北京大学湍流国家重点实验室介绍

北京大学湍流研究国家重点实验室是在我国著名科学家、国际湍流模式理论奠基人周培源先生的支持与倡导下,于1995年底通过科技部验收而正式成立的．实验室成立以来,继承和发扬了长期积累的学科优势,在科学研究、基础建设、合作交流等方面精耕细作,目前已经成为国内湍流研究的中心,并在国际湍流界具有一定影响．实验室顺应非线性科学与复杂系统科学研究的不断深入,将湍流问题逐步纳入到复杂系统这一更高层次的研究领域中去,2001年经科技部批准,实验室更名为“湍流与复杂系统国家重点实验室”．     

OPERATIONAL MODAL ANALYSIS SCHEMES USING CORRELATION TECHNIQUE

For some large-scale engineering structures in operating conditions, modal parameters estimation must base itself on response-only data. This problem has received a considerable amount of attention in the past few years. It is well known that the cross-correlation function between the measured responses is a sum of complex exponential functions of the same form as the impulse response function of the original system. So this paper presents a time-domain operating modal identification global scheme and a frequency-domain scheme from output-only by coupling the cross-correlation function with conventional modal parameter estimation. The outlined techniques are applied to an airplane model to estimate modal parameters from response-only data.     

A MIXED MODE FRACTURE CRITERION BASED ON THE MAXIMUM TANGENTIAL STRESS IN BRITTLE INCLUSION

A closed-form solution for predicting the tangential stress of an inclusion located in mixed mode Ⅰ and Ⅱ crack tip field was developed based on the Eshelby equivalent inclusion theory. Then a mixed mode fracture criterion, including the fracture direction and the critical load, was established based on the maximum tangential stress in the inclusion for brittle inclusioninduced fracture materials. The proposed fracture criterion is a function of the inclusion fracture stress, its size and volume fraction, as well as the elastic constants of the inclusion and the matrix material. The present criterion will reduce to the conventional one as the inclusion having the same elastic behavior as the matrix material. The proposed solutions are in good agreement with detailed finite element analysis and measurement.     

FATIGUE GROWTH MODELING OF MIXED-MODE CRACK IN PLANE ELASTIC MEDIA

This paper presents an extension of a displacement discontinuity method with cracktip elements （a boundary element method） proposed by the author for fatigue crack growth analysis in plane elastic media under mixed-mode conditions. The boundary element method consists of the non-singular displacement discontinuity elements presented by Crouch and Starfield and the crack-tip displacement discontinuity elements due to the author. In the boundary element implementation the left or right crack-tip element is placed locally at the corresponding left or right crack tip on top of the non-singular displacement discontinuity elements that cover the entire crack surface and the other boundaries. Crack growth is simulated with an incremental crack extension analysis based on the maximum circumferential stress criterion. In the numerical simulation, for each increment of crack extension, remeshing of existing boundaries is not required because of an intrinsic feature of the numerical approach. Crack growth is modeled by adding new boundary elements on the incremental crack extension to the previous crack boundaries. At the same time, the element characteristics of some related elements are adjusted according to the manner in which the boundary element method is implemented. As an example, the fatigue growth process of cracks emanating from a circular hole in a plane elastic plate is simulated using the numerical simulation approach.     

迷宫密封湍流增阻的数值研究

迷宫密封是依靠节流间隙中的节流过程和迷宫空腔内的动能耗散过程来实现密封的,这两个流体流动过程从本质上规定了迷宫密封的密封性能。通过密封内部流动本质的研究,合理利用湍流流动的规律和特点,能够发挥密封内部湍流的增阻作用,提高密封性能。本文数值计算了一种锯齿型径向迷宫密封的内部流动,得到了密封内部的流动形态,揭示了密封内部流动与密封性能的关系。结果表示,密封存在一个最佳的齿插入长度。在最佳齿插入长度时,尽管节流间隙较大,但由于节流间隙内流动收缩和偏转,间隙的有效通流面积却很小,泄漏介质可以在间隙中获得较大的速度,在迷宫空腔内形成正确的耗散涡流,密封具有最佳的密封效果。所以,充分发挥湍流增阻的作用,可以突破节流间隙的微小尺寸限制,实现较大间隙下的良好密封。这不仅会给迷官密封的制造、安装及运行等带来很大的方便,而且可以为发展新的迷宫密封技术奠定理论基础。     

Particle behavior in homogeneous isotropic turbulence

Direct numerical simulations were conducted to investigate the behavior of heavy particles in homogeneous isotropic turbulence. The present study focused on the effect of particle inertia and drift on the autocorrelations of the particle velocity and the fluid seen by particles and the dispersion characteristics of particles. The Lagrangian integral time scale of particles monotonically increased as the magnitude of the particle response time increased, while that of the fluid seen by particles remained relatively constant; it reached a maximum when the particle response time was close to the Kolmolgorov time scale of the flow. Particle dispersion increased as the particle inertia increased for small particles, while for larger particles, it decreased as particle inertia increased; particle eddy diffusion coefficient was maximal, and greater than that of the fluid by about 30%, at the preferential concentration. The concentration field of the particles with τp/τk≈1.0 showed that particles tend to collect in regions of low vorticity (high strain) due to preferential concentration. As the drift velocity of a particle is increased it crosses the paths of fluid elements more rapidly and will tend to lose correlation with its previous velocity faster than a fluid element will. And the correlation of particle velocities along the drift direction is more persistent than that perpendicular to the direction of drift. Simulations also showed that the continuity effect and the crossing-trajectory effect are weakened for particles with infinite inertia.     

有压管道双孔板水流流场的数值模拟

利用数值计算的方法对多级孔板消能器内湍流流场进行模拟，并将所得的结果与实验进行了比较。这些比较表明，数值计算的方法是可靠的。     

复杂气固两相系统的微观结构

流化床中的气固两相流动是一个高度复杂的非线性混沌系统。本文利用激光粒子动态分析仪（ＰＤＡ）得到的循环流化床中颗粒脉动速度信号,采用ＦＦＴ分析了脉动信号的宽频谱特征,在此基础上应用小波法分析了脉动信号的动态特征,得到了颗粒脉动速度的微观结构,指出颗粒脉动速度的非线性特性是流化床具有混沌特性的根源,且在不同的尺度上颗粒脉动速度表现出各向异性的特征。     

部分预混超声速燃烧火焰面模式研究综述

采用部分预混火焰面模式建模与仿真已经成为超声速燃烧数值研究的主要趋势之一.系统回顾了超声速燃烧火焰面模式的发展历程,针对其应用的两个基本问题进行总结:一是火焰面模式在超声速燃烧中的物理存在问题,二是超声速燃烧火焰面模式的建模问题.重点分析了火焰面模式应用于超声速燃烧的难点,提出了超声速燃烧火焰面模式建模应兼顾的问题.     

Experimental study on correlation between turbulence and sound in a subsonic wind tunnel

In this paper, the effects of turbulence on sound generation and velocity fluctuations due to pressure waves in a large subsonic wind tunnel are studied. A trip strip located at different positions in the contraction part or at one position in the diffuser of a large wind tunnel is used to investigate the aforementioned phenomenon, and the results indicate that the trip strip has significant effects on sound reduction. The lowest turbulence intensity and sound are obtained from a trip strip with a diameter of 0.91 mm located either at X/L = 0.79 or at X/L = 0.115 in the wide portion of the contraction. Furthermore, the effect of monopole, dipole and quadrupole sources of aerodynamic noise at different velocities is investigated, and it is demonstrated that the contribution of the monopole is dominant, while the shares due to the dipole and quadrupole remain less important. In addition, it is found that the sound waves have a modest impact on the measured longitudinal turbulence and are generated essentially by eddies.