度含气非饱和沙质沉积物声学特性获取平台,在有界空间中应用多水听器反演方法首次获取了含气非饱和沙质沉积物300—3000 Hz频段内的声速数据(79—142 m/s),并同时利用双水听器法获取了同一频段的数据(112—121 m/s).在声波频率远低于沉积物中最大气泡的共振频率时,根据等效介质理论,将孔隙水和气泡等效为一种均匀流体,改进了水饱和等效密度流体近似模型.模型揭示了气泡对沉积物低频段声学特性的影响规律,理论上解释了沉积物中声速的降低.通过分析模型预报声速对模型参数的敏感性,根据测量得到的声速分布反演得到了沉积物不同区域的气泡体积分数,气泡体积分数从1.07%变化到2.81%.改进的模型为沉积物中气泡体积分数估计提供了一种新方法.     

FEM calculation of an acoustic field in a sonochemical reactor

The spatial distribution of the acoustic amplitude in a sonochemical reactor has been numerically calculated using the finite element method (FEM). In the FEM program, the acoustic field in a sonochemical reactor is coupled with the vibration of the reactor's wall. The present calculations have revealed that the thin (thick) glass or stainless steel wall is nearly a free (rigid) boundary and that the glass wall is freer than the stainless steel wall. The influence of the attenuation coefficient of ultrasound on the acoustic field has also been studied in order to see the effect of bubbles on the acoustic field. As the attenuation coefficient increases, the vibration amplitude of the reactor's wall becomes smaller and the acoustic emission from the vibrating wall becomes weaker. The qualitative feature of the spatial pattern of sonochemiluminescence from an aqueous luminol solution has been reproduced by the calculation when the attenuation coefficient is in the range of 0.5-5m(-1). When the attenuation coefficient is less than about 0.05 m(-1), the standing wave pattern of an acoustic field in the liquid is very complex due to the acoustic emission from the vibrating wall. The present calculations have also revealed that some stripes of pressure antinodes have also been disconnected when the radius of the transducer is much smaller than the side length of the vibrating plate. The dependence of the acoustic field on the liquid height is also discussed.     

Theoretical prediction of the yield of strong oxides under acoustic cavitation

Considering liquid viscosity, surface tension, and liquid compressibility, the effects of dynamical behaviors of cavitation bubbles on temperature and the amount of oxides inside the bubble are numerically investigated by acoustic field,regarding water as a work medium. The effects of acoustic frequency, acoustic pressure amplitude, and driving waveforms on bubble temperature and the number of oxides inside the bubbles by rapid collapse of cavitation bubbles are analysed.The results show that the changes of acoustic frequency, acoustic pressure amplitude, and driving waveforms not only have an effect on temperature and the number of oxides inside the bubble, but also influence the degradation species of pollution,which provides guidance for improving the degradation of water pollution.