Dynamic behavior and sound transmission analysis of a fluid-structure coupled system using the direct-BEM/FEM

A direct-BEM/Fem method was proposed to analyze the vibration and acoustic radiation characteristics of a submerged structure. Model parameters of the structure and the fluid-structure interaction due to surrounding water were analyzed by using FEM and direct BEM. Vibration velocity of the outer hull surface and underwater sound pressure were computed through modal superposition technique. The direct-BEM/FEM method was first validated by analyzing a submerged cylindrical shell, then was used to analyze the vibro-acoustic behavior of a submarine stern structure. The results have demonstrated the direct-BEM/FEM method is more effective than FEM in computing the underwater sound radiation of the stern structure.     

Analysis of a perforated panel for the correction of low frequency resonances in medium size rooms

This work illustrates an engineering approach for the analysis of plywood/fibreglass perforated panels, to be used for the linearisation of the frequency response of medium size rooms below 200 Hz. The sound absorption coefficient of these panels is computed, by means of coupled acoustical/structural finite element (FEM) analyses, as a function of the frequency. The comparison between the results given by a complete numerical model of the panel, and those provided by a simplified numerical model, in which only one hole and surrounding material (wood, air, and fibreglass) are described, shows that the second computational approach provides essentially the same results as the former one, at a quite reasonable cost. The FEM results allow us to understand some details about the air motion inside the panel, and to define suitable assumptions for the construction of a simplified analytical model of the panel. Both the numerical and the analytical results are finally compared to experimental data obtained by means of pressure-particle velocity measurements.     

铝电解槽内电-磁-流场的耦合仿真方法及应用

基于电磁流体力学的基本理论,应用等效电阻网络法与有限元相结合计算铝电解槽电流场,标量电位法和双标量磁位法计算铝电解槽磁场,k-ε双方程模型计算电磁力驱动下的铝液流动,并与工业测试结果进行比较.对200 kA侧部四点进电的铝电解槽进行计算与测试,其结果吻合良好;水平磁场形成一个顺时针的漩涡,垂直磁场沿长轴和短轴呈反对分布;铝液流动呈现出沿长轴方向排列的4个漩涡.仿真磁场误差小于10.0%,流场误差不超过5.0%.     

Experimental study of an FEM with a microwave system of a new type

Operability of the wide-band reflector based on the effect of microwave beams multiplication was tested in the “hot” experiment. A microwave system of the new type including such a reflector was used in a free electron maser (FEM)-oscillator with a guide magnetic field. The high-current accelerator “Sinus-6” forming a 500 keV/120 A electron beam was used as the FEM driver. The double-resonance regime, when both the undulator and the cyclotron resonance conditions were fulfilled simultaneously, provided maximum output power 7 MW and electron efficiency 12% at the designed frequency 45 GHz     

激光辐照受拉铝板破坏行为的时空多尺度数值模拟

推导耦合过渡区内参变量信息交换的元/网格动量传递多尺度算法,建立离散元与有限元耦合时空多尺度计算模型,并应用于激光辐照下受拉铝板破坏行为的数值模拟中.通过对比有限元计算模型、空间多尺度计算模型与时空多尺度计算模型在激光辐照下受拉铝板破坏算例的模拟结果,验证离散元与有限元耦合时空多尺度计算模型的准确性和数值计算高效率优势.使用该多尺度计算模型从宏观和细观尺度对铝板破坏行为进行数值模拟,模拟结果与实验结果基本一致.     

颗粒体侵彻数值计算方法研究

分别采用连续体有限元方法、离散元方法、离散颗粒有限元方法和有限元-离散元混合方法,对钢球以100 m/s和200 m/s的速度侵彻细沙石回收靶过程进行了数值计算,通过对4种方法的计算精度和计算效率的比较分析,表明有限元-离散元方法更适合于颗粒材料侵彻问题的数值计算。     

A novel Galerkin-like weakform and a superconvergent alpha finite element method (SαFEM) for mechanics problems using triangular meshes

A carefully designed procedure is presented to modify the piecewise constant strain field of linear triangular FEM models, and to reconstruct a strain field with an adjustable parameter α. A novel Galerkin-like weakform derived from the Hellinger–Reissner variational principle is proposed for establishing the discretized system equations. The new weak form is very simple, possesses the same good properties of the standard Galerkin weakform, and works particularly well for strain construction methods. A superconvergent alpha finite element method (SαFEM) is then formulated by using the constructed strain field and the Galerkin-like weakform for solid mechanics problems. The implementation of the SαFEM is straightforward and no additional parameters are used. We prove theoretically and show numerically that the SαFEM always achieves more accurate and higher convergence rate than the standard FEM of triangular elements (T3) and even more accurate than the four-node quadrilateral elements (Q4) when the same sets of nodes are used. The SαFEM can always produce both lower and upper bounds to the exact solution in the energy norm for all elasticity problems by properly choosing an α. In addition, a preferable-α approach has also been devised to produce very accurate solutions for both displacement and energy norms and a superconvergent rate in the energy error norm. Furthermore, a model-based selective scheme is proposed to formulate a combined SαFEM/NS-FEM model that handily overcomes the volumetric locking problems. Intensive numerical studies including singularity problems have been conducted to confirm the theory and properties of the SαFEM.     

有限元/边界元耦合方法在海洋生物目标强度特性研究的应用*

目标强度特性是海洋生物声学识别与资源量评估的重要依据,其中,基于近似几何体和声阻抗特性的理论模型法是研究海洋生物目标强度的重要手段。由于对几何形态近似处理以及数值求解方法的限制,传统理论模型对声波频率、入射方位以及目标声阻抗、形态尺寸等均有各自不同的适用范围,单一模型难以满足不同种类或同一种类但不同尺寸海洋生物的目标强度求解。本文尝试将逐渐见诸应用的有限元/边界元耦合方法用于海洋生物目标强度特性研究,分别以球形生物、纺锤形鱼类尾明角灯鱼(Ceratoscopelus warmingii)和细长形浮游动物南极大磷虾(Euphausia superba)为例进行仿真计算,并与相适应的经典理论模型进行对比分析。结果表明,对于球形生物,有限元/边界元耦合方法与解析模型的目标强度频响曲线完全吻合;对于纺锤形鱼类,有限元/边界元耦合方法可有效弥补基于模态级数解的形变圆柱体模型在中低频和两端入射时的准确性问题;对于细长形浮游动物,有限元/边界元耦合方法与畸变波玻恩近似模型高度吻合。综上,有限元/边界元耦合方法对多种海洋生物目标强度求解均具有较好的适用性,未来有待进一步结合实验测定进行验证。     

Experimental study of coaxial free-electron maser based on two-dimensional distributed feedback

The first experimental study of a coaxial free-electron maser (FEM) based on two-dimensional (2D) distributed feedback is presented. A new type of cavity formed with coaxial 2D surface photonic band gap structures was used. The FEM was driven by a large diameter (7 cm), high-current (500 A), annular electron beam of energy 475 keV. By tuning the amplitude of the undulator or guide magnetic field, modes associated with the different band gaps of the 2D structures were excited. The -band coaxial FEM generated 15 MW of radiation with a 6% conversion efficiency, in excellent agreement with theory.     

Investigation of field electron microscope tips bombarded with microparticles at limited field emission currents

Mo, Ta, W, and Re field electron microscope (FEM) tips, bombarded with microparticles at limited field emission currents (400 MΩ resistor in series with the FEM tip) were investigated by means of FEM, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Two groups of tips could be distinguished: One group had a slight tip radius increase to a maximum of 2.5 μm and microcraters were formed along the tip shank. The other group had no detectable tip radius change; however, there was microcrater formation near the tip apex area. FEM patterns showed surface contamination clearly. Heating such contaminated tips and tips where phosphorescent material (Zn:Cd)S had been deposited in less than monolayer concentration by evaporation from a heating coil, resulted in similar sequences of FEM patterns. A new type of microcrater with smooth crater lips could be detected in both groups. These results support the combined microparticle-field emission mechanism [1], which was proposed earlier to be responsible for melting cap and microcrater formation. Excerpts were presented at the 19th Field Emission Symposium, Urbana, Illinois (USA), August 1972.     

A combined three-dimensional finite element and scattering matrix method for the analysis of plane wave diffraction by bi-periodic,multilayered structures

A three-dimensional finite element method (FEM) for the analysis of plane wave diffraction by a bi-periodic slab is described and implemented. A scattering matrix formalism based on the FEM allows the efficient treatment of light reflection and transmission by multilayer bi-periodic structures, and the computation of Bloch modes of three-dimensional arrays. Numerical simulations, which show the accuracy and flexibility of the FEM, are presented.     

Electric and magnetic losses modeled by a stable hybrid with explicit–implicit time-stepping for Maxwell’s equations

A stable hybridization of the finite-element method (FEM) and the finite-difference time-domain (FDTD) scheme for Maxwell’s equations with electric and magnetic losses is presented for two-dimensional problems. The hybrid method combines the flexibility of the FEM with the efficiency of the FDTD scheme and it is based directly on Ampère’s and Faraday’s law. The electric and magnetic losses can be treated implicitly by the FEM on an unstructured mesh, which allows for local mesh refinement in order to resolve rapid variations in the material parameters and/or the electromagnetic field. It is also feasible to handle larger homogeneous regions with losses by the explicit FDTD scheme connected to an implicitly time-stepped and lossy FEM region. The hybrid method shows second-order convergence for smooth scatterers. The bistatic radar cross section (RCS) for a circular metal cylinder with a lossy coating converges to the analytical solution and an accuracy of 2% is achieved for about 20 points per wavelength. The monostatic RCS for an airfoil that features sharp corners yields a lower order of convergence and it is found to agree well with what can be expected for singular fields at the sharp corners. A careful convergence study with resolutions from 20 to 140 points per wavelength provides accurate extrapolated results for this non-trivial test case, which makes it possible to use as a reference problem for scattering codes that model both electric and magnetic losses.     

抛物方程基于自然边界归化的耦合法

将冯康和余德浩提出的自然边界归化方法^[1~4]应用于求解抛物方程初边值外区域问题,提出一种自然边界元与有限元耦合算法。先将控制方程对时间进行离散化,得到关于时间步长的离散化格式,给出圆外域上的自然积分方程,基于此研究抛物方程无界区域问题的自然边界元与有限元耦合法,最后给出相应的数值例子。     

ITER校正场底部线圈结构的有限元分析

采用有限元法(FEM)研究了ITER装置中超导校正场底部线圈 (BCC)结构的力学性能。针对BCC结构各组合部件三个方向尺寸的巨大差异,将BCC结构均匀化模型和局部细化模型分步组合,通过有限元分析获得了整体结构和薄弱部位各材料的位移场和应力场。针对BCC结构提出了支撑部位螺栓中预紧力的确定方法,用有限元法得出合理的螺栓预紧力,可用于指导BCC结构的安装。     

类He离子基态的Hartree-Fock有限元计算

FEM直至七十年代中期才应用于原子分子问题。已有工作虽不多,却说明FEM对一维和二维问题是有效的,但应用于n(≥3)电子原子问題至少在目前是不现实的。我们提出一种FEM与HF方法相结合的方法(HFFEM),在DPS8机上计算了类He离子的基态能量与平均值<rⁿ>,能量计算值与实验值的误差在0.3%~0.03%之间。这种HFFEM可推广应用于各种原子与分子问题。     

某稳瞄转塔结构的有限元模态分析与试验验证

结构动态效应是影响稳瞄系统稳定精度的关键因素。目前有限元模态分析与试验模态分析结合的方法是分析结构动力学特性的最有效途径之一。以某型稳瞄具两轴四框架结构为研究对象,首先利用有限元方法进行了模态预分析。根据仿真结果获取的振型特征,对两轴四框架的外环进行模态测试,给出了前8阶模态的固有频率、振型和模态阻尼比。模态仿真与测试的结果相互验证表明:有限元仿真可实现两轴四框架结构固有频率相对误差小于15%的精度要求,在测点密度足够的条件下振型相关性良好。     

New technology for the design of advanced ultrasonic transducers for high-power applications

A new high-frequency ultrasonic transducer for wire bonding has been conceived, designed, prototyped and tested. In the design phase an advanced approach was used and established. The method is based on the two basic principles of modularity and iteration. The transducer is decomposed to its elementary components. For each component an initial design is obtained with finite elements method (FEM) simulations. The simulated ultrasonic modules are then built and characterized experimentally through laser-interferometry measurements and electrical resonance spectra. The comparison of simulation results with experimental data allows the parameters of FEM models to be iteratively adjusted and optimized. The achieved FEM simulations exhibit a remarkably high-predictive potential and allow full control on the vibration behavior of the ultrasonic modules and of the whole transducer. The new transducer is fixed on the wire bonder with a flange whose special geometry was calculated by means of FEM simulations. This flange allows the converter to be attached on the wire bonder not only in longitudinal nodes but also in radial nodes of the ultrasonic field excited in the horn. This leads to a nearly complete decoupling of the transducer to the wire bonder, which has not been previously obtained. The new approach to mount ultrasonic transducers on a welding-device is of major importance not only for wire bonding but also for all high-power ultrasound applications and has been patented.     

Coupled FEM/BEM for control of noise radiation and sound transmission using piezoelectric shunt damping

In this paper, we present a coupled finite element/boundary element method (FEM/BEM) for control of noise radiation and sound transmission of vibrating structure by passive piezoelectric techniques. The system consists of an elastic structure (with surface mounted piezoelectric patches) coupled to external/internal acoustic domains. The passive shunt damping strategy is employed for vibration attenuation in the low frequency range. The originality of the present paper lies in evaluating the classically used FEM/BEM methods for structural–acoustics problems when taking account smart systems at the fluid–structure interfaces.     

Field emission microscopy of adsorption and desorption of residual gas molecules on a carbon nanotube tip

Field emission of electrons from multiwall carbon nanotubes has been investigated by field emission microscopy (FEM) in ultra-high vacuum. A carbon nanotube, at the end of which at least six pentagons exist to make a closed cap, gives an FEM pattern consisting of bright pentagonal rings if the nanotube surface is clean. Adsorption of residual gas molecules is observed as bright spots in the FEM pattern, giving rise to an abrupt increase in the emission current. Adsorbed molecules seem to reside preferentially on the pentagonal sites where the strong electric field is concentrated. A heat cleaning of the emitter at about 1300 K allows the molecules to desorb, and the nanotube emitter recovers its original clean surface. It has been revealed that the adsorption and desorption of gas molecules are responsible for stepwise fluctuation of the emission current.     

A hybrid approach for predicting the distribution of vibro-acoustic energy in complex built-up structures

Finding the distribution of vibro-acoustic energy in complex built-up structures in the mid-to-high frequency regime is a difficult task. In particular, structures with large variation of local wavelengths and/or characteristic scales pose a challenge referred to as the mid-frequency problem. Standard numerical methods such as the finite element method (FEM) scale with the local wavelength and quickly become too large even for modern computer architectures. High frequency techniques, such as statistical energy analysis (SEA), often miss important information such as dominant resonance behavior due to stiff or small scale parts of the structure. Hybrid methods circumvent this problem by coupling FEM/BEM and SEA models in a given built-up structure. In the approach adopted here, the whole system is split into a number of subsystems that are treated by either FEM or SEA depending on the local wavelength. Subsystems with relative long wavelengths are modeled using FEM. Making a diffuse field assumption for the wave fields in the short wave length components, the coupling between subsystems can be reduced to a weighted random field correlation function. The approach presented results in an SEA-like set of linear equations that can be solved for the mean energies in the short wavelength subsystems.