Generation of localized magnetic fields by dynamos with conducting surroundings

The investigation of kinematic dynamos embedded in electrically conducting space leads to an eigenvalue problem whose spectrum contains a continuous as well as a discrete part. It is shown that the continuous spectrum has no relevance to the dynamo problem. The determination of the discrete spectrum can be regarded in analogy to the related quantum mechanical problem. Die Untersuchung kinematischer Dynamos, die in ein elektrisch leitendes Medium eingebettet sind, führt auf ein Eigenwertproblem, dessen Spektrum aus einem kontinuierlichen und einem diskreten Anteil besteht. Es wird bewiesen, daß das kontinuierliche Spektrum keine Bedeutung für das Dynamoproblem hat. Die Bestimmung des diskreten Spektrums kann in Analogie zur entsprechenden quantenmechanischen Aufgabe gesehen werden.     

凹陷地形对Rayleigh面波传播影响的研究

利用局域离散波数法,研究了地形对近地表爆炸源产生的Rayleigh面波的影响,展示了Rayleigh面波穿越地形过程中波的传播进程,研究了穿越地形前后Rayleigh面波能量、频率的变化,并讨论了该变化与地形起伏程度的关系,从中可见,Rayleigh面波穿越地形时,激发出新的体波和面波,并且越陡的地形对面波传播的阻碍性越强,且面波高频部分能量损失的越多.     

几种离散格网模型的几何稳定性分析

对仅从剖分单元的形状和面积的变形研究,不能完全表达格网模型几何稳定性作了分析,并提出了从研究格网结点空间分布均匀性在多分辨率剖分过程中的变化出发,分析格网系统稳定性的研究思路,并选择能直接反映格网结点分布特性的四个统计量作为格网结点均匀性的评价指标。对几种离散格网系统的几何稳定性进行比较和评价,分析了不同格网生成方式对稳定性的影响,最后,在此研究基础之上,提出了进一步研究的工作方向。     

Modelling dynamic breakage in rock blocks with different elongation and flatness ratios upon impact

The two crucial shape factors (elongation ratio and flatness ratio) of brittle particles may influence the dynamic breakage of brittle particles upon impact. Hence, three-dimensional discrete element method simulations of brittle rock blocks with different shapes upon normal impact were performed. The simulated results indicate that the elongation ratio, that is, ratio of width to length and flatness ratio, that is, ratio of thickness to width can significantly affect the breakage of brittle rock blocks. Three fracture mechanisms, that is, fragmentation, horizontal tensile fracture and vertical tensile fracture, were revealed, which determine the dynamic breakage of rock blocks. The fragmentation results in numerous single-sphered fragments with velocities even larger than 2 times of the initial velocities. Fragmentation can provide a buffering effect at high impact velocities of larger than 4 m/s. With an increasing elongation ratio or flatness ratio, the phenomenon of fragmentation gradually disappears. The reflection of a compression stress wave results in horizontal tensile fracture. The expansion in the plane perpendicular to the impact velocity results in vertical tensile fracture.     

SEISMIC GROUND MOTION SYNTHETICS OF THE 1989 LOMA PRIETA EARTHQUAKE

A discrete wave number approach in conjunction with a propagator-based formalism is used to synthesize the Loma Prieta earthquake ground motion at both the near and the far field, taking into account all kinds of seismic waves (body and surface). A bilaterally propagating shear slip over a rectangular fault is used to describe the seismic source mechanism, while the earth model is based on geological profiles of the Santa Cruz mountain area and consists of three layers overlaying a half-space. The synthesized ground motion is first compared with actual records from the Loma Prieta earthquake and the agreement between the two is found to be satisfactory, as far as magnitude, duration and essential wave form characteristics are concerned. Then, ground motions are synthesized and plotted at a dense grid of observer locations over a large area around the epicenter, at different time instants. Using such plots, it is possible to study the generation ana propagation of different kinds of seismic waves, the spatial variability of ground motion, as well as the development of the permanent gound deformation.     

多维地震作用下钢筋混凝土建筑结构的抗连续倒塌仿真分析

为了提高钢筋混凝土建筑结构的抗震性能,分析多维地震作用下钢筋混凝土建筑结构的抗连续倒塌能力,结合钢筋混凝土建筑结构特性、节点构造特点以及其在多维地震作用下的破坏机理,采用离散单元法建立结构连续倒塌的理论模型,对建筑结构连续倒塌过程进行数值模拟。基于数值模拟化结果,通过备用荷载路径法,实现建筑结构的抗连续倒塌分析。仿真实验结果得出,所提方法能实现对建筑结构抗连续倒塌的准确分析,且在多维地震作用下建筑结构扭转的幅度明显变大,结构顶层位移发散状态显著,不同楼层会产生不同的层间位移以及薄弱部位,建筑结构的抗连续倒塌性能随着失效构件位置的提升而增强。     

Effective location of active control devices for building vibrations caused by periodic excitation acting on intermediate storey

In order to investigate ways of reducing vibrations of building structures subjected to excitation acting on intermediate storey, active vibration controls are conducted with active control devices installed on different floors of the structure, and the effective location of control devices is also investigated. In this paper, we propose a new ‘Discrete‐Optimizing Control Method’ for vibration control. The control forces are determined analytically which makes the ‘discrete‐index function’ minimum. Through numerical simulation, the Discrete‐Optimizing Control Method is proved to be an effective control method. The response reduction effects are best when the control devices are concentrated on the adjacent three floors of the vibration source. Copyright © 2000 John Wiley & Sons, Ltd.     

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Analysis of mechanical behaviour and internal stability of granular materials using discrete element method

Granular materials like sand are widely used in civil engineering. They are composed of different sizes of grains, which generate a complex behaviour, difficult to assess experimentally. Internal instability of a granular material is its inability to prevent the loss of its fine particles under flow effect. It is geometrically possible if the fine particles can migrate through the pores of the coarse soil matrix and results in a change in its mechanical properties. This paper uses the three‐dimensional Particle Flow Code (PFC3D/DEM) to study the stability/instability of granular materials and their mechanical behaviour after suffusion. Stability properties of widely graded materials are analysed by simulating the transport of smaller particles through the constrictions formed by the coarse particles under the effect of a downward flow with uniform pressure gradient. A sample made by an initially stable material according to the Kenney & Lau geometrical criterion was divided into five equal layers. The classification of these layers by this criterion before and after the test shows that even stable granular materials can lose fine particles and present local instability. The failure criterion of eroded samples, in which erosion is simulated by progressive removal of fine particles, evolves in an unexpected way. Internal friction angle increases with the initial porosity, the rate of lost fine particles and the average diameter D₅₀. Copyright © 2016 John Wiley & Sons, Ltd.