Theoretical study of a phased-array system using the diffraction model

Abstract

The transformation of a diffusion photon density wave (DPDW) in an infinite homogenous medium with an absorber for a phased-array system is studied theoretically using the diffraction model. The theoretical results agree with the experiment results. The results prove that a small object in a highly scattering medium can be detected and localized precisely by amplitude cancellation phase modulation spectroscopy (PMS).     

Analysis of temperature-dependent magnetization in Sm-Fe-Ccompounds using a two-sublattice model

The temperature dependence of magnetization in Sm₃Fe ₂₀C_x (x =0.3, 0.6, 0.8) and Sm₂Fe_16.5C_1.0 intermetallic compounds with rhombohedral structure has been analyzed using molecular field theory. On the basis of a two-sublattice model, the molecular field coefficients are calculated using a numerical fitting method. The Curie temperature and the Fe-Fe, Sm-Fe, Sm-Sm magnetic interaction energies for the compounds are determined from these coefficients. The results show that the increase of the Curie temperature (T_c) with the increase of carbon content x is attributed mainly to the enhancement of the Fe-Fe exchange interaction energy caused by carbon atoms added     

Theoretical study of molecular aggregation in Langmuir-Blodgett films using a dipolar interaction model

We present a dipole-dipole interaction model based on the classical electrostatic theory in the Langmuir-Blodgett (LB) films to calculate the state of aggregation of rod-like molecules with permanent electronic dipole moment. The dependence of the molecular aggregation spectral properties on the structure parameters in the Z-type LB films, such as the intra-layer molecular separation distance, the adjacent layer separation distance, the molecular orientation angle and the layer number of LB films have been studied. It is found that the H-J-type transition of the aggregation could be realized by modulating the structure parameters of the LB films. The theoretical results show a qualitative agreement with experimental data.     

Differential equation model for accommodation magnetization

We use the differential equation method of computing the accommodation magnetization in a modified Preisach model. We present the properties of this model for a Gaussian medium, and show that the resulting model has neither the congruency property nor the deletion property.     

Combination hysteresis model for accommodation magnetization

We describe an accommodation hysteresis model based on a combination of history-dependent and history-independent models. The amount of accommodation is changed by varying the weights of the models employed in the resulting model. We illustrate the operation of the proposed model with accommodating cycles attached to the major loop. The model can be useful in further investigations of the accommodation phenomenon, as we show by using complex trajectories constructed by it. It can be incorporated in magnetodynamic calculations and circuit analysis.     

The angular distribution of the magnetization in magnetic recording tapes

The angular distribution of the magnetization in a recording tape in, for example, the remanent state is obtained by fitting realistic models of the distribution to experimental data. The results on a wide range of tapes show that the distribution (assumed to take a double Gaussian form) tends to be cylindrically symmetrical about the tape axis. Values of the angular standard deviation usually lie between 0.4 and 0.8 radians.     

Theoretical study of the defect distribution of trivalent cation impurities in MgO

Atomistic simulation techniques have been used to calculate defect energies for a range of trivalent cation impurities in MgO single crystals. From these, values of the association energies have been estimated for impurity-vacancy aggregates ranging in size from simple impurityvacancy monomers, containing one impurity ion, to large clusters containing up to 24 impurity ions. For the MgO:Cr³⁺ system, these energies have been incorporated in a mass-action analysis and predictions made of the dependence of the equilibrium distribution of defects on temperature and on the nominal concentration of the dopant.     

Fast Preisach-based magnetization model and fast inverse hysteresismodel

Fast computational methods for Preisach-based models and their inverses are presented. The methods are based on a differential equation approach to computing a sequence of magnetization values due to a sequence of applied fields. The method used to speed up calculations can be applied to any Preisach model. The Della Torre, Oti, Kadar model was used here as an illustration. Sequential computations for the magnetization model are substantially faster than for standard Preisach models with comparable output error. Computations for the inverse hysteresis model are even faster than for the model. Using the inverse, open loop control of magnetic hysteresis is simulated, showing hysteretic material tracking a desired magnetization in a linear manner for both major loops and minor loops, with less than 2% error in inversion for the waveforms used. The effect of misidentification of material parameters on operation of the inverse is also investigated through simulations     

Theoretical analysis of micro-machined ultrasonic transducer using a simple 1-D model

Micro-machined ultrasonic transducers (MUT) appear as an attractive alternative to standard bulk transducers mainly based on PZT ceramic actuators. However, the simulation of these new devices requires one to take correctly into account their operating conditions. Particularly, most of the MUT structures are periodic, associating a very large number of elementary actuators excited in phase. Furthermore, the development of an equivalent to the Mason model for MUTs would help in the promotion of this new kind of transducers. In this work, we propose a very simple model based on the material resistance theory to describe the flexural motion of a MUT. It is associated with a periodic Green's function development to take into account radiation in water. Basic working principles of MUT then are deduced from computing results, which coincides with already published data on that topic.     

Theoretical impact failure distribution of granules

A two-parameter Weibull distribution is presented that predicts the number of undamaged granules after impact with a rigid surface (platen) as a function of the impact velocity. The Weibull distribution is fitted, using experimental results from the breakage of fertilizer granules, to find the two parameters. The number of undamaged granules can be found using the velocity and angle of impact. A theoretical model derived by Maxim et al. is used to define a failure velocity as the criterion for failure. This model allows the theoretical prediction of the impact velocity required to induce 63.2% failure (c parameter in the Weibull distribution) from knowledge of measurable granule and platen material properties.     

橡胶隔振器非线性动力学模型理论与实验研究

基于静态压缩实验、广义胡克定律和应变能密度推导了描述橡胶超弹性的本构关系,进而获得了橡胶隔振器静态刚度模型;在此基础上,提出基于动态激振实验的动态刚度模型,该模型由橡胶隔振器静态刚度、位移系数和频率系数组成。在此基础上,对单自由度质量-隔振器系统进行数值仿真和实验测试,二者获得的系统的固有频率基本吻合,验证了所提出的橡胶隔振器动力学模型的正确性。研究结果为获得橡胶隔振器静、动态刚度模型提供了简洁、实用的思路,并为研究含有隔振器的动力系统的动力学特性提供了理论基础。     

Equations of motion of the magnetization vector in the nutation coil of a nuclear magnetic flowmeter

The dependence of the nutation line shape in a nuclear magnetic flowmeter on the magnetic-field inhomogeneity in the zone of nutation-coil arrangement and the time of occurrence of the magnetized liquid under the action of a radio-frequency field in the coil has been studied by experimental and theoretical methods. The obtained experimental and theoretical data are compared, and directions of investigations to come are outlined.     

The study of a maintenance float model with gamma failure distribution

This paper considers an operational system made up of a number of independent and identical units. A given number, N, must be in service and it is supported by a maintenance float. We estimate the float factors using a Taylor series approximation of the gamma distribution for failures and a log normal function for repairs. From the model developed for the gamma case, the exponential, erlang-2 and degenerate cases are obtained. It is shown that the maintenance float factors approach asymptotic values for large N, This paper extends the previous research on maintenance float modelling and shows that the maintenance float factors are independent of N for large values of N. This property is used to obtain a simpler formula for maintenance float factor for the gamma failure distribution. Unlike the formulae obtained for the cases of exponential and Weibull failure distributions, the gamma failure yields a more complex formula. The asymptotic model considerably reduces the computational efforts involved.     

Theoretical study of electron concentration distribution in positive column glow discharge with longitudinal gas stream

A solution is obtained to the system of equations describing the positive column of a glow discharge in a cylindrical channel with a longitudinal gas stream. Expressions are derived for calculating the electron concentration and the electric field intensity.Translated from Inzhenerno-Fizicheskii Zhur'nal, Vol. 43, No. 4, pp. 585–589, October, 1982.     

Rotation of the magnetization vector of a monodomain particle under the action of a high-frequency field pulse

We obtained a numerical solution of the modified Hilbert equation describing rotation of the magnetization vector in a high-frequency field of large amplitude. Based on this solution, we have studied rotation of the magnetization vector of a spherical ferromagnetic monodomain particle, possessing a cubic anisotropy, from the direction parallel to an easy magnetization axis to the perpendicular direction under the action of a high-frequency magnetic field pulse. The amplitudes and the interval of frequencies of the magnetic field capable of rotating the particle magnetization vector are determined.     

Two-level Voltage-controlled magnetization switch using a ferromagnetic semiconductor resonant-tunneling diode

It is predicted from a simple analytic theory of quantum transport, coupled to the mean-field theory for dilute magnetic semiconductor ferromagnetism, that a resonant-tunneling diode with a ferromagnetic semiconductor well can be engineered to function as a two-level magnetic switch with a large magnetization swing controlled by the applied bias across the device. Self-consistent transport and electrostatics simulations, together with the aforementioned mean-field theory, are used to illustrate a single, sharp transition of the Curie temperature of the system from its equilibrium value to nearly zero for a suitable choice of device parameters.     

Theoretical investigation of modes of compression of a spherical vapor bubble using a simplified model

The evolution of a single spherical vapor bubble in unbounded liquid is considered. The pressure in the bubble is related to its volume by the polytropic relation. The minimal value of polytropic exponent is taken to be zero, which corresponds to isothermal vapor bubble. It is demonstrated that diverse modes of compression of bubble are possible in the case of polytropic exponent of less than unity. Special features of evolution of bubble in these modes and the boundaries between the modes are investigated.     

利用支持向量回归插值减少全息面测量点数方法的试验研究

近场声全息要获得较高的重建精度必须在全息面上大孔径范围内进行密集的数据采集,需要大量的测量点,测量任务十分繁重。因此在保证重建精度的前提下研究如何有效地减少全息面测量点数对NAH的意义十分重大。提出一种利用支持向量回归对全息面进行插值的方法,该方法可以在全息面传声器数目较少时通过插值提高重建精度,因而可大幅地减少全息面的测量点数。试验表明该方法是有效的。