The propagation of a nonlinear sound wave in an unconsolidated granular medium

The propagation of a high-intensity sound wave in an unconsolidated medium is considered. Dissipation effects are taken into account on the basis of Buckingham’s theory of a relaxation mechanism of sound attenuation in a saturated sediment. The nonlinear evolution equation for the relaxing medium is obtained, and the solutions of this equation are analyzed. The second-harmonic generation in such a medium decays, as does the linear sound wave of the same frequency. The stationary weak shock profile has a specific form due to relaxation effects.     

Non-local treatment of hopping conduction — Application to ultrasonic attenuation

A non-local theory of hopping conduction is developed on the basis of the relaxation mode theory. An application of this theory to the ultrasonic attenuation is made, in which the attenuation by particles hopping on a doubly periodic lattice is investigated in the presence of the piezoelectric coupling. It turns out that in case of particle-hopping on a lattice with equivalent sites, the attenuation formula of Hutson and White can be reproduced when the lowest order approximation inq, the wave number, is taken into account.     

Low-frequency sound absorption and attenuation in marine medium

Original experimental data are analyzed on the low-frequency sound attenuation in the Mediterranean, Black, and Baltic Seas, Sea of Japan, and the north-western region of the Pacific Ocean. In these regions, waters significantly differ in their temperatures and salinities. The analysis is aimed at obtaining an expression for calculating the low-frequency absorption coefficient in sea water. The analysis uses the previously published data on the measured (by the temperature discontinuity method) low-frequency relaxation times associated with boron present in sea water. The dependence of the absorption on the pH value (which was revealed in the 1970s) and the experimental data on sound absorption at frequencies higher than 5–10 kHz are also taken into account. As a result of the analysis based on the assumption that low-frequency relaxation takes place, an expression is proposed that relates the low-frequency absorption to the temperature, salinity, and pH value and equally well describes the experimental frequency dependences of attenuation for the four regions at hand (except for the Baltic Sea). Increased attenuation coefficients are noticed for shallow seas and deep-water regions where waters are influenced by intense currents, strait zones, and zones of mixing waters of different origin, i.e., for the ocean areas where, in addition to the attenuation, sound scattering by inhomogeneities of the marine medium and sound energy leakage into the sea floor are significant.     

Sound propagation in saturated gas-vapor-droplet suspensions with droplet evaporation and nonlinear relaxation

The Sound attenuation and dispersion in saturated gas-vapor-droplet mixture in the presence of evaporation has been investigated theoretically. The theory is based on an extension of the work of Davidson [J. Atmos. Sci. 32(11), 2201-2205 (1975)] to accommodate the effects of nonlinear particle relaxation processes of mass, momentum and energy transfer on sound attenuation and dispersion. The results indicate the existence of a spectral broadening effect in the attenuation coefficient (scaled with respect to the peak value) with a decrease in droplet mass concentration. It is further shown that for large values of the droplet concentration the scaled attenuation coefficient is characterized by a universal spectrum independent of droplet mass concentration.     

Frequency dependence of acoustic properties of aqueous glucose solutions in the VHF/UHF range

The bioultrasonic spectroscopy system was employed for measurements of velocity and attenuation coefficient of glucose solutions in the VHF/UHF range. The relation between the slope of the square of velocity and the relaxation parameters, and the relation between the frequency exponent on attenuation coefficient and the relaxation parameters are investigated. In order to carry out numerical calculations, a model for a single relaxation process is employed, wherein the attenuation coefficient is expressed as (A/( 1 + (f/falpha)2) + B)f2 where falpha is the attenuation relaxation frequency, and A and B are constants. The numerical calculations show that the slope of the square of the velocity is determined uniquely by the velocity relaxation frequency fv and v(infinity)2 - v(0)2 where v0 is the zero-frequency velocity and v(infinity) is the infinite-frequency velocity, and that the frequency exponent on the attenuation coefficient is determined uniquely by falpha and A/B. For experimental considerations, the velocities and the attenuation coefficients of 5, 15, and 25% concentration aqueous solutions of glucose were measured in the frequency range 20 to 700 MHz. The data for the 5 and 15% aqueous solutions can be explained using the single relaxation model. However, the data for the 25% aqueous solution suggest the existence of multirelaxation processes.     

多原子分子气体中声波弛豫衰减谱的重建算法

提出了一种宽频率范围的弛豫衰减谱重建算法,并采用基于SSH理论的方法和基于实验数据的方法估计气体的有效弛豫时间.通过该算法得到了包括氮气、甲烷、氧气、二氧化碳和水蒸气在内的多种多原子分子混合气体的声衰减谱,研究的声波频率范围从1Hz到10GHz.与预测弛豫衰减的DL模型的结果比较表明,该算法获得的弛豫衰减谱结果与之相符,其预测精度取决于对分子弛豫过程的正确认识.另外该算法还被用于几种混合气体中水蒸气和二氧化碳含量的分析,其结果表明弛豫衰减谱可被用于定量分析多原子分子气体的成分组成,这使得实现高灵敏度地检测气体成分的智能声气体传感技术成为可能. 关键词： 声弛豫衰减 有效弛豫时间 重建算法 声气体传感器     

Anisotropy of the phonon-phason dynamics and the pinning effect in icosahedral AlPdMn quasicrystals

A minimal model of the phonon-phason dynamics in icosahedral quasicrystals with inclusion of the pinning effect is suggested. Resonant attenuation of low-frequency acoustic waves in the temperature range corresponding to thermal activation of phasons is considered. In the long-wave length limit, the velocity of acoustic phonons is isotropic; however, the phonon-phason coupling causes anisotropy of the velocity and of the attenuation of acoustic waves with small wave vectors. These effects manifest themselves most strongly at an acoustic wave frequency close to the inverse relaxation time of phasons with the same wave vector. The pinning effect can cause a significant decrease in the anisotropy of the velocity and of attenuation of acoustic waves.     

The flipped longitudinal polarization sequence

By flipping the longitudinal magnetization with a chain of 180° pulses it is possible to effectively restore the effects of relaxation so that the same longitudinal magnetization is periodically recovered. The pulse sequence for achieving this, called Flipped LOngitudinal Polarization (FLOP), can be incorporated into any pulse sequence whenever it is desired to stop the attenuation in longitudinal magnetization caused by relaxation. We illustrate its use for fast, single-shot measurements of the longitudinal relaxation time and for three-dimensional T₁ mapping.     

Thermodynamics of a continuous medium with electric and magnetic dipoles

The thermodynamics of an electrically charged, multicomponent fluid with spontaneous electric and magnetic dipoles is analysed in the presence of electromagnetic fields. Taking into account the chemical composition of the current densities and stress tensors leads to three types of irreversible terms: scalars, vectors and pseudo-vectors. The scalar terms account for chemical reactivities, the vectorial terms account for transport and the pseudo-vectorial terms account for relaxation. The linear phenomenological relations, derived from the irreversible evolution, describe notably the Lehmann and electric Lehmann effects, the Debye relaxation of polar molecules and the Landau-Lifshitz relaxation of the magnetisation. This formalism accounts for the thermal and electric magnetisation accumulations and magnetisation waves. It also predicts that a temperature gradient affects the dynamics of magnetic vortices and drives magnetisation waves.     

Sound propagation in an anharmonic metal : III. The collision-free regime

The electron contribution to the phonon damping is calculated by solving the generalized transport equations derived in I. In particular, the collision-free regime and the transition from the collision-dominated to the collision-free regime are investigated. The solution of the transport equations is a generalization of the one given in II and it is valid also outside the collision-dominated regime. No single-relaxation-time approximation is used and the collision operator is taken into account in full. Certain higher-order relaxation times are taken into account only approximately. It is shown, however, that this approximation only affects the attenuation in a narrow frequency range around the transition from the collision-dominated to the collision-free regime. Even there, its influence is weak as demonstrated by some numerical results for potassium.     

Electron relaxation and transport in nanostructured and bulk silica

Processes of ballistic and hot electron relaxation in extended bulk as well as nanostructured silica have been analyzed by means of a phonon-based scattering model and respective Monte-Carlo computer simulation. Optical as well as acoustic phonons are taken into account. Trajectories of electrons and their energy attenuation in nanostructured silica are additionally affected by scattering processes at the grain boundaries between the nanoparticles, i.e. by surface phonon as well as potential scattering. Moreover, a flatter conduction band and a higher effective electron mass have been taken into account too. According to these calculations, electrons with an initial energy of several eV, but still below the valence band ionization threshold, were thermalized in 50–300 fs increasing with the silica grain size from 1 nm up to bulk material. The electron emission probabilities over the surface barrier into vacuum are extended up to depths of 60–100 nm, respectively, increasing with enhancement by an electric field.     

气体声弛豫过程中有效比热容与弛豫时间的分解对应关系

声在多原子分子气体中传播所引起的弛豫过程是探索气体特性的重要方面. 本文通过研究气体声弛豫过程中振动自由度与平动自由度(V-T)以及振动自由度之间(V-V)的分子能量转移模型, 给出了有效比热容与弛豫时间的分解对应关系及其通用获得方法. 该分解模型与现有的声弛豫模型相比, 反映了分解后的V-T 和V-V弛豫过程中振动比热容与弛豫时间的对应关系, 并发现了较高能级是引起对应声弛豫过程的决定因素. 将基于该分解模型获得的气体声弛豫衰减谱经碰撞直径微调改进后, 比现有理论更接近实验数据, 其结果证明了该分解对应关系的正确性和合理性.     

Bulk viscosity and ultrasonic attenuation in liquid metals

Ultrasonic attenuation in simple liquid metals has been investigated using the thermodynamic theory of relaxation processes incorporating the concept of a two-state model for the liquid near the melting point. Agreement of the results with the experimental values of the ultrasonic attenuation and bulk viscosity indicates that this might be an appropriate approach to explain the excess attenuation of ultrasonic waves in liquid metals.     

Endogenous versus exogenous shocks in complex networks: an empirical test using book sale rankings

We study the precursory and recovery signatures accompanying shocks in complex networks, that we test on a unique database of the Amazon.com ranking of book sales. We find clear distinguishing signatures classifying two types of sales peaks. Exogenous peaks occur abruptly and are followed by a power law relaxation, while endogenous peaks occur after a progressively accelerating power law growth followed by an approximately symmetrical power law relaxation which is slower than for exogenous peaks. These results are rationalized quantitatively by a simple model of epidemic propagation of interactions with long memory within a network of acquaintances. The observed relaxation of sales implies that the sales dynamics is dominated by cascades rather than by the direct effects of news or advertisements, indicating that the social network is close to critical.     

A new approach for extracting phase effect based on in-line X-ray phase imaging

In clinical applications, the X-ray spectra from tungsten are continuous, and include both attenuation and phase effects for in-line X-ray imaging. A new approach for extracting the pure phase effect from the mixed images is presented. In this work, a normalized imaging formula independent of the light intensity is derived for the polychromatic case. Taking both photoelectric and Compton interactions into account, the attenuation effects at 70 and 50 kVp for borosilicate pyrex, air, and water are accurately calculated and shown to be stable across these substances. The attenuation effect at the interface can be greatly weakened by the subtraction of the attenuation effects of two normalized images, thus revealing details of the phase effect. The experimental results show that this approach is valid.     

饱和砂岩的滞弹性弛豫衰减特征及微观机理的探索

应用Metravib热机械分析仪,以饱和岩石进行正弦波加载的方式, 分别对三种不同孔隙度的泵油饱和彭山砂岩、泵油和甘油饱和自贡长石砂岩进行了衰减实验研究,得到衰减的热弛豫规律.据此规律求得它们的激活能和原子振动频率, 其激活能和原子的振动频率比间隙原子的激活能和振动的频率低, 此现象用饱和砂岩中产生的缺陷原子簇的整体振动比单个或孤立的原子的振动频率低做出了解释. 在饱和岩石的晶粒间界缺陷处参与扩散的是固体原子、液体及气体原子. 并得出随孔隙度和黏滞系数增大,衰减强度和激活能增大,原子的振动频率增高,弛豫时间减小的结论. 在正弦波应力作用下,由多种矿物晶体胶结而成的饱和砂岩是一种多晶、多相的固体, 由于内部结构复杂、缺陷广布,产生弛豫衰减是普遍现象.砂岩中存在点缺陷、位错、 晶界及孪晶界面等许多缺陷及缺陷间的相互作用都可以产生弛豫型衰减峰. 用饱和砂岩中特有的饱和液体及砂岩内部结构的复杂性解释了饱和砂岩的衰减机理, 很自然地将其宏观衰减特征与微观结构紧密连在一起.饱和砂岩中的各种缺陷、 相界等会导致多重弛豫,使它们的弛豫衰减峰变宽,分布参数增大.这项研究既具有理论意义,也具有实用价值.     

Diffusion and relaxation effects in general stray field NMR experiments

We analyze the evolution of magnetization following any series of radiofrequency pulses in strongly inhomogeneous fields, with particular attention to diffusion and relaxation effects. When the inhomogeneity of the static magnetic field approaches or exceeds the strength of the RF field, the magnetization has contributions from different coherence pathways. The diffusion or relaxation induced decay of the signal amplitude is in general nonexponential, even if the sample has single relaxation times T(1), T(2) and a single diffusion coefficient D. In addition, the shape of the echo depends on diffusion and relaxation. It is possible to separate contributions from different coherence pathways by phase cycling of the RF pulses. The general analysis is tested on stray field measurements using two different pulse sequences. We find excellent agreement between measurements and calculations. The inversion recovery sequence is used to study the relaxation effects. We demonstrate two different approaches of data analysis to extract the relaxation time T(1). Finite pulse width effects on the timing of the echo formation are also studied. Diffusion effects are analyzed using the Carr--Purcell--Meiboom--Gill sequence. In a stray field of a constant gradient g, we find that unrestricted diffusion leads to nonexponential signal decay versus echo number N, but within experimental error the diffusion attenuation is still only a function of g(2)Dt(3)(E)N, where t(E) is the echo spacing.     

Spin relaxation in fluids

The effect of spin relaxation in fluids on perturbed angular distributions is discussed. The first example, deals with dynamical perturbations resulting from collision-induced charge-transfer process in gases. A brief discussion of relaxation effects caused by molecular motion in non-metallic liquids is followed by a presentation of a new approach to the theory of quadrupole relaxation in liquid metals.