主动悬架与EPS集成控制系统操纵稳定性试验

在主动悬架与EPS集成控制系统动力学模型建立、控制策略设计及其仿真计算分析基础上,为进一步验证集成控制系统车辆的抗侧倾能力,进行了车速为50 km/h和60 km/h的蛇行试验,试验结果表明,相对于被动系统,车速为50 km/h时经过集成控制后的侧向加速度峰值和标准差分别下降了21.4 %和15.9 %,车身侧倾角峰值和标准差分别下降了13.4 %和15.1 %,有效提高了车辆转弯时的操纵稳定性能。     

Nonreversible homoclinic snaking

Homoclinic snaking refers to the sinusoidal ‘snaking’ continuation curve of homoclinic orbits near a heteroclinic cycle connecting an equilibrium E and a periodic orbit P. Along this curve the homoclinic orbit performs more windings about the periodic orbit. Typically, this behaviour appears in reversible Hamiltonian systems. Here we discuss this phenomenon in systems without any particular structure. We give a rigorous analytical verification of homoclinic snaking under certain assumptions on the behaviour of the stable and unstable manifolds of E and P. We show how the snaking behaviour depends on the signs of the Floquet multipliers of P. Further we present a nonsnaking scenario. Finally, we show numerically that these assumptions are fulfilled in a model equation.     

Divergence and flutter of shear deformable aircraft swept wings subjected to roll angular velocity

The aeroelastic modeling and instability of shear deformable swept wings under roll angular velocity is investigated. The structural wing model was originally developed by Librescu and consists of non-classical effects such as warping inhibition and transverse shear flexibility. This model is used to study divergence and flutter instabilities when the aircraft wing is subjected to a roll moment created during a maneuver. The aeroelastic governing equations and boundary conditions are determined via Hamilton’s variational principle. The resulting partial differential equations are transformed into a set of eigenvalue/boundary value equations through the Extended Galerkin approach and solved by numerical integration. The effects of roll angular velocity, sweep angle, and wing aspect ratio on divergence and flutter speed are presented for classic and shear deformable wings. Validations of selected results against the previous publications are also supplied. Results indicate that roll angular velocities have a significant influence on the static and dynamic aeroelastic instability region.     

空间桁架梁固有振动的能量变分解析解

目前人们对空间桁架梁的振动特性研究得较少,该文首先采用Timoshenko梁的连续化模型来模拟空间桁架梁,推导得到了空间桁架梁的等代抗弯刚度和等代抗剪刚度,并采用能量变分法对空间桁架梁的固有振动进行分析,给出空间桁架梁的竖向振动频率和振型的解析解。然后采用有限元软件ANSYS对几种不同算例进行模拟,通过模态分析得到空间桁架梁的频率与振型。将能量变分法求得的频率解析解与有限元分析求得的频率数值解进行对比,结果基本上是一致的,将两种方法得到的振型对比,结果吻合良好。该文所提出的能量变分解析解可供空间桁架梁的工程设计参考。     

Variational density matrix approach to the temperature-dependent elementary excitation spectrum of two-dimensional liquid4He

Using the variational density matrix method, we obtain a temperature-dependent elementary excitation spectrum for two-dimensional liquid⁴He. For more precise results, we use a Jastrow-Feenberg-type trial wave function and include the contribution of elementary diagrams within the hypernetted chain approximation. The behavior of the excitation spectrum as a function of the temperature and density in two dimensions is similar to that of the bulk system, but has a smaller roton minimum. The roton minimum of the excitation spectrum decreases with increasing temperature and increases with increasing density at low densities but decreases at large densities. The results agree well with Monte Carlo calculations and are closer than pevious theories to experimental measurements of⁴He film adsorbed on substrates.     

Genetic algorithm-guided deep learning of grain boundary diagrams: Addressing the challenge of five degrees of freedom

Grain boundaries (GBs) often control the processing and properties of polycrystalline materials. Here, potentially transformative research is represented by constructing GB property diagrams as functions of temperature and bulk composition, also called “complexion diagrams,” as a general materials science tool on par with phase diagrams. However, a GB has five macroscopic (crystallographic) degrees of freedom (DOFs). It is essentially a “mission impossible” to construct property diagrams for GBs as a function of five DOFs by either experiments or modeling. Herein, we combine isobaric semi-grand canonical ensemble hybrid Monte Carlo and molecular dynamics (hybrid MC/MD) simulations with a genetic algorithm (GA) and deep neural network (DNN) models to tackle this grand challenge. The DNN prediction is ∼10⁸ faster than atomistic simulations, thereby enabling the construction of the property diagrams for millions of distinctly different GBs of five DOFs. Notably, excellent prediction accuracies have been achieved for not only symmetric-tilt and twist GBs, but also asymmetric-tilt and mixed tilt-twist GBs; the latter are more complex and much less understood, but they are ubiquitous and often limit the performance properties of real polycrystals as the weak links. The data-driven prediction of GB properties as function of temperature, bulk composition, and five crystallographic DOFs (i.e., in a 7D space) opens a new paradigm.     

Determination of the initial condition in parabolic equations from integral observations

We study the problem of determining the initial condition in parabolic equations with time-dependent coefficients from integral observations which can be regarded as generalizations of point-wise interior observations. Our approach is new in the sense that for determining the initial condition we do not assume that the data available in the whole space domain at the final moment or in a subset of the space domain during a certain time interval, but some integral observations during a time interval. We propose a variational method in combination with Tikhonov regularization for solving the problem and then discretize it by finite difference splitting methods. The discretized minimization problem is solved by the conjugate gradient method and tested on computer to show its efficiency. Also as a by-product of the variational method, we propose a numerical scheme for estimating the degree of ill-posedness of the problem.     

Variational‐based modeling of micro‐electro‐elasticity with electric field‐driven and stress‐driven domain evolutions

Recently, increasing interest in so‐called functional or smart materials with electromechanical coupling has been shown such as ferroelectric piezoceramics. These materials are characterized by microstructural properties, which can be changed by external stress and electric field stimuli, and hence find use as the active components in sensors and actuators. The electromechanical coupling effects result from the existence and rearrangement of microstructural domains with uniformly oriented electric polarization. The understanding and efficient simulation of these highly nonlinear and dissipative mechanisms, which occur on the microscale of ferroelectric piezoceramics, are a key challenge of the current research. This paper does not offer a substantially new physical model of these phenomena but a new mathematical modeling approach based on a rigorous exploitation of rate‐type variational principles. This provides a new insight in the structure of the coupled problem, where the governing field equations appear as the Euler equations of a variational statement. We outline a variational‐based micro‐electro‐elastic model for the microstructural evolution of both electrically and mechanically driven electric domains in ferroelectric ceramics, which also incorporates the surrounding free space. To this end, we extend recently developed multifield incremental variational principles of electromechanics from local to gradient‐extended dissipative response and specialize it by a Ginzburg–Landau‐type phase field model, where the thickness of the domain walls enters the formulation as a length scale. This serves as a natural starting point for a canonical compact, symmetric finite element implementation, considering the mechanical displacement, the microscopic polarization, and the electric potential induced by the polarization as the primary fields. The latter is defined on both the solid domain and a surrounding free space. Numerical simulations treat domain wall motions for electric field‐driven and stress‐driven loading processes, including the expansion of the electric potential into the free space. Copyright © 2012 John Wiley & Sons, Ltd.     

Nonlocal variational theory and optimal control problems

This paper seeks to portray the nonlocal variational theory as a variational framework of sufficient generality to encompass a broad class of optimal control problems. A general constrained nonlocal variational problem is posed. For this problem, a sufficient condition for an absolute minimum is obtained in a simple manner. This condition yields a uniform formalism for solving a wide class of constrained variational problems. The resulting formalism is such that it embodies the nonlocal analogues of the well known Weyl-Hamilton-Jacobi and Lagrange formalisms. A similar formalism is then constructed to cover problems wherein the constraints are specified by a set of inequalities. The results of this paper subsume many of the recent results pertaining to sufficiency theory.The sufficient conditions developed are then specialized to a number of optimal control problems 1associated with distributed systems. The problems include certain unorthodox optimal control problems which have spurred a great deal of interest in recent years. The analysis shows that the recent results obtained in the optimal control theory by function space methods can also be obtained formally, in a more direct manner, by the usual tool of the calculus of variations.     

A SPACE–TIME FINITE ELEMENT METHOD FOR THE EXTERIOR STRUCTURAL ACOUSTICS PROBLEM: TIME-DEPENDENT RADIATION BOUNDARY CONDITIONS IN TWO SPACE DIMENSIONS

A time-discontinuous Galerkin space–time finite element method is formulated for the exterior structural acoustics problem in two space dimensions. The problem is posed over a bounded computational domain with local time-dependent radiation (absorbing) boundary conditions applied to the fluid truncation boundary. Absorbing boundary conditions are incorporated as ‘natural’ boundary conditions in the space–time variational equation, i.e. they are enforced weakly in both space and time. Following Bayliss and Turkel, time-dependent radiation boundary conditions for the two-dimensional wave equation are developed from an asymptotic approximation to the exact solution in the frequency domain expressed in negative powers of a non-dimensional wavenumber. In this paper, we undertake a brief development of the time-dependent radiation boundary conditions, establishing their relationship to the exact impedance (Dirichlet-to-Neumann map) for the acoustic fluid, and characterize their accuracy when implemented in our space–time finite element formulation for transient structural acoustics. Stability estimates are reported together with an analysis of the positive form of the matrix problem emanating from the space–time variational equations for the coupled fluid-structure system. Several numerical simulations of transient radiation and scattering in two space dimensions are presented to demonstrate the effectiveness of the space–time method.     

弹性接触问题的最小二乘混合变分形式

采用最小二乘法对弹性接触问题提出了一种新的混合变分形式，对该变分问题解的存在唯一性进行了论证。     

声学多普勒测速仪标校技术研究

为了提高声学多普勒测速仪输出速度的准确度,安装过程中测速基阵与载体之间的偏差角不可忽略,安装偏差角包括航向偏角,横摇角及纵摇角三类。介绍了一种三维空间上的多普勒标定技术,通过高精度的GPS导航仪以及多普勒测速仪对海底测速,利用速度比值差校准航偏角。通过纵向剖面的几何关系,从航偏角出发进而获得纵摇角和横摇角的大小,完成了三维方向上多普勒测速仪的校准,使多普勒测速仪坐标系与载体坐标系能够进行精确转换,从而提高了声学多普勒测速仪输出速度的准确度。外场试验较好地证明了该方法的有效性,分析结论看出在二维平面上,造成误差的原因主要在于安装偏角的航向偏角,而在三维空间上,尤其垂向速度,误差主要由纵摇角和横摇角产生。该方法可以快速地对三维安装偏角进行校准,运算量小,并且在对海水测速后续研究中可以形成一套体系。     

Element-free Galerkin methods

An element-free Galerkin method which is applicable to arbitrary shapes but requires only nodal data is applied to elasticity and heat conduction problems. In this method, moving least-squares interpolants are used to construct the trial and test functions for the variational principle (weak form); the dependent variable and its gradient are continuous in the entire domain. In contrast to an earlier formulation by Nayroles and coworkers, certain key differences are introduced in the implementation to increase its accuracy. The numerical examples in this paper show that with these modifications, the method does not exhibit any volumetric locking, the rate of convergence can exceed that of finite elements significantly and a high resolution of localized steep gradients can be achieved. The moving least-squares interpolants and the choices of the weight function are also discussed in this paper.     

基于异步消息记录(CMLA)的卷回恢复算法

为了最大限度地减少检测点设置和卷回恢复的时间，空间开销，提出了基于异步消息记录的卷回恢复算法，同以前的算法相比较，该算法一方面节省了用于进程之间同步的时间开销，另一方面检测点设置及卷回过程中只涉及少量的相关进程，并且减少了保存检测点的空间开销，文中对算法的正确性进行了证明，并通过模拟实验给出了算法的开销。     

Transient thermal stresses of work roll by coupled thermoelasticity

A numerical method, based on a two-dimensional plane strain model, is developed to predict the transient responses (that include distributions of temperature, thermal deformation, and thermal stress) of work roll during strip rolling by coupled thermoelasticity. The method consists of discretizing the space domain of the problem by finite element method first, and then treating the time domain by implicit time integration techniques. In order to avoid the difficulty in analysis due to relative movement between work roll and its thermal boundary, the energy equation is formulated with respect to a fixed Eulerian reference frame. The effect of thermoelastic coupling term, that is generally disregarded in strip rolling, can be considered and assessed. The influences of some important process parameters, such as rotational speed of the roll and intensity of heat flux, on transient solutions are also included and discussed. Furthermore, since the stress history at any point of the roll in both transient and steady state could be accurately evaluated, it is available to perform the analysis of thermal fatigue for the roll by means of previous data.     

Banach空间中-类非线性变分包含问题解的具混合误差项的Ishikawa迭代逼近

本文在实自反Banach空间的框架下,研究了一类具有Lispschitz条件的强增生型变分包含解的存在性、唯一性及其具有混合误差项的Ishikawa迭代程序的收敛性问题。在适当的条件下,证明了该迭代序列强收敛于变分包含问题的唯一解。其结果改进和推广了引文中相应的结果。     

Dynamics of sub-5-nm air-bearing sliders in the presence of electrostatic and intermolecular forces at the head-disk interface

A head-disk interface (HDI) model has been used to investigate the effect of the intermolecular and electrostatic forces on the static and dynamic performance of air-bearing sliders. It is seen that the intermolecular and electrostatic forces increase the level of flying height modulations at low flying heights, which in turn results in dynamic instability of the system similar to what has also been observed in experiments. In the analysis a stiffness matrix is defined to characterize the stability in the vertical, pitch, and roll directions. Fly height diagrams are used to examine the multiple equilibriums that exist for low flying sliders. In addition, the simulation results are compared with previously published experiments for validation.     

Influence of damage on the plastic instability of sheet metals under complex strain paths

During the sheet metal forming operation, internal damage occurs as a result of nucleation growth and coalescence of cavities around particles. This phenomenon limits the strains which can be achieved before the appearence of localized necking. In this paper, damage is represented by initially equi-axed cavities and a void growth model is extended and linearized for complex strain paths. For a given void distribution, a statistical study pointed out the existence of weak sections in the material leading to localized plastic flow. The influence of the physical parameters of voids on the forming limit diagrams is shown.     

基于卷积型原理的时空样条元

结构动力分析是工程设计中的重要组成部分,传统动力分析不能全面反映动力初值特征,而Gurtin变分原理则被认为是目前唯一能全面反映动力初值特征的变分原理.本文以Gurtin变分原理为基础,空间和时间上同时采用样条元,建立了计算梁的动力初值问题的样条元法。算例表明, 与现有的分析方法相比,本文具有更高的精度。