Wave propagation and Landau-type damping in liquids

Intermolecular forces are modeled by means of a modified Lennard-Jones potential, introducing a distance of minimum approach, and the effect of intermolecular interactions is accounted for with a self consistent field of the Vlasov type. A Vlasov equation is then written and used to investigate the propagation of perturbations in a liquid. A dispersion relation is obtained and an effect of damping, analogous to what is known in plasmas as “Landau damping”, is found to take place.     

Glass transition and random close packing above three dimensions

Motivated by a recently identified severe discrepancy between a static and a dynamic theory of glasses, we numerically investigate the behavior of dense hard spheres in spatial dimensions 3 to 12. Our results are consistent with the static replica theory, but disagree with the dynamic mode-coupling theory, indicating that key ingredients of high-dimensional physics are missing from the latter. We also obtain numerical estimates of the random close packing density, which provides new insights into the mathematical problem of packing spheres in large dimensions.     

丛聚的含气泡水对线性声传播的影响

含气泡水内气泡的空间分布会对线性声传播产生影响,导致实验结论与理论预测存在较大偏差.为解决这一问题,将准晶体近似引入到自洽方法中,导出了考虑空间分布时多分散含气泡水的等效声波波数.考虑到含气泡水内,气泡间存在小范围的聚集趋势(简称丛聚现象),在此基础上引入Neyman-Scott点过程描述了含气泡水内气泡的丛聚现象.分析发现,丛聚时,声速、声衰减的峰值将受到抑制,并向低频偏移,且抑制和频偏现象会随丛聚加剧而变强;随频率远离峰值段,丛聚对声传播的影响逐渐减弱.此外,考虑到空间分布的统计信息提取对相关研究的精确与否起到重要作用,引入了一种比例无偏估计,通过该方法获得了仿真环境下丛聚含气泡水模型的相速度及衰减系数,该建模及统计方法也可为相关实验工作提供理论基础.     

Metallic liquids and glasses: atomic order and global packing

In this Letter, we have revealed the common structural behavior of metallic glasses through scrutinizing the evolution of pair distribution functions from metallic liquids to glasses and statistically analyzing pair distribution functions of 64 metallic glasses. It is found that the complex atomic configuration in metallic glasses can be interpreted globally as a combination of the spherical-periodic order and local translational symmetry. The implications of our study suggest that the glass transition could be visualized mainly as a process involving in local translational symmetry increased from the liquid to glassy states.     

Excluded volumes of clusters in tetrahedral particle packing

We investigate the excluded volumes of clusters in tetrahedral particle packing using an ideal tetrahedron model and Monte Carlo simulation. Both the influences of the size and topology of clusters on the excluded volume are studied. We find that the excluded volumes of the dimer composed of two tetrahedra and the wagon wheel composed of five tetrahedra are relatively lower than other cluster forms. For large clusters, the excluded volume decreases when the topology of a cluster approaches the wagon-wheel geometry. The results give an explanation to the cluster distribution which demonstrates that the dimer and wagon wheel are the dominative cluster forms in the packing structure of tetrahedra.     

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.     

Optical visualization of non-linear acoustic propagation in cavitating liquids

Non-linearity effects on sound propagation induced by cavitation bubbles are investigated. The convergence of an acoustic wave due to the interaction with the microbubbles produced in the cavitation zone is shown experimentally. In these conditions the theoretical analysis shows that the self-focusing primarily depends on the effective microbubble volume fraction. This fraction turns out to be about 10^?6 with a corresponding self-focusing distance of about 9 cm in the Fraunhofer region of a plane circular transducer.     

Robust propagation direction of stresses in a minimal granular packing

By employing the adaptive network simulation method, we demonstrate that the ensemble-averaged stress caused by a local force for packings of frictionless rigid beads is concentrated along rays whose slope is consistent with unity: forces propagate along lines at 45 degrees to the horizontal or vertical. This slope is shown to be independent of polydispersity or the degree to which the system is sheared. Further confirmation of this result comes from fitting the components of the stress tensor to the null stress “constitutive equation”. The magnitude of the response is also shown to fall off with the -1/2 power of distance. We argue that our findings are a natural consequence of a system that preserves its volume under small perturbations. Received 30 April 2001 and Received in final form 16 August 2001     

Ultrasound wave propagation in glass-bead packing under isotropic compression and uniaxial shear

The axial-stress dependence of sound wave velocity in granular packing is experimentally investigated with tri-axial and uni-axial devices. Preparing samples by repetitive loadings and unloadings in a range of 20 kPa–1000 kPa, we find that the axial-stress dependence of sound wave velocity approaches the Hertz scaling with an exponent of 1/6 for large axial stresses( 400 kPa). Weak deviation from the Hertz scaling is seen at low stresses. Repetitive axial loadings slightly reduce this deviation, and sound velocities increase nonlinearly approaching some saturated values. Velocities for uni-axial case are found slightly to be bigger than those for tri-axial isotropic compression case. These effects are discussed in the frameworks of granular solid hydrodynamics(GSH) and effective medium theory(EMT), which indicate that they cannot be explained with density nor Janssen ratio only. Dissipation occurring during wave propagation may be a non-negligible factor.     

Noise signal propagation in soft biological tissues

Mathematical models are formulated that discribe linear and nonlinear wave propagation in biological tissues. The basis of the method is evolutionary integro-differential equations with a kernel that takes into account the specific properties of tissue. An equation is obtained for the correlation function of acoustic noise in a medium with memory. The procedure for calculating the correlation function by the given type of kernel and noise spectrum at the entrance to the medium is described. It is shown that in different tissue, there is a difference in the laws of decrease in full intensity of a wideband signal with distance. It is demonstrated that the nonlinear equation in the limiting cases of ??short-?? and ??long-term?? memory reduces to equations that have been well studied in statistical nonlinear acoustics.     

液体中激光等离子体冲击波波前传播特性研究及测试

将空气中球对称冲击波衰减波前传播公式推广到非完全中心对称情况，根据对光学阴影法对激光等离子体冲击波波前测试数据的计算分析，提出液体中点源激光等离子体冲击波旋转椭球面波前传播公式.并且用声学方法对水中和酒精中的激光等离子体冲击波波前进行实验测试，结果表明测试结果与计算公式相吻合. 关键词： 激光 等离子体冲击波 旋转椭球面     

Radiation transfer in absorbing-scattering liquids—II. Comparisons of measurements with predictions

Comparisons have been made between measured and predicted results for the radiation field in both highly absorbing and scattering aqueous suspensions with a highly reflecting or absorbing bottom. Predictions were based on a discrete ordinate solution to the equation of transfer, with the requisite properties (extinction coefficient, absorption coefficient and scattering phase function) obtained from measurements performed on samples extracted from the suspensions. All trends in the data were predicted by the model, although poor numerical agreement was obtained for suspensions of large albedo and extinction coefficient. The disagreement is attributed to the effect of multiple scattering on the property measurements.     

Radiation field propagation in low-contrast single-mode optical waveguides

In short waveguides used in integrated optical circuits, the radiation field is often present in the guiding region of a waveguide as well as the guided modes. We show that this can be investigated efficiently by extracting the radiation part from the total input field and simulating its evolution along the waveguide by use of the beam propagation method. The results are presented for a single-mode waveguide and a waveguide junction excited by a Gaussian beam. For a noncentred excitation, a considerable fraction of the radiation power is still in the guiding region after propagation over several tens of micrometres, and can be coupled to guided modes past the junction.     

The long time and Brownian limits for tagged particle motion in liquids

Formally exact theories of tagged particle motion in liquids are developed, based upon kinetic theory for hard spheres and mode coupling for smooth potentials. It is shown that the resulting equations are tractable in the long time and Brownian limits. The coefficient of the long time tail of the velocity correlation function is seen to differ from its low-density form by only the replacement of the low-density viscosity and diffusion constant by the true viscosity and diffusion constant. In the Brownian limit, the slip Stokes-Einstein law is obtained, with the true viscosity. The relation to other work is discussed.Supported by NSF Grant No. CHE81-11422 and by a Dreyfus Teacher-Scholar grant to TK.     

Universal nature of particle displacements close to glass and jamming transitions

We examine the structure of the distribution of single particle displacements (van Hove function) in a broad class of materials close to glass and jamming transitions. In a wide time window comprising structural relaxation, van Hove functions reflect the coexistence of slow and fast particles (dynamic heterogeneity). The tails of the distributions exhibit exponential, rather than Gaussian, decay. We argue that this behavior is universal in glassy materials and should be considered the analog, in space, of the stretched exponential decay of time correlation functions. We introduce a dynamical model that describes quantitatively numerical and experimental data in supercooled liquids, colloidal hard spheres, and granular materials. The tails of the distributions directly explain the decoupling between translational diffusion and structural relaxation observed in glassy materials.     

Radiation transfer in absorbing-scattering liquids—I. Radiance and flux measurements

Radiation measurements have been made in highly absorbing and scattering aqueous suspensions for which interface reflectivities, as well as suspension properties, can significantly influence results. Measurements were made for the spectral band from 0.40 to 0.68 μm, using probes which determined vertical and directional distributions of the radiance, as well as the vertical distribution of the downwelling flux. Agreement between the separate measurements has established confidence in the experimental procedures, and the data have been used to delineate the effect of the suspension opacity, albedo and bottom reflectivity on the radiation field.