Phase coexistence induced by a defensive reaction in a cellular automaton traffic flow model

Traffic flow modeling is an elusive example for the emergence of complexity in dynamical systems of interacting objects. In this work, we introduce an extension of the Nagel-Schreckenberg (NaSch) model of vehicle traffic flow that takes into account a defensive driver’s reaction. Such a mechanism acts as an additional nearest-neighbor coupling. The defensive reaction dynamical rule consists in reducing the driver’s velocity in response to deceleration of the vehicle immediately in front of it whenever the distance is smaller than a security minimum. This new mechanism, when associated with the random deceleration rule due to fluctuations, considerably reduces the mean velocity by adjusting the distance between the vehicles. It also produces the emergence of bottlenecks along the road on which the velocity is much lower than the road mean velocity. Besides the two standard phases of the NaSch model corresponding to the free flow and jammed flow, the present model also exhibits an intermediate phase on which these two flow regimes coexist, as it indeed occurs in real traffics. These findings are consistent with empirical results as well as with the general three-phase traffic theory.     

Disorder effect on the traffic flow behavior

The effects of some disorders, on the traffic flow behavior, are studied numerically. Especially, the effect of mixture of vehicles of different velocities and/or lengths, the effects of different drivers reactions, the position and the extraction rate of off-ramp in the free way. Using a generalized optimal velocity model, for a mixture of fast and slow vehicles, we have investigated the effect of delay times τ _f and τ _s on the fundamental diagram. It is Found that the small delay times have almost no effect, while, for sufficiently large delay time τ _s , the current profile displays qualitatively five different forms, depending on τ _f , τ _s and the fractions f _f and f _s of the fast and slow cars, respectively. The velocity (current) exhibits first-order transitions at low and/or high densities, from freely moving phase to the congested state, and from congested state to a jamming one, respectively. The minimal current appears in intermediate values of τ _s . Furthermore there exist, a critical value of τ _f above which the meta-stability and hysteresis appear. The effects of disorder due to drivers behaviors have been introduced through a random delay time τ allowing the car to reach its optimal velocity traffic flow models with open boundaries. In the absence of the variation of the delay time Δτ, it is found that the transition from unstable to meta-stable and from meta-stable to stable state occur under the effect of the injecting and the extracting rate probabilities α and β respectively. Moreover, the perturbation of the traffic flow behavior due to the off-ramp has been studied using numerical simulations in the one dimensional cellular automaton traffic flow model with open boundaries. When the off-ramp is located between two critical positions i _c1 and i _c2 the current remains constant (plateau) for β_0c1 < β₀ < β_0c2, and the density undergoes two successive first order transitions: from high density to plateau current phase and from average density to the low one. In the case of two off-ramps, these transitions occur only when the distance between ramps, is smaller than a critical value.     

考虑行车状态的一维元胞自动机交通流模型

在Nagel Schrekenberg单车道元胞自动机交通流模型（简称NS模型）的基础上，考虑车辆之间的相对运动薛郁等提出了一种改进的单车道元胞自动机交通流模型（简称改进的NS模型）.通过两种情况列出了改进的NS模型存在不尽周严的地方，随之在新模型中引入了行车状态 变量和反馈规则，从而控制车辆出现倒车和刹车过急等现象.通过计算机对新模型进行模拟 ，发现减速概率和车流密度对车流状态的演化影响很大，当减速概率高（如道路条件差）时 ，即使车流密度低，车流也会出现局部堵塞状态；而当减速概率一定时，随着车流密度增加 ，车流的运动相与堵塞相发生了全局性的交替出现，此时类似于波的波峰和波谷的传播.与 改进的NS模型相比较，新模型模拟的车流量较高，说明新模型减少了车流的总体停滞状态. 关键词： 交通流 元胞自动机 行车状态 反馈规则     

Magnetization curves of a spin- bilayer system (A2) (ApB1−p)1 (B)2 with disordered interfaces

A calculation is presented for the component magnetizations of an infinite multilayer Ising system, consisting periodically of two layers of spin- A ions, two layers of spin- B ions, and a disordered layer interface in between that is characterized by a random arrangement of A and B ions like a two-dimensional A_pB_1−p alloy. The system is a simple cubic Ising-type structure with a coordination number z = 6. The model is general for ferro- and for antiferromagnetic A-B exchange couplings. The A-A and B-B exchange couplings are regarded as ferromagnetic. An effective field theory that goes beyond mean field, is employed to calculate the bulk-like transition temperature, the different component magnetizations as well as the total bulk-like magnetization. The component magnetizations are calculated for different realistic model values of ferro- and antiferromagnetic A-B exchange constants, as a function of temperature and of the concentration parameter p that characterizes the disorder in the interface. We show that the presence of a disordered interface may significantly affect the component and total magnetizations. In particular, for the case of antiferromagnetic exchange couplings, it is shown that the system can acquire a compensation temperature for certain domains of values of the concentration parameter p in the disordered interface.     

一维元胞自动机随机交通流模型的理论分析与计算机实验

在一维局部作用元胞自动机(CA)交通流模型中，引入刹车噪声与产生、消失概率，得到一个完全随机的CA交通流模型．利用平衡自旋理论对该模型的研究表明，当P_in＝P_out≠0时，初始密度分布不影响系统的最终状态，求得ρ_ｔ＝0.5；当满足条件P_in＋P_b＝1与P_in＝P_out时，得到简单的“线性”模型，该模型在条件｜1－2P_in｜＝1下，表现出长程相关，导致严重交通“阻塞”的出现．理论结果与计算机实验一致． 关键词：     

一种改进的一维元胞自动机交通流模型及减速概率的影响

在Nagel-Schrekenberg单车道元胞自动机交通流模型的基础上,考虑车辆之间的相对运动以及车辆减速概率对交通状态的影响,提出了一种改进的单车道元胞自动机交通流模型.并以该模型进行计算机模拟,结果表明,在车流状态的演化过程中,通过确定减速概率与车辆密度的指数v关系来控制车流量,不同的v值车流量不同,对车辆运动出现堵塞相的相变点有影响.当v约为0.75时,模拟结果与实测结果符合.随着车辆密度的增加,车辆的局域聚集程度加大,平均速度下降增大,将出现不稳定的车辆聚集的堵塞相.在车辆的运动过程中,车流的运 关键词： 交通流 元胞自动机 减速概率 堵塞相     

The effect of elevated reactant temperatures on soot nanostructures in a coflow diffusion ethylene flame

The relationship between soot surface growth, soot nanostructure and reactant temperature (T_r) in a coflow diffusion ethylene flame was investigated with multiple experimental techniques. The T_r was raised by heating the coflow air. Three cases, with 300K, 473K, and 673K T_r, respectively, were studied. Laser-induced Incandescence revealed that increasing T_r promotes soot formation. Although soot primary particle diameter (d_p) also increases with T_r, the increase in d_p slows down after 473K T_r, suggesting that there is a deceleration in soot surface growth. Transmission Electron Microscopy (TEM) imaging showed that increased T_r promotes soot aggregation and yields larger and more mature primary particles. The assessment of the Selected Area Electron Diffraction (SAED) patterns indicated that, at 673K T_r, there is a growth of lattice planes. Raman spectroscopy revealed further structural details. By assessing the band intensity ratios, soot for the T_r of 673K has more curved nanostructures. The deceleration of soot surface growth may be explained by surface aging, which is characterized by an increase in curved nanostructures.     

Exact determination of the two-point cluster function for one-dimensional continuum percolation

The two-point cluster functionC ₂(r ₁,r ₂) provides a measure of clustering in continuum models of disordered many-particle systems and thus is a useful signature of the microstructure. For a two-phase disordered medium,C ₂(r ₁,r ₂) is defined to be the probability of finding two points at positionsr ₁ andr ₂ in thesame cluster of one of the phases. An exact analytical expression is found for the two-point cluster functionC ₂(r ₁,r ₂) of a one-dimensional continuumpercolation model of Poisson-distributed rods (for an arbitrary number density) using renewal theory. We also give asymptotic formulas for the tail probabilities. Along the way we find exact results for other cluster statistics of this continuum percolation model, such as the cluster size distribution, mean number of clusters, and two-point blocking function.     

速度限制对交通流的影响

运用元胞自动机模型研究单车道路面上设立限速区对交通流的影响.计算机模拟结果显示,在交通流与密度的基本图中存在饱和交通流量,其大小取决于限速区的最大速度.限速区的设立导致不同相的分离.在确定性的模型中,当车辆密度较低时,存在两种不同的自由流相;而当车辆密度较高时,出现最大交通流相和自由流相.在随机模型中,车辆密度较高时,出现最大交通流相-堵塞相-自由流.对交通流以及临界密度等量给出一些解析结果.     

Joint densities for random walks in the plane

We point out the existence of computationally convenient techniques for calculating the joint probability density for the position of a Pearson random walk after n steps. A new Fourier-Bessel function expansion for p_n(r, θ) is developed for this purpose which does not require radial symmetry, but does require that p_n(r, θ) = 0 when r exceeds some maximum radius, R.     

交通灯控制下主干道的交通流研究

用元胞自动机模型模拟二维交通流.通过交叉口设置的红绿灯，研究交通激波的形成和传播 ；对于一定的红绿灯周期，交通流量出现多个极值现象；在交叉口间隔相同的情况下，对于 一定的红绿灯周期，在一定的车辆密度范围内，交通流量是一个与密度无关的常量；在车辆 密度较高的情况下，交叉口间距大于某一值后，交通流量保持恒定值. 关键词： 元胞自动机模型 交通流 交通激波     

Thick skin in neutron/proton-rich sodium isotopes

Nucleon (both neutron and proton) density distributions of the chain of sodium isotopes are calculated using a semi-phenomenological model of nuclear density which incorporates correctly the asymptotic behaviour and the behaviour near the centre. The experimental charge root-mean-square radii and the single neutron and proton separation energies, required as input, are used. The calculated interaction cross-sections using these densities in the Glauber model agree well with the experiment. The calculated neutron rms radii r _n and the nuclear skin thickness ( r _n - r _p) closely agree with the corresponding experimental values and also are consistent with the Relativistic Hartree-Bogoliubov (RHB) calculations. Received: 24 April 2001 / Accepted: 28 June 2001     

Coulomb energies and nuclear radii in the energy density formalism

The relation between Coulomb displacement energies,ΔE _c , andΔr=r _n -r _p , the difference between the rms radii of neutrons and protons in nuclei, is investigated within the energy density formalism (EDF). The variational equation, obtained by minimizing the Coulomb plus symmetry energies, is solved assuming the symmetry interaction is a simple functional of the local nuclear matter density. Varying parameters of the model, rather unique relation betweenΔE _c andΔr is obtained (within ±50 keV).ΔE _c isindependent ofr _ex, the rms radius of the excess neutrons distribution. Using the experimental values ofr _p and adjusting the model to reproduce the recent data onΔr (Δr∽~0.05 fm for⁴⁸Ca and²⁰⁸Pb), which are significantly smaller than those obtained from current Hartree-Fock calculations, the calculatedΔE _c agree with the experimental results. Using the value ofΔr～0.05 fm and the experimental values ofr _ex, a small compression (<0.02 fm) of the proton core in the analogue state relative to its parent state emerges, thus contributing an additional electrostatic term to the Coulomb displacement energy. The size of this relative core-compression effect depends on the values assumed forΔr andr _ex, it increases with the decreasing ofΔr and the increasing ofr _ex. IfΔr～0.05 fm the effect is large enough to remove the long standing Coulomb energy anomaly. The main result of the present work is the correlation betweenΔE _c andΔr, suggesting that the difficulties of current Hartree-Fock calculations in reproducing isotope shifts ofr _p , the small value ofr _n ?r _p and the values ofΔE _c may all be different manifestations of some missing residualp n effective interaction.     

The random field Ising model with an asymmetric trimodal probability distribution

The Ising model in the presence of a random field is investigated within the mean field approximation based on Landau expansion. The random field is drawn from the trimodal probability distribution P(h_i)=pδ(h_i−h₀)+qδ(h_i+h₀)+rδ(h_i), where the probabilities p,q,r take on values within the interval [0,1] consistent with the constraint p+q+r=1 (asymmetric distribution), h_i is the random field variable and h₀ the respective strength. This probability distribution is an extension of the bimodal one allowing for the existence in the lattice of non magnetic particles or vacant sites. The current random field Ising system displays second order phase transitions, which, for some values of p,q and h₀, are followed by first order phase transitions, thus confirming the existence of a tricritical point and in some cases two tricritical points. Also, reentrance can be seen for appropriate ranges of the aforementioned variables. Using the variational principle, we determine the equilibrium equation for magnetization, solve it for both transitions and at the tricritical point in order to determine the magnetization profile with respect to h₀.     

射频感应耦合Ar-N2等离子体物理特性的Langmuir探针测量及理论研究

分别通过Langmuir探针测量和动力学模型模拟方法研究了射频感应耦合Ar-N₂等离子体中电子能量分布、电子温度、电子密度等物理量随N₂含量的变化规律.实验研究结果表明：电子能量分布呈现出非Maxwell型分布,并由双温分布向三温分布过渡;电子温度在不同的气压下随N₂含量的增加呈现出不同的变化规律.在放电气压小于1.3 Pa时,电子温度随N₂含量的增加而下降;当气压大于1.3 Pa时,电子温度随N₂关键词： 感应耦合等离子体 2混合气体放电')" href="#">Ar-N₂混合气体放电 电子能量分布 Langmuir探针     

二维电子体系在高密度下的稳定性

本文计算了高密度的二维电子体系的边缘能(将二维体系沿某一直线解离成两片时,形成单位长度新边缘所需要的能量)。结果发现,当r_ss^(c)(约0.415)时,边缘能变负,从而表明在高密度下,二维电子气的基态有可能发生不稳。我们分别讨论了二维非束缚的电子气和束缚的电子气基态的稳定性,并在一个简化的模型下给出了束缚的电子气基态稳定性的判据。 关键词：     

Electro-optic properties of LiKSO4 and LiK1-xRbxSO4 crystals

1-x Rb_xSO₄ for x varying between 0 and 0.50 were investigated by an ac modulation method based upon the Sénarmont arrangement. The electro-optic coefficient r_c, and for the first time the coefficients r₄₁ and r₅₁ were determined with good accuracy in pure LiKSO₄ around room temperature at a wavelength of 633 nm. The frequency dependence of the electro-optic coefficient r_c was measured in the frequency range from 1 kHz up to 1 MHz. The values of the factor of merit n_e ³r_c of the mixed crystals LiK_1-xRb_xSO₄ were obtained for the first time, for the concentrations x=0.05, 0.10, 0.20, and 0.50. The electro-optic properties are shown to be affected by the random presence of the rubidium ions in this disordered system. Received: 16 March 1998/Revised version: 2 June 1998     

The effects of overtaking strategy in the Nagel-Schreckenberg model

Based on the Nagel-Schreckenberg (NS) model with periodic boundary conditions, weproposed the NSOS model by adding the overtaking strategy (OS). In our model, overtakingvehicles are randomly selected with probability q at each time step, and the successful overtakingis determined by their velocities. We observed that (i) traffic jams still occur in theNSOS model; (ii) OS increases the traffic flow in the regime where the densities exceedthe maximum flow density. We also studied the phase transition (from free flow phase tojammed phase) of the NSOS model by analyzing the overtaking success rate, order parameter,relaxation time and correlation function, respectively. It was shown that the NSOS modeldiffers from the NS model mainly in the jammed regime, and the influence of OS on thetransition density is dominated by the braking probability p.     

Growth, percolation, and correlations in disordered fiber networks

This paper studies growth, percolation, and correlations in disordered fiber networks. We start by introducing a 2D continuum deposition model with effective fiber-fiber interactions represented by a parameterp which controls the degree of clustering. Forp=1 the deposited network is uniformly random, while forp=0 only a single connected cluster can grow. Forp=0 we first derive the growth law for the average size of the cluster as well as a formula for its mass density profile. Forp>0 we carry out extensive simulations on fibers, and also needles and disks, to study the dependence of the percolation threshold onp. We also derive a mean-field theory for the threshold nearp=0 andp=1 and find good qualitative agreement with the simulations. The fiber networks produced by the model display nontrivial density correlations forp<1. We study these by deriving an approximate expression for the pair distribution function of the model that reduces to the exactly known case of a uniformly random network. We also show that the two-point mass density correlation function of the model has a nontrivial form, and discuss our results in view of recent experimental data on mss density correlations in paper sheets.     

Analysis of Phase Transition in Traffic Flow based on a New Model of Driving Decision

Different driving decisions will cause different processes of phase transition in traffic flow.To reveal the inner mechanism, this paper built a new cellular automaton (CA) model,based on the driving decision (DD). In the DD model, a driver's decision is divided intothree stages: decision-making, action, and result. The acceleration is taken as a decisionvariable and three core factors, i.e. distance between adjacent vehicles, their own velocity,and the preceding vehicle's velocity, are considered. Simulation results show that the DDmodel can simulate the synchronized flow effectively and describe the phase transitionin traffic flow well. Further analyses illustrate that various density will cause the phasetransition and the random probability will impact the process. Compared with the traditional NaSch model, the DD model considered the preceding vehicle's velocity, the deceleration limitation, and a safe
distance, so it can depict closer to the driver preferences on pursuing safety, stability and fuel-saving and has strong theoreticalinnovation for future studies.