Calibration of the stray field gradient by a heteronuclear method and by field profiling

Heteronuclear and field-profiling stray field (STRAFI) techniques are used to calibrate the STRAFI gradient. Both methods compare very favorably indeed with the conventional method of calibration which uses a standard with a known self-diffusion constant. The distinct advantages of the techniques presented here are that the constraints on both sample purity and sample temperature that are inherent to the conventional method are completely eliminated. The accuracy of the heteronuclear method typically matches that of the conventional method with a pure sample and temperature stability to within 0.4°C. The field-profiling method is more accurate than the heteronuclear method in the form that it is presented here.     

The intensity coherence function of time for partially saturated acoustic propagation through ocean internal waves

The intensity coherence function of time for partially saturated acoustic propagation through internal waves is calculated with a method that is improved over previous treatments. Two specific improvements are introduced: the usual expansion in (1/lambdaphi2) is carried out to a higher order, and then the terms of that expansion are calculated with a new perturbative method. The method is applied to propagation without a sound channel, for both phase-screen and continuous-medium cases. The validity of the new perturbative method is estimated by calculating the next order error terms. Accuracies at the few-percent level are found. The new analytic formulas are also corroborated with numerical integration. Finally, the method is applied to a specific ocean-acoustic experiment [Azores Fixed Acoustic Range (AFAR)]. In order to achieve good agreement with experiment it will be necessary to add an accurate treatment of the sound channel to the present perturbation method.     

A second-order accurate numerical method for the two-dimensional fractional diffusion equation

Spatially fractional order diffusion equations are generalizations of classical diffusion equations which are used in modeling practical superdiffusive problems in fluid flow, finance and others. In this paper, we present an accurate and efficient numerical method to solve a fractional superdiffusive differential equation. This numerical method combines the alternating directions implicit (ADI) approach with a Crank–Nicolson discretization and a Richardson extrapolation to obtain an unconditionally stable second-order accurate finite difference method. The stability and the consistency of the method are established. Numerical solutions for an example super-diffusion equation with a known analytic solution are obtained and the behavior of the errors are analyzed to demonstrate the order of convergence of the method.     

积分节点迎风偏移的节点积分无单元Galerkin方法

建立求解稳态对流-扩散方程的-种稳定、高效的无单元Galerkin方法.该方法计算积分时采用基于局部Taylor展开的节点积分,并根据对流占优的程度对积分节点进行自适应迎风偏移.与传统的使用稳定化的无单元Galerkin方法相比,该方法是-种不依赖于背景网格积分的纯无网格方法,具有更好的稳定性和较高的计算效率,其程序实施更为简便.     

Variational approach to configuration interaction

A variational method is developed to determine configuration interaction wave functions. The method is straightforward and is applied to a pairing Hamiltonian with constant matrix elements, for which exact eigenvalues are available. Comparisons are made with the exact results. Calculations can be carried out to any desired degree of accuracy. The method is also applied to a Hamiltonian that has neutron-neutron, proton-proton, and neutron-proton pairing. No difficulties are found in extending the method to this Hamiltonian that has many collective modes. In practice, the method scales linearly with , where is the number of variational basis states.     

基于MCNP计算结果的快速物理插值方法（英）

在核辐射科学研究中经常使用MCNP蒙卡计算程序,但其计算结果只是给定位置的物理量。为了快速获得计算点附近的感兴趣位置处的值,本研究建立了一种3点插值计算方法。首先分析了粒子在材料中输运的物理规律,给出了满足3点插值的空间范围,建立了插值计算模型。然后利用距离平方反比权重法,结合粒子衰减规律,推导了插值计算公式。最后设计了计算机程序,并用X射线屏蔽后注量的MCNP计算结果进行验证。通过和直接采用的距离平方反比权重法和克里金插值法对比,考虑物理作用机制的该计算方法精度更高,且误差在10%以内。     

纳米尺度流动速度计算的新方法

采用流体力学中流量与流速的计算和分子动力学相结合的方法,模拟液态氩在纳米通道内的三维Poiseuille流动和驱动方腔流动,计算流体流速.结果表明:平板形纳米通道内,该方法求得的流速与传统分子动力学方法所求流速基本吻合,可以用该方法计算不同壁面情况下的流速;对于纳米方腔通道内的流体,在不同模型下两种方法计算出的流速分布大致相同,但是其边界速度差别很大,在边界的速度计算方面新方法的精确度更高,收敛速度比传统方法快.     

定向耦合法测量非标准接口元件驻波系数

 利用网络分析仪，根据需要在设定的频率范围进行扫频测量，网络分析仪的端口1输出的微波信号经过微波传输转换器进入与待测非标准元件匹配连接的连接段（如波导）；网络分析仪的端口2连接定向耦合器，用于监测传输段内的反射微波信号。首先，在非标准接口端面连接短路面，通过网络分析仪测量反射信号在设定频点的相对幅度值；然后去掉短路面，在非标准接口端面连接待测元件，再次测量反射信号在特定频点的相对幅度值；最后根据本文推导的公式得出驻波系数。该方法的测量误差与定向耦合器的方向性（方向性系数越大越好）和待测元件驻波系数有关。将该方法运用在非标准接口匹配负载驻波系数的测量中，定向耦合器方向性系数取为40 dB，测得其驻波系数小于1.2，误差小于20%。该方法简便可行，可以用于测量常用的非标准接口元件尤其是非标准低驻波系数元件的驻波系数。     

一类奇异复本征方程组的数值计算

利用基本解矩阵法数值求解一类带有奇异点的复本征方程组,并对奇异点的消除和复本征值的确定及数值不稳定性等问题进行了讨论,编制了求解程序代码,并应用于离子温度梯度(ITG模或η_i模)驱动不稳定性研究的数值模拟.实例计算表明,数值结果与理论分析完全吻合.     

NON-LINEAR STEADY STATE VIBRATIONS OF BEAMS EXCITED BY VORTEX SHEDDING

In this paper the non-linear vibrations of beams excited by vortex-shedding are considered. In particular, the steady state responses of beams near the synchronization region are taken into account. The main aerodynamic properties of wind are described by using the semi-empirical model proposed by Hartlen and Currie. The finite element method and the strip method are used to formulate the equation of motion of the system treated. The harmonic balance method is adopted to derive the amplitude equations. These equations are solved with the help of the continuation method which is very convenient to perform the parametric studies of the problem and to determine the response curve in the synchronization region. Moreover, the equations of motion are also integrated using the Newmark method. The results of calculations of several example problems are also shown to confirm the efficiency and accuracy of the presented method. The results obtained by the harmonic balance method and by the Newmark methods are in good agreement with each other.     

Baseline subtraction technique in the frequency–wavenumber domain for high sensitivity damage detection

This paper suggests a method for high-sensitivity damage detection. The method is based on pitch-catch measurements of Lamb waves combined with a baseline subtraction technique in the frequency–wavenumber domain. Small amplitude converted modes, generated during the interaction of propagating waves with damage, can thus be detected with minimal a priori information regarding their expected location in the frequency–wavenumber plane. This method is applied in the present paper to a case of notches with varied depth. Finite element simulations are carried out in the temporal domain to mimic results obtainable in real-world experiments. Two cases are studied, namely when each of the two pure fundamental modes are incident on a notch. The advantages of the method are detailed. The procedure to implement this method is described in the context of Structural Health Monitoring (SHM) or Non-Destructive Testing (NDT).     

多曲面反射体的计算机辅助设计及优化

研究了自由曲面反射式照明系统的设计理论和方法.根据反射器的尺寸、位置以及配光要求按点光源设计得到曲面型值点初始坐标后,将双三次B样条曲面引入照明系统的设计,反算出控制顶点.给出了空间任意一条入射光线与反射器区块的交点及其曲面法线向量的求法,为了提高光路追迹的效率,针对多曲面反射体给出了一种快速判断相交区块的算法.探讨了用阻尼最小二乘法对B样条曲面反射器进行优化设计的方法,通过优化运算修正控制顶点Z轴坐标,使得曲面在使用扩展光源时光形分布尽可能满足设计要求.     

Homogenization of periodic bi-isotropic composite materials

In this paper, we present a new method for homogenizing the bi-periodic materials with bi-isotropic components phases. The presented method is a numerical method based on the finite element method to compute the local electromagnetic properties. The homogenized constitutive parameters are expressed as a function of the macroscopic electromagnetic properties which are obtained from the local properties. The obtained results are compared to Unfolding Finite Element Method and Maxwell–Garnett formulas.     

径向基函数插值方法在动网格技术中的应用

分析用于非结构网格的弹簧比拟和用于结构网格的无限插值动态网格方法在实际应用中的优缺点,提出无需网格连接关系的基于径向基函数(radial basis functions,RBF)插值的动网格技术并编制相应的网格运动计算程序.以二维菱形翼的旋转运动及三维菱形翼的柔性变形为例,分析不同基函数和紧支半径的选取对网格质量及计算效率的影响,并通过与弹簧比拟方法的对比验证了RBF方法的有效性.结果表明:RBF方法数据结构简单,计算效率高,适应大变形能力强,可以有效地实现计算流体力学中的网格运动问题.     

Identification of nonlinear multi-degree-of freedom structures based on Hilbert transformation

The modeling method and identified method adapted to multi-degree-of-freedom structures with strucrural nonlinearities are established. The component mode synthesis method is used to establish the nonlinear governing equations by extending the connected relationships. Based on the modeling method, the Hilbert transform method is applied to identify the nonlinear stiffness of multi-degree-of-freedom structures. Nonlinear analysis and identification of a typical folding wing configuration with three freeplay hinges are investigated. The nonlinear governing equation is established based on present methods and the computing results of different stiffness are checked by finite element programming. In order to illustrate the influence of the nonlinearities, the frequency response characteristics of the structure are analyzed and Hilbert transform is performed. The Hilbert transform identification method is utilized to identify the nonlinear stiffness of nonlinear hinges in the time domain and several parametric studies are performed. In addition, the comparison of response is made to illustrate the feasibility of the methods. The results show that the extending component mode synthesis method in the present work can be used to establish the governing equation with structural nonlinearities. Based on the modeling method, the Hilbert transform identified method can be extended to multi-degree-of-freedom structures accurately.     

Analysis of force characteristics of a superconducting ball in a given magnetic field

The electromagnetic force characteristics along Z direction of a superconducting ball levitated by spherical coils with shaping blocks are calculated based on a semi-analytical method. The calculating results from the semi-analytical method are compared with the finite element analysis (FEA) method through a calculation example. The method can be applied to further analysis of dynamic characteristics and parameter optimization in the suspension system.     

圆镜共焦腔激光横模的传输矩阵法模拟和分析

共焦腔是一般稳定腔的特例,由于等价性,共焦腔中自再现模的振幅分布、损耗和光斑尺寸常常被用于讨论一般稳定腔。为了分析具有大菲涅尔数的圆镜共焦腔,本文利用传输矩阵方法对腔模进行计算和讨论。研究结果表明:传输矩阵方法的结果与Fox-Li积分迭代法的结果一致,并且传输矩阵方法实用于大菲涅尔数共焦腔。     

A fast estimation of sonar cross section of acoustically large and complex underwater targets using a deterministic scattering center model

In this paper, a fast method is proposed to estimate the sonar cross section of acoustically large and complex underwater targets such as submarines and torpedoes. The proposed method is based on a deterministic scattering center model which constructs scattering center database by using a combining method of physical optics and geometric optics and then reconstructs sonar cross section patterns from that database with a polynomial interpolation with respect to the incident angle. The parametric studies to find appropriate intervals of the reference incident angle are systematically carried out for simple targets such as a flat square plate and an orthogonal dihedral. Moreover, to validate the proposed method, sonar cross sections of real-like targets such as a pressure hull and an idealized submarine are calculated. The comparisons show that the results by the proposed method are in good agreement with those by the direct calculation.     

Impact on short length bars

A finite element method for the solution of an axisymmetric wave propagation problem of a bar of finite length is proposed. The results are obtained for a uniform bar with (i) a displacement input and (ii) a stress input. Also results are obtained for a discontinuous bar with stress input. They are compared with the existing finite difference solutions and experimental results. It is observed that the results are in close agreement. The study is then extended to the case of bars with an hemispherical end. The advantage of using the finite element method for such problems is indicated. Finally the results for a composite bar comprising a hallow cylinder and hemisphere are obtained by using the finite element method. Experiments are conducted to verify these results.     

面向杆系结构的离散元异步长计算方法

为了提升离散元法处理连续介质问题的计算效率, 提出一种基于重叠颗粒的离散元分区异步长计算方法来处理连续介质问题。该方法采用颗粒分割将连续介质划分为若干子分区, 各分区采用velocity-Verlet积分格式求解运动方程。相邻分区通过重叠颗粒构成局部耦合区域, 边界数据传递过程中不涉及插值和截断过程。数值算例表明, 该方法在保证高精度的同时有效地降低了计算时间。在实际应用中可以有针对性地将连续介质划分为不同尺度颗粒的分区, 根据问题规模及分区颗粒尺度特性采用不同时间步长, 节省存储空间且大幅提升计算效率。