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.     

有黏条件气泡声散射特性和衰减谱数值研究*

通过研究有黏条件下的气泡散射模型,数值分析水中单气泡声散射特性,进一步结合Beer-Lambert定律将其扩展到多气泡体系的声衰减预测。数值结果表明,随着谐波阶数递增,散射强度分布数值结果趋于稳定且前向散射增强。同时发现,无因次尺寸参量ka=0.1为过渡区与纯散射区的分界线,且在共振区间具有明显的吸收效应。对多气泡体系的声衰减预测也表明,ka 0.1时,该文气泡散射模型声衰减计算与经典ECAH模型结果吻合,在低浓度条件下声衰减谱随着剪切黏度的增加呈增宽趋势,且与体积浓度成正比例递增。模型预测的声衰减随粒径、声波频率、体积浓度分布数值特征能够为颗粒两相体系粒径及浓度表征提供理论依据。     

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。     

高密度超细颗粒高浓度悬浊液超声衰减的数值模拟分析

以拓展耦合相ECP理论模型为基础,通过对高浓度超细二氧化钛-水悬浊液和玻璃微珠-水悬浊液中超声衰减的数值计算分析,讨论了高密度差异颗粒两相介质在高浓度情况下,超声频率、颗粒粒径大小、颗粒浓度对超声波衰减的影响,为高密度超细颗粒在高浓度悬浊液中颗粒粒度和浓度的超声波测量提供理论支持。     

纤维复合媒质中的超声衰减

为了精确估计纤维复合煤质中散射所引起的超声衰减，必须计及纤维之间的多次散射现象本文提出了纤维复合煤质中任一点有效入射声场的计算方法当超声纵波传播方向与纤维轴线相垂直时，散射横波并不影响有效入射声场；导出了考虑多次散射后超声衰减系数的表达式；数值计算了玻璃／金属铝复合媒质中纵波衰减系数随频率的变化关系 关键词：     

二维圆盘颗粒体系声学行为的数值研究

数值测量了卸载过程中二维单分散圆盘颗粒系统的横波、纵波声速、声衰减系数、非线性系数随压强的变化以及声衰减系数随频率的变化.结果表明,二维(2D)圆盘颗粒体系的横波、纵波声速均随压强呈分段幂律标度:当压强P10~(-4)时,横波、纵波声速随压强的增大而减小;当P10~(-4)时,有v_t~P~(0.202),v_l~P~(0.338).进一步得到其剪切模量和体积模量的比值G/B也随压强呈幂律标度,G/B~P~(-0.502),暗示在低压强下,与三维(3D)球形颗粒体系类似,2D圆盘颗粒体系也处于L玻璃态.水平激励和垂直激励下2D圆盘颗粒系统的衰减系数随频率变化也呈现分段行为:当频率f0.05时,衰减系数不随f变化;当f0.05时,横波纵波的衰减系数α~f;当f0.35时,横波衰减系数α_T~f~2,纵波衰减系数α_L~f~(1.5).此外,竖直水平激励下的2D圆盘颗粒系统的非线性系数和衰减系数随压强也呈现与声速类似的分段规律:当P10~(-4)时,横波非线性系数β_T~P~(-0.230),其余都不随压强变化.当P10~(-4)时,两者均随压强增大呈幂律减小:β_T~P~(-0.703),β_L~P~(-0.684),α_T~P~(-0.099),α_L~P~(-0.105).进而得到2D圆盘颗粒系统中散射相关的特征长度?~*随压强呈幂律标度,当P10~(-4)时,?~*~P~(-0.595);当P10~(-4)时,?~*~P~(0.236).     

外加液滴条件下固体细颗粒声凝并特性

外加声场激励下固体细颗粒的声凝并在燃烧污染物超低排放中具有应用潜力,添加大粒径液滴有望提高细颗粒声凝并效果.本文从颗粒运动、碰撞、凝并与反弹的动力学过程出发,基于直接模拟蒙特卡罗方法,建立气相中液滴与固体颗粒共存的气-液-固三相体系的声凝并模型,对外加液滴条件下固体细颗粒声凝并的过程和效果开展数值模拟.将模拟结果与实验相对比,验证模型可靠性.在此基础上,探究外加液滴条件下细颗粒声凝并动力学行为,考察外加液滴直径和数目浓度对细颗粒声凝并效果的影响规律.结果表明,外加液滴条件下,固体细颗粒迅速与大粒径液滴凝并,形成液-固混合相颗粒,细颗粒凝并效率显著提升.外加液滴直径和数目浓度是影响细颗粒声凝并的重要因素,随着直径的增大或数目浓度的提高,细颗粒凝并效率增大,而增幅趋于减小.研究结果可为复杂颗粒系统凝并模型的建立提供理论基础,并可为燃烧源细颗粒超低排放提供方法指导.     

固井水泥传输函数及其含水成份对声速和声衰减影响的实验研究

本文采用实验方法初步研究了固井水泥的超声传输函数、声衰减和频率的关系以及含水饱和度对声速和衰减的影响。实验中我们对若干种不同型号、水灰比和固化温度的水泥样品进行了声传输函数测量,测量频段为1.5MHz—4.5MHz。并使用中心频率为1.25MHz探头测量了固井水泥含水饱和度对声速和声衰减的影响。结果表明,在本测量频段内,各种固井水泥的声衰减系数主要与频率一次方成正比。随着固化温度的增加,固井水泥的声衰减系数有增大的趋势,水灰比由40％增至50％时,衰减系数有减小的趋势。固井水泥含水饱和度变化对声衰减影响剧烈,当含水饱和度由100％降至80％时,声衰减增加最快。含水饱和度变化对固井水泥声速度的影响不大。     

格子气的声振荡衰减

采用格子气数值方法模拟了共振腔中气体声振荡衰减过程．计算结果表明：格子气中声振荡衰减的规律和经典理论结果一致；衰减系数随频率增加而减小，并且随密度增加而减小．该结果可以用来说明一些大学统计物理教科书中的一个经典例子存在不完善之处．本中采用了一种新的扭转9-bit格子气模型和并行计算方法，这些结果对今后研究格子气中的热声问题有参考价值．     

超细颗粒悬浊液中声衰减和声速的数值分析研究

为研究应用超声测量超细颗粒的粒度和浓度,本文由描述声波在悬浊液中传播的Allegra &Hawley数学模型出发,通过两个算例的数值模拟计算,研究了二氧化钛－水,铁粉-水两种超细颗粒悬浊液中声波传播的衰减和相速度,分析了声波的频率、颗粒尺寸和浓度对衰减和相速度的影响规律,并讨论了计算模型对不同物性参数的敏感程度。     

Simulation of ultrasonic attenuation in theelastic mixing particle system

For the study of predicting ultrasonic attenuation of elastic, spherical mixing particles in the liquid-solid two-phase system, the Monte Carlo method(MCM) is introduced,serving as a probability and statistics technique to evaluate the inside ultrasonic events during the ultrasound propagation. On the basis of ultrasonic scattering and aborption, the continuous ultrasonic waves are represented as discrete and independent phonons. By recording the scattering events, tracing the trajectory of a moving phonon and calculating the number of phonons that finally reach the receiving transducer, the ultrasonic attenuation coefficient is obtained to be a frequence-dependent spectrum. Numerical investigations have been carried out to predict and compare the ultrasonic attenuation for a solid-liquid two-phase system with a single type particle. After verifing its feasibility, such a method is then appalied into mixing particle system, where the mixing iron particles and glass beads with various ratios are set as examples for the purpose of predicting ultrasonic attenuation for the monodisperse and polydisperse mixing particle systems. The results of MCM, the ECAH model, the Lloyd Berry(LB) model and the Waterman model match well when the particle volume concentration is lower than 10%,corresponding to iron particles and glass beads respectively. In the case of two-phase system with mixing particles, it is shown that as the particle volume concentration increases to 10%, the variation of the ultrasonic attenuation coefficient with mixing ratio yields a nonlinear tendency.The physical properties of particles can also influence ultrasonic attenuation significantly.     

Analysis of ultrasonic attenuation in particle-reinforced plastics by a differential scheme

Attenuation of ultrasonic longitudinal waves in some particle-reinforced polymer composites is studied theoretically by a micromechanical model based on a differential (incremental) scheme. A set of differential equations is established by which the attenuation spectrum of the composite can be computed from the known properties of viscoelastic matrix and elastic particles. For a composite reinforced with glass particles with radius 0.15 mm, the proposed scheme is shown to predict the attenuation in better agreement with the foregoing experimental results than the previous simplistic independent scattering model. Based on this scheme, the dependence of the longitudinal attenuation spectrum of a particulate polymer composite on the wavelength-to-particle radius ratio and the particle volume fraction is examined in detail. It is then shown theoretically that the attenuation of the composite decreases monotonically with the particle volume fraction when the particle radius is sufficiently small compared to the incident wavelength, while it shows non-monotonic particle-fraction dependence when the ratio of the particle radius to the wavelength is larger. To examine this theoretical finding from an experimental point of view, the longitudinal attenuation in a glass-particle-reinforced polyester composite with particle radius 0.0225 mm is measured for different particle volume fractions. The measured attenuation characteristics are shown to support the qualitative features of the theoretical prediction.     

Analysis of ultrasonic attenuation in particle-reinforced plastics by a differential scheme

Attenuation of ultrasonic longitudinal waves in some particle-reinforced polymer composites is studied theoretically by a micromechanical model based on a differential (incremental) scheme. A set of differential equations is established by which the attenuation spectrum of the composite can be computed from the known properties of viscoelastic matrix and elastic particles. For a composite reinforced with glass particles with radius 0.15 mm, the proposed scheme is shown to predict the attenuation in better agreement with the foregoing experimental results than the previous simplistic independent scattering model. Based on this scheme, the dependence of the longitudinal attenuation spectrum of a particulate polymer composite on the wavelength-to-particle radius ratio and the particle volume fraction is examined in detail. It is then shown theoretically that the attenuation of the composite decreases monotonically with the particle volume fraction when the particle radius is sufficiently small compared to the incident wavelength, while it shows non-monotonic particle-fraction dependence when the ratio of the particle radius to the wavelength is larger. To examine this theoretical finding from an experimental point of view, the longitudinal attenuation in a glass-particle-reinforced polyester composite with particle radius 0.0225 mm is measured for different particle volume fractions. The measured attenuation characteristics are shown to support the qualitative features of the theoretical prediction.     

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.     

Polydisperse particle size characterization by ultrasonic attenuation spectroscopy in the micrometer range

The theoretical advantages of ultrasonic attenuation spectroscopy for particle size are currently not fully utilized. Especially in the region of larger particles, there is a lack of experimental confirmation of applicable models which may be used to infer particle sizes from measured attenuation spectra. With the present work, an attempt is made to supply experimental data, obtained with a commercially available ultrasonic attenuation spectrometer, and model calculations, which are based on the resonant scattering theory. It is shown that measured attenuation results for various combinations of disperse and continuous phase for both polydisperse emulsions and suspensions are reproducible by calculation. The approach is further examined for suspensions of porous particles. Here, the resonant scattering approach is combined with the Biot model for poroelasticity to obtain attenuation results with several fractions of titania aggregates, differing in particle size and pore diameter. The results indicate that the theory of resonant scattering is a valid approach if applied to particle size characterization in the large particle limit.     

雾霾粒子的紫外光散射特性研究

紫外光与雾霾粒子发生散射后,其散射信道特性能够反映雾霾粒子的相关物理信息,利用无线紫外光单次和多次散射信道模型,采用Mie散射和T矩阵理论分析了霾粒子在不同形态和浓度下的紫外光散射信道特性,以及散射角对散射光强的影响,并完成了紫外光在雾霾环境下的实测。通过理论及仿真分析,得到了不同霾粒子形态下的紫外光通信路径损耗以及光强分布。结果表明：紫外光直视通信方式下,路径损耗随着霾粒子浓度的增大而增大,且通信质量差于晴朗天。非直视通信方式中,在短距离通信时,高霾浓度下的路径损耗小于中低霾浓度,然而随着通信距离的继续增大,高雾霾浓度下的通信质量急剧下降,低霾浓度下通信质量最终达到最优,且距离为200 m时通信质量能优于晴朗环境。当通信距离相同时,三种雾霾浓度下的紫外光散射光强分布均随着散射角的增大而减小,当散射角继续增大并超过90°时,低霾浓度下的散射光强最大。主要原因是虽然散射角继续增大,但是有效散射体体积逐渐减小,因此低霾浓度下的散射光强较大。且当粒子粒径相同时,球形粒子的衰减较非球形粒子大。雾霾环境下实测结果与仿真结果相类似,证明了仿真结果的正确性,并在一定程度上证明了实际大气中雾霾非球形粒子多于球形粒子。     

Anisotropic attenuation of transverse ultrasound in cubic crystals of Ge,Si, and diamond with various isotopic compositions

The attenuation of transverse ultrasound in germanium, silicon, and diamond crystals is considered with allowance for competing isotopic and anharmonic scattering processes. The dependence of the attenuation of transverse ultrasound on the direction of the wave vector of quasi-transverse phonons is analyzed within an anisotropic continuum model. The Landau—Rumer mechanism is considered for anharmonic scattering processes. Given the second-and third-order elastic moduli, the parameters are found determining ultrasonic absorption in the above crystals with various degrees of isotopic disorder. The attenuation coefficients of transverse ultrasound associated with isotopic and anharmonic scattering processes are shown to have qualitatively different angular dependences. Therefore, from studying the anisotropic attenuation of ultrasound in cubic crystals, one can determine the dominant mechanism of ultrasonic absorption in isotopically modified crystals.     

A study on measuring distribution of nanoparticle size based on ultrasonic attenuation spectrum

Through varying the ultrasonic path length,the ultrasonic attenuation spectrum of the nanometer particle suspensions is measured.Based on the McClements and BLBL model, the Optimum Regularization Technique is employed to inverse the distribution of particle size for nanometer particles.Experiments are carried out to measure the distribution of particle size of aqueous nanometer titanium dioxide suspension with volume fraction of 1%.The ultrasonic measurement results show good consistence with those from TEM(Transmission Electron Microscope) and centrifugal sedimentation particle size analyzers,which indicate that measuring distribution of the particle size of nanometer particles with the ultrasonic attenuation spectrum is feasible and reliable.     

基于超声波测量技术的颗粒尺寸分布模型的研究

提出了一种利用单颗粒背向散射信号来进行粒径分布估算的超声波测量技术,系统阐述了其 理论基础及基本工作原理。被测颗粒的散射信号幅度与它在探测区中的位置、颗粒尺寸和形状有关。 通过理论分析,建立了一个可用于描述散射信号幅度概率分布与颗粒系尺寸分布之间关系的数学模 型,对该模型的反演计算便可得到颗粒系的尺寸分布信息。为了避免求解过程中的病态系数矩阵问 题,将模型求解转化成了易于解决的最优化问题。仿真实例结果验证了该技术的可行性。