混合弹性颗粒体系声衰减数值模拟

采用概率统计方法——蒙特卡罗方法,建立一种预测液固两相体系中混合弹性球形颗粒声衰减的理论模型。在单颗粒声散射和吸收的基础上,将连续超声波抽象离散化为大量独立的声子,追踪声散射过程,通过统计接收器探测声子数最终确定声衰减系数。采用数值方法对单一球形颗粒的液固两相体系中声衰减进行预测和比较,确定了该方法的可行性后将该方法推广到混合颗粒体系中,对玻璃微珠/铁粉构成的混合颗粒及多分散混合颗粒体系进行数值研究。结果表明:在体积浓度低于10%时,蒙特卡罗法预测得玻璃微珠或铁粉颗粒声衰减和ECAH,Lloyd和Berry,Waterman等建立的模型结果吻合。对于混合颗粒构成的两相体系,算例中,随着体积浓度增大到10%,声衰减系数随混合颗粒数目比的变化呈现出了非线性的变化,同时颗粒物性也会影响不同组分颗粒对声衰减的贡献,算例中铁粉颗粒比玻璃微珠对声衰减的影响更大。      

基于蒙特卡罗原理的混合颗粒三相体系声衰减计算模型研究

从单个固体和液滴颗粒的声吸收和散射特性计算入手,基于概率统计的蒙特卡罗方法(MCM),将声波以离散化的声子加以处理,通过追踪其运动历程并进行事件统计,建立一种气体介质中球形混合颗粒的声衰减预测模型.对空气中铝粉颗粒和亚微米级水滴的声衰减分别计算和验证后,将模型推广至含有混合颗粒的三相体系,对铝粉和液滴构成的单、多分散混合颗粒体系进行数值研究.结果表明:两类颗粒的声吸收特性差异明显,其散射声压均随颗粒无因次尺寸kR的增加呈现从后向散射占主要地位逐渐过渡到前向增强的趋势.气液固混合颗粒三相体系中,颗粒类型对于声衰减影响明显、且随浓度的增加不同颗粒的衰减贡献不再遵循随混合比的线性递变关系;对于多分散体系而言,声衰减谱受平均粒径影响较大,对于粒径分布宽度参数则不敏感.模型可进一步结合数学反演形成混合颗粒体系测量的理论基础.     

基于小斜率近似的深海海面混响

收发设备在海面附近的深海混响实验中,多途时延使得最先到达水听器的为海面混响信号,且不受海底散射声场的干扰.本文利用射线理论描述深海声传播的格林函数,采用粗糙界面一阶小斜率近似方法描述全角度海面散射,给出海面混响声场的表达式.同时考虑了海面表层气泡散射的贡献,获得了海面混响理论.通过数值仿真数据和深海实验数据的比较对海面混响模型进行验证,分析了不同接收深度、频率下的海面混响强度衰减趋势.结果表明:低海况条件下,低频海面混响由粗糙界面散射主导,气泡散射可以忽略,随频率升高,气泡散射对海面混响的贡献逐渐增大,海面附近收发深度的小幅变化对混响衰减曲线的影响不明显.基于该模型提出一种反演海面粗糙界面谱参数的方法,数值计算结果验证了该模型能够在风速已知的前提下,通过海面混响数据提取海面粗糙界面谱参数.     

超声法测量高浓度矿浆两相流粒径分布和浓度

研究了超声衰减谱法和超声多次回波反射法测量高浓度矿浆两相体系粒径分布和浓度。实验测量了多个浓度下硫矿浆在3～6 MHz频率下的声衰减谱,结合反演算法由实验数据计算得到矿浆两相流粒径分布。同时测量了不同浓度下硫矿的声阻抗值,由超声多次回波反射法计算得到浓度。粒径分布与浓度测量得到的结果与标准配置值较为吻合,表明超声法适用于高浓度矿浆两相流粒径分布和浓度的在线检测。     

非球形气泡的超声定量检测

超声是检测不透明液体中气泡的有效方法,声散射模型是超声反演技术的核心.经典气泡散射模型通常是基于球形假设及ka≤1 (a为气泡半径,k为入射波的波数),然而实际应用中这些假设并不总能得到满足.本研究针对非球形气泡及ka偏离假设情况,提出一种超声反演定量方案.建立不受ka约束的球形气泡级数背散射f模型,将其与经典Medwin (ka?1)和Anderson (ka≈1)散射f模型进行对比,发现ka偏离引发的散射截面效应仅体现在散射高阶共振峰位置及大小差异上.据此提出:可通过曲线拟合法解决散射截面σ_bs/(πa²)与ka间的多值问题,同时用当量半径a^*对非球形气泡尺寸进行量化.具体首先利用非球形气泡背散射信号的频域信息测定其散射截面σ_bs,再根据σ_bs与非球形当量半径a^*间拟合曲线进行反演,同时利用回波时域信息勾画气泡形状轮廓.反演结果通过高速摄影定量结果进行检验.结果表明:气泡沿之字形路径上升过程中产生非球形形变,当9≤kr₀≤35时,...     

超声脉冲背向散射法测量悬浊液浓度的研究

本文报导了在不同浓度和不同粒径的水泥浆悬浊液中超声脉冲背向散射讯号的数字特征,主要是幅域和频域特征.对于不同的浓度范围,可以取不同的背向散射讯号特征量(如散射声强或散的衰减)来表征是浊液的浓度和粒径.对于低浓度范围,相对散射声强与浓度有较好的线性关系,对于高浓度范围,声强的散射衰减与浓度有较好的线性关系,高浓度与低浓度之间存在着过渡区域,在此区域内,散射声强随浓度的增加与散射衰减随浓度的增加呈动态平衡,因而在过渡区域内散射声强将与浓度无关,并与频率或粒径有关,当频率一定时,可确定粒径分布.     

超声脉冲背向散射法测量悬浊液浓度的研究

本文报导了在不同浓度和不同粒径的水泥浆悬浊液中超声脉冲背向散射讯号的数字特征，主要是幅域和频域特征。对于不同的浓度范围，可以取不同的背向散射讯号特征量（如散射声强或散射衰减）来表征悬浊液的浓度和粒径，对于低浓度范围，相对散射声强与浓度有较好的线性关系，对于高浓度范围，声强的散射衰减与浓度有较好的线性关系，高浓度与低浓度之间存在着过渡区域，在此区域内，散射声强随浓度的增加与散射衰减随浓度的增加呈动态平衡，因而在过渡区域内散射声强将与浓度无关，并与频率或粒径有关，当频率一定时，可确定粒径分布。     

水中气泡上的体散射函数的模拟与计算

基于几何光学的基本原理,推导出一种可以计算水中大尺度气泡上光散射角度与强度的关系式。推导中避免引入衰减因子G,较Davis模型更为简单。该模型可应用于光在水中单个气泡上散射的数值计算。最后,模拟计算了平行光束入射水中气泡的体散射函数曲线,发现水中气泡的前向散射远大于后向散射;当气泡半径在远大于光波波长的前提下变化时,气泡上散射光强分布规律与气泡半径无关;而介质相对折射率的增大会削弱前向散射而增强后向散射光强。     

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。     

超声衰减法测量悬浊液中颗粒粒度和浓度

研究了由颗粒悬浊液中3个频率下声衰减系数测量颗粒粒度分布和浓度的方法。用FFT对测得的1,2和5 MHz 3个频率的声信号进行降噪后得到声幅度和声衰减系数,再根据描述不同频率下声衰减系数与颗粒粒径及浓度关系的ECAH模型,运用Philip-Twomey-NNLS算法和改进Chahine算法求解相关的第一类Fredholm方程,反演出颗粒的尺寸分布和浓度。用该方法对两种不同尺寸体积浓度为5％的玻璃微珠-水悬浊液颗粒进行测量,结果与显微镜法结果吻合。这表明:测量3个频率下的声衰减信号,结合Philip-Twomey-NNLS或改进的Chahine算法,可在较高浓度条件下较准确地测得颗粒粒度和浓度,从而简化了现有的高频率宽频带测量方法。     

声波在含气泡液体中的线性传播

为了探讨含气泡液体对声波传播的影响, 研究了声波在含气泡液体中的线性传播. 在建立含气泡液体的声学模型时引入气泡含量的影响,建立气泡模型时引用 Keller的气泡振动模型并同时考虑气泡间的声相互作用,得到了经过修正的气泡振动方程. 通过对含气泡液体的声传播方程和气泡振动方程联立并线性化求解,在满足 (ω R₀)/c << 1 的前提下,得到了描述含气泡液体对声波传播的衰减系数和传播速度. 通过数值分析发现,在驱动声场频率一定的情况下,气泡含量的增加及气泡的变小均会导致衰减系数增加和声速减小;气泡的体积分数和大小一定时, 驱动声场频率在远小于气泡谐振频率的情况下,声速会随驱动频率的增加而减小; 气泡间的声相互作用对声波传播速度及含气泡液体衰减系数的影响不明显.最终认为气泡的大小、 数量和驱动声场频率是影响声波在含气泡液体中线性传播的主要因素. 关键词： 含气泡液体 线性声波 声衰减系数 声速     

Consistency in statistical moments as a test for bubble cloud clustering

Frequency dependent measurements of attenuation and/or sound speed through clouds of gas bubbles in liquids are often inverted to find the bubble size distribution and the void fraction of gas. The inversions are often done using an effective medium theory as a forward model under the assumption that the bubble positions are Poisson distributed (i.e., statistically independent). Under circumstances in which single scattering does not adequately describe the pressure field, the assumption of independence in position can yield large errors when clustering is present, leading to errors in the inverted bubble size distribution. It is difficult, however, to determine the existence of clustering in bubble clouds without the use of specialized acoustic or optical imaging equipment. A method is described here in which the existence of bubble clustering can be identified by examining the consistency between the first two statistical moments of multiple frequency acoustic measurements.     

Study on the multiple scattering effects of ultrasound contrast agents

In this paper, the multiple scattering of interacting encapsulated microbubbles in suspensions is calculated using two novel approaches--Kargl's effective medium approach and Ye and Ding's approach of 2nd-order correction. Two types of contrast agents with bubbles of different sizes and concentrations are chosen for our numerical simulations. One is Albunex, which is depicted by Church and has a size range of several microns, and the other is sodium laureate solution (before fractionation) described by Soetanto and Chan and has an average size of 35.5 microm. The numerical results from these two approaches are compared with that from the linear approximation. It is found that the multiple scattering effects on attenuation and dispersion of sound in suspensions are evident in the cases of high bubble volume fractions, basically greater than the order of 1 x 10(-4), and much more obvious for larger bubbles with average size of tens of microns.     

Sound waves in a liquid with polydisperse vapor–gas bubbles

A mathematical model is presented for the propagation of plane, spherical, and cylindrical sound waves in a liquid containing polydisperse vapor–gas bubbles with allowance for phase transitions. A system of integro-differential equations is constructed to describe perturbed motion of a two-phase mixture, and a dispersion relation is derived. An expression for equilibrium sound velocity is obtained for a gas–liquid or vapor–liquid mixture. The theoretical results agree well with the known experimental data. The dispersion curves obtained for the phase velocity and the attenuation coefficient in a mixture of water with vapor–gas bubbles are compared for various values of vapor concentration in the bubbles and various bubble distributions in size. The evolution of pressure pulses of plane and cylindrical waves is demonstrated for different values of the initial vapor concentration in bubbles. The calculated frequency dependence of the phase sound velocity in a mixture of water with vapor bubbles is compared with experimental data.     

Ultrasonic Measurement of Sub-Micron Particles

Based on technology described in a Du Pont patent for an offline system, we have developed a prototype in-line ultrasonic cell for measuring particle size distribution. We have used this cell to verify the Allegra-Hawley model of ultrasonic attenuation in dilute (< 5 % vol) slurries of sub-micron ceramic particles, and we are developing a model that can cope with the multiple scattering effects occuring at higher concentrations. In this paper we present the results of attenuation measurements for ultrasound (2–50 MHz) in slurries with concentrations ranging from 0.5 010 to 38 010 (vol). The attenuation is proportional to slurry concentration up to 5 010 (vol) and is predicted by single scattering models. Above approximately 100% concentration, the attenuation is actually lower than expected. For the dilute slurries we find excellent agreement between our measurements and the Allegra-Hawley calculations, and the effects of the particle size distribution are evident in the ultrasonic attenuation spectrum. These ultrasonic data can be inverted to determine the particle size and concentration in aqueous slurries of sub-micron particles.     

次Bjerknes力作用下气泡的体积振动和散射声场

利用Lagrange方程得到了次Bjerknes力作用下气泡的体积振动方程,并探讨了次Bjerknes力作用下不同参数对气泡体积振动振幅和振动初相位的影响,研究了振动初相位差为π和0的气泡对在液体中形成的散射声场特征.结果表明:次Bjerknes作用力下,相邻气泡半径、气泡间距、多方指数均能影响气泡的体积振动振幅,气泡对的均衡半径、气泡间距和驱动频率则对气泡振动初相位产生明显影响;相距很近、相位相差为π的两个气泡的散射声压与气泡体积振动振幅、气泡间距、驱动频率和振动初相位有关,随声场距离成反比减小,与声场位置有关,其平均散射声功率是单个孤立气泡的1/6(kd_(12))~2半径相同、相距很近、相位相同的两个6气泡的散射声压与气泡振动初相位、体积振动振幅、气泡间距、驱动频率有关,随声场距离成反比减小,其平均散射声功率是单个孤立气泡的4倍.     

基于多重散射强度和偏振特征的舰船尾流气泡激光探测方法

研究多重散射效应对舰船尾流气泡群光散射强度和偏振特征的影响是舰船光尾流探测以及新型光自导鱼雷研究的基础. 基于矢量Monte Carlo方法建立了舰船尾流气泡群激光后向探测仿真模型, 重点研究了尾流气泡群的多重散射机理,分析了多重散射效应、尾流气泡群密度对回波信号强度和偏振特征的影响规律. 基于粒子碰撞重要性抽样的基本思想, 在传统能量接收方法的基础上, 提出了回波光子偏振贡献接收方法和回波信号偏振信息统计方法, 解决了小视场系统光子返回概率低无法形成回波能量的难题. 构建了模拟尾流气泡群激光散射强度和偏振探测实验平台, 从实验的角度验证了模拟结果的准确性. 实验和模拟结果的一致性表明, 利用回波强度、偏振信息可表征气泡群距离、密度信息, 从而可对舰船尾流特别是低密度尾流进行高精度的探测和辨识. 关键词： Monte Carlo 偏振 多重散射 气泡     

Counting bubbles acoustically: a review

The acoustical resonance characteristics of bubbles are the key to their detection by backscatter, excess attenuation and sound speed dispersion in bubbly liquids. The linear, lumped-constant, behaviour of single bubbles is reviewed and formulated in terms of the physical constants of the medium liquid, the bubble gas and the interface between. Acoustical techniques for bubble counting are discussed using in-situ experiments at sea as an example.     

Measurements of bubbles in sea water by nonstationary sound scattering

Methods for the characterization of bubbles in sea water by acoustic scattering are analyzed. Nonstationary linear and nonlinear sound scattering methods are proposed. The transient linear and nonlinear sound scattering allows the scattering by resonant gas bubbles to be distinguished from the scattering by other microinhomogeneities. The application of parametric arrays in oceanic experiments, together with the broadband frequency analysis of the backscattering coefficient, allows information about bubbles in sea water to be obtained. Experimental results on sound scattering and gas bubble distribution functions are presented for different conditions in the ocean.     

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.