流面理论探讨

本文从三维流动的物理现象和偏微分方程组的理论分析出发,讨论了流面模型,流面方程组的类型和流面交叉迭代的收敛问题。指出在超音速条件下流面理论的局限性。      

低频微幅偏航与侧滑振动机翼的低速升力面理论

本文利用与机翼固连的坐标系,提出偏航振动机翼低速绕流的数学问题,对翼后蛇形尾涡作了仔细研究.在低频振动的情况下,利用非定常涡格法进行数值求解.本方法可以用来计算机翼因偏航与侧滑振动而引起的滚转力矩气动导数.某些导数的计算结果与实验结果作了比较.     

连续梁的有限差分解

1.基本理论1.1 差分概念 由文献[1]知,对于图1所示梁的挠曲线 y=f(x),一阶导数和二阶导数的差分表达式为     

具有圆锥型内外壁的叶栅内三元流动近似直接解

在作了回转流面为圆锥面的假定以后,得出了十分简洁的运动方程的形式,其中不包含沿流线的导数项,与目前常用的S_1、S_2流面运动方程比较,不仅使计算大大简化,而且在计算时,可同时计及气流径向,及回转方向的变化,使我们近似地得出了三元流场的直接解。由于忽略了叶栅内流线的径向波动.这只能是一种近似解法,特别适合于轴流式叶轮机械的叶栅内部计算。      

叶轮机械沿任意曲线运动方程的通用形式及其应用

本文从[1]提出的叶轮机械内气体流面的概念出发,推导出了叶轮机械内理想气体沿任意曲线运动方程的通用形式。由这一通用形式可以方便地得到各类流面计算问题的各种特殊形式。讨论了运动方程推导和应用中的若干问题。对于S_1流面为回转面及S_2流面为任意空间曲面的情况编写了电子计算机计算程序。在S_1流面的求解中提出了一种满足下游区周期性条件的简化处理方法,在S_2流面的求解中还给出了一种手算的近似方法。介绍了一些例题的计算结果。      

桥梁颤振气动导数在均匀流与紊流作用下的统一辨识理论研究

桥梁断面颤振气动导数的准确识别是桥梁空气动力稳定性研究的基础,一直是桥梁风工程研究领域的前沿课题。基于工况模态分析理论,本文首先导出了紊流条件下桥梁颤振气动导数辨识的完整方法;其次,将上述数据处理方式运用到由均匀流条件引起的自由衰减振动中,得出与紊流情况类似的结果。本文提出的方法能够用同样的辨识原理、同一个数据处理程序统一地对紊流和均匀流下的桥梁气动导数进行辨识;最后,利用数值仿真算例验证上述理论。     

An important fundamental concept of °uid mechanics

不可压缩流体是流体力学的一个重要基本概念,但在流体力学教学的过程中,不可压缩流体与不可压缩均质流体这两个概念经常被混淆,在一些流体力学的教科书中,这两个概念也常被混为一谈,其关键是对物质(随体)导数这个概念理解不正确. 本文对不可压缩流体与不可压缩均质流体这两个重要的流体力学概念进行了辨析,并以此为切入点,介绍了如何理解物质(随体)导数、时间导数、空间导数等概念的区别与关系,澄清了一些教科书中的错误论述.     

关于相对矢导数的研讨

关于相对矢导数的研讨蒲致祥（西北工业大学，西安７１００７２）在矢量力学分析中，矢量的相对导数（简称相对矢导数）有重要应用，但一般文献对它的阐述很简略和局限。为了对这一理论应用得正确和更有效，相关的一些概念和方法值得进一步研讨。基矢为ｅ＿ｓ（ｓ＝１，２...     

关于相对矢导数的研讨

关于相对矢导数的研讨蒲致祥（西北工业大学，西安７１００７２）在矢量力学分析中，矢量的相对导数（简称相对矢导数）有重要应用，但一般文献对它的阐述很简略和局限。为了对这一理论应用得正确和更有效，相关的一些概念和方法值得进一步研讨。基矢为ｅ＿ｓ（ｓ＝１，２...     

离面客观散斑法

本文提出了一种可同时获得面内位移和离面位移一阶导数的方法——离面客观散斑法,该法具有功能多、装置简单、无需隔振、对刚体位移不敏感等特点。文中论述了此法的基本原理,应用傅里叶光学理论和空间散斑的运动规律推导了计算公式,研制的表面处理技术操作简便,增强条纹反差效果十分明显,实验值与计算结果符合较好。     

Simulation of three-dimensional incompressible flows in generalized curvilinear coordinates using a high-order compact finite-difference lattice Boltzmann method

In the present study, a high-order compact finite-difference lattice Boltzmann method is applied for accurately computing 3-D incompressible flows in the generalized curvilinear coordinates to handle practical and realistic geometries with curved boundaries and nonuniform grids. The incompressible form of the 3-D nineteen discrete velocity lattice Boltzmann method is transformed into the generalized curvilinear coordinates. Herein, a fourth-order compact finite-difference scheme and a fourth-order Runge-Kutta scheme are used for the discretization of the spatial derivatives and the temporal term, respectively, in the resulting 3-D nineteen discrete velocity lattice Boltzmann equation to provide an accurate 3-D incompressible flow solver. A high-order spectral-type low-pass compact filtering technique is applied to have a stable solution. All boundary conditions are implemented based on the solution of the governing equations in the 3-D generalized curvilinear coordinates. Numerical solutions of different 3-D benchmark and practical incompressible flow problems are performed to demonstrate the accuracy and performance of the solution methodology presented. Herein, the 2-D cylindrical Couette flow, the decay of a 3-D double shear wave, the cubic lid-driven cavity flow with nonuniform grids, the flow through a square duct with 90° bend and the flow past a sphere at different flow conditions are considered for validating the present computations. Numerical results obtained show the accuracy and robustness of the present solution methodology based on the implementation of the high-order compact finite-difference lattice Boltzman method in the generalized curvilinear coordinates for solving 3-D incompressible flows over practical and realistic geometries.     

Analytical formulation of 2-D aeroelastic model in weak ground effect

This paper deals with the aeroelastic modeling and analysis of a 2-D oscillating airfoil in ground effect, elastically constrained by linear and torsional springs and immersed in an incompressible potential flow (typical section) at a finite distance from the ground. This work aims to extend Theodorsen theory, valid in an unbounded flow domain, to the case of weak ground effect, i.e., for clearances above half the airfoil chord. The key point is the determination of the aerodynamic loads, first in the frequency domain and then in the time domain, accounting for their dependence on the ground distance. The method of images is exploited in order to comply with the impermeability condition on the ground. The new integral equation in the unknown vortex distribution along the chord and the wake is solved using asymptotic expansions in the perturbation parameter defined as the inverse of the non-dimensional ground clearance of the airfoil. The mathematical model describing the aeroelastic system is transformed from the frequency domain into the time domain and then in a pure differential form using a finite-state aerodynamic approximation (augmented states). The typical section, which the developed theory is applied to, is obtained as a reduced model of a wing box finite element representation, thus allowing comparison with the corresponding aeroelastic analysis carried out by a commercial solver based on a 3-D lifting surface aerodynamic model. Stability (flutter margins) and response of the airfoil both in frequency and time domains are then investigated. In particular, within the developed theory, the solution of the Wagner problem can be directly achieved confirming an asymptotic trend of the aerodynamic coefficients toward the steady-state conditions different from that relative to the unbounded domain case. The dependence of flutter speed and the frequency response functions on ground clearance is highlighted, showing the usefulness of this approach in efficiently and robustly accounting for the presence of the ground when unsteady analysis of elastic lifting surfaces in weak ground effect is required.     

Turbulence microscale variation due to interaction with shock wave

A speckle photographic technique is used for visualizing the planar distribution of the refractive deflection angles of light transmitted through a compressible turbulent flow. Double exposure speckle photography is applied for the statistical analysis of such flows. Quantitative visualization of the planar distribution of the refractive deflection angles of the light transmitted through the compressible turbulent flow is performed. Turbulent structures are visible in the patterns of the deflection angles isolines. The 2-D correlation functions of these deflection angles are constructed and analyzed. A mathematical procedure for solving Erbeck-Merzkirch integral transformation is described and the 3-D density correlation functions are evaluated using 2-D correlation functions of deflection angles of the light passed through the turbulence. Received 14 November 1999 / Accepted 3 June 2000     

Asymptotic construction of Reissner-like composite plate theory with accurate strain recovery

The focus of this paper is to develop an asymptotically correct theory for composite laminated plates when each lamina exhibits monoclinic material symmetry. The development starts with formulation of the three-dimensional (3-D), anisotropic elasticity problem in which the deformation of the reference surface is expressed in terms of intrinsic two-dimensional (2-D) variables. The variational asymptotic method is then used to rigorously split this 3-D problem into a linear one-dimensional normal-line analysis and a nonlinear 2-D plate analysis accounting for classical as well as transverse shear deformation. The normal-line analysis provides a constitutive law between the generalized, 2-D strains and stress resultants as well as recovering relations to approximately but accurately express the 3-D displacement, strain and stress fields in terms of plate variables calculated in the plate analysis. It is known that more than one theory may exist that is asymptotically correct to a given order. This nonuniqueness is used to cast a strain energy functional that is asymptotically correct through the second order into a simple “Reissner-like” plate theory. Although it is not possible in general to construct an asymptotically correct Reissner-like composite plate theory, an optimization procedure is used to drive the present theory as close to being asymptotically correct as possible while maintaining the beauty of the Reissner-like formulation. Numerical results are presented to compare with the exact solution as well as a previous similar yet very different theory. The present theory has excellent agreement with the previous theory and exact results.     

Boundary Integral Procedures for Unsaturated Flow Problems

Abstract. A novel numerical scheme based on the singular integral theory of the boundary element method. (BEM) is presented for the solution of transient unsaturated flow in porous media. The effort in the present paper is directed in facilitating the application of the boundary integral theory to the solution of the highly non-linear equations that govern unsaturated flow. The resulting algorithm known as the Green element method (GEM) presents a robust attractive method in the state-of -the-art application of the boundary element methodology. Three GEM models based on their different methods of handling the non-linear diffusivity, illustrate the suitability and robustness of this approach for solving highly non-linear 1-D and 2-D flows which would have proved cumbersome or too difficult to implement with the classical BEM approach.     

Computation of 3-D Turbulent Boundary Layers Using the V2F Model

The V2F model makes use of the standard k– model, but extends it by incorporating near-wall turbulence anisotropy and non-local pressure-strain effects, while retaining a linear eddy viscosity assumption. It has the attraction of fewer equations and more numerical robustness than Reynolds stress models. The model is presented in a form that is completely independent of distance to the wall. This formalism is well suited to complex, 3-D, multi-zone configurations. It has been applied to the computation of two complex 3-D turbulent flows: the infinitely swept bump and the appendage-body junction; some preliminary results for the flow in a U-bend are also presented. Despite the use of a linear, eddy viscosity formula, the V2F model is shown to provide excellent predictions of mean flow quantities. The appendage-body test case involves very complex features, such as a 3-D separation and a horseshoe vortex. The V2F simulations have been shown to successfully reproduce these features, both qualitatively and quantitatively. The calculation of the complex flow structure inside and downstream of the U-bend also shows very promising results.     

The general stress strain relation of soils involving the rotation of principal stress axes

1.IntroductionThestudyofsoils'constitutiverelationismostlylimitedinsuchStressstatethattherotationofprincipalstressaxesisabletobeignored.Soonlythevalueofprincipalstressneedtobeconsideredandthedirectionofprincipal'stressisregardedtobeunchangeable.Onthiscondition,theprincipalaxesof'stressincrement,strainincrementjstressandstrainarethesame.Therelevantexperimentsandengineeringsshowthattherotationofprincipalstressaxeswillgeneratesignificantplasticdeformationandthenoncoaxalityofstressandstrain.First…     

Reduction of dimensions in random,elastic soil medium

The paper deals with an application of the plane strain analysis in a stochastic three-dimensional soil medium. In a framework of random elasticity theory, the geostatical state of stresses and the problem of a unit force acting in a statistically homogeneous half-space are considered. Only the modulus of elasticity is considered to be random and is modelled as a three-dimensional (3-D) homogeneous random field. As the result of imposed constrains due to the plane strain assumption the additional body and surface forces are induced. In order to determine them, additional equations must be introduced. The equations in a form of constrain relations are proposed in this paper. These equations are also valid for a case of uniformly distributed external loading.First, the two-dimensional (2-D) problem and its reduction to the uni-axial strain state, for the gravity forces and uniform, unlimited surface loading is considered. Then, it is generalised into a 2-D schematization of the 3-D state. Next, the problem of a unit force acting in a statistically homogeneous half-space is considered. For a 3-D state of stress and strain the resulting stresses are compared with those for a 2-D state. These stresses for the multidimensional state of strain and stress are presented as a sum of two components. The first one reflects plane strain state stresses and is given in a form of a 3-D random field. This term allows for incorporating a spatial, 3-D soil variability into a two-dimensional analysis. The second component can be treated as a correction term and it represents the longitudinal influence of a 3-D analysis.Some numerical results are presented in this paper. The proposed method can be regarded as a framework for further research aiming at application to a variety of geotechnical problems, for which the plane strain state is assumed.