Mode-coupling effects in simple fluids measured by neutron scattering

Summary After a summary of the relevant results from mode-coupling theory a review is given of the neutron scattering experiments that have given direct evidence for the existence of mode-coupling effects in simple fluids. The problem of determining the velocity autocorrelation function from neutron data is discussed in connection with experimental data on liquid sodium and on high-density hydrogen. Paper presented at the workshop ?Highlights on Simple Liquids?, held in Turin at ISI on 1–3 May, 1989.     

Long-range effects in an elementary cellular automaton

We present evidence that one of the elementary one-dimensional cellular automata in the sense of Wolfram (rule 22 in Wolfram's notation) involves very complex long-range effects, similar to a critical phenomenon. This is in contrast to superficial evidence that would suggest that this rule leads to fairly simple behavior.     

Long-range correlations in kinetic theory

It is shown that macroscopic correlations in a fluid are conserved for macroscopically long times. The equations of conservation can be written in a form independent of the density of the fluid and are therefore valid for a liquid as well as for a gas. The possibility of developing a kinetic theory of turbulence on the basis of these equations (along the lines of V. N. Zhigulev and of S. Tsugé) is indicated.The contents of this paper formed part of the Ph.D. thesis submitted by the author under the supervision of Prof. Harold Grad to the Department of Mathematics, New York University and issued as NYU-Courant Institute of Mathematical Sciences Technical Report MF-72, October 1973.     

Neutron scattering studies of simple fluids

Summary Neutron scattering may be used in a variety of ways to study simple fluids, and four examples will be given. In the first the nature of the forces causing the structure of noble-gas fluids will be examined. Secondly the structure of homonuclear diatomic liquids will be considered including studies of molecular bonding. Thirdly, the structure of a relatively complicated atomic fluid, sulphur, will be reviewed. And in the fourth the properties of a model Lorentz gas—a dilute mixture of hydrogen in fluid argon—will be described. These examples show both the power and diversity of neutron results, especially when they are combined with accurate computer simulations of these fluids. Proposals for future experiments in this field will be discussed. Paper presented at the workshop ?Highlights on Simple Liquids?, held in Turin at ISI on 1–3 May, 1989.     

Collision-induced absorption in dense mixtures of simple fluids

Summary Collision-induced absorption (CIA) spectroscopy is a probe of the dynamical properties of dense systems. Two simple cases of CIA spectra are here discussed: the translational band observed in solutions of a noble gas in liquid Ar and the rotational lines of H₂ observed in solutions of H₂ in Ar. In the first case, the analysis of the translational band allows one to derive the characteristic frequency ω₀, associated with the rattling component of the motion of the impurity in the Ar medium. In the second case, the analysis of the rotational lines allows one to derive the parameter δ, that characterizes the density narrowing effect. In the high-density region, the parameter δ is directly related to the diffusive motion of the Ar atoms around the impurity. Paper presented at the workshop ?Highlights on Simple Liquids?, held in Turin at ISI on 1–3 May, 1989.     

Long-range spatial correlations in a simple diffusion model

A simple anisotropic diffusion model, according to semiphenomenological arguments, exhibits long-ranged spatial correlations in uniform stationary states.     

Long-range correction for dipolar fluids at planar interfaces

A slab-based long-range correction for dipolar interactions in molecular dynamics simulation of systems with a planar geometry is presented and applied to simulate vapour–liquid interfaces. The present approach is validated with respect to the saturated liquid density and the surface tension of the Stockmayer fluid and a molecular model for ethylene oxide. The simulation results exhibit no dependence on the cut-off radius for radii down to 1 nm, proving that the long-range correction accurately captures the influence of the dipole moment on the intermolecular interaction energies and forces as well as the virial and the surface tension.     

Patterns in passive optical systems with boundary effects

Pattern formation is closely related to system boundary conditions in nonlinear dynamic systems.apart from the system control parameters,To avoid complexity,system boundary conditions are usually considered to be infinite or periodic,and the initial conditions spatially homogeneous,But it is not always the case in real situations,or sometimes periodic boundary conditions are not exact.To show the important and interesting boundary effects in real pattern formation,we suggest a simple universal boundary condition in a typical optical pattern formation system.Numerical simulations of the passive optical system show that pattern characteristics such as distribution symmetry,peak number,structure strength,evolution course and stability are all greatly influenced by the system boundary conditions.     

Fluctuations in fluids out of thermal equilibrium

After a brief review of dynamic correlations in equilibrium fluids, we consider the long-range correlations between the fluctuations in a fluid subjected to a large stationary temperature gradient. These long-range correlations enhance and modify the Rayleigh spectrum of the fluid. We elucidate that the modifications of the Rayleigh line are determined by the coupling of the entropy fluctuations to the transverse velocity fluctuations. Recent attempts to test the theoretical predictions with the aid of light-scattering experiments are discussed.     

Shock formation in mixtures of fluids

Summary We examine the problem of weak-discontinuity propagation in mixtures of two ideal fluids. The presence of exchange of momentum reduces or enhances the time for shock formation depending on the mechanism with whom the exchange of momentum takes place. Numerical evaluations are also presented for mixtures of nitrogen and oxygen simulating dry-air models.

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Riassunto Si esamina il problema della propagazione di discontinuità deboli in miscele binarie di fluidi ideali. La presenza di scambio di momento può ridurre o allungare il tempo di formazione dell’urto a seconda del meccanismo con cui lo scambio stesso ha luogo. Si presentano risultati numerici di miscele di azoto e ossigeno simulanti modelli di aria secca.

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Резюме Мы исследуем проблему распространения слабых разрывов в смесях двух идеальных газов. Обмен импульсом уменьшает или увеличивает время образования ударной волны, в зависимости от механизма который определяет обмен импульсом. Приводятся численные расчеты в случае смеси азота и кислорода, которые представльют модели сухого воздуха.

     

Translational light scattering in simple liquids

Summary A short review of translational light scattering of simple liquids is reported. The microscopic cross-section is compared with the equivalent expression for neutron scattering experiments (Van Hove, 1954). A survey of the fundamental experiments is reported and the possible application to the study of the dynamics of simple dense systems is analysed. The relevant approximations are critically discussed and the limits of the technique are established. The review mainly deals with classical or almost classical liquids, but also some extensions to quantum systems are considered. paper presented at the workshop ?Highlights on Simple Liquids?, held in Turin at ISI on 1–3 May 1989.     

Long-range correlations and generic scale invariance in classical fluids and disordered electron systems

Long-ranged, or power-law, behavior of correlation functions in both space and time is discussed for classical systems and for quantum systems at finite temperature, and is compared with the corresponding behavior in quantum systems at zero temperature. The origin of the long-ranged correlations is explained in terms of soft modes. In general, correlations at zero temperature are of longer range than their finite-temperature or classical counterparts. This phenomenon is due to additional soft modes that exist at zero temperature.     

长程表面等离子体的增强效应

研究了双层金属薄膜构型中构型参数对长程表面等离子体的影响,并发现了衰减全反射激发方法下长程表面等离子体的增强效应.以特征矩阵算法为基础,通过数值计算构型的反射谱,研究构型参数的变化对反射谱的影响.发现由于衰减全反射激发方法中耦合器的存在导致的非对称特性,会使双层金属薄膜构型中的长程表面等离子体拥有本征模式特性以外的有趣特性,如长程模式得到增强而另一支受到抑制,从而使能量更为集中在希望被激发的一支.研究结果对非对称激发构型中的长程表面等离子体研究具有启发意义.     

计算机网络的长程相关特性

针对一种计算机网络模型，利用节点排队长度累计量的均方涨落函数，研究了网络节点在时间上的长程相关特性.结果表明，随着负载的增加，网络节点数据包排队长度在时间上由自由流状态的不相关或短程相关逐渐演变为临界和拥塞时的长程相关，关联范围逐渐增大，长程关联特性开始显现.在自由流状态时，节点的不相关或短程相关，并且有一致的数值为0.5的幂指数这一典型特征.而在临界状态时，节点数据包排队长度长程相关，有大于0.5的幂指数为特征.并且随网络规模的增大，节点间的群体作用逐渐显著，幂指数呈下降趋势. 关键词： 计算机网络 相变 长程相关 幂律     

A kinetic theory of dense fluids

A new kinetic equation is developed which incorporates the desirable features of the Enskog, the Rice-Allnatt, and the Prigogine-Nicolis-Misguich kinetic theories of dense fluids. Advantages of the present theory over the latter three theories are (1) it yields the correct local equilibrium hydrodynamic equations, (2) unlike the Rice-Allnatt theory, it determines the singlet and doublet distribution functions from the same equation, and (3) unlike the Prigogine-Nicolis-Misguich theory, it predicts the kinetic and kinetic-potential transport coefficients. The kinetic equation is solved by the Chapman-Enskog method and the coefficients of shear viscosity, bulk viscosity, thermal conductivity, and self-diffusion are obtained. The predicted bulk viscosity and thermal conductivity coefficients are singular at the critical point, while the shear viscosity and self-diffusion coefficients are not.     

Long-range correlations in two-dimensional spatio-temporal seismic fluctuations

We analysed the scaling behaviour of the two-dimensional (2-D) sequence (Δs, Δt) of the 1981–1998 southern California seismicity, where Δs is the distance between two consecutive earthquakes (jump) and Δt is their interevent interval. The 2-D seismic spatio-temporal fluctuations were investigated by means of the detrended fluctuation analysis (DFA), well-known methodology used to detect scaling behaviour in observational time series possibly affected by nonstationarities. The estimated scaling exponents α_DFA, larger than 0.5, indicate the presence of persistent long-range correlations in the 2-D sequence analysed. The variation of the scaling exponent with the increase of threshold magnitude shows a two-fold behaviour: in the range between 1.5 (the completeness magnitude of the catalog) and 3.0, the scaling exponent is quite constant and denoting a flicker-noise dynamics; while for magnitudes larger than 3.0 it decreases with the increase of magnitude, indicating a tendency toward a 2-D space–time Poissonian process for large events.     

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.     

Nonlinear and nonlocal transport effects in simple fluids

Correlation function formulae for transport coefficients of 2nd order in arbitrarily dense fluids are derived, using a modified Chapman-Enskog solution of the Liouville equation. Some static correlations are neglected. Approximate evaluation for dilute gases gives essentially the same results as the solution of Boltzmann's equation. As an application higher order transport effects in the critical region are estimated. It is conjectured that they are apparent in sound absorption and the line width of Rayleigh scattering if (T?T_c)/T _c?10^?3.     

An atomic scale characterization of coupled grain boundary motion in silicon bicrystals

The mechanical response of symmetric tilt grain boundaries (GBs) in silicon bicrystals under shear loading are characterized using molecular dynamics simulations. It is seen that under shear, high-angle GBs namely Σ5 and Σ13 having a rotation axis [0 0 1] demonstrate coupled GB motion, such that the displacement of grains parallel to the GB interface is accompanied by normal GB motion. An atomic-scale characterization revealed that concerted rotations of silicon tetrahedra within the GB are the primary mechanisms leading to the coupled GB motion. Interestingly, so far, this phenomenon has only been examined in detail for metallic systems. A distinguishing feature of the coupled GB motion observed for the silicon symmetric tilt bicrystals as compared to metallic bicrystals is the fact that in the absence of shear, spontaneous coupled motion is not observed at high temperatures.     

Characterizing the relative role of low-frequency and turbulent processes in the nocturnal boundary layer through the analysis of two-point correlations of the wind components

The study presents an analysis of two-point correlations between time series of nocturnal atmospheric wind, obtained from two micrometeorological towers, 45 m horizontally apart, each equipped with two sonic anemometers, 2.5 m vertically apart. It focuses on the scale dependence of the two-point correlations obtained from sensors vertically and horizontally separated. In particular, the role of low-frequency non-turbulent processes in the correlations is assessed, and compared to that of the turbulent scales of motion. The vertical correlations of the streamwise and vertical wind components show little dependence on the turbulence intensity, but those of the spanwise component decrease appreciably as it gets more turbulent. Multiresolution decomposition shows that the two-point correlations become increasingly dominated by low-frequency scales as it gets less turbulent, and that such large-scale processes are largely reduced in fully turbulent conditions. It is also shown that the vertical correlations of the spanwise wind component is negative for very small time scales. Horizontal two-point correlations obtained at the 45 m separation distance between the towers are almost entirely dominated by low-frequency motions, regardless of the turbulence intensity, but the magnitude of such correlations decreases with increasing turbulence intensity for any wind components. A comparison between the horizontal two-point correlations and autocorrelations taken with a time lag given by the ratio of the horizontal separation to the mean wind component in the direction that connects the two towers leads to the conclusion that the statistical properties of turbulence are often preserved over the horizontal distance, despite the lack of turbulence correlations for that separation.