阴极金属微凸起电场增强因子数值模拟

 利用有限元方法对无穷大平板二极管中阴极表面金属微凸起的微观电场增强因子进行了计算研究,给出了微凸起微观电场增强因子随凸起参数的变化规律,拟合得到了微观电场增强因子的简单实用的经验表达式,并与文献中的近似公式进行了比较。模拟的结果表明,对锥状球头微凸起的电场增强因子,文献给出的近似公式误差较大,应避免使用。     

Numerical simulation of the magnetoresistance effect controlled by electric field in p–n junction

The magnetoresistance effect of a p–n junction under an electric field which is introduced by the gate voltage at room temperature is investigated by simulation. As auxiliary models, the Lombardi CVT model and carrier generationrecombination model are introduced into a drift-diffusion transport model and carrier continuity equations. All the equations are discretized by the finite-difference method and the box integration method and then solved by Newton iteration.Taking advantage of those models and methods, an abrupt junction with uniform doping is studied systematically, and the magnetoresistance as a function of doping concentration, SiO_2 thickness and geometrical size is also investigated. The simulation results show that the magnetoresistance(MR) can be controlled substantially by the gate and is dependent on the polarity of the magnetic field.     

Modeling and simulation for the field emission of carbon nanotubes array

To optimize the field emission of the infinite carbon nanotubes (CNTs) array on a planar cathode surface, the numerical simulation for the behavior of field emission with finite difference method was proposed. By solving the Laplace equation with computer, the influence of the intertube distance, the anode–cathode distance and the opened/capped CNT on the field emission of CNTs array were taken into account, and the results could accord well with the experiments. The simulated results proved that the field enhancement factor of individual CNT is largest, but the emission current density is little. Due to the enhanced screening of the electric field, the enhancement factor of CNTs array decreases with decreasing the intertube distance. From the simulation the field emission can be optimized when the intertube distance is close to the tube height. The anode–cathode distance hardly influences the field enhancement factor of CNTs array, but can low the threshold voltage by decreasing the anode–cathode distance. Finally, the distribution of potential of the capped CNTs array and the opened CNTs array was simulated, which the results showed that the distribution of potential can be influenced to some extent by the anode–cathode distance, especially at the apex of the capped CNTs array and the brim of the opened CNTs array. The opened CNTs array has larger field enhancement factor and can emit more current than the capped one.     

Analysis of mode shapes of a rigidly backed cylindrical foam using three-dimensional finite element acoustical analysis

In this study, the three-dimensional finite element frequency domain acoustical analysis is used to determine the modal shapes of cylindrical foam with a rigid backing and subjected to a unit normal incidence impulsive sound pressure loading while placed in the impedance tube. The acoustic results predicted for the foam are validated by data from the two-microphone acoustic measurements, and good agreement between the measured and predicted acoustic results is observed. The mode shapes of the incident face of the foam at a low frequency, resonant and anti-resonant frequencies as well as the frequency that occurring the peak loss modulus are illustrated. It is found that the modal behaviors of the cylindrical foam are dominated by the fluid, although the acoustic properties of the cylindrical foam are also influenced by the circumferential edge constraints and the modal movements of the solid skeleton.     

Analysis of nonlinear electric field of hvdc wall bushing with a finite element approach

The present research intends to establish a numerical model, on the basis of a theoretical analysis, for describing and analyzing the electric field of High Voltage Direct Current (HVDC) wall bushing that demonstrates highly nonlinear characteristics. The wall bushing is subjected high voltage with nonlinear electric field and the relationship between the electric field intensity and the resistance of the insulators of the wall bushing is highly nonlinear. With a parameter design language of a Finite Element Analysis software package for carrying out the numerical calculations, the effects of the nonlinearity on the electric field can be well taken into consideration in performing the numerical assessment. A technique utilizing the numerical iteration is developed for quantifying the electric intensity of the electric field. With the model and the iteration technique established, the nonlinear characteristics of the HVDC wall bushing can be investigated with efficiency.     

Numerical treatment of acoustic problems with the smoothed finite element method

We incorporated a cell-wise acoustic pressure gradient smoothing operation into the standard compatible finite element method and extended the smoothed finite element method (SFEM) for 2D acoustic problems. This enhancement was especially useful for dealing with the problem of an arbitrary shape with violent distortion elements. In this method, the domain integrals that involve shape function gradients can be converted into boundary integrals that involve only shape functions. Restrictions on the shape elements can be removed, and the problem domain can be discretized in more flexible ways. Numerical results showed that the proposed method achieved more accurate results and higher convergence rates than the corresponding finite element methods, even for violently distorted meshes. The most promising feature of SFEM is its insensitivity to mesh distortion. The superiority of the method is remarkable, especially when solving problems that have high wave numbers. Hence, SFEM can be beneficially applied in solving two-dimensional acoustic problems with severely distorted elements, which, in practice, have more foreground than regularity mesh.     

ITER������Ϸ����������Ч�ܵ�����Ԫ����

通过Ansoft Maxwell有限元分析软件,用封闭圆桶模型对ITER送气系统阀门箱的磁屏蔽进行了计算。计算,获得了屏蔽层厚度、上端开孔及侧面狭缝对磁屏蔽的影响方式及规律,给出了一种具有合理参数的可能设计。     

Determination of displacement distributions in bolted steel tension elements using digital image techniques

Digital imaging methods have found a great interest in various engineering fields to study stress-deformation characteristics of materials. Recent enhancements in visual instrumentation with the availability of cost-effective hardware and software products make the digital imaging techniques a viable tool to replace direct strain or displacement measurement methods in engineering applications. In this study, deformation characteristics of bolted steel connections are investigated by calculating in-plane displacement distributions using digital image correlation method (DIC). Validation of the method is presented by comparing the strains measured in standard tension specimens using electrical resistance strain gages and the DIC method. Finite element analysis of the connection specimen is also performed to compare the in-plane displacement distributions calculated from both methods. Results from the validation process indicate that the strains obtained from the DIC method compare well with the results of strain gages. The findings also indicate that the displacement distributions calculated from the finite element method may differ from those of the DIC method in terms of distribution pattern, and the location and magnitude of the extreme values of displacements. It is suggested that the proposed method can be used to determine the in-plane displacement distributions for the bolted connections, hence to evaluate their deformation characteristics under loading.     

Spatial and temporal thermal analysis of acousto-optic deflectors using finite element analysis model

Thermal effects greatly influence the optical properties of the acousto-optic deflectors (AODs). Thermal analysis plays an important role in modern AOD design. However, the lack of an effective method of analysis limits the prediction in the thermal performance. In this paper, we propose a finite element analysis model to analyze the thermal effects of a TeO₂-based AOD. Both transducer heating and acoustic absorption are considered as thermal sources. The anisotropy of sound propagation is taken into account for determining the acoustic absorption. Based on this model, a transient thermal analysis is employed using ANSYS software. The spatial temperature distributions in the crystal and the temperature changes over time are acquired. The simulation results are validated by experimental results. The effect of heat source and heat convection on temperature distribution is discussed. This numerical model and analytical method of thermal analysis would be helpful in the thermal design and practical applications of AODs.     

Microscopic model of the THz field enhancement in a metal nanoslit

We discuss the strong THz-field enhancement effect in a metal slit of dozens of nanometers sizes reported recently. Proposed simple microscopic model considers electric charges induced at the edges of the slit by a polarized incident wave. These charges contribute then to the field in the slit. The model is capable of explaining peculiarities of the field enhancement phenomenon such as an inverse frequency dependence of the enhancement factor. It provides closed-form expressions for the enhancement factor and field mapping inside the slit having only one fitting parameter. The model predicts influence of the slit shape on the field enhancement.     

Extraction of isotropic points using simulated isoclinics obtained by photoelasticity-assisted finite element analysis

A method for extracting isotropic points in structures using simulated isoclinics obtained from a combination of photoelastic experiments and finite element analysis (FEA) is presented. This method is divided into two parts. The first part involves the confirmation of the boundary condition necessary for FEA using isochromatics obtained by photoelastic experiments. The second part involves the determination of isotropic points using simulated isoclinics obtained by FEA under the boundary condition confirmed by photoelastic experiments. This method is applied to a ring and T-shaped plates subjected to a compressive load. The results show that the isotropic points in the ring and T-shaped plates could be accurately and easily extracted by this method.     

Effects of inclined magnetic field on mixed convection in a nanofluid filled double lid-driven cavity with volumetric heat generation or absorption using finite element method

A computational analysis has been performed on mixed convection in a double sided lid-driven cavity in the presence of volumetric heat generation or absorption. Effects of inclined magnetic field are also studied. The governing parameters are solved via Galerkin weighted residual finite element method in space and the Crank–Nicolson in time. Governing parameters are nanoparticle volume fraction (0.0?≤???≤?0.04), Richardson number (0.01?≤?Ri?≤?10), internal heat generation or absorption parameter $(?10\le q\le 10),$ inclination angle of magnetic field (0°?≤?γ?≤?90°) and Hartmann number (0?≤?Ha?≤?100). It is observed that the highest heat transfer is obtained in case of the maximum value of heat absorption. As a further finding, heat transfer decreases with increasing of Hartmann number and increases with increasing of nanoparticle volume fraction.     

Effects of the triple junction types on the grain boundary carbide precipitation in a nickel-based superalloy,a statistical analysis

The morphology evolution of carbide precipitated on grain boundary nearby different triple junctions in grain boundary engineering (GBE) treated nickel-based Inconel Alloy 690 aged at 715°C for different time was investigated by scanning electron microscopy and electron backscatter diffraction. The results show that, the diversity of triple junction types was increased by GBE significantly. The size and morphology of grain boundary carbide were not only affected by the grain boundary character, but also the nearby grain boundary character at the triple junction. The higher Σ values of the nearby grain boundaries, the larger carbide precipitated on the other grain boundary. Based on the experimental results, the effects of grain boundary characters and triple junction types on the carbide precipitation behaviours are discussed.     

Identification of the piezoelectric material coefficients using the finite element method with an asymptotic waveform evaluation

A rapid identification of the piezoelectric material constants for a piezoelectric transducer is proposed. The validity of a three-dimensional finite element routine was confirmed experimentally. The asymptotic waveform evaluation (AWE) was adopted for a fast frequency sweep of the finite element analysis. The three-dimensional finite element method with an AWE and a design sensitivity method was used for a material inversion scheme of piezoelectric transducers. In order to confirm the inversion routine of the material constants, the mechanical displacements, which mean the mode shape, were calculated along the vertical and lateral position of the sample transducer.     

EAST���ϲ��Ͼ�Ե����������Ԫ����

用有限元法对EAST装置磁体系统的高性能氦气密复合材料绝缘子性能作了全面的分析.结果表明:绝缘子可满足承受EAST装置耐电压15kV的使用要求;热应力对绝缘子的绝缘层影响最大,外力对绝缘层应力的影响可忽略;EAST复合材料绝缘子具有优良的低温力学性能.     

Numerical simulation of laser-generated ultrasound in non-metallic material by the finite element method

The use of a pulsed laser for the generation of the elastic waves in non-metallic materials in the thermoelastic regime is investigated by using finite element method (FEM), taking into account not only thermal diffusion and the finite spatial and temporal shape of the laser pulse, but also optical penetration and the temperature dependence of material properties. The optimum finite element model is established based on analysis of two important parameters, meshing size and time step, and the stability of solution. Temperature distributions and temperature gradient fields in non-metallic material for different time steps are obtained, this temperature field is equivalent to a bulk force source to generate ultrasonic wave. The laser-generated ultrasound waveforms at the epicenter and surface acoustic waveforms (SAWs) are obtained and the influence of optical penetration into the material on the temperature field and the ultrasound waveforms are analyzed. The numerical results indicate that the heat penetration into non-metallic material is caused mainly by the optical penetration, and the ultrasound waveforms, especially the shape of the precursor, are strongly dependent on the optical penetration depth into non-metallic material.     

Quantification of blood flow velocity in stenosed arteries by the use of finite elements: an observer-independent noninvasive method

Interventions for peripheral arterial disease should be designed to treat a physiological rather than an anatomic defect. Thus, for vascular surgeons, functional information about stenoses is as important as the anatomic one. In case of finding a stenosis by the use of magnetic resonance angiography, it would be a matter of particular interest to derive automatically and directly objective information about the hemodynamic influence on blood flow, caused by patient-specific stenoses. We developed a methodology to noninvasively perform numerical simulations of a patient's hemodynamic state on the basis of magnetic resonance images and by the means of the finite element method. We performed patient-specific three-dimensional simulation studies of the increase in systolic blood flow velocity due to stenoses using the commercial computational fluid dynamic software package FIDAP 8.52. The generation of a mesh defining the flow domain with a stenosis and some simulation results are shown.     

The design of a high power ultrasonic test cell using finite element modelling techniques

This paper will describe the application of a finite element (FE) code to design a test cell, in which a single transducer is used to generate acoustic cavitation. The FE model comprises a 2-D slice through the centre of the test cell and was used to evaluate the generated pressure fields as a function of frequency. Importantly, the pressure fields predicted by FE modelling are used to indicate the position of pressure peaks, or 'hot-spots', and nulls enabling the systems design engineer to visualise both the potential cavitation areas, corresponding to the 'hot-spots', and areas of low acoustic pressure. Through this design process, a rectangular test cell was constructed from perspex for use with a 40 kHz Tonpilz transducer. A series of experimental measurements was conducted to evaluate the cavitation threshold as a function of temperature and viscosity/surface tension, for different fluid load media. The results indicate the potential of the FE design approach and assist the design engineer in understanding the influence of the fluid load medium on the cell's ability to produce a strong cavitation field.     

Numerical simulation of laser-generated surface acoustic waves in the transparent coating on a substrate by the finite element method

The optimum finite element model in the system consisting of a transparent coating and an opaque substrate is established based on the analysis of two important parameters: meshing size and time step, and the stability of solution. Taking into account the temperature dependence of material properties, the transient temperature and temperature gradient field are obtained. According to the thermoelastic theory, this temperature gradient field can be taken as a buried bulk source to generate ultrasonic wave. The surface acoustic waves (SAWs) are obtained. The influence of the coating thickness on the SAWs is analyzed. The model provides a useful tool for the determination of modes which are generated by a laser source in transparent coating on opaque substrate. The surface skimming longitudinal wave exists for the multiple oscillations and it charges from unipolar waveforms to dipolar.     

An alternative method for using bipolar junction transistors as a radiation dosimetry detector in breast cancer treatment

Silicon photodetectors and MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistors) are frequently used devices for measuring ionizing radiation in health physics instrumentation. The Bipolar Junction Transistor (BJT) is not a typical device used as a detector for measuring some physical quantities in radiotherapy beams due to its loss of sensitivity to ionizing radiation, as a consequence of radiation damage in the silicon semiconductor substrate. Actually, the know-how of the BJT characteristic curves and its response to ionizing radiation leads us to suggest an alternative method to estimate the radiation dose value in breast cancer treatments. The BJT parameter to be evaluated before and after the irradiation procedure is the BJT amplification factor, also called DC gain β. In this work, the study was done using a BJT known as Darlington type, within an Alderson Rando anthropomorphic phantom. Darlington transistors have very high gain and this feature allowed that the BJT gain changes to be correlated with the dose of the radiation beam. The results indicate that this new method could be an alternative option to estimate the dose value in the phantom for measurements in breast cancer radiotherapy.