Levitation and lateral forces between a point magnetic dipole and a superconducting sphere

The dipole–dipole interaction model is employed to investigate the angular dependence of the levitation and lateral forces acting on a small magnet in an anti-symmetric magnet/superconducting sphere system. Breaking the symmetry of the system enables us to study the lateral force which is important in the stability of the magnet above a superconducting sphere in the Meissner state. Under the assumption that the lateral displacement of the magnet is small compared to the physical dimensions of our proposed system, analytical expressions are obtained for the levitation and lateral forces as a function of the geometrical parameters of the superconductor as well as the height, the lateral displacement, and the orientation of the magnetic moment of the magnet. The dependence of the levitation force on the height of the levitating magnet is similar to that in the symmetric magnet/superconducting sphere system within the range of proposed lateral displacements. It is found that the levitation force is linearly dependent on the lateral displacement whereas the lateral force is independent of this displacement. A sinusoidal variation of both forces as a function of the polar and azimuthal angles specifying the orientation of the magnetic moment is observed. The relationship between the stability and the orientation of the magnetic moment is discussed for different orientations.     

Analysis of self-excited forces for a box-girder bridge deck through unsteady RANS simulations

This paper presents the results of two-dimensional numerical simulations of the flow field around a trapezoidal box-girder bridge section with later cantilevers, experiencing small-amplitude heaving or pitching harmonic oscillations. Unsteady Reynolds-averaged Navier–Stokes equations are solved in conjunction with an eddy-viscosity and an explicit algebraic Reynolds stress model. Flutter derivatives are determined and compared with wind tunnel results, showing fairly good agreement. The degree of sharpness of the deck lower edges is found to play a key role in the aeroelastic behavior of the profile. In particular, the bridge section fully behaves as a bluff body and is prone to low-reduced-wind-speed torsional galloping in the case of perfectly sharp edges. By contrast, the presence of a small radius of curvature in the section lower corners changes the nature of the instability to coupled flutter and significantly postpones the stability threshold, in line with a quasi-streamlined body behavior. Moreover, a wide sensitivity study is carried out, investigating the influence on the self-excited forces of the amplitude of oscillation, mean angle of attack and Reynolds number. In particular, the numerical simulations for the geometry with smooth lower edges highlight the regime transition occurring when the Reynolds number is varied, with significant effects on the flutter derivatives. Finally, the numerical flow visualizations provide a physical explanation of some phenomena observed in the wind tunnel experiments.     

浅析大学物理中摩擦力难学成因及其对策

摩擦力是大学物理学教学的难点之一。文章基于教学实践,剖析了摩擦力难学的形成原因,并给出了相应的解决办法,以促进教学效果的提高。     

Scalable gas-phase processes to create nanostructured particles

The properties of nanoparticles are often different from those of larger grains of the same solid material because of their very large specific surface area. This enables many novel applications, but properties such as agglomeration can also hinder their potential use. By creating nanostructured particles one can take optimum benefit from the desired properties while minimizing the adverse effects. We aim at developing high-precision routes for scalable production of nanostructured particles. Two gas-phase synthesis routes are explored. The first one - covering nanoparticles with a continuous layer - is carried out using atomic layer deposition in a fluidized bed. Through fluidization, the full surface area of the nanoparticles becomes available. With this process, particles can be coated with an ultra-thin film of constant and well-tunable thickness. For the second route - attaching nanoparticles to larger particles - a novel approach using electrostatic forces is demonstrated. The micron-sized particles are charged with one polarity using tribocharging. Using electrospraying, a spray of charged nanoparticles with opposite polarity is generated. Their charge prevents agglomeration, while it enhances efficient deposition at the surface of the host particle. While the proposed processes offer good potential for scale-up, further work is needed to realize large-scale processes.     

盾构隧道同步注浆浆液压力变化规律研究

基于浆液流体的连续方程和运动方程,建立了浆液刚进入盾尾空隙时的环向压力分布模型;并推导了在距离注浆孔较远距离处的浆液压力梯度与上浮力的关系式。结合实际工程监测数据验证了理论分析的合理性。分析结果表明:盾构机推进时,注浆点附近的浆液环向压力分布主要由注浆孔位置和注浆孔压力及浆液重度决定;停止推进时,浆液在固结效应下压力逐渐消散,竖直方向上压力梯度逐渐减小;随着远离注浆点,浆液的压力梯度逐步减小并趋于稳定,稳定时的压力梯度主要由上浮力的大小决定。     

Ⅲ—Ⅳ级六杆机构的传力特性分析及曲柄存在准则

本文讨论了Ⅲ级六杆机构的曲柄存在准则及其使用方法；研究了几种六杆机构的传力特性；得出几条重要结论，提出用“合力臂”评价六杆机构传力特性的方法；并对常用六杆机构的实用性作出了定性的评价。     

基于时空分析的城市边缘区耕地演变机理研究——以武汉市黄陂区为例

城市边缘区是土地利用最复杂和最活跃的区域,其耕地演变一直是国内外学者关注的焦点．在地理信息系统技术的支持下,利用耕地面积变化的时间序列模型、耕地动态度模型、耕地重心变化模型等,对城市边缘区耕地的时空演变特征进行定量研究,并应用主成分分析法,选取18个因素对其驱动因子进行相关分析．结果表明,地处城市边缘区的黄陂区耕地面积的变化主要是受社会经济的快速发展、人口增长、农业结构调整及耕地比较效益的影响．     

Orbital dynamics in the photogravitational restricted four-body problem: Lagrange configuration

We study the effect of the radiation parameter in the location, stability and orbital dynamics in the Lagrange configuration of the restricted four-body problem when one of the primaries is a radiating body. The equations of motion for the test particle are derived by assuming that the primaries revolve in the same plane with uniform angular velocity, lying at the vertices of an equilateral triangle. The insertion of the radiation factor in the restricted four-body problem, let us model the dynamics of a test particle orbiting an astrophysical system with an active star. The dynamical mechanisms responsible for the smoothening on the basin structures of the configuration space is related to the decrease in the total number of fixed points with increasing values of the radiation parameter. In our model of the Sun-Jupiter-Trojan Asteroid system, it is found that despite the solar radiation pressure, there exist two stable libration points.     

Non-linear interaction of water waves with submerged obstacles

The interaction of two-dimensional water waves with a fixed submerged cylinder is studied using a finite difference scheme with boundary-fitted co-ordinates. A mixed Eulerian–Lagrangian (MEL) formulation is used to satisfy the fully non-linear free surface conditions. The diffraction of small-amplitude water waves by a cylinder is examined for various wavelengths and amplitudes of the incident wave. Fourier analyses of the incident and diffracted waves are performed to determine their spectra. An example of a large-amplitude wave breaking over a cylinder is also studied. The non-linear numerical solutions are compared with those of experiments and linear theory where appropriate.     

影响汽车侧翻的悬架因素分析

利用具有可变形悬架和轮胎的汽车侧翻模型，分析了悬架及轮胎等因素对汽车侧翻的影响，推导出包括轮距变化、举升效应、轮胎转动惯性力矩等因素在内的汽车侧翻阈值公式，突出影响汽车侧翻的悬架因素，为评估悬架对汽车侧翻稳定性影响提供了理论依据。