不确定系统控制的自稳定域方法的推广

基于已有的为构造不确定系统的连续反馈控制律而提出的自稳定域的理论,给出了推广的系统的自稳定域的概念及其构造方式的理论证明,使得零点成为系统自稳定域的内点,从而系统自稳定域不再仅限于第2、第4象限,保证了由此得到的系统在零点的连续性.基于这些理论结果,进一步给出了一般的2阶不确定系统控制的自稳定域以及相应的连续控制项的构造方法.并在文章最后给出了仿真计算的实例.     

面向PIM的三级域分解规则研究

域分解在PIM(Platform Independent Model)建模中具有先导作用。目前对域分解尚没有一个被业内普遍接受的方法,给PIM建模带来了不确定性,间接地提高了MDA(Model Driven Architecture)方法的应用成本。鉴于此,提出一种三级域分解规则,将域分解问题划分为三个层级,首先依据系统类型进行域分解,再通过BCE(Boundary Control Entity)模式进行二级分解,最后进行三级整合求精。通过六个不同类型系统的实例进行验证,提出域分解的评价指标。实例验证的结果表明,依据该规则产生的域分解结果具有迭代次数少、场景覆盖全的特点,能较好地满足PIM建模的需要。     

分布式环境下基于角色的访问控制

本文通过扩展权限的定义:给权限增加一个标示位来区别主体、访问的客体属于相同域和属于不同域时的权限,这样解决了采用对等角色时授予给非本域主体过大权限的问题.同时主体在访问非本域的客体时可以申请临时角色,这样避免了仅仅采用对等角色去访问非本域客体的简单化,更有利于最小权限的实现.通过这两点的改进,使基于角色的访问控制模型更加适合分布式访问控制的特点.     

鱼雷自导系统中目标数目的波束域检测方法

针对在鱼雷自导系统中应用波束域子空间类高分辨方法时，需要已知目标数目的问题，该文提出了波束域目标数目检测方法。该方法将模型阶数选择的信息理论准则推广到波束域处理。当波束形成矩阵满足各列正交时，可以由波束域协方差矩阵的特征值计算得到波束域信息理论准则。使波束域信息理论准则最小化的值即判定为目标数目的估计值。该方法直接利用子空间类方法特征分解的结果，计算量小。计算机仿真实验结果证实了该方法能够正确估计空间间隔较小的多个目标的数目。     

基于比例边界问断谱元法的无穷域亚音速圆柱绕流数值模拟

将比例边界坐标插值方法引入谱元法,构成比例边界谱单元;为了增加计算的稳定性,将节点布置在单元内部;用若干无限谱元离散计算域,用间断有限元方法对无穷域Euler方程亚音速圆柱绕流问题进行了数值模拟;计算结果的误差很小,显示了计算方法的可行性.     

域自适应学习研究进展

传统的机器学习假设测试样本和训练样本来自同一概率分布. 但当前很多学习场景下训练样本和测试样本可能来自不同的概率分布. 域自 适应学习能够有效地解决训练样本和测试样本概率分布不一致的学习问题,作为 机器学习新出现的研究领域在近几年受到了广泛的关注. 鉴于域自适应学习技术 的重要性,综述了域自适应学习的研究进展. 首先概述了域自适应学习的基本问 题,并总结了近几年出现的重要的域自适应学习方法. 接着介绍了近几年提出的 较为经典的域自适应学习理论和当下域自适应学习的热门研究方向,包括样例加 权域自适应学习、特征表示域自适应学习、参数和特征分解域自适应学习和多 源域自适应学习. 然后对域自适应学习进行了相关的理论分析,讨论了高效的度 量判据,并给出了相应的误差界. 接着对当前域自适应学习在算法、模型结构和 实际应用这三个方面的研究新进展进行了综述. 最后分别探讨了域自适应学习在 特征变换和假设、训练优化、模型和数据表示、NLP 研究中存在的问题这四个方面 的有待进一步解决的问题.     

面向专色印刷的色域匹配算法

针对专色印刷色域与图像色域不匹配的问题,提出一种色域映射算法.首先分析专色色域的几何特征,发现常规色域匹配方法不适用;进而对明度、色调和饱和度三属性分而治之,采用非线性压缩、灰度轴模拟和彩度裁剪实现色域匹配.实验结果表明,该算法获得的图像匹配结果在颜色连续性和视觉效果方面均有明显的提高.     

控制燃料受限下离散线性系统能控域的研究*

本文讨论控制燃料受限下离散线性系统能控域问题。文中籍助于矩量论方法与线性系统分解技巧得到了能控域的结构性定理,该定理除与系统的Jordan分解有关外,还与状态矩阵的特征值有关。此外,文中还给出了判别任给状态是否属于能控域的显式判别条件。这些结果在实际应用中是有用的。     

连续TS模糊控制系统稳定吸引域估计方法的进一步研究

研究了连续TS模糊控制系统稳定性判别条件与平衡点稳定吸引域估计方法的进一点改进。首先基于模糊李亚普诺夫函数,给出了连续TS模糊控制系统的稳定性充分条件;该条件更一步减小了判别的保守性。其次基于模糊李亚普诺夫函数,进一步改进了连续TS模糊控制系统渐近稳定平衡点稳定吸引域的估计方法;该方法简单实用,最大限度地估计稳定吸引域大小。实例验证了该方法的有效性和实用性。     

基于约束的可能性XML条件计算

将约束引入到可能性XML时,因分布节点值的不确定性,XML文档无法检验对涉及分布节点的约束的合法性。在一些场景中,要返回给用户符合约束的查询结果和概率,通常没有考虑去除不可能域分布来做条件计算,得出的并不是准确概率。为此,提出一种可能性XML的概念,给出可能性XML中可能域和约束的有效表达,通过提出的方案解决条件计算和准确概率计算的问题。实验结果表明,该算法的效率较高,条件计算后的结果更能被用户接受。     

Stokes外问题的一种重叠型区域分解法

本文讨论了平面无界区域上Stokes问题的重叠型区域分解法.利用混合元方法求解内子区域问题得到速度和压力,再用Poisson积分公式解出外子区域的速度和压力,如此交替迭代克服区域无界性并按原始变量求出原问题的数值解.根据投影理论证明重叠型区域分解法的几何收敛性.最后给出数值例子.     

On a method for direct numerical simulation of shear layer/compression wave interaction for aeroacoustic investigations

Direct numerical simulation (DNS) of a spatially developing mixing layer was performed. The compressible three-dimensional Navier-Stokes equations were solved for pressure, velocities and entropy for this flow using a compact finite-difference scheme of sixth-order accuracy in space, combined with Runge-Kutta three-step time advancement. On one of the transverse boundaries of the box-shaped domain, a compression wave profile was imposed in pressure and velocity components via a wave decomposition of the governing equations, in order to study the interaction of an isolated weak shock wave entering the domain with the mixed subsonic/supersonic shear layer. This flow situation is found along the shear layer of supersonic, imperfectly expanded jets containing a shock cell structure. In the present work, an isolated compression-expansion structure constitutes the model problem. The domain setup and the boundary conditions were chosen such as to allow analysis of the sound field generated by the turbulent flow and the shock-turbulence interaction. The numerical method used to impose the boundary conditions and solve the compressible Navier-Stokes equations, and the choice of numerical parameters, are described in detail. Some results on the two-dimensional and three-dimensional flow field computed are presented as well.     

Solution domain decomposition method for viscous incompressible flow

A solution domain decomposition method is developed for steady state solution of the biharmonic-based Navier-Stokes equations. It consists of a domain decomposition in conjunction with Chebyshev collocation for spatial discretization. The interactions between subdomains are effectively decoupled by means of a superposition of auxiliary solutions to yield a set of independent elementary problems which can be solved concurrently on multiprocessor computers. Assessments are carried out to a number of test problems including the two-dimensional steady flow in a driven square cavity. Illustrative examples indicate a good performance of the proposed methodology which does not affect the convergence and stability of the discretization scheme. Spectral accuracy is retained with absolute error decaying in an exponential fashion. The numerical solutions for the driven cavity compare favorably against previously published numerical results except for a slight overprediction in the vertical velocity component at Reynolds number of 400. TheC ³ continuity is speculated to be its cause.     

双螺杆挤出机流场数值模拟中流道进出口边界条件的探讨

在对双螺杆挤出机流场的数值模拟中，流道进出口边界条件的设置一直是一个颇具争议的问题。由于事先无法获得计算域进出口平面上的真实边界条件，研究人员在进行双螺杆挤出机的流场分析时，大都采用放松边界条件。为了考察放松边界条件对双螺杆挤出机流场数值模拟结果的影响，本文采用聚合物流动分析软件POLYFLOW，在流量恒定的前提下对双螺杆挤出机流道进出口给定三种不同分布形式的速度边界条件，对其流场进行了数值模拟。数值计算结果表明，在体积流量恒定的条件下，流道进出口不同分布形式的速度边界条件对流场的影响主要集中在进出口附近区域，但对离进出口边界较远的流场影响很小。一般而言，当计算域所对应的螺杆较长时，可以忽略流道进出口的放松边界条件所引起的误差；当计算域较短时，不宜直接采用放松边界条件，而应根据螺杆的实际构型．在计算域的进出口增加适当长度的发展段。     

Pressure bubbles stabilization features in the Stokes problem

The standard finite element approximation using equal-order-linear-continuous velocity–pressure variables is enriched with velocity and pressure bubble functions to model the Stokes problem. We show by static condensation that these bubble functions give rise to a stabilized method involving least-squares forms of the momentum and of the continuity equations. In particular, pressure bubbles play a key role in explaining the addition of the least-squares form of the continuity equation in a stabilized method for Stokes.     

Finite element solution for advection and natural convection flows

A new equal order velocity—pressure finite element procedure is presented for the calculation of 2-D viscous, incompressible flows of a recirculating nature. As in the finite difference procedures, velocity and pressure e uncoupled and the equations are solved one after the other. In this splitting-up method, an auxilary velocity field is computed first, which accounts for all contributions to the acceleration, except pressure, and satisfies the velocity boundary conditions. Then, the final velocities are evaluated by adding to the auxilary velocities pressure contributions which are computed to satisfy the continuity equation. The effectiveness is illustrated via example problems of 2-D advection and natural convection flows.     

A direct spectral domain decomposition method for the computation of rotating flows in a T-shape geometry

This paper presents a direct domain decomposition method, coupled with a Chebyshev collocation approximation, for solving the incompressible Navier-Stokes equations in the vorticity-streamfunction formulation. The method is based on the influence matrix technique used to treat the lack of vorticity boundary conditions on no-slip walls as well as to enforce the continuity conditions at the interfaces between adjacent subdomains. The multi-domain approach is proposed in order to extend the use of spectral approximations to non-rectangular geometries and singular solutions. It is applied to the computation of a four domain configuration, corresponding to a forced throughflow in a rotating channel-cavity system which is important in air cooling devices and cannot be modeled by single-domain spectral approximations.     

On a collocation B-spline method for the solution of the Navier–Stokes equations

A novel B-spline collocation method for the solution of the incompressible Navier–Stokes equations is presented. The discretization employs B-splines of maximum continuity, yielding schemes with high-resolution power. The Navier–Stokes equations are solved by using a fractional step method, where the projection step is considered as a Div–Grad problem, so that no pressure boundary conditions need to be prescribed. Pressure oscillations are prevented by introducing compatible B-spline bases for the velocity and pressure, yielding efficient schemes of arbitrary order of accuracy. The method is applied to two-dimensional benchmark flows, and mass lumping techniques for cost-effective computation of unsteady problems are discussed.     

Reflection and transmission of plane waves at an interface between two fluids

The propagation, reflection, and transmission of a plane wave through a column of two fluids with a material discontinuity is studied by three methods: a mixed finite element formulation with both pressure and velocity at a point taken as independent variables, and a scaled and an un-scaled acoustic pressure formulation in which only the pressure at a point is taken as an independent variable. It is found that when mass densities of two fluids are close to each other, the un-scaled acoustic pressure formulation gives reasonable results. However, when the speeds of sound in two fluids are close to each other but their mass densities are quite different, and for cases where the first fluid has high impedance relative to that of the second fluid, a mixed or scaled pressure formulation is necessary. Without the mixed or scaled pressure formulation, the continuity conditions at the interface between two fluids are not well satisfied for the un-scaled pressure formulation. The consideration of viscosity of the two fluids and using a dispersion correction method in the time integration scheme in the mixed formulation slightly improves results.     

Optimizing predictive models for leaf chlorophyll concentration based on continuous wavelet analysis of hyperspectral data

Recent studies have demonstrated that the decomposition of hyperspectral data using wavelet analysis is able to generate wavelet coefficients that can be used for estimating leaf chlorophyll (chl) concentrations. However, there is considerable scope for refining such techniques and this study addresses this issue by identifying the optimal spectral domain for use in constructing predictive models. Leaf reflectance spectra were simulated with the PROSPECT model (a model of leaf optical properties spectra) using randomly selected values for the input parameters. From reflectance and first derivative spectra different spectral wavelength domains were extracted, ranging from 400–450 to 400–2500 nm, using increments of 50 nm for the upper wavelength limit. Using the data for each wavelength domain, continuous wavelet decomposition was applied using 53 different wavelets, in turn. The resulting wavelet coefficients, from scales 1 to 128, were used as independent factors to construct predictive models for leaf chl concentration. Wavelet coefficients (at a specific scale generated by a given wavelet) in the chl absorption region remain constant when using spectral wavelength domains of 400–900 nm and broader, but narrower domains cause variability in the coefficients. Lower scale wavelet coefficients (scales 1–32) contain little information on chl concentration and their predictive performance does not vary with the spectral wavelength domain used. The higher scale wavelet coefficients (scales 64 and 128) can capture information on chl concentration, and predictive capability increases rapidly when the spectral wavelength domains vary from 400–700 to 400–900 nm but it can decrease or fluctuate for broader domains. In terms of accuracy and computational efficiency, models derived from the spectral wavelength domain 400–900 nm which use wavelet coefficients from scale 64 are optimal and a range of wavelet functions are suitable for performing the decomposition. The importance of optimizing the spectral wavelength domain highlighted by these findings has broader significance for the use of wavelet decomposition of hyperspectral data in quantifying other vegetation biochemicals and in other remote sensing applications.