探讨振动激励下湿颗粒物质中的断层行为

研究了竖直振动激励下湿颗粒物质中的一种断层行为,实验表明断层块的运动具有周期性,其周期是激振周期的整数倍,倍率强烈地依赖于约化加速度Г,随着Г的加大,倍率阶梯式增加。采用蹦球模型对其进行了动力学分析,其数值解结果对于倍率为三倍和四倍的周期运动对应的Г值与实验结果吻合较好,对于倍率为两倍的周期运动对应的Г值比实验值小,最后定性地解释了偏小的原因。     

振动激励下的颗粒物质的有效隆起力

振动激励下的颗粒物质的有效隆起力陈伟中魏荣爵（南京大学声学研究所近代声学国家重点实验室南京·２１００９３）直到８０年代末，有关颗粒物质的研究才引起人们的高度重视，发现了许多令人感兴趣的现象［１］，如颗粒大小分离，声传播反常，隆起和对流［２～８］等等，...     

振动下颗粒物质系统中纽结孤立波的实验观察

实验表明,垂直振动下条件的颗粒物质可以呈现出诸如隆起、对流、分层及表面波动等现象[1].其中表面波动的现象,在三维和二维系统中都已有报导[2][3][4].在我们关于二维系统所作的工作中,除了已知的f/2和f/4波动外(f为激励频率)[3],我们还观察到了扭结孤立波及其他的扭结现象.本文将主要介绍我们在这方面的工作.     

垂直激励下颗粒物质的质心动力学特性

颗粒物质是一种中微观的物质存在方式,这种存在方式非常普遍,与工农业生产,人们生活都有着密切的关系.长期以来人们对这种媒质的特性研究一直都有着浓厚的兴趣.但由于颗粒物质具有的非常独特的非线性、耗散以及非平衡的特殊性质,对于这种媒质到目前为止还没有一个系统的理论来描述.颗粒物质本身由固体组成,有些情况下则会出现类似液体的流动、对流和表面波动,在较大的激发下还可能会表现得类似于气体[1].     

常数激励与简谐激励联合作用下Duffing系统的非线性振动

以Duffing系统为研究对象,探讨常数激励与简谐激励联合作用下系统的骨架曲线及幅频响应特性,重点考察常数激励的影响。采用谐波平衡法求解该系统的振动方程,得到幅频响应关系,并给出了骨架曲线以及周期解的稳定性分析。采用幅频响应曲线和骨架曲线表征系统的基本动力学性质,讨论了常数激励和简谐激励幅值对系统幅频曲线性态和骨架曲线形态的影响。研究发现,系统振动响应中直流分量与谐波分量的振幅同步变化,但变化趋势相反。此外,两者骨架曲线的形态均是先向左微偏后转为向右弯曲,因此,在某些参数条件下,对应一个激励频率的周期解可能有5组,其中3组为稳定性,2组为不稳定解。进一步通过增大常数激励发现其能够对该系统造成的“刚度增强”效应,但同时也会伴随着愈加显著的“刚度渐软”特性。相应地,对于特定的简谐激励幅值,随着常数激励的增大,系统的幅频曲线能够由纯硬特性转变为软硬特性共存,甚至纯软特性。但是,在大尺度下观察,常数激励对系统骨架曲线的影响主要表现在骨架曲线根部的形态上,即随着简谐激励幅值的增大,常数激励对系统共振频率的影响变弱,不同常数激励下的骨架曲线趋于一致。     

大跨越导线在局部激励下微风振动的波动特性

微风振动常造成架空导线疲劳断股断线、金具脱落等破坏.考虑大跨越导线所处风场的切变特性,采用局部激励模型研究大跨越导线的微风振动机理,建立了长柔导线在档内任意局部位置激励下的动力学模型,基于导线系统的小阻尼特性,得到了局部周期激励下长柔导线波动的格林函数解;依据振动波沿展向的衰减特性,将导线内的波动过程分为驻波振动、行波...     

混沌激励下振动系统的非线性参数识别

本文对基于时-频相结合的非线性振动系统的参数识别问题进行了研究。首先建立含非线性参数单自由度振动系统的力学模型,将已知非线性系统产生的混沌响应作为该系统的激励, 假定其响应有若干不稳定的周期轨道组成,从混沌响应的状态空间中提取出近似周期轨道, 采用谐波平衡法识别出系统的参数,然后对识别出的参数进行误差分析。最后通过数值模拟,验证了混沌信号作为激励源对非线性系统进行参数识别的可行性。     

随机激励下高维包装振动系统的可靠性分析

研究了两自由度包装振动系统在受高斯白噪声外激励下的可靠性.利用由随机平均法导出的关于系统的独立运动积分的随机平均方程,通过系统本身的特性以及对漂移与扩散系数作出的一些假定,给出了系统的可靠性函数以及首通损坏的条件转移概率密度函数所满足的后向柯尔莫哥洛夫方程和福克-普朗克方程,并进一步详细地讨论了其边界与可解性条件.数值结果表明,关于系统的可靠性函数以及首通损坏的条件转移概率密度函数方面的结果是比较合理的,这也表明文中的分析方法是正确的.     

参外激励作用下非线性振动系统的混沌

用多尺度法给邮了ｖａｎ ｄｅｒ Ｐｏｌ－Ｄｕｆｆｉｎｇ振子在参数激励与外部激励联合作用下的主亚谐联合共振条件，用Ｍｅｌｎｉｋｏｖ方法研究了与之对应的平衡系统的同、异宿轨道相变的条件，证明了原系统的的振幅与相位存在类随机运动，并结合数值结果给出由概周期运动通向混沌的两种方式。     

一类振动系统在平稳Gauss激励下的响应分析

本文提出了一类线性振动系统X(t) 2ζω_0X(t)十ω_0~2X(t)=F(t),在工程振动中常见的一种功率谱密度为φ_0/(ω~2 α~2)的Gauss平稳激励下,系统响应的时域解和频域解。并且证明了初始响应是一个演变的随机过程,指出响应的初始破坏的可能性是不容忽视的。今后的工作是进一步提出减少这种初始破坏的具体方案。     

Characterization of the energy bursts in vibrated shallow granular systems

We study the recently reported energy bursts that take place in a granular system confined to a vertically vibrated shallow box containing two types of grains of equal size but different mass (Rivas in Phys Rev Lett 106:088001?C1?C088001?C4, 2011). In a quasi one dimensional configuration, it is possible to characterize the propagating fronts. The rapid expansion and the subsequent compression of the energy bursts take place at roughly constant velocities. The expansion velocity is 40 times larger than the compression velocity. Starting from an initially segregated configuration it is possible to determine the instants at which the energy bursts begin and the mechanisms that trigger them. Two mechanisms are identified: an oblique collision of a heavy grain with a light one in contact with one of the horizontal walls and a slow destabilization produced by light grains that are surrounded by heavy ones.     

Harmonic patterns in fine granular vibrated beds

We examine the spontaneous formation of numerous previously unreported patterns in deep beds of fine grains vibrated at frequencies ranging from 10 to 100 Hz and accelerations ranging from 1 to 14 times gravity. Similarly to shallow beds, we find stripes, closed cells and labyrinthine patterns. In addition, we observe traveling waves, cannibalizing cells and other behaviors that are unexpected and remain unexplained. All of these patterns vary spatiotemporally, and unlike shallow bed patterns, are harmonic rather than subharmonic. Received: 17 May 1999     

The temperature of a vibrated granular gas

The temperature of a granular medium is defined, in a manner analogous to that of a molecular gas, as the root mean squared kinetic energy of the grains. Using a novel two-layer granular experiment, we have made a systematic study of the granular temperature as a function of driving parameters in order to determine whether the behavior of the granular temperature is truly analogous to the temperature of a molecular gas. By examining the behavior of this particular system in detail, where two fluctuating granular media are in thermal contact, we also suggest general conditions under which such analogies can be extended to other driven systems.     

Sinking and recirculation of large intruders in vertically vibrated granular beds

A series of experiments are described in which air-immersed grains within different containers are exposed to vertical vibrations. When the peak acceleration relative to gravity exceeded Γ = 6 at 49 Hz), tilted cavitating beds were formed. Then, when a large, low-density object was put onto the free surface, the vigorous granular convection dragged it into the bed. The large object was not being dragged into the bed at Γ > 6 when the air pressure inside the container fell well below1 Torr. These findings, as well as those from computer simulations, confirm the key role played by large air cavities such as swallow holes in submerging large, low-density objects into a granular bed.     

Velocity correlations between a vibrated granular gas and a piston on top

The stationary state eventually reached by a vibrated granular gas with a piston on the top, and in presence of gravity, is considered. The relationship between the second velocity moments of the piston and of the gas just below it is investigated by using event driven molecular dynamics simulations. It is concluded that velocity correlations between the piston and the gas play a crucial role in determining the velocity fluctuations of the piston, that can be much larger than estimated by energy equipartition.     

Effect of the confinement on the properties of ultrasonic vibrated granular gases

The properties of a ultrasonic vibrated granular gas are investigated by using an event-driven Molecular Dynamics simulation of inelastic hard spheres that are fluidized by a vibrating bottom wall (sonotrode) in boxes with different geometries showing either no or an increased confinement. It appears that geometry controls dramatically not only the distribution of wall impacts and their velocity and impact angles, but also more fundamental properties of the granular gas such as temperature and density. For geometries displaying an acute angle (prism), stacking is found, which induces a strong temperature and density gradient. The underlying origin of such phenomena for the granular gas arises from the spatial limitation, confinement, combined with the inelastic collisions induced by closely located side walls. It underscores the possibility that kinetic and thermodynamic properties of granular gases can be changed by simple geometries inducing spatial limitation.     

A discrete element study of settlement in vibrated granular layers: role of contact loss and acceleration

This paper deals with the vibration of granular materials due to cyclic external excitation. It highlights the effect of the acceleration on the settlement speed and proves the existence of a relationship between settlement and loss of contacts in partially confined granular materials under vibration. The numerical simulations are carried out using the Molecular Dynamics method, where the discrete elements consist of polygonal grains. The data analyses are conducted based on multivariate autoregressive models to describe the settlement and permanent contacts number with respect to the number of loading cycles.     

Waves of dilatancy in a granular material with incompressible granules

As a densely packed granular material begins to flow through a constriction, dilatant waves are often observed. These waves initiate at the constriction and propagate outward into the granular material with an increase in the porosity of the material behind the wave front. As a model for dilatant waves, we consider curved waves carrying a jump in the gradient of porosity in the context of the continuum theory for the flow of granular materials presented by Goodman and Cowin. Their speed of propagation is determined and it is shown that such waves carry a jump in the stress acting on planes transverse to their direction of propagation. For the case when a wave of dilatancy propagates into a uniform region at rest, an explicit formula for the amplitude of the wave is derived for waves of general shape and then specialized to give results for plane, cylindrical, and spherical waves. In this case, the wave is purely longitudinal.     

Error analysis and applications in transportation systems

This paper presents an overview of the field of error analysis. Section 1 shows why discussions about human error are relevant for societal safety. With regard to safety research, it is important to predict abnormal events. At the machine side, reliability studies proved their value, but to predict failures in the human factor has been shown to be very difficult. Therefore, problems in how to define the notion of human error (Section 2) and how to classify different types of error (Section 3) are discussed. Some researchers started to use systematical classifications of human error types based on a recent, hierarchical model of human task performance. The outline of the model is presented. Examples of error analysis studies from the field of transportation research (Section 4) provide some basic suggestions on how to reduce error rates. Some conclusions on error control are given in Section 5. The responsibility of managers and system designers in this respect is strongly emphasized.