Dynamical diversity and metastability in a hindered granular column near jamming

Granular media jam into a panoply of metastable states. The way in which these states are achieved depends on the nature of local and global constraints on grains; here we investigate this issue by means of a non-equilibrium stochastic model of a hindered granular column near its jamming limit. Grains feel the constraints of grains above and below them differently, depending on their position. A rich phase diagram with four dynamical phases (ballistic, activated, logarithmic and glassy) is revealed. The statistics of the jamming time and of the metastable states reached as attractors of the zero-temperature dynamics is investigated in each of these phases. Of particular interest is the glassy phase, where intermittency and a strong deviation from Edwards' flatness are manifest.     

Jamming and flow of random-close-packed spherical bubbles: an analogy with granular materials

Wet foams are random-close-packed assemblies of approximately spherical gas bubbles in a liquid. We report rheological experiments with this material, showing that even though the stiffness and frictional interactions of bubbles strongly distinguish them from solid spherical grains, jamming and flow of wet foams and granular materials are governed by closely analogous laws.     

Granular fingers on jammed systems: new fluidlike patterns arising in grain-grain invasion experiments

In this Letter we report spontaneous pattern formation in dense granular assemblies confined to a Hele-Shaw cell and quasistatic regime. Varied unexpected patterns, ranging from rounded to fingered, are observed due to the displacement of one granular material by another. Computer simulations reproduce the major features observed in these experiments. Two mechanisms are responsible for the pattern formation: crystallization of the injected grains and plastic deformation of the displaced grains. The experiment suggests analogies with viscous fingering and jamming transition experiments.     

Stress field in granular systems: loop forces and potential formulation

The transmission of stress through a marginally stable granular pile in two dimensions is exactly formulated in terms of a vector field of loop forces, and thence in terms of a single scalar potential. This leads to a local constitutive equation coupling the stress tensor to fluctuations in the local geometry. For a disordered pile of rough grains this means the stress tensor components are coupled in a frustrated manner. In piles of rough grains with long range staggered order, frustration is avoided and a simple linear theory follows. We show that piles of smooth grains can be mapped onto a pile of unfrustrated rough grains, indicating that the problems of rough and smooth grains may be fundamentally distinct.     

剪切振动下湿颗粒的力学谱

在恒温25 ℃剪切振动条件下,测量不同水分含量的NaCl湿颗粒体系的力学谱（能量耗散tanφ和剪切模量G）.研究发现,随着剪切振幅增大,NaCl湿颗粒体系的剪切模量G和能量耗散tanφ都表现出类似于干颗粒体系的阻塞（Jamming）转变行为.随着体系中水含量的增大,湿颗粒体系的剪切模量G和能量耗散tanφ在质量分数约等于11%的临界水浓度下均出现一个峰值,且峰位与应变振幅无关,表明此时颗粒之间主要的作用力发生了变化.     

Bubble rearrangement duration in foams near the jamming point

We investigate the dynamics of bubble rearrangements in coarsening foams, using a time-resolved multiple light scattering technique. We measure the average duration of such events as a function of the foam confinement pressure. Rearrangements slow down as the pressure is decreased toward the jamming point. Our results are explained by a scaling law based on the balance of pressure and Darcy flow, highlighting an analogy between wet foams with mobile interfaces and suspensions of hard grains.     

Coupled structural and magnetic properties of ferric fluoride nanostructures: Part II,a Monte Carlo–Heisenberg study

We present a numerical study of the magnetic structure of nanostructured iron fluoride, using the Monte Carlo Metropolis simulated annealing technique and a classical Heisenberg Hamiltonian with superexchange angle dependent interactions. The parameters are adjusted on experimental results, and the atomic structure and topology taken from a previous atomistic model of grain boundaries in the same system. We find perfect antiferromagnetic crystalline grains and a disordered magnetic configuration (speromagnetic) at the grain boundary, in agreement with experimental features. Both the lowest magnetic energy and the rate of magnetic frustration are found to be dependent on the relative disorientation of crystalline grains, i.e. on the cationic topology. We conclude on possible extensions of the model.     

Speckle visibility spectroscopy and variable granular fluidization

We introduce a dynamic light scattering technique capable of resolving motion that changes systematically, and rapidly, with time. It is based on the visibility of a speckle pattern for a given exposure duration. Applying this to a vibrated layer of glass beads, we measure the granular temperature and its variation with phase in the oscillation cycle. We observe several transitions involving jammed states, where the grains are at rest during some portion of the cycle. We also observe a two-step decay of the temperature on approach to jamming.     

Jamming and fluctuations in granular drag

We investigate the dynamic evolution of jamming in granular media through fluctuations in the granular drag force. The successive collapse and formation of jammed states give a stick-slip nature to the fluctuations which is independent of the contact surface between the grains and the dragged object, thus implying that the stress-induced collapse is nucleated in the bulk of the granular sample. We also find that while the fluctuations are periodic at small depths, they become "stepped" at large depths, a transition which we interpret as a consequence of the long-range nature of the force chains.     

Effective temperature and jamming transition in dense, gently sheared granular assemblies

We present extensive computational results for the effective temperature, defined by the fluctuation-dissipation relation between the mean square displacement and the average displacement of grains, under the action of a weak, external perturbation, of a sheared, bi-disperse granular packing of compressible spheres. We study the dependence of this parameter on the shear rate and volume fractions, the type of particle and the observable in the fluctuation-dissipation relation. We find the same temperature for different tracer particles in the system. The temperature becomes independent on the shear rate for slow enough shear suggesting that it is the effective temperature of the jammed packing. However, we also show that the agreement of the effective temperature for different observables is only approximate, for very long times, suggesting that this defintion may not capture the full thermodynamics of the system. On the other hand, we find good agreement between the dynamical effective temperature and a compactivity calculated assuming that all jammed states are equiprobable. Therefore, this definition of temperature may capture an instance of the ergodic hypothesis for granular materials as proposed by theoretical formalisms for jamming. Finally, our simulations indicate that the average shear stress and apparent shear viscosity follow the usual relation with the shear rate for complex fluids. Our results show that the application of shear induces jamming in packings whose particles interact by tangential forces.     

激光雷达干扰效果评估方法研究

研究了光电干扰对测距和跟踪测量激光雷达干扰效果的评估方法.在深入分析激光雷达光电干扰机理的基础上,提出了相应的干扰效果评估准则.对激光测距机的距离欺骗干扰效果采用测距误差-干扰成功率准则或准测率准则定量评估,对测距机的激光盲干扰效果依据盲激光对测距准确度、测距功能的影响程度进行评估.对于跟踪测量激光雷达,根据实施干扰后其跟踪误差是否超出正常跟踪准确度的三倍判定一次干扰是否有效,再依据多次试验的干扰成功率的大小评估干扰效果.     

激光致盲干扰效果评估方法研究

研究了激光致盲干扰设备对光电制导系统、光电成像系统和激光测距机干扰效果的评估方法,分别提出了相应的干扰效果评估准则。对于光电制导系统,可以根据实施激光致盲干扰后制导武器的脱靶量是否超出正常制导精度的三倍或杀伤半径判定干扰是否有效,并根据干扰成功率的大小划分干扰等级。对观瞄用光电成像系统的干扰效果,可以按照致盲激光对成像功能的损伤、破坏程度划分为四个等级;对跟踪用光电成像系统的干扰效果可以根据实施干扰后成像系统能否输出有效跟踪脱靶量,以及脱靶量是否超出正常跟踪精度的三倍判定干扰是否有效。对激光测距机的干扰效果可以依据致盲激光对测距机测距精度、测距能力的影响程度划分为四个等级。     

基于低轨卫星的分布式超宽带电磁脉冲对地面接收机干扰技术

随着抗干扰技术的不断发展和进步,以阻塞式和欺骗式干扰为代表的传统干扰技术面临挑战。为此,提出了一种基于低轨卫星的分布式超宽带电磁脉冲干扰技术,相比于传统干扰机,超宽带电磁脉冲干扰是一种新型电磁攻击体制。首先,理论推导了重频超宽带电磁脉冲的功率谱;其次,对分布式干扰技术可行性进行分析,并计算了基于低轨卫星平台的分布式干扰所需的发射功率;最后,开展了针对导航接收机低噪放的超宽带电磁脉冲效应实验,并利用STK(Satellite Tool Kit)设计了中低纬度下用于搭载超宽带电磁脉冲干扰机的低轨卫星星座布局。实验结果表明,UWB电磁脉冲可以使低噪声放大器出现暂时增益压缩现象,脉宽为0.7 ns的单脉冲可以使导航信号经过低噪声放大器后被压制近400 ns,重频形式下可以实现信号的完全压制。因此,基于低轨卫星的分布式超宽带电磁脉冲干扰体系可以有效增强干扰效果,有望实现目标区域的全覆盖。     

About the influence of friction and polydispersity on the jamming behavior of bead assemblies

We study the jamming of bead assemblies placed in a cylindrical container whose bottom is pierced with a circular hole. Their jamming behavior is quantified here by the median jamming diameter, that is the diameter of the hole for which the jamming probability is 0.5. Median jamming diameters of monodisperse assemblies are obtained numerically using the Distinct Element Method and experimentally with steel beads. We obtain good agreement between numerical and experimental results. The influence of friction is then investigated. In particular, the formation of concentric bead rings is observed for low frictions. We identify this phenomenon as a boundary effect and study its influence on jamming. Relying on measures obtained from simulation runs, the median jamming diameter of bidisperse bead assemblies is finally found to depend only on the volume-average diameter of their constituting beads. We formulate this as a tentative law and validate it using bidisperse assemblies of steel beads.     

Nanomagnetism in nanocrystalline multiferroic bismuth ferrite lead titanate films

Multiferroics conventionally refer to the materials exhibiting co-existing electric, magnetic, and structure order parameters. Interplay between ferroelectricity, magnetism, and ferroelasticity in a single phase makes multiferroics truly multifunctional providing control over magnetic and electric ordering by applying electric and magnetic fields, respectively. Incorporation of multiferroic-based components into nanoscale applications will enable additional degrees of freedom in manipulating with spin and charge not easily attainable otherwise. Multiferroic bismuth ferrite lead titanate has been chemically synthesized in form of nanocrystalline films. The morphology of the films revealed a single perovskite phase confined within crystalline grains of few tens of nm in size. The films were found to exhibit ferroelectricity and ferromagnetism with characteristic electric polarization and magnetization hysteresis loops, transformations associated with spin reorientation in an external magnetic field and the spin-glassy behavior well above the room temperature. High degree of magnetic frustration and disorder in the spin system spatially confined in the nanograins, distribution of the grains anisotropy axis, inter-grain interactions, and the effects of uncompensated spins on the large effective surface/interface favored by the nanocrystalline morphology were assumed to be responsible for the anomalous magnetic properties and glassy dynamics in the films.     

一种基于粒子滤波的雷达抗欺骗干扰方法

针对雷达的抗欺骗干扰问题,利用欺骗干扰信号强于目标信号的特点,基于粒子滤波进行了抗欺骗干扰研究。当存在欺骗干扰时,粒子滤波中各粒子的重要性权值会明显减小,据此可以检测干扰,并对受到干扰的数据点进行置零处理,使得欺骗干扰不再与匹配滤波器匹配,从而达到抑制干扰的目的。设计的粒子滤波算法不需要估计系统状态转移函数、系统量测噪声,从而使得算法更具实用性。仿真结果表明,该算法能有效地抑制欺骗干扰,且对干信比不敏感。     

单CCD彩色相机激光干扰模型及外场干扰实验

开展了多波段激光(750~970 nm)对彩色CCD成像系统的外场干扰实验,测得了不同辐照条件下对外场1.3 km处彩色CCD成像系统的干扰效果;建立了彩色CCD相机的激光干扰模型,对实验结果进行了理论验证与分析。理论与实验结果表明:强激光对彩色CCD成像系统的干扰效果明显,CCD靶面出现了明显的光饱和和串扰现象,光饱和区域的形成是由激光束进入光学系统后发生衍射效应造成的;到靶激光功率密度越强,CCD靶面光饱和面积越大,激光干扰效果越好;单波段750 nm激光作用下,到靶功率密度为4.2 kW/cm²,CCD靶面的光饱和面积为0.88 mm×0.97 mm;多波段激光(750~970 nm)作用下,到靶功率密度为20.7 kW/cm²,CCD靶面发生全靶面饱和现象;仿真结果与实验结果基本一致,证明了理论模型的正确性。对远场干扰能力计算结果表明:随着干扰距离的增加,到靶功率密度减小,激光干扰效果变差。     

基于蒙特卡洛方法的四元探测器干扰效果分析

正交四元探测器是第3代红外导引头的核心传感器,为了对该型导引头实施有效干扰,探讨了其探测工作机理,有针对性地提出影响其自动增益控制(AGC)电路工作的干扰策略,对干扰实施过程进行了仿真分析。采用蒙特卡洛方法模拟了在干扰存在频差,干扰占空比改变,干扰相位变化条件下的平均有效干扰率。仿真结果表明:以同频或倍频、0.25占空比、较高功率激光可影响导引头AGC电路正常工作,适时通过调整干扰相位可以影响干扰效果,最终破坏其信号提取逻辑,实现欺骗干扰。分析结果对于激光定向红外对抗系统的研制具有一定理论指导意义。     

激光箔条云对激光导引头的干扰效果研究

激光箔条云对激光有很高的散射率,光电对抗中越来越多地采用激光箔条云作为激光无源假目标实施无源干扰,或作为激光漫反射体与激光有源干扰设备配合使用,实施激光角度欺骗干扰。基于箔条云对激光的散射原理,分析了两种干扰方式的原理,设计了激光箔条云对激光导引头的有源角度欺骗干扰实验方法,给出了所需测试设备及布站要求。结果表明,干扰效果理想。该干扰方法具有灵活、机动、快速等特点,可作为一种激光有源角度欺骗干扰的新手段。     

人工自旋冰中阻挫现象的研究进展

磁学中的阻挫现象近些年来是实验及理论研究的热点.本文就磁学中阻挫研究的新进展进行了介绍.描述了用显微刻蚀制备的人工自旋冰的研究,讨论了其潜在的应用前景以及计算机模拟的结果.