离焦散斑技术研究Kirchhoff板在动载作用期的弯曲

本文使用动态离焦散斑技术,测试板的动态弯曲获得成功。并推导了Kirchhoff板动态变形的有关分析,提出了由最大斜率确定材料常数的思想。显然,这是同离焦散斑方法所测数据直接相关的处理手法。     

基于改进布谷鸟算法识别瞬态热传导问题的导热系数

基于改进布谷鸟算法反演瞬态热传导问题随温度变化的导热系数.采用Kirchhoff变换将非线性热传导问题转换为线性热传导问题,使用边界元法求解瞬态热传导正问题.将导热系数的反演转化为函数表达式中未知参数的反演,使用改进布谷鸟算法求解未知参数.与共轭梯度法相比,改进布谷鸟算法对迭代初值不敏感;与布谷鸟算法相比,改进布谷鸟算法迭代次数大大减少.数值算例表明对改进布谷鸟算法,增加测点数量迭代次数增加;增加鸟巢数量迭代次数减少;减小测量误差计算结果更精确,同时迭代次数更少.数值算例验证了改进布谷鸟算法反演导热系数的准确性和有效性.     

基于Galerkin法研究应力波作用下复合材料板的动力学失稳

基于Kirchhoff薄板理论和Hamilton原理,考虑应力波效应,对含初始几何缺陷的三边简支、一边固支的复合材料板,建立了振动控制方程,得到了其屈曲临界荷载表达式。采用MATLAB编程进行数值计算,讨论了初始几何缺陷、初相位、铺层角度、屈曲模态阶数及铺层层数对板屈曲临界荷载的影响。结果表明:复合材料板的屈曲临界载荷随临界长度增大、铺设厚度减小、初始几何缺陷系数增大、振型函数初相位减小而减小。此外,复合材料板的各层铺层角度与荷载作用方向的夹角越小,屈曲临界载荷越大,当板的对称铺设层数达到7层时,临界荷载趋于稳定。     

冲击载荷下混凝土本构模型构建研究

 在对混凝土动态力学性能和现有本构模型综合分析的基础上,构建了一个新的适用于冲击响应问题数值分析的混凝土本构模型。该本构模型全面考虑了压力、应力第三不变量、变形的硬化和软化、应变率强化以及拉伸损伤等各个影响因素。将其加入LTZ-2D程序,确定了本构模型参数,对混凝土靶板的穿透问题进行了数值验证分析。计算得到的弹体剩余速度同实验结果基本一致,同时得到了混凝土靶板破裂的计算图像。计算结果及其分析表明,所构建的本构模型能够较好地反映冲击载荷作用下混凝土动态响应的主要特性。     

各向异性有耗介质板介电系数和电导率的反演

简述了各向异性介质FDTD方法，并用FDTD方法分析了三维各向异性有耗介质板的瞬态后向散射.根据各向异性介质板后向散射与入射电磁波极化方向有关的特点，利用其后向RCS的谐振特性和后向散射场的时域波形特点反演有耗介质板的横向介电系数和电导率.数值模拟结果表明本反演方法可行，且方便、快捷. 关键词： 各向异性板 FDTD方法 有耗介质 谐振 反演     

轴对称射流气动声场的数值模拟

构造了Kirchhoff积分和CFD计算相结合的算法,采用高精度差分格式对不同喷口马赫数的轴对称射流进行数值模拟,并以此作为近场声源,运用Kirchhoff积分方法研究远场气动噪声.数值结果表明,远场噪声具有方向性,噪声声压在离开对称轴20°处达到最大值.随着传播距离增大,噪声方向性逐渐减弱.     

退火铝合金中Portevin-Le Chatelier效应的数值模拟研究

通过综合考虑沉淀动力学和动态应变时效机理，建立了一套具有明确物理内涵的唯象本构模型.并在对不同加载应变率下的单轴拉伸实验的数值模拟中，得到了3种类型的Portevin-Le Chatelier效应应力曲线.计算结果与实验数据的较好吻合，验证了该模型的有效性. 关键词： Portevin-Le Chatelier效应 动态应变时效 数值模拟     

二阶层级自相似四边形蜂窝动力压缩行为数值模拟

运用ABAQUS对在冲击载荷作用下二阶自相似四边形蜂窝面外方向的动态压缩行为进行了数值模拟,分析了其在冲击载荷作用下的动态压缩过程,并对孔径尺寸、冲击速度和基体材料屈服强度对其压缩模式及应力-应变关系的影响进行了研究。数值模拟表明:孔径较小的二阶自相似四边形层级蜂窝在冲击载荷作用下更易出现整体坍塌的压缩失效模式;随着基体材料屈服强度增大,平台应力值随之增加,但对平台长度没有影响;随着冲击速度的增加,冲击端平台应力值和平台长度都有所增加。     

冲击载荷下土的动态力学性能研究

 利用分离式霍普金森压杆（Split Hopkinson Pressure Bar,SHPB）技术,研究了土体在不同应变率条件下的冲击动态力学性能,发现土体有明显的应变率效应,与静载相比,冲击载荷下土的动强度和动模量均有很大的提高。根据实验曲线的特征,以一根线性弹簧和两个不同松弛时间的Maxwell体并联的粘弹性模型来表达土体的损伤型粘弹性本构模型,两个Maxwell体分别表示土体的低应变率响应和高应变率响应,模型的数值拟合曲线与实测动态本构曲线具有较好的一致性。拟合参数表明,土体对低应变率的响应与混凝土相同,对高应变率的敏感性远远高于混凝土。     

线性滤波法高速解调技术中的测量有效性研究

光纤光栅在不均匀应变场中会由于其反射谱的展宽而带来测量误差.线性滤波法是一种适合于高频瞬变信号的解调方法, 在光纤光栅参数取典型值时, 使用传输矩阵法对线性滤波法解调光纤光栅在不均匀应变场中反射谱的过程进行了数值模拟.模拟结果表明, 固定应变峰值为3000 με, 应变波波长大于0.0025 m时, 测量相对误差不超过3 %;固定应变波长0.05 m, 应变波幅值小于10000 με时, 应变误差不超过0.25 %.使用Hopkinson压杆装置对数值模拟结果进行了验证, 结果与数值分析结论较为吻合.     

Response of a plate to a transient pressure wave load

The dynamical behaviour of a Kirchhoff plate under a moving pressure wave is investigated. The results are presented as dynamic amplification factors (DAF). The influence of the load parameters (propagation speed, pressure wave duration) and the plate parameters (slenderness, aspect ratio, damping, etc.) is discussed. Finally results for two examples are presented.     

Dynamic instability of composite plates subjected to non-uniform in-plane loads

In this paper, the dynamic instability of a shear deformable composite plate subjected to periodic non-uniform in-plane loading is studied for four sets of boundary conditions. The static component and the dynamic component of the applied periodic in-plane loading are assumed to vary according to either parabolic or linear distributions. Initially, the plate membrane problem is solved using the Ritz method to evaluate the plate in-plane stress distributions within the prebuckling range due to the applied non-uniform in-plane edge loading. Subsequently using the evaluated stress distribution within the plate, the equations governing the plate instability boundaries are formulated via Hamilton's variational principle. Employing Galerkin's method, these partial differential equations are reduced into a set of ordinary differential equations (Mathieu type of equations) describing the plate dynamic instability behaviour. Following Bolotin's method, the instability regions are determined from the boundaries of instability, which represents the periodic solution of the differential equations with period T and 2T to the Mathieu equations. The instability regions are determined for uniform, linear and parabolic dynamic in-plane loads using first-order and second-order approximations. Numerical results are also presented to bring out the effects of span to thickness ratio, shear deformation, aspect ratio, boundary conditions and static load factor on the instability regions.     

Solution of the inverse radiative load problem in a two-dimensional system

An inverse radiation analysis is presented for estimating the temperature and the heat load distributions of the heating surface from the temperature and the heat flux measurements of the heated object. The Monte Carlo method is employed to solve the direct radiation problem. The inverse radiation problem is solved using the conjugate gradient and singular value decomposition methods. The measured data are simulated by adding random errors to the exact solution of the direct problem. The effects of the measurement errors on the accuracy of the inverse analysis are investigated. The study shows that the heat load distribution of the heating surface can be estimated accurately for the exact and noisy data. And the conjugate gradient method is better than the singular value decomposition method since the former can obtain more accurate results if the measurement errors are the same.     

Dynamics of a rectangular plate resting on an elastic foundation and partially in contact with a quiescent fluid

In this study, a method of analysis is presented for investigating the effects of elastic foundation and fluid on the dynamic response characteristics (natural frequencies and associated mode shapes) of rectangular Kirchhoff plates. For the interaction of the Kirchhoff plate–Pasternak foundation, a mixed-type finite element formulation is employed by using the Gâteaux differential. The plate finite element adopted in this study is quadrilateral and isoparametric having four corner nodes, and at each node four degrees of freedom are present (one transverse displacement, two bending moments and one torsional moment). Therefore, a total number of 16 degrees-of-freedom are assigned to each element. A consistent mass formulation is used for the eigenvalue solution in the mixed finite element analysis. The plate structure considered is assumed clamped or simply supported along its edges and resting on a Pasternak foundation. Furthermore, the plate is fully or partially in contact with fresh water on its one side. For the calculation of the fluid–structure interaction effects (generalized fluid–structure interaction forces), a boundary element method is adopted together with the method of images in order to impose an appropriate boundary condition on the fluid's free surface. It is assumed that the fluid is ideal, i.e., inviscid, incompressible, and its motion is irrotational. It is also assumed that the plate–elastic foundation system vibrates in its in vacuo eigenmodes when it is in contact with fluid, and that each mode gives rise to a corresponding surface pressure distribution on the wetted surface of the structure. At the fluid–structure interface, continuity considerations require that the normal velocity of the fluid is equal to that of the structure. The normal velocities on the wetted surface of the structure are expressed in terms of the modal structural displacements, obtained from the finite element analysis. By using the boundary integral equation method the fluid pressure is eliminated from the problem, and the fluid–structure interaction forces are calculated in terms of the generalized hydrodynamic added mass coefficients (due to the inertial effect of fluid). To asses the influences of the elastic foundation and fluid on the dynamic behavior of the plate structure, the natural frequencies and associated mode shapes are presented. Furthermore, the influence of the submerging depth on the dynamic behavior is also investigated.     

The dynamic stiffness matrix of two-dimensional elements: application to Kirchhoff's plate continuous elements

This paper describes a procedure for building the dynamic stiffness matrix of two-dimensional elements with free edge boundary conditions. The dynamic stiffness matrix is the basis of the continuous element method. Then, the formulation is used to build a Kirchhoff rectangular plate element. Gorman's method of boundary condition decomposition and Levy's series are used to obtain the strong solution of the elementary problem. A symbolic computation software partially performs the construction of the dynamic stiffness matrix from this solution. The performances of the element are evaluated from comparisons with harmonic responses of plates obtained by the finite element method.     

AN INVERSE AEROACOUSTIC PROBLEM ON ROTOR WAKE/STATOR INTERACTION

An inverse aeroacoustic model on rotor wake/stator interaction is proposed based on the linearized Euler equations. The sound field is related to the pressure distribution on the stator surface in the form of a Fredholm integral equation of the first kind which is a well-known ill-posed problem. Since the sound field is fully specified, numerical inversion allows the reconstruction of the pressure distribution on the stator surface. For solving the discrete ill-posed problem, the singular-value decomposition technique coupled with the Tikhonov regularization method is applied to stabilize the solution. The optimal regularization parameter is chosen by the generalized cross-validation criterion and the discrete Picard condition is employed to analyze the ill-posedness of the inverse problem. Numerical results show that the reconstruction is fairly good when the signal-to-noise ratio is not very low. The results become inaccurate when the noise dominates over the observer signal. In addition, numerical results also indicate the importance of the reduced frequency. The higher the reduced frequency, the better the reconstruction results.     

反向响应过程抗负载干扰PID控制回路性能评估

反向响应过程广泛存在于化工领域中,但针对反向响应过程的性能评估研究较少。针对一类二阶时滞反向响应过程,通过幂级数展开的方法简化了对象模型,然后研究其抗负载干扰PID控制回路性能评估问题。根据DS-d控制原理获取期望闭环传递函数以及PID控制器参数,推导干扰模型为阶跃情况下的PID控制回路累积绝对误差(IAE)性能基准值,并将其推广到斜坡输入以及一般输入情形,为不同干扰类型情况下的反向响应过程建立通用化的IAE性能评价标准。仿真结果验证了所提性能评估算法的正确性以及通用性。     

Three-dimensional steady state Green function for a layered isotropic plate

The elastodynamic response of a layered isotropic plate to a source point load having an arbitrary direction is studied in this paper. A decomposition technique is developed within each homogeneous isotropic lamina to simplify the general three-dimensional plane-wave propagation problem as a separate plane-strain problem and an anti-plane-wave propagation problem. The accuracy of computation is assured by cross-checking the numerical results by different methods. Results are checked numerically for a vertical point load acting on a homogeneous and a layered plate by using a hybrid method. On the other hand, results are checked for a horizontal point load by using dynamic reciprocal identities. Results are presented for both a homogeneous as well as a layered plate.     

Nonlocal continuum-based modeling of a nanoplate subjected to a moving nanoparticle. Part I: Theoretical formulations

The potential applications of nanoplates in energy storage, chemical and biological sensors, solar cells, field emission, and transporting of nanocars have been attracted the attentions of the nanotechnology community to them during recent years. Herein, the later application of nanoplates from nonlocal elastodynamic point of view is of interest. To this end, dynamic response of a nanoplate subjected to a moving nanoparticle is examined within the context of nonlocal continuum theory of Eringen. The fully simply supported nanoplate is modeled based on the nonlocal Kirchhoff, Mindlin, and higher-order plate theories. The non-dimensional equations of motion of the nonlocal plate models are established. The effects of moving nanoparticle's weight and existing friction between the surfaces of the moving nanoparticle and nanoplate on the in-plane and out-of-plane vibrations of the nanoplate are incorporated into the formulations of the proposed models. The eigen function expansion and the Laplace transform methods are employed for discretization of the governing equations in the spatial and the time domains, respectively. The analytical expressions of the dynamic deformation field associated with each nonlocal plate theory are obtained when the moving nanoparticle traverses the nanoplate on an arbitrary straight path (an opened path) as well as an ellipse path (a closed path). The dynamic in-plane forces and moments of each nonlocal plate model are also derived. Furthermore, the critical velocity and the critical angular velocity of the moving nanoparticle for the proposed models are expressed analytically for the aforementioned paths. Part II of this work consists in a comprehensive parametric study where the effects of influential parameters on dynamic response of the proposed nonlocal plate models are scrutinized in some detail.     

Fast analytical approximation for arbitrary geometries in diffuse optical tomography

Diffuse optical tomography is a novel imaging technique that resolves and quantifies the optical properties of objects buried in turbid media. Typically, numerical solutions of the diffusion equation are employed to construct the tomographic problem when media of complex geometries are investigated. Numerical methods offer implementation simplicity but also significant computation burden, especially when large three-dimensional reconstructions are involved. We present an alternative method of performing tomography of diffuse media of arbitrary geometries by means of an analytical approach, the Kirchhoff approximation. We show that the method is extremely efficient in computation times and consider its potential as a real-time three-dimensional imaging tool.