同心旋转圆柱间隙流场中纤维取向的动态模拟

本文对两同心旋转圆柱间隙形成的流场以及处于流场中的纤维运动和取向进行了数值研究. 在贴体坐标网格下求解了流场控制方程, 得到了流场中的速度、压力等物理量. 研究了两同心圆柱同速反向旋转以及仅内层圆柱旋转这两种情况下的纤维运动和取向状态. 得到了处于这两种情况下的纤维在流场中从静止到开始运动和取向直至最终达到稳定状态的动态详细过程. 结果表明, 当两个圆柱同速反向旋转时, 纤维运动与取向也相应的呈现两层结构; 而仅内圆柱旋转时, 纤维运动与取向呈单层结构. 在两种情况下, 纤维均沿流线方向运动和取向. 讨论了纤维长径比对纤维取向的影响, 结果表明随着纤维长径比的增加, 纤维沿流线取向的取向度逐渐增强.     

介观圆筒持续电流——紧束缚近似下的严格解

证明了含Ａｈａｒｏｎｏｖ－Ｂｏｈｍ型磁通的空心圆筒中环行电流的单电子理论计算公式，它与圆筒形状及载流子属性（电荷与质量）无关，在紧束缚近似下采用完整晶体模型求得能量和电流的严格解，并研究了电流随磁通变化的周期往，以及持续电流的幅值随筒高与筒厚增大的可能性，获得一系列新结果。 关键词：     

Steady hydromagnetic flow between two porous concentric circular cylinders

The problem considered here is to study the MHD effects on the steady flow of an incompressibe viscous conducting fluid through two concentric porous non-conducting infinite circular cylinders, rotating in various ways with uniform angular velocities in presence of a radial magnetic field. It is supposed that the rate of suction at the inner cylinder is equal to the rate of injection at the outer.     

Undulated cylinders of charged diblock copolymers

We study the cylinder to sphere morphological transition of diblock copolymers in aqueous solution with a hydrophobic block and a charged block. We find a metastable undulated cylinder configuration for a range of charge and salt concentrations which, nevertheless, occurs above the threshold where spheres are thermodynamically favorable. By modeling the shape of the cylinder ends, we find that the free-energy barrier for the transition from cylinders to spheres is quite large and that this barrier falls significantly in the limit of high polymer charge and low solution salinity. This suggests that observed undulated cylinder phases are kinetically trapped structures.     

The Green's function of the Poisson equation on the non-concentric annular region

In this article the two-dimensional Poisson equation is considered in the region between two non-concentric circular cylinders. Upon introducing bipolar coordinates the corresponding Green's function is found in form of a simple and rapidly converging series which can be formally summarized as a closed-form. Based on this result we additionally provide the Green's function for the conducting cylinder which is oriented parallel to a ground plane as well as for the case of two conducting cylinders.     

Scattering from a coated nihility circular cylinder placed in chiral metamaterial

The scattering of electromagnetic plane wave from a coated nihility circular cylinder placed in chiral metamaterial is investigated. The nihility cylinder is coated with chiral or chiral-nihility metamaterial and the medium hosting the coated cylinder is also considered as chiral or chiral-nihility metamaterial. The vector wave equation is used to find out the solution for the fields scattered from the concentric chiral ciruclar cylinders immersed in chiral metamaterial. Inner chiral cylinder is reduced to nihility cylinder taking permittivity and permeability approaching to zero with chirality parameter equal to zero. Coated perfect electric conductor (PEC)/perfect magnetic conductor (PMC) are obtained by taking very large value of permittivity/permeability of the inner cylinder. Numerical results for fields reflected from coated nihility cylinder are compared with those obtained for coated PEC/PMC cylinder. Our results for the special case of dielectric coated nihility/PEC cylinders in free space are shown to be in agreement with already published results thus validating the scheme of analysis.     

Calculating the forced response of cylinders and cylindrical shells using the wave and finite element method

The dynamic response of circular cylinders can be obtained analytically in very few (and simple) cases. For complicated (thick or anisotropic) circular cylinders, researchers often resort to the finite element (FE) method. This can lead to large models, especially at higher frequencies, which translates into high computational costs and memory requirements. In this paper, the response of axially homogenous circular cylinders (that can be arbitrarily complex through the thickness) is obtained using the wave and finite element (WFE) method. Here, the homogeneity of the cylinder around the circumference and along the axis are exploited to post-process the FE model of a small rectangular segment of the cylinder using periodic structure theory and obtain the wave characteristics of the cylinder. The full power of FE methods can be utilised to obtain the FE model of the small segment. Then, the forced response of the cylinder is posed as an inverse Fourier transform. However, since there are an integer number of wavelengths around the circumference of a closed circular cylinder, one of the integrals in the inverse Fourier transform becomes a simple summation, whereas the other can be resolved analytically using contour integration and the residue theorem. The result is a computationally efficient technique for obtaining the response to time harmonic, arbitrarily distributed loads of axially homogenous, circular cylinders with arbitrary complexity across the thickness.     

Numerical Simulation of Two-Dimensional Flow over Three Cylinders by LatticeBoltzmann Method

The numerical simulation using the multiple relaxation time lattice Boltzmann method (MRT-LBM) is carried out for the purpose of investigating the two-dimensional flow around three circular cylinders. Among these three circular cylinders, one of the three cylinders on which a forced in-line vibrating is used to do this research and attempt to find out the effects of the movingcylinder and the other two rigid cylinders on the wake characteristics and vortex formation. As a benchmark problem to discuss the problem of lift coefficient r.m.s for these cylinders with spacing ratios T/D between other rigid side-by-side cylinders, and the calculation is carried out with two compared cases at Reynolds number of 100, two of the cylinders are rigid and the other one is an in-line vibrated cylinder lying downstream, in addition, forced vibrating amplitude and frequency are A/D= 0.5 and f_ν=0.4 (where A is the forced amplitude, D is the cylinder diameter, and f_ν stands for the vibrating frequency, respectively). The calculated results not onlyindicate that the spacing ratios T/D (T is the center-to-center spacing between the two upstream cylinders) have influence on the wake patterns and the formation of vortex shedding, but also analyze the lift coefficient r.m.s for the three cylinders with the spacing ratios S/D (where S is the center-to-center spacing between the center of upstream two side-by-side cylinders and downstream cylinder).     

串列双圆柱时均流场特性分析

对Re=12000,间距比L/D=1.167,2.333,3.500和4.667的串列双圆柱后方速度场进行了实验测量,分析串列双圆柱后方不同剖面处的速度分布规律和湍流度分布规律.并通过流函数理论模型对小间距比串列双圆柱后方流场进行了分析,得出如下结论:与单圆柱相比,当串列双圆柱间距比较小时,后方圆柱的自由剪切层明显向内...     

Interaction of acoustic scatterers

The interaction of scatterers under steady-state acoustic irradiation is studied for the case of scatterers in the form of elliptic cylinders taken as an example. The angular scattering characteristics of two interacting cylinders are calculated and compared with the angular characteristics of a single cylinder in a wide frequency band and in a wide angular range of irradiation. The parameters of interacting bodies (the angle of irradiation, the size with respect to the wavelength, and the distance between the bodies) at which the interaction is negligibly small are determined.     

Snaking states on a cylindrical surface in a perpendicular magnetic field

We calculate electronic states on a closed cylindrical surface as a model of a core-shell nanowire. The length of the cylinder can be infinite or finite. We define cardinal points on the circumference of the cylinder and consider a spatially uniform magnetic field perpendicular to the cylinder axis, in the direction South-North. The orbital motion of the electrons depends on the radial component of the field which is nonuniform around the circumference: it is equal to the total field at North and South, but vanishes at the West and East sides. For a strong field, when the magnetic length is comparable to the radius of the cylinder, the electronic states at North and South become localized cyclotron orbits, whereas at East and West the states become long and narrow snaking orbits propagating along the cylinder. The energy of the cyclotron states increases with the magnetic field whereas the energy of the snaking states is stable. Consequently, at high magnetic fields the electron density vanishes at North and South and concentrates at East and West. We include spin-orbit interaction with linear Rashba and Dresselhaus models. For a cylinder of finite length the Dresselhaus interaction produces an axial twist of the charge density relative to the center of the wire, which may be amplified in the presence of the Rashba interaction.     

Charge transfer between percolation levels in a system with an artificial,strongly disordered potential

We observe a hysteresis in the magnetoresistance of mesoscopic samples with a disordered antidot lattice. When the Fermi level lies in between two Landau levels, the Faraday electric field induces a change in the charge of two closed, intercalated electron trajectories in the artificial strongly disordered potential. The total transferred charge is approximately equal to 40e. Depending on whether this transfer is towards the inner or outer closed trajectory (which is determined by the direction of the magnetic field sweep) the potential barrier for trajectories that traverse the sample will increase or decrease.     

激波诱导双层气柱演化的偏心效应研究

实验与数值研究了平面激波诱导下双层气柱的演化规律.利用肥皂膜技术生成了3种双层气柱,通过固定内、外层气柱半径,改变内层气柱在流向方向上的位置,研究了双层气柱偏心效应对流场演化的影响.结果表明,当内层气柱向上游偏移时,外层气柱的上游界面在后期会产生朝向上游的"射流";当内层气柱向下游偏移时,下游两道界面会较早地耦合在一起...     

通道中串列旋转圆柱的格子Boltzmann-虚拟区域方法的模拟

通过结合格子Boltzmann方法(LBM)和虚拟区域(Fictitiou sDomain)思想,建立格子Boltzmann-虚拟区域(LB-DF/FD)方法.采用两套网格系统,欧拉网格用于流体,拉格朗日网格用于固体.原有的LBM在计算运动固体的受力方面存在数据振荡,LB-DF/FD方法改进了此缺陷.为验证该方法,模拟圆柱绕流、圆形颗粒在无限长通道中平动及在无限大流场中转动三种情况,结果与其他数值解及理论解符合得很好.利用该方法模拟低雷诺数下通道中串列旋转圆柱周围的流场,分析圆柱间距(g)及雷诺数(Re)对流场结构的影响.给出Re=0.001,0.1和10下,0.2≤g≤8.0的流线结构、圆柱升力、阻力以及力矩等数值结果.结果表明,g对流场的结构及圆柱的受力有显著影响,Re对圆柱阻力及Stokes单元数目的影响较大.     

Transition of free-surface flow modes in taylor-couette system

This paper presents our numerical and experimental results of the bifurcation found in Taylor-Couette system with a free surface. The lengths of the two concentric cylinders are finite and their axes are parallel to the direction of the gravitational force. When the end walls of the cylinders are fixed and stationary, numerical and experimental studies have shown that the flow has multiple patterns depending on the cylinder lengths and the Reynolds numbers. Experimental studies on flows with free surfaces also gave various flow modes. Our result shows that the measured and predicted time-dependent displacements of the free surface are in favorable agreement. In case of the cylinder length comparable with the gap width between the cylinders, gradual accelerations of the inner cylinder bring the normal mode flows with one, three and five toroidal vortices. The exchanges of stabilities between these flow modes are summarized in a phase diagram.     

The Force Acting on a Cylinder in a Ring Flow of a Viscous Fluid with a Small Eccentric Displacement

A system consisting of two circular cylinders one inside the other with parallel axes is considered. The outer cylinder of radius R₂ is fixed, and the inner cylinder of radius R₁ rotates with a sufficiently large angular velocity. The region between the cylinders is filled with an incompressible viscous fluid and, in the case of coaxial cylinders, Couette flow along circular trajectories arises. Upon an eccentric small displacement of the axis of the inner cylinder, the symmetry of the flow is disturbed and a force exerted on the inner cylinder by the fluid is created. Within the ideal fluid model, the force depends linearly on the transverse velocities and accelerations of the cylinder. In a viscous fluid, the force depends on the previous motion of the cylinder. It is expressed in terms of the velocity functional by analogy with the Basset force acting on a ball moving in a viscous fluid with a variable velocity.     

Measurement of diameter of metal cylinders using a sinusoidally vibrating interference pattern

A method for measuring external diameters of metal cylinders is proposed in which a sinusoidally vibrating interference pattern (SVIP) of 100-μm period is used as an exact scale. Lights from the end-points of a metal cylinder are extracted with a spatial filtering in an imaging system to form a cross-sectional image of the metal cylinder. On the image a sinusoidally phase-modulated signal owing to the SVIP is detected with a CCD image sensor to measure the phases of the SVIP at the two end-points of the cylinder. The coordinates of the two end-points are obtained from the phases at the two end-points, the phase distribution of the SVIP, and the coordinates of the pixels of the CCD image sensor. Metal cylinder diameters of 7.99, 8.00, and 9.00 mm are measured along their length directions with an error less than 1 μm.     

Integrated approach to the electronic interaction of swift heavy ions with solids and gases

The two most prominent processes of ion-matter interaction are the energy loss and charge exchange, whereby the first depends on the charge state of the ion as determined by the latter. However, up to now there has existed no closed calculation of these two major processes. We herewith present, within an integrated approach, a Monte Carlo calculation which intrinsically entails target ionization/excitation, as well as charge exchange and projectile ionization/excitation on equal footing, especially extended to the situation of ion-solid interaction.     

Lattice Boltzmann simulation of flow around two,three and four circular cylinders in close proximity

Cross-flows around two, three and four circular cylinders in tandem, side-by-side, isosceles triangle and square arrangements are simulated using the incompressible lattice Boltzmann method with a second-order accurate curved boundary condition at Reynolds number 200 and the cylinder center-to-center transverse or/and longitudinal spacing 1.5D, where D is the identical circular cylinder diameter. The wake patterns, pressure and force distributions on the cylinders and mechanism of flow dynamics are investigated and compared among the four cases. The results also show that flows around the three or four cylinders significantly differ from those of the two cylinders in the tandem and side-by-side arrangements although there are some common features among the four cases due to their similarity of structures, which are interesting, complex and useful for practical applications. This study provides a useful database to validate the simplicity, accuracy and robustness of the Lattice Boltzmann method.

     

基于浸入边界-多松弛时间格子玻尔兹曼通量求解法的流固耦合算法研究

针对流固耦合问题,发展了基于浸入边界-多松弛时间格子玻尔兹曼通量求解法(immersed boundary method multi-relaxation-time lattice Boltzmann flux solver,IB-MRT-LBFS)的弱耦合算法.依据多尺度Chapman-Enskog展开,建立不可压宏观方程状态变量和通量与格子玻尔兹曼方程中粒子密度分布函数之间的关系;采用强制浸入边界法处理流固界面使固壁表面满足无滑移边界条件,根据修正的速度求解动量方程力源项;结构运动方程采用四阶龙格-库塔法求解.格子模型与浸入边界法的引入使流固耦合计算可以在笛卡尔网格下进行,无需生成贴体网格及运用动网格技术,简化了计算过程.数值模拟了单圆柱横向涡激振动、单圆柱及串列双圆柱双自由度涡激振动问题.结果表明,IB-MRT-LBFS能够准确预测圆柱涡激振动的锁定区间、振动响应、受力情况以及捕捉尾流场结构形态,验证了该算法在求解流固耦合问题的有效性和可行性.