Finite-difference time-domain model for the feeding gap of coaxial probe driven antennas

In the finite-difference time-domain (FDTD) method, a simple and realistic feed model for coaxial probe driven antennas is proposed here. The feed zone of the antenna may be considered as an equivalent source in view of the antenna theory and a load port in view of the transmission line theory. The proposed feed model is constructed by combining the infinitesimal-gap source condition of the antenna and the equivalent load condition of the feed line. It leads to perform no additional FDTD cell modelling of the line. The transient reflected voltage and the input impedance of cylindrical monopole antennas fed by coaxial lines are calculated numerically and then compared with the accurate measurement and a full fine-grid. The FDTD results of the proposed model have a good agreement with the measured data and the fine-grid results.     

Dual-grid finite-difference time-domain scheme for the fast simulation of surrounded antennas

A new finite-difference time-domain (FDTD) multiresolution strategy for surrounded antenna analysis is presented. The dual-grid FDTD (DG-FDTD) is divided into two FDTD simulations using different grids. Indeed, the antenna is firstly simulated without its environment using a finely discretised FDTD in order to determine its main characteristics and save its primary radiation. In a second step, this saved field is used as the excitation of a coarse FDTD to simulate the antenna with its environment. The application of the DG-FDTD to an ultra wide-band problem is discussed, and the DG-FDTD turns out to be an accurate and fast tool to simulate various antenna configurations. Furthermore, this method remains stable along the computational time, and is easy to implement in a classical FDTD scheme     

柱坐标系下浅海传播声场时域有限差分预报方法

直角坐标系下的时域有限差分方法在水声传播仿真时由于计算量太大而较少应用。文章利用时域有限差分方法推导了柱坐标系下的声波方程的近似表达式,结合复频移完全匹配层建立浅海声传播模型,在减少计算量的同时准确预报了传播信号声压的时域波形、传播声场的时空演变过程以及频域传播损失曲线。在柱坐标系下使用时域有限差分方法仿真Pekeris传播环境声场并与简正波和波数积分模型进行对比,分析了时域有限差分水声传播模型的适用范围。结果表明,时域有限差分方法仿真浅海中近程传播声场精度较高。模型的稳定性与时间和空间网格大小有关,声源频率越高,空间和时间网格划分越小,计算量越大。数值离散带来的频散误差累积会导致远场传播声场计算不准确,因此时域有限差分水声传播模型更适用于低频中近程声场计算。     

Development of a numerical cancellous bone model for finite-difference time-domain simulations of ultrasound propagation

The trabecular frame in cancellous bone has numerous porous spaces of various sizes and shapes. Their continual arrangement changes with position in the bone. Assuming that the complicated pore space is the aggregation of spherical pores, in this study, the trabecular structure was analyzed using a three-dimensional (3-D) X-ray microcomputed tomography (muCT) image. Analysis involved a 3-D cancellous bone model developed for numerical simulations of ultrasound propagation. In this model, the trabecular structure was simplified by regularly arranging spherical pores in a solid bone. Using a viscoelastic, finite-difference, time-domain (FDTD) method with the simplified cancellous bone model, ultrasound pulse waveforms propagating through cancellous bone were simulated in two cases of the propagations parallel and perpendicular to the main trabecular orientation. The porosity dependences of the propagation properties, attenuation, and propagation speed were derived from the simulated waveforms. Comparisons with simulated results using the realistic cancellous bone model reconstructed from a 3-D muCT image, assisted to further validate this simplified model.     

Efficient finite-difference time-domain scheme for light scattering by dielectric particles: application to aerosols

We have examined the Maxwell-Garnett, inverted Maxwell-Garnett, and Bruggeman rules for evaluation of the mean permittivity involving partially empty cells at particle surface in conjunction with the finite-difference time-domain (FDTD) computation. Sensitivity studies show that the inverted Maxwell-Garnett rule is the most effective in reducing the staircasing effect. The discontinuity of permittivity at the interface of free space and the particle medium can be minimized by use of an effective permittivity at the cell edges determined by the average of the permittivity values associated with adjacent cells. The efficiency of the FDTD computational program is further improved by use of a perfectly matched layer absorbing boundary condition and the appropriate coding technique. The accuracy of the FDTD method is assessed on the basis of a comparison of the FDTD and the Mie calculations for ice spheres. This program is then applied to light scattering by convex and concave aerosol particles. Comparisons of the scattering phase function for these types of aerosol with those for spheres and spheroids show substantial differences in backscattering directions. Finally, we illustrate that the FDTD method is robust and flexible in computing the scattering properties of particles with complex morphological configurations.     

Thermophysical model of metal-vapor lasers with discharge chambers of cylindrical and coaxial geometry

A thermophysical model and a technique for calculating the gas temperature in the active volume of a metal-vapor laser and of the construction elements of discharge chambers are presented. Results of comparative thermal calculations of the coaxial and cylindrical discharge chambers of the laser are given. The temperature dependences of the gas thermal conductivity, radial distribution of heat release, and the radiative heat exchange between the coaxial cylinders are taken into account. Formulas for calculation of the central-insert temperature have been derived. The possibility of eliminating superheating of the working substance and enhancing output characteristics of large-volume copper vapor lasers due to the use of a coaxial construction of discharge chambers has been demonstrated numerically.     

Accurate modeling of dielectric interfaces by the effective permittivities for the fourth-order symplectic finite-difference time-domain method

The fourth-order finite-difference time-domain (FDTD) method using a symplectic integrator propagator can calculate the propagation of the electromagnetic waves with very low dispersion error in the region of a constant or smoothly varying index profile. An additional technique is required for the problem with the discontinuous dielectric interfaces. We derived the third-order effective permittivities at dielectric interfaces for the fourth-order FDTD method in the case of 2D TE polarization. As the required accuracy level is increased, the memory resources used by the fourth-order FDTD method with the effective permittivities are reduced severalfold or more compared with the standard FDTD method. The accurate performance of the proposed method is demonstrated through numerical examples.     

Comparison of Cartesian grid configurations for application of the finite-difference time-domain method to electromagnetic scattering by dielectric particles

Two grid configurations can be employed to implement the finite-difference time-domain (FDTD) technique in a Cartesian system. One configuration defines the electric and magnetic field components at the cell edges and cell-face centers, respectively, whereas the other reverses these definitions. These two grid configurations differ in terms of implication on the electromagnetic boundary conditions if the scatterer in the FDTD computation is a dielectric particle. The permittivity has an abrupt transition at the cell interface if the dielectric properties of two adjacent cells are not identical. Similarly, the discontinuity of permittivity is also observed at the edges of neighboring cells that are different in terms of their dielectric constants. We present two FDTD schemes for light scattering by dielectric particles to overcome the above-mentioned discontinuity on the basis of the electromagnetic boundary conditions for the two Cartesian grid configurations. We also present an empirical approach to accelerate the convergence of the discrete Fourier transform to obtain the field values in the frequency domain. As a new application of the FDTD method, we investigate the scattering properties of multibranched bullet-rosette ice crystals at both visible and thermal infrared wavelengths.     

Slip and energy dissipation in cylindrical friction joints loaded by impact-induced waves

Cylindrical friction (CYLF) joints subjected to impact-induced wave (IIW) loading are studied theoretically and experimentally. The system considered consists of a long, straight, cylindrical and linearly elastic rod which is impacted axially at one end by a cylindrical and linearly elastic hammer. A body is attached at the other end by means of a CYLF joint. The conditions are such that the wave propagation is 1-D in the hammer and the rod and can be neglected in the attached body which is therefore treated as rigid. Slip and energy dissipation in the CYLF joint due to IIW loading are determined as functions of three dimensionless parameters which represent the rod-to-hammer length ratio, the mass of the attached body and the static frictional force. In the general case the problem is solved numerically. In the case of a semi-infinite rod the problem is also solved analytically. The agreement between numerical and analytical solutions is good. There is also fair agreement between theoretical and experimental results.     

Dugdale model for circumferential through-cracks in pipes loaded by bending

The paper contains an analytical solution to the title problem by means of the semi-membrane theory of cylindrical shells. Results include the crack opening displacements as a function of circumferential angle, the fracture parameter J, and the effects of the crack on the compliance of the pipe.

---

Résumé L'étude comporte une solution analytique du modèle de Dugdale appliqué au cas envisagé, en recourant á la théorie des semi-membranes dans les enveloppes cylindriques. Les résultats donnent la relation entre les déplacements d'ouverture de la fissure et l'angle circonférentiel, le paramètre de rupture J, ainsi que les effects de la fissure sur la compliance du tube.

     

Bandwidth enhancement of stacked dielectric resonator antennas excited by a coaxial probe: an experimental and numerical investigation

An experimental and numerical investigation about bandwidth enhancement of dielectric resonator antennas (DRAs) using multiple DRAs, arranged according to a 'stacked' configuration is reported. The antenna consists of two cylindrical discs of the dielectrics Cr ^{Fe O (CRFO) and Fe Cu Ti O (FCTO) stacked vertically, placed above a ground plane and excited by a coaxial probe. The lateral edge of the cylinder should be placed against the feed probe, which excites the HEM11d mode. The numerical procedure is performed through a software package based on the finite-element method. An excellent agreement between theoretical and experimental results is observed. The concept of increasing the bandwidth of the DRA by stacking is verified. For the first time, the temperature coefficient of resonant frequency (t ) was also measured for CRFO and FCTO ceramics.}     

Criterion for the onset of a shock-wave flow in a gas driven by harmonic oscillations of a piston in a closed cylindrical tube

A criterion for the onset of a shock wave motion of a gas column driven by resonance harmonic oscillations of a piston in a closed tube is formulated based on an analysis of the results of experiments and theoretical investigations. The validity of the proposed criterion in a broad range of parameters has been verified within the framework of numerical integration of a nonstationary system of narrow-channel gasdynamics equations written in a divergent form.     

物理模型与高斯过程融合驱动的残余应力疲劳状态评估

振动是金属构件疲劳失效的重要因素,残余应力可以表征金属构件疲劳状态.然而残余应力在构件疲劳过程中演化行为复杂,传统寿命模型通常适用于残余应力缓慢松弛过程,且较少考虑初始残余应力、冷作硬化、材料差异性等影响,评估误差大.该研究融合Kodama物理模型和基于高斯过程的数据驱动方法,建立了物理模型和高斯过程融合驱动的疲劳状态...     

Stress intensity factors for curvilinear cracks loaded by bending and torsional moments

The geometry of a thin flat plate containing a curvilinear crack of finite size is investigated. The plate is loaded by uniform bending and torsional moments at infinite distance from the crack contour, while distributed normal bending moments and torques exist at the upper and lower crack surfaces. The bending stress intensity factors for the curvilinear crack are calculated on the basis of classical plate theory. The displacements and internal moments are represented by two complex analytic functions. Extra conditions are imposed to ensure the univalence of the displacements, which is not evident because the plate comprises a multiply connected domain due to the presence of the crack. A linearization with respect to the crack-curvature function has been performed and the bending stress intensity factors are calculated as the first-order solutions for slightly curved cracks. The results are illustrated with a few examples, such as uniform loading configurations and the geometry of a crack along a circular arc. The loading of thin flat plates by a combination of tensile forces and bending moments is also investigated. In analogy with the variation of the stress components over the cross section of the plate, two combined stress intensity factors are introduced having the same dependence on the perpendicular coordinate and being related to the symmetric and anti-symmetric stress intensity factors of the separate plane-stress and bending problems. The resulting energy release rate is shown to be in full agreement with known results in the literature.     

National primary standard for the unit of power of electromagnetic oscillations in waveguides and coaxial lines at frequencies of 0.03–37.5?GHz

The design of a modernized standard for reproduction and transfer of the unit of ultrahigh frequency power in waveguides and coaxial channels and some studies of this system are described.     

小比例柱面网壳振动台试验及动力响应概率模型

针对单层网壳实验费用高、周期长及地震变异性大等问题,开展了一小比例单层柱网壳振动台试验模型试验,并基于该模型试验统计了考虑地震动变异性时单层柱面网壳的动力响应概率模型。参考孔洞胶接节点装配式单层球面网壳模型制作方法制作了一小比例单层柱面网壳模型;在此基础上,分别选用TAFT波、人工地震波和简谐波对该单层柱面网壳模型开展一系列振动台试验研究,包括弹性阶段的动力特性测试和弹塑性状态下的动力倒塌测试;立足于现场实测数据,基于通用有限元分析软件ABAQUS建模、并开展相应数值仿真分析,总结分析单层柱面网壳模型动力特性与破坏特征;基于40组动力实测数据研究地震变异性对单层柱面网壳动力特性的影响,并初步拟合出地震动变异性与该结构动力响应的概率模型。该研究对开展系列单层网壳振动台试验定性研究及空间网壳结构抗震规范的修订提供参考。