Penetration into a granular bed in the presence of upward gas flows

We present a numerical study on the penetration of spherical projectiles into a granular bed in the presence of upward gas flows. Due to the presence of interstitial fluid, the force chains between particles in the granular bed are weakened significantly, and this distinguishes the penetration behavior from that in the absence of fluid. An interesting phenomenon, namely granular jet, is observed during the penetration, and the mechanism for its formation and growth is attributed to the merging of granular vortices generated by the interaction between the intruder and primary particles. Moreover, both the final penetration depth and the maximum diameter of the crater are found to follow a power-law dependence with the impact velocity, and the maximum height reached by the granular jet tends to increase linearly as the impact velocity increases, agreeing well with the experimental results reported in the literature.     

Very Long Baseline Interferometry — a high precision tool for geodesy and astrometry

Very Long Baseline Interferometry is able to provide a direct geometrical tie to the extragalactic radio sources which represent the best possible realization of an inertial system. By this token, VLBI can measure Earth rotation and orientation as well as precise station positions and their velocities without involving the gravity field of the Earth. In the broader context of Earth observation and the monitoring of geodynamic processes, this one and many more unique features have allowed the VLBI technique to achieve pioneering feats such as the determination of present-day plate tectonic motions, post glacial rebound, sub-daily Earth rotation variations and parameters of general relativity. In this contribution, a brief outline of the VLBI technique and the models used in geodetic data analysis will be given before some of the most important achievements to date will be passed in review and future developments will be indicated.     

DEM analysis of passive arching in a shallow trapdoor under eccentric loading

This study analyzed the passive arching effect under eccentric loading by developing a series of trapdoor discrete numerical models. The numerical models were validated by comparison with laboratory test results. The deformation pattern, soil arching ratio, force chain distribution, and coordination number under various surcharge magnitudes and deviation distances were analyzed. The numerical results showed that the deformation diagram of soil particles can be divided into three zones: principal displacement zone, transition zone, and static zone. With an increase in the surcharge magnitude, the range of the principal displacement zone decreased, but the range of the transition region increased. The curve of the soil arching ratio on the trapdoor can be divided into three phases, which can be well characterized by the tangent modulus. The passive arching effect is degraded by a surcharge. The ultimate soil arching ratio could be approximated as a W-shaped distribution along the +x-direction. With an increase in the trapdoor displacement, the force chain on the trapdoor gradually expanded outward to form an inverted funnel shape. The most powerful force on the trapdoor was mainly distributed on its edge. The average coordination number decreased gradually as the trapdoor moved upward.     

基于分数阶Fourier变换的结构瞬时频率识别北大核心CSCD

为识别时变信号的瞬时频率,由分数阶Fourier变换定义推导出了一般信号的频率与单一变量旋转角度α的关系式,从理论上解释了分数阶Fourier变换本质上是一种普通Fourier变换结合伸缩平移窗的算法,进而在分数阶Fourier域建立了非平稳信号瞬时频率的一般表达式,实现了结构瞬时频率的识别.采用任意非线性调频信号仿真算例和三自由度有阻尼时变结构系统的数值算例对提出的方法进行了比较分析.结果表明,该文提出的方法与理论值吻合良好,并具有一定的抗噪性,验证了方法的可靠性和实用性,可以应用于时变结构瞬时频率的识别.     

The theoretical study of the efficiency of diffusion deposition of nanoaerosols in the extended range of the Peclet numbers

The efficiencies of the diffusion deposition of nanoaerosols for a single fiber for the models of aerosol filter and wire mesh screen are studied numerically in the extended range of the Peclet number Pe. The rectangular periodic cell model for fluid flow and convective-diffusive transport of small aerosol particles is used. Most of the previous theoretical and experimental studies of single fiber diffusion deposition efficiency were for the case of Pe > 1. The array with uniform square or chess grid of fibers and of a row of circular cylindrical fibers are considered as the filter and wire mesh screen models. The flow and particles transport equations are solved numerically using the Boundary Element Method.The obtained numerical data are used to derive the approximate formulas for the deposition efficiency in the entire range of the Peclet number for the various porosities of the filter medium or distances between fibers in a wire mesh screen. The derived dependencies take into account nonlinearity of the deposition efficiency at the low Peclet numbers. The obtained analytical dependencies compare well with the numerical and experimental data.