人工扰动信号在湍流边界层中的衰减

将人工扰动引入湍流流场,使用功率谱分析方法,研究边界层外层的较大频率范围内的人工扰动信号沿流向和法向的衰减,获得了人工扰动在湍流边界层中的衰减趋势．     

有序波状扰动对壁湍流相干结构的作用

本文在湍流边界层外层引入了正弦波状扰动;实验结果表明扰动波波幅沿流向是衰减的,衰减率与 Landahl(1967)的线性理论结果定性一致。本文发现在扰动波沿流向的演化过程中,出现以扰动波频率为基频的高次谐波。外层单一频率的扰动会减小内层的猝发平均周期,影响内层的流动结构。     

应用共轴型二维激光测速系统测量孔板管流的湍流特性

本文发表了一种共轴型二维激光测速系统,可同时测量由三束入射光组成的平面内的二维速度分量。讨论了主要的测量误差并提出了一种修正共轴分量角度偏差的方法。应用该系统详细测量了单孔板和双孔板管流的轴向和径向平均速度。湍流度和雷诺切应力分布,表明来流条件对孔板下游的湍流特性有强烈影响。     

吹吸扰动对壁湍流相干结构的影响

采用热线风速仪测量受吹吸扰动的壁湍流边界层的流向速度,用傅里叶变换和子波变换研究吹吸扰动对壁湍流能谱的影响,结果显示施加的低频扰动使边界层内层大尺度结构的能量减少,小尺度结构的能量有所增强,远离壁面时扰动强度逐步衰减直到在外层中消失;通过VITA法和子波变换法检测猝发事件,表明该扰动降低了猝发强度,使猝发周期延长,条件平均速度波形的幅值降低、持续时间变短,说明扰动明显抑制了相干结构的猝发过程. 利用子波变换可以实现湍谱分析,能有效检测猝发中的湍流结构,是一种客观的分析工具.     

大动脉中血液流动的中性扰动

本文在长波假定下用流体力学线性稳定性理论对大动脉中血液流动求出了一种中性扰动。结果表明，心室或瓣引起的扰动在一定条件下有可能量一种中性扰动，它可以沿着动脉管无变形地传播到动脉远端。这说明也许有可能提供一种通过在远离心脏的某些浅表动脉部位检测这种中性成动以了解心脏的某些功能的新方法。     

管内湍流旋流的数值计算

本文用有限差分法对直管内的湍流旋流进行了数值模拟。计算中采用Ｂｏｕｓｓｉｎｅｓｑ湍流涡粘性假设的基本思想和Ｋ－ε双方程模型来求解雷诺应力各分量。为了反映旋流中湍流转输的非均匀性和各向异性特征，对雷诺应力各分量及与之相主尖的各湍流粘性系数分别进行计算。计算结果表明该模型能较好地反映直管内湍流旋流的流动结构。     

气固两相流动的湍流脉动扩散数学模型及其应用

本文提出了气固两相流动的湍流扩展数学模型，本模型用k-ε双方程模型求解气相湍流场，并根据气流脉动的频谱、能谱曲线提出了随机富工级数来模拟气相脉动速度，用拉氏方法描述颗粒的运动，故称为脉动频谱随机颗粒轨道模型。本文还给出了本模型在气固多相射流和流化床内应用的实例。     

PANS模型在空化湍流数值计算中的应用

采用一种基于标准k-ε模型改进的局部时均化模型（Partially-Averaged Navier-Stokes Model,PANS）,并应用于空化流动计算。控制不同的模型参数,分别对绕平头轴对称回转体和Clark-Y型水翼的空化流动进行模拟,并与实验结果进行对比。结果表明：PANS模型中未分解湍动能比率fk的取值对预测空化流动的数值计算精度有重要影响,改变fk的取值可实现对不同滤波尺度范围内的求解;随着fk值的减小PANS的预测精度逐步提高,能在相对较大范围内求解较小尺度的湍流运动过程中,预测到湍流运动中强烈的非定常特性;同时可以比较准确地预测空化流场结构和动力特性。     

含活塞的管内湍流的数值模拟

应用两方程模式和 SIMPLER 算法对含活塞的管内复杂湍流进行了数值模拟.活塞直径和外管内径之比为0.8,雷诺数为6.05×10~4.数值模拟得到了三个回流区.下游回流的分离-再附长度是活塞直径的1.5倍.间隙段存在流速超越现象,但不存在后台阶流动计算中的核心区现象.文中给出了数值模拟的详细成果.     

轴流压气机失速初始扰动的研究进展

在失速主动控制思想的推动下,对轴流压气机失速初始扰动的研究一直是叶轮机械非定常流领域的热点问题之一,同时也是一个尚未认识清楚的难点问题. 本文从失速初始扰动的理论模型、实验研究和机理分析3个方面对轴流压气机失速初始扰动的研究进展进行了回顾.对目前失速初始扰动研究中的集中问题,如低速和高速环境下初始扰动的试验检测方法, 初始扰动类型,以及初始扰动出现的内在流体动力学机理等问题进行了总结. 在此基础上,讨论了叶尖区域的复杂流动特性和失速初始扰动的内在联系,并指出了失速初始扰动研究的发展趋势, 认为今后应进一步深化如下问题:初始扰动形式影响因素的系统研究, 高速压气机中初始扰动新形式研究,初始扰动产生的流体动力学物理机制及其与压气机的设计和运行参数之间的关联性研究.      

Effect of the pattern of initial perturbations on the steady pipe flow regime

The influence of the inlet flow formation mode on the steady flow regime in a circular pipe has been investigated experimentally. For a given inlet flow formation mode the Reynolds number Re* at which the transition from laminar to turbulent steady flow occurred was determined. With decrease in the Reynolds number the difference between the resistance coefficients for laminar and turbulent flows decreases. At a Reynolds number approximately equal to 1000 the resistance coefficients calculated from the Hagen-Poiseuille formula for laminar steady flow and from the Prandtl formula for turbulent steady flow are equal. Therefore, we may assume that at Re > 1000 steady pipe flow can only be laminar and in this case it is meaningless to speak of a transition from one steady pipe flow regime to the other. The previously published results [1–9] show that the Reynolds number at which laminar goes over into turbulent steady flow decreases with increase in the intensity of the inlet pulsations. However, at the highest inlet pulsation intensities realized experimentally, turbulent flow was observed only at Reynolds numbers higher than a certain value, which in different experiments varied over the range 1900–2320 [10]. In spite of this scatter, it has been assumed that in the experiments a so-called lower critical Reynolds number was determined, such that at higher Reynolds numbers turbulent flow can be observed and at lower Reynolds numbers for any inlet perturbations only steady laminar flow can be realized. In contrast to the lower critical Reynolds number, the Re* values obtained in the present study, were determined for given (not arbitrary) inlet flow formation modes. In this study, it is experimentally shown that the Re* values depend not only on the pipe inlet pulsation intensity but also on the pulsation flow pattern. This result suggests that in the previous experiments the Re* values were determined and that their scatter is related with the different pulsation flow patterns at the pipe inlet. The experimental data so far obtained are insufficient either to determine the lower critical Reynolds number or even to assert that this number exists for a pipe at all.     

Degradation effects of dilute polymer solutions on turbulent drag reduction in pipe flows

An important practical problem in the application and study of drag reduction by polymer additives is the degradation of the polymer, for instance due to intense shearing, especially in recirculatory flow systems. Such degradation leads to a marked loss of the drag-reducing capability of the polymer.Three different polymer types were tested on degradation effects in a closed pipe flow system. The polymers used were Polyox WSR-301, Separan AP-273 and Superfloc A-110, dissolved in water in concentrations of 20 wppm each. The flow system consisted of a 16.3 mm pipe of 4.25 m length. Two different pumps were used: a centrifugal pump and a disc pump. Different solution-preparation procedures were tried and the experiments were performed at different flow rates.Superfloc A-110 proved to be both the most effective drag reducer and most resistant to degradation. Because of very fast degradation, Polyox WSR-301 was found to be unsuitable for being used as a drag reducer in re-circulatory systems. The disc pump proved to be much better suited for pumping the polymer solutions than the centrifugal pump. The degradation curve of the combination Superfloc/disc pump showed a plateau-like region with reasonable drag reduction, which makes it possible to perform (laser Doppler) measurements under nearly constant circumstances during a sufficient time.     

湍流管径问题算法的改进

本文采服无量纲参数（ｇＪＯ＾３／ｖ＾５）及ＫｓＶ／Ｑ）由最小二乘法以指数函数拟合Ｐｒａｎｄｔｌ及Ｋａｒｍａｎ公式，得出分别适用于光滑区和粗糙区的管径算式。再对以上两式作粘性关联偶合，得出适用于湍流过渡区的管径算式。可直接求出管径，避免繁复试算和迭代。计算结果与Ｐｒａｎｄｔｌ，Ｋａｒｍａｎ及Ｃｏｌｅｌｒｏｏｋ－Ｗｈｉｔｅ等经典公式能精确吻合，较Ｓｗａｍｅｅ－Ｊａｉｎ湍流管径算式误差减少一半以上。     

圆管过渡流态的间歇因子

用激光测速方法研究圆管流动的湍流间歇现象．实验表明,间歇湍流首先在管壁发生,逐渐向下游扩张．随着雷诺数的增加,间歇因子γ＝０．５的转捩界面逐渐向入口推移,一直到Ｒｅ＝９８８７,整个管流才变成间歇湍流和充分湍流     

Experiments in Turbulent Pipe Flow with Polymer Additives at Maximum Drag Reduction

In this paper we report on (two-component) LDV experiments in a fully developed turbulent pipe flow with a drag-reducing polymer (partially hydrolyzed polyacrylamide) dissolved in water. The Reynolds number based on the mean velocity, the pipe diameter and the local viscosity at the wall is approximately 10000. We have used polymer solutions with three different concentrations which have been chosen such that maximum drag reduction occurs. The amount of drag reduction found is 60–70%. Our experimental results are compared with results obtained with water and with a very dilute solution which exhibits only a small amount of drag reduction. We have focused on the observation of turbulence statistics (mean velocities and turbulence intensities) and on the various contributions to the total shear stress. The latter consists of a turbulent, a solvent (viscous) and a polymeric part. The polymers are found to contribute significantly to the total stress. With respect to the mean velocity profile we find a thickening of the buffer layer and an increase in the slope of the logarithmic profile. With respect to the turbulence statistics we find for the streamwise velocity fluctuations an increase of the root mean square at low polymer concentration but a return to values comparable to those for water at higher concentrations. The root mean square of the normal velocity fluctuations shows a strong decrease. Also the Reynolds (turbulent) shear stress and the correlation coefficient between the stream wise and the normal components are drastically reduced over the entire pipe diameter. In all cases the Reynolds stress stays definitely non-zero at maximum drag reduction. The consequence of the drop of the Reynolds stress is a large polymer stress, which can be 60% of the total stress. The kinetic-energy balance of the mean flow shows a large transfer of energy directly to the polymers instead of the route by turbulence. The kinetic energy of the turbulence suggests a possibly negative polymeric dissipation of turbulent energy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Analysis of Streamwise Velocity Fluctuations in Turbulent Pipe Flow with the Use of an Ultrasonic Doppler Flowmeter

Data collected from several studies of experimental and numerical nature in wall-bounded turbulent flows and in particular in internal flows (channel and pipe flows, Mochizuki and Nieuwstadt [1]) at different Reynolds numbers R ⁺(Ru _*/ν), indicate that: (i) the peak of the rms-value (normalized by u _*) of the streamwise velocity fluctuations (σ _u ⁺|_peak) is essentially independent of the Reynolds number, (ii) the position of the rms peak value (y ⁺|_peak) is weakly dependent of the Reynolds number, (iii) the skewness of the streamwise velocity fluctuations (S _u ) is close to zero at the position in which the variance has its peak. A series of measurements of streamwise velocity fluctuations has been performed in turbulent pipe flow with the use of an Ultrasonic Doppler Velocimeter and our results support those reported in [1]. This revised version was published online in July 2006 with corrections to the Cover Date.     

Direct numerical simulation of turbulent pipe flow

Fully developed turbulent pipe flow at low Re-number is studied by means of direct numerical simulation (DNS). In contrast to many previous DNS's of turbulent flows in rectangular geometries, the present DNS code, developed for a cylindrical geometry, is based on the finite volume technique rather than being based on a spectral method. The statistical results are compared with experimental data obtained with two different experimental techniques. The agreement between numerical and experimental results is found to be good which indicates that the present DNS code is suited for this kind of numerical simulations.