汽车悬架控制臂液压衬套动态特性实测与计算分析

在汽车悬架系统中,为了满足其低频振动控制和高频隔振性能的要求,对控制臂衬套动态特性的要求是不一致的。常见的橡胶衬套很难提供在低频振动控制时所需要的大阻尼。液压衬套是一种可在一个较大的频率范围内提供大阻尼和大刚度的振动控制元件,目前它在悬架系统中得到了广泛的应用。介绍了可在径向或轴向方向提供大阻尼和大刚度的两种结构型式的液压衬套,实验测试了它们的静态特性、动刚度和阻尼及其与激振频率和激振振幅的相关性;建立了液压衬套动特性计算分析的集总参数模型,利用该模型计算分析了一液压衬套的动刚度和滞后角,计算结果和实验值的对比结果证明了模型的正确性。最后给出了液压衬套在解决由于路面或车轮不平衡激励而引起方向盘振动问题的应用实例。     

二自由度振子型的二维LR声子晶体弯曲振动带隙研究

为了抑制板件的低频振动,提出了一种由二自由度振子周期性排列的二维LR (Locally Resonant)声子晶体,实现了低频范围内的双带隙。运用平面波展开法计算了该声子晶体结构的带隙特性,并通过有限元法和样件试验得到的振动传递特性验证了带隙计算结果的准确性。通过对含二自由度振子的元胞模态振型计算,进一步揭示了该结构低频双带隙的产生机理;振子的平动和转动的两种振型相互耦合作用可以形成两个低频弯曲波带隙,并根据简化模型推导出双带隙起始和截止频率计算方法;分析了弹簧刚度、弹簧距离、振子质量和转动惯量对双带隙的影响。结果表明,通过合理选择设计参数可以获得低频范围内的双带隙特性,为声子晶体在汽车板件多频减振的应用研究上提供了新的方法。     

激励力施加方式对结构振动特性影响研究

为了更合理的预测空压机结构振动特性,对CZ60/30型往复式压缩机进行有限元数值模拟,研究了激励力施加方式对机体振动特性的影响。首先建立空压机机体有限元模型,并计算空压机机体受到的主要激励力。在进行活塞侧推力和气体力对气缸内壁的激励加载时,按照等活塞行程和等运行时间两种方式将一个运动周期分成10部分,分析不同激励力施加方式对计算结果的影响。结果显示等活塞行程分段方式仿真结果更能反应空压机低频振动特性,实验结果与仿真结果也较为吻合,证明了计算方法的可行性。     

开关磁阻电动机振动特性的有限元分析

本文建立了开关磁阻电动机(SR电机)振动特性的二维和三维有限元模型,利用有限单元法计算了SR电机的振动模态和固有频率,对各种模型计算结果进行了对比分析,研究了SR电机定子结构对其振动模态和固有频率的影响,并与模态实验的结果进行了比较.     

深海波导中目标低频声散射特性研究

针对深海环境中目标的主动探测问题,建立了深海波导中目标低频声散射仿真的简正波耦合边界元理论模型。首先仿真了深海波导中Munk声速剖面条件下的声传播特性,然后根据深海波导中的声传播特性,仿真计算了声源位于不同深度时,波导中目标低频散射回波强度随声源与目标之间水平距离变化的特性。仿真结果表明,当声源深度为100 m(近海面)与1 400 m(声道轴)时,受完全声道的影响,在会聚区附近范围内散射回波强度较大;声源深度为4 900 m(近海底)时,受直达波与一次海面反射波的影响,在中近距离(小于40 km)范围内散射回波强度较大;对于接收水听器而言,置于临界深度以下时主动探测的距离更远。     

基于声学隧道效应的低频目标强度增强技术

高分子聚合物(Polymethyl Methacrylate,PMMA)材料的瑞利(Rayleigh)波相速度低于水中声速,是一类亚音速波,其声散射特性及形成机理与金属材料存在一定差异。根据弹性球散射声场简正级数解,对比分析了相同尺寸的PMMA球和钢球的目标强度,发现PMMA球在低频段具有较大的目标强度。采用亚音速波声学隧道效应和表面环绕波传播共振特性,对PMMA球中亚音速Rayleigh波形成的低频目标强度增强进行了物理机理解释和分析。最后,开展了PMMA球和钢球声散射水池试验,验证了PMMA球具备较大的低频目标强度以及分析结果的准确性。     

低频振动对塑性成形表面微观形貌的影响

目的 通过对塑性成形界面施加纵向和法向低频振动,探究不同振动参数对成形过程中表面质量的影响规律。方法 采用自主设计的低频振动发生器完成纵向振动作用下的摩擦实验和法向振动作用下的压缩实验,统计材料表面变形区域的平均粗糙度,评价成形质量。结果 在干摩擦条件下,不同频率(0~50 Hz)的纵向小振幅(0.1 mm)振动有利于提高成形界面的表面质量,变形区域的表面粗糙度与频率成反比,但是较大振幅(0.4 mm)的纵向振动会增加接触面的材料磨损,降低表面质量;不同频率(0~50 Hz)和不同振幅(0~0.4 mm)的法向振动均有利于提高干摩擦成形界面的表面质量,在0.2 mm振幅条件下表面粗糙度下降最为明显,振幅超过0.2 mm后,增加振幅对提升表面质量的影响不明显。结论 低频振动对成形界面微观形貌影响较大,合理优化低频振动工艺参数可显著提高成形界面的表面质量。     

开关磁阻电动机振动特性的有限元分析

本文建立了开关滋阻电动机(SR电机)振动特性的二维和三维有限元模型，利用有限单元法计算了SR，电机的振动模态和固有频率，对各种模型计算结果进行了对比分析，研究了SR电机定子结构对其振动模态和固有频率的影响，并与模态实验的结果进行了比较。     

水下圆柱壳低频声辐射特性及有源控制

利用振动模态及声辐射模态分析水下有限长圆柱壳低频模态辐射特性。计算各阶周向振动模态对辐射声功率贡献;将各阶周向模态下轴向振动模态分为奇、偶模态组,分析低频范围内振动模态组与声辐射模态对应关系;以主导声辐射模态声功率为目标函数对水下有限长圆柱壳低频声辐射进行有源控制。结果表明,低频范围内水下简支圆柱壳受径向点力激励时,仅前几阶周向振动模态对辐射声功率有贡献;同一周向振动模态下轴向为奇(偶)振动模态组产生的声功率与具有相同周向阶数而轴向为偶(奇)声辐射模态产生的声功率对应。通过控制前几阶主导声辐射模态即可完成对水下有限长壳体低频辐射噪声抑制。     

可变外激励下制动器低频振动的实验与数值仿真

在实验和数值仿真的基础上,利用三自由度模型来预测制动系统的动力响应,即可变外激励下的制动器低频振动。实验中制动器参数可调,外激励可变。采用事件驱动法对制动器低频振动进行数值仿真,实验和数值仿真中均可测得制动器低频振动,证明数值模拟结果可靠。通过实验与数值仿真观测制动系统参数变化以及外激励对其低频振动的影响,发现制动系统低频振动对其自身参数改变十分敏感;同时,选取适当形式的外激励可以抑制制动器的低频振动。     

Defect characterization using an ultrasonic array to measure the scattering coefficient matrix

Ultrasonic nondestructive evaluation is used for detection, characterization, and sizing of defects. The accurate sizing of defects that are of similar or less size than the ultrasonic wavelength is of particular importance in assessing structural integrity. In this paper, we demonstrate how measurement of the scattering coefficient matrix of a cracklike defect can be used to obtain its size, shape, and orientation. The scattering coefficient matrix describes the far field amplitude of scattered signals from a scatterer as a function of incident and scattering angles. A finite element (FE) modeling procedure is described that predicts the scattering coefficient matrix of various cracklike defects. Experimental results are presented using a commercial 64-element, 5 MHz array on 2 aluminum test samples that contain several machined slots and through thickness circular holes. To minimize the interference from the reflections of neighboring defects, a subarray approach is used to focus ultrasound on each target defect in turn and extract its scattering coefficient matrices. A circular hole and a fine slot can be clearly distinguished by their different scattering coefficient matrices over a specific range of incident angles and scattering angles. The orientation angles of slots directly below the array are deduced from the measured scattering coefficient matrix to an accuracy of a few degrees, and their lengths are determined with an error of 10%.     

Isogeometric FEM‐BEM coupled structural‐acoustic analysis of shells using subdivision surfaces

We introduce a coupled finite and boundary element formulation for acoustic scattering analysis over thin‐shell structures. A triangular Loop subdivision surface discretisation is used for both geometry and analysis fields. The Kirchhoff‐Love shell equation is discretised with the finite element method and the Helmholtz equation for the acoustic field with the boundary element method. The use of the boundary element formulation allows the elegant handling of infinite domains and precludes the need for volumetric meshing. In the present work, the subdivision control meshes for the shell displacements and the acoustic pressures have the same resolution. The corresponding smooth subdivision basis functions have the C¹ continuity property required for the Kirchhoff‐Love formulation and are highly efficient for the acoustic field computations. We verify the proposed isogeometric formulation through a closed‐form solution of acoustic scattering over a thin‐shell sphere. Furthermore, we demonstrate the ability of the proposed approach to handle complex geometries with arbitrary topology that provides an integrated isogeometric design and analysis workflow for coupled structural‐acoustic analysis of shells.     

Computational Prediction of Micro-crack Induced Ultrasound Attenuation in CFRP Composites

A computational study is presented of ultrasound attenuation arising from linear elastic scattering by matrix micro-cracking in carbon fiber reinforced polymer (CFRP) composites. The model considers ultrasound propagation in a unidirectionally reinforced laminate containing randomly distributed matrix micro-cracks, configured such that neither the cracked laminate nor wavefield display dependence on the spatial direction of fiber reinforcement. The resulting 2D scattering formulation is solved using the boundary element method (BEM). Scatter-induced ultrasound attenuation is computed using a fully-interacting scattering matrix, as well as approximate multiple scattering formulations. Through comparison to the fully-interacting computation, the validity of level 1 scattering (independent scattering approximation), level 2 scattering (single neighbor interaction), and level 3 scattering (two neighbor interaction) is noted as a function of micro-crack density. A single parameter estimation of attenuation dependence on micro-crack density is obtained by fitting exponential dependence to the fully-interacting scattering results. Additionally, this single parameter is shown to be extracted equally well by fitting over the limited validity range of the independent scattering formulation. When uniform crack morphology is assumed, the exponential fit to the independent scattering approximation yields a practical estimation of micro-crack induced attenuation over the full range of micro-crack density through consideration of forward scattering by a single micro-crack. Comparison of results to limited experimental data, and to an independent alternate computational approach, lends plausibility to the study’s conclusions.     

某小型潜航器的声散射特性研究

运用目标回声强度预报的板块元方法,研究某小型潜航器的水下回声特性。计算并分析了入射波频率和方位对其声散射特性的影响规律;利用板块元法得到的散射特性,根据频域间接法仿真出水下声成像结果;完成外场试验,将试验结果与计算结果进行对比,吻合较好。研究表明潜航器中的桅杆结构对其散射有较明显影响。     

快速预估水下圆形角反射体散射声场的修正声束弹跳方法

针对水下圆形角反射体散射声场计算速度问题,在声束弹跳法基础上,提出了一种快速预估散射声场的修正声束弹跳法。对组成圆形三面角反射体的弧形边缘进行离散化,相邻离散点与角反射体顶点构成板块元,并与声源点构成入射声束,利用几何声学计算每条声束在角反射体反射面上的反射,同时得到反射面上每条声束"照射"后再次构成的板块元,用物理声学方法计算所有板块元的散射声场,叠加求和得到整个角反射的散射声场。通过与原始的声束弹跳法计算结果的对比,两者计算结果一致,修正声束弹跳法降低了计算量。     

Longitudinal wave scattering from rough crack-like defects

The roughness of crack-like defects affects ultrasonic wave scattering and this, in turn, affects defect detection and characterization. The first part of this paper is concerned with the efficient numerical modeling of scattering from rough cracks, i.e., a finite element local scattering (FELS) model. The scattered field is presented in the form of a scattering matrix, which describes the far-field scattering coefficient for all possible combinations of incident and scattering directions. The scattering matrices for many different realizations of rough cracks are simulated using both a FELS model and a model based on the Kirchhoff approximation. It is shown that the difference between scattering matrices extracted from the Kirchhoff model and the FELS model is less than 8%, for rough cracks with a standard deviation less than 0.3 wavelengths and a correlation length longer than 0.5 wavelengths, at incident and scattering angles ranging from -80° to 80° relative to the normal direction of the mean surface. Because the Kirchhoff model is significantly more efficient than the FELS model, it is used for subsequent simulations in which many realizations of rough cracks are studied to gain insight into the statistical nature of the scattering process. In line with previous work, a distinction is made between the coherent and diffuse contributions to the overall scattered field, in which the former represents the ensemble average over multiple surface realizations. The coherent and diffuse contributions of scattered field from various types of rough cracks are simulated. It is shown that surface roughness directly affects the coherent contribution to scattering behavior, whereas the diffuse contribution is affected by both surface roughness and correlation length, especially for rougher cracks.     

Boundary element method calculation for coherent electromagnetic scattering from two and three dielectric spheres

A quadratic, isoparametric boundary element formulation has been used to calculate the multiple scattering of electromagnetic waves from systems of two and three dielectric spheres. Extinction efficiency results for the scattering of a plane wave are presented for variations of the separation of the two spheres in three kinds of orientations of the system with respect to the incident wave. These have been verified against analytical calculations based on Mie's theory and calculations by other authors. The results demonstrate a large side scattering resonance (the so-called specular resonance). Agreement between the results establishes the boundary element method as a very powerful tool for solving multiple scattering problems because the method applies to arbitrarily shaped objects having a homogeneous dielectric constant in any configuration. To illustrate the versatility of the method, scattering from three spheres is calculated.     

Integral Equation Method for Evaluation of Eddy-Current Impedance of a Rectangular,Near Surface Crack Inside a Cylindrical Hole

An integral equation method for solving the eddy-current nondestructive evaluation problem of a flat, rectangular, near surface crack inside of a cylindrical hole in a conducting material is presented. The method involves expanding the Green’s tensor, the incoming field, and the jump in electric potential over the crack in suitable basis functions. Here, plane waves, cylindrical waves, and basis functions related to the Chebyshev polynomials, are used. The way of discretization in this method leads to a formulation where the scattering is defined by a scattering matrix, independent of the incoming field. This presents an advantage, when conducting numerical simulations, since the scattering matrix does not have to be recalculated for every probe position. The numerical calculations are straightforward to perform and model predictions are compared with finite element results.     

基于板块元的水面舰船远场声目标强度预报方法

提出了一种基于板块元的水面舰船水下远场声目标强度数值预报方法,首先根据水面舰船水下船体及附体的型线进行等效建模,采用板块元方法进行网格划分,建立了水面舰船特有的复杂附体二次反射、海面散射效应的数值模型,将各板块元的声散射视为一种"滤波",各板块元的散射声传输函数具有不同幅度、相位、时延,将其叠加之和进行反傅氏变换后,获得水面舰船波形级的远场声目标强度及方位分布特征。通过分析比较典型声呐探测信号下水面舰船的声目标强度,验证了该方法的有效性,为优化与控制水面舰船目标强度,分析水面舰艇水声对抗的作战效能提供了物理依据。     

Reflection and scattering characteristics of reflectors in SAW tags

As a foundation of an optimal design for SAW tags,the reflection, scattering, and transmission of the reflector composed of a few electrodes are discussed. A source regeneration method based on Green's function and the finite element method/boundary element method is used to obtain the reflection coefficient and the scattering coefficient for the short-circuited and open circuit reflectors. Examples are presented to show how one can use structure variance to satisfy the requirements for SAW tag design according to the reflection and scattering characteristics.