一种颗粒系超声衰减谱的改进计算方法

Epstein和Carhart,Allegra和Hawley先后研究建立起的超声谱颗粒粒径测量的经典理论模型——ECAH,针对直接求解ECAH理论模型预测颗粒两相系中物理量声衰减系数时出现的困难,通过构造与Bessel（Hankel）函数有关的新变量,改造原有系数矩阵,降低矩阵的条件数,避免了数值溢出问题,拓展该模型的计算范围至频率100MHz和毫米级颗粒,通过与BLBL（Bouguer-Lambert-Beer-Law）散射方法的比较验证了文中提出的改进方法的有效性。     

Asymmetric hysteresis loops of systems of bistable nanoscopic wires

A system of bistable magnetic nanowires of diameter D = 57 nm, length L = 115 nm, magnetization M = 370 emu/cm3 is simulated. The probability distribution of the switching fields of the wires is Gaussian, with mean Hs = 710 Oe and standard deviation u(Hs) = 105 Oe. The wires are randomly distributed on a plane, with directions parallel (OX) or perpendicular (OY) to the axis direction where the magnetic field is applied. The magnetostatic interaction between the wires leads to an asymmetry of the hysteresis loop of the system. Namely, we obtain different curves M(H) for ascending and descending magnetic field. This behaviour is due to the wires which are perpendicular to the applied magnetic field. The directions of their magnetic moments remain often unchanged during the hysteresis experiment, and their contribution to the magnetic moment measured along OX is equal to zero. However, they interact with the wires parallel to the field, and this interaction influences the magnetic state of the parallel wires. The effect can be useful when we are interested in tailoring the shape of the hysteresis loop. Also, a given magnetic configuration of the wires parallel to OY produces a unique asymmetry of the hysteresis loop of the wires parallel to OX. Once an additional field is applied along OY, this unique state is destroyed. The effect can be useful for safety systems.     

A vector model for magnetic hysteresis based on interacting dipoles

A model for magnetic hysteresis is described in which the applied field intensity is free to rotate as well as vary in magnitude. The model is an infinite lattice of ellipsoidal Stoner-Wolfarth particles withN^{3} = 8,27,64,125,216...independent and not necessarily identical particles per unit cell. Techniques for making this simple yet very general model computationally tractable are described. The model is intended primarily for studying the magnetic recording process in particulate storage media.     

单根多楔带传动系统带横向振动的计算方法

以一典型的由驱动轮、从动轮、张紧器和多楔带组成的三轮-带传动系统为研究对象,建立了三轮-带传动系统轮-带耦合振动计算的数学模型。模型中,带简化为纵向运动伯努利-欧拉梁,各轮和张紧器简化为刚性旋转元件。应用边界值问题(BVP)求解技术计算了带横向位移的稳态解;带的横向振动位移为时间与空间的函数,为求解方便,应用迦辽金法将其离散为时间函数和空间函数之积,计算了各带的横向振动。实验测试了一三轮-带传动系统中带的横向振动和轮的旋转振动。论文对计算结果与实测结果进行了对比,结果表明,计算值与实测值吻合较好,从而验证了模型和论文计算方法。文中计算带横向振动的方法,对计算发动机前段附件驱动系统等复杂的多楔带传动系统动态特性具有参考价值。     

A reproducing kernel particle method for meshless analysis of microelectromechanical systems

Many existing computer-aided design systems for microelectromechanical systems require the generation of a three-dimensional mesh for computational analysis of the microdevice. Mesh generation requirements for microdevices are very complicated because of the presence of mixed-energy domains. Point methods or meshless methods do not require the generation of a mesh, and computational analysis can be performed by sprinkling points covering the domain of the microdevice. A corrected smooth particle hydrodynamics approach also referred to as the reproducing kernel particle method is developed here for microelectromechanical applications. A correction function that establishes the consistency and the stability of the meshless method is derived. A simple approach combining the constraint elimination and the Lagrange multiplier technique is developed for imposition of boundary conditions. Numerical results are shown for static and dynamic analysis of microswitches and electromechanical pressure sensors. The accuracy of the meshless method is established by comparing the numerical results obtained with meshless methods with previously reported experimental and numerical data.     

Calculation of hysteresis loops in face-centered cubic thin films

A total energy equation for a single-crystalline face-centered cubic thin film with its close-packed (111) plane parallel to the film plane is used to calculate in-plane and out-of-plane hysteresis loops at various values of the cubic magnetocrystalline anisotropy constant (K₁). In-plane loops show that the easy and hard axes are the <112> and <110> directions, respectively, and that the coercivities, which are nearly independent of the sign of K₁, are quite low over the K₁ values due to the small variation of the magnetocrystalline anisotropy energy during the field sweep. The out-of-plane loops are similarly shaped to those of typical thin films, except for the appearance of a small hysteresis loop near the origin, which is attributed to an irreversible magnetization jump from one easy axis to another in the (111) plane. The magnetic parameters such as K₁ are estimated by comparing the theoretical results for in-plane and out-of-plane hysteresis loops with the experimental results measured for an epitaxial Co thin film. The value of K₁ is estimated to be − 1.2 × 10⁶ erg/cm³, which is significantly higher than that of − 6×10⁵ erg/cm³ for bulk Co, but is in good agreement with the values of − 1.3 to − 1.6 × 10⁶ erg/cm³ reported for Co thin films.     

Wedge simulation method for calculating the reliability of linear dynamical systems

In this paper the problem of calculating the probability of failure of linear dynamical systems subjected to random excitations is considered. The failure probability can be described as a union of failure events each of which is described by a linear limit state function. While the failure probability due to a union of non-interacting limit state functions can be evaluated without difficulty, the interaction among the limit state functions makes the calculation of the failure probability a difficult and challenging task. A novel robust reliability methodology, referred to as Wedge-Simulation-Method, is proposed to calculate the probability that the response of a linear system subjected to Gaussian random excitation exceeds specified target thresholds. A numerical example is given to demonstrate the efficiency of the proposed method which is found to be enormously more efficient than Monte Carlo Simulations.     

A magnetic amplifier for the visual presentation of low-frequency hysteresis loops of very small ferromagnetic samples

This paper describes a magnetic amplifier based on the principle of permeability tuning, and intended for the amplification of small, low-impedance signals obtained from a small ferromagnetic sample (e.g., a ferrite memory core or a thin film). The apparatus is able to produce a good display of a flux as low asDeltaphi = 10^{-12}V/s at 50 c/s. In this case, the signal power applied to the amplifier is of the order of 10^-16watts.     

Critical superparamagnetic/single-domain grain sizes in interacting magnetite particles: implications for magnetosome crystals

Magnetotactic bacteria contain chains of magnetically interacting crystals (magnetosome crystals), which they use for navigation (magnetotaxis). To improve magnetotaxis efficiency, the magnetosome crystals (usually magnetite or greigite in composition) should be magnetically stable single-domain (SSD) particles. Smaller single-domain particles become magnetically unstable owing to thermal fluctuations and are termed superparamagnetic (SP). Previous calculations for the SSD/SP threshold size or blocking volume did not include the contribution of magnetic interactions. In this study, the blocking volume has been calculated as a function of grain elongation and separation for chains of identical magnetite grains. The inclusion of magnetic interactions was found to decrease the blocking volume, thereby increasing the range of SSD behaviour. Combining the results with previously published calculations for the SSD to multidomain threshold size in chains of magnetite reveals that interactions significantly increase the SSD range. We argue that chains of interacting magnetosome crystals found in magnetotactic bacteria have used this effect to improve magnetotaxis.     

Magnetoplasmon modes in the response of a mesoscopic particle of a semimetal or a nonmagnetic insulator

The macroscopic features of the behavior of a mesoscopic particle in a magnetic field are investigated for mesoscopic particles possessing the properties of a semimetal or a nonmagnetic insulator. It is shown that for a fixed value of the magnetic field the frequency of the Alfve’n magnetoplasma mode falls off monotonically (as 1/R) as the radius of the particle increases. Pis’ma Zh. Tekh. Fiz. 23, 16–21 (December 12, 1997)     

Phase-sensitive voltmeter for determining hysteresis loops by means of an alternating current

Conclusions Tests of an experimental model of the voltmeter made at the VNIIM have shown that it is suitable for measuring the mean values of voltages with a form factor varying from 1.11 to 2.5. The mean referred error of the voltmeter determined at a frequency of 1000 cps, does not exceed ±2%. Additional frequency errors in the range of 400 to 10,000 do not exceed ±2%.     

A method for calculating stress intensities in bimaterial fracture

A numerical method is presented for obtaining the values of K^* ₁,K ^* _II and K^* _III in the elasticity solution at the tip of an interface crack in general states of stress. The basis of the method is an evaluation of theJ-integral by the virtual crack extension method. Individual stress intensities can then be obtained from further calculations ofJ perturbed by small increments of the stress intensity factors. The calculations are carried out by the finite element method but minimal extra computations are required compared to those for the boundary value problem. Very accurate results are presented for a crack in the bimaterial interface and compared with other methods of evaluating the stress intensity factors. In particular, a comparison is made with stress intensity factors obtained by computingJ by the virtual crack extension method but separating the modes by using the ratio of displacements on the crack surface. Both techniques work well with fine finite element meshes but the results suggest that the method that relies entirely on J-integral evaluations can be used to give reliable results for coarse meshes.

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Résumé On présente une méthode numérique en vue d'obtenir les valeurs de K^* ₁, K^* _II et K^* _III relatives à la solution élastique d'application à l'extrémité d'une fissure d'interface sujette à un état de contraintes général. La méthode repose sur l'évaluation de l'intégraleJ par la technique d'extension virtuelle de la fissure. On peut ensuite obtenir les intensités de contraintes individuelles à partir de calculs deJ subséquents, correspondant à des perturbations introduites par de petits accroissements des facteurs d'intensité de contraintes.Les calculs sont accomplis par la méthode des éléments finis, mais, par rapport aux calculs à mettre en oeuvre dans le problème des valeurs aux limites, il ne faut procéder qu'à quelques calculs supplémentaires.On présente des résultats très précis pour le cas d'une fissure dans un interface entre deux matériaux, et on les compare avec ceux provenant d'autres méthodes d'évaluation des facteurs d'intensité de contraintes.En particulier, on fait une comparaison pour des facteurs d'intensité de contraintes obtenus en calculant J par la méthode d'extension virtuelle d'une fissure, mais en séparant les modes selon le rapport des déplacements de la surface de la fissure.Les deux techniques fonctionnent de manière satisfaisante avec des maillages fins d'éléments finis; cependant, les résultats suggèrent que la méthode qui repose entièrement sur les évaluations de l'intégraleJ peut être utilisée afin d'obtenir des résultats fiables dans les réseaux à mailles grossières.

     

一种非分层海洋中的声线计算方法

在浅海环境中,声速剖面和海底深度在水平方向上的变化都会对声传播产生较大的影响。将声速剖面用前几阶经验正交函数来表示,在一定范围的海域各阶经验正交函数系数可以近似为随水平距离线性变化,提出了一种非分层海洋中的声线计算方法,可以计算到达接收水听器的本征声线和传播时间。该方法计算速度快,计算精度较高,可以用于海洋中的声速剖面快速反演。     

钢铝结构连接耦合损耗因子计算方法研究

统计能量法已广泛用于解决高频振动与噪声问题,耦合损耗因子是其关键参数之一。研究基于统计能量法、有限元法和功率输入法,提出一种钢铝连接耦合损耗因子数值计算方法,并运用MATLAB编写了计算软件。通过对含过渡接头的T型连接结构进行数值和实验研究,验证了该法的可靠性。这对提高钢铝混合结构的噪声预报精度及指导其结构声学设计具有重要意义。