后掠机翼低速流动转捩位置的升华法测量

报导了在西北工业大学低湍流度风洞中采用升华法测量后掠翼上表面流动转捩位置及其随攻角变化而改变情况,并参照后掠翼压力分布的实验结果进行分析.本文涉及的升华法,是根据边界层的层流区和湍流区流态差别来判断边界层转捩的一种测量方法.升华法的测量结果表明:在小攻角范围内,转捩位置逐渐提前,当达到某一攻角时,转捩快速接近前缘.这一结果和压力分布的实验结果相吻合,说明升华法能够较准确地显示转捩线位置.由于影响后掠翼转捩位置的因素较多,诸如边界层内横流的不稳定性、壁面的干扰、后掠翼的三维流动效应等,致使出现了攻角为4°时转捩位置呈现出一条斜线的现象.     

壁面温度在多参数下对平板边界层的影响研究

为了得到壁面温度在不同来流速度、不同湍流强度条件下对边界层转捩与减阻的影响规律,本文采用Transitionk-kl-ω模型对低来流速度下无压力梯度的光滑平板进行了数值模拟。结果表明,随着来流速度的升高,壁温升高所起到的减阻效果更好,即高来流速度对壁面温度更为敏感。当来流处于中高湍流强度下时,壁温升高能起到推迟转捩的作用,且随着湍流强度的升高,转捩推迟的效果越好,但减阻效果正好相反;当来流处于低湍流强度下时,壁温升高会使得转捩提前发生。壁温升高抑制了边界层内流体的脉动程度,使得层流的稳态不易被破坏,流动更加稳定;同时,壁温升高使得边界层内流体的速度梯度减小,从而降低了壁面摩擦系数,故壁温升高能起到推迟边界层转捩与减阻的作用。     

TRPIV MEASUREMENT OF DRAG-REDUCTION IN THE TURBULENT BOUNDARY LAYER OVER RIBLETS PLATE

采用高时间分辨率粒子图像测速技术对沟槽壁面平板湍流边界层速度矢量场的时间序列及其统计量进行了实验测量,讨论了在同一来流速度下沟槽壁面对平均速度剖面﹑雷诺切应力及湍流强度的影响. 用流向速度分量的多尺度空间局部平均结构函数辨识壁湍流多尺度相干结构,用条件采样和相位平均技术提取壁湍流多尺度相干结构喷射和扫掠事件的脉动速度、展向涡量的二维空间拓扑形态. 结果表明,与同材料光滑壁面对比,沟槽壁面实现了10.73%的摩阻减小量;沟槽壁面湍流边界层湍流强度及雷诺切应力皆比光滑平板湍流边界层对应统计量小,说明沟槽壁面有效降低了湍流边界层内流体的脉动. 通过比较壁湍流相干结构猝发事件各脉动速度分量与展向涡量的空间分布特征,肯定了沟槽壁面的减阻效果,发现沟槽壁面通过抑制相干结构猝发事件实现减阻.     

绕振荡水翼流动及其转捩特性的数值计算研究简

通过对比标准k-ω SST 湍流模型和基于标准k-ω SST 湍流模型修正的γ-Re_θ 转捩湍流模型对绕振荡NACA66 水翼流动的数值计算结果与实验结果,对水翼振荡过程的水动力特性和流场结构变化进行了分析研究. 结果表明：与标准k-ω SST 湍流模型的数值计算结果相比,基于标准k-ω SST 湍流模型修正的γ-Re_θ 转捩湍流模型能有效预测绕振荡翼型流场结构和水动力特性,捕捉流场边界层发生的流动分离和转捩现象;绕振荡水翼的流动过程可分为5 个特征阶段,当来流攻角较小时,在水翼前缘发生层流向湍流的转捩现象,水翼动力特征曲线出现变化拐点;随着来流攻角的增大,顺时针尾缘涡逐渐形成并向水翼前缘发展;当攻角较大时,前缘涡分离导致动力失速,水翼的动力特征曲线出现大幅波动;水翼处于顺时针向下旋转阶段,绕水翼的流动状态逐渐由湍流过渡为层流.     

锥体效应对超音速湍流边界层统计特性的影响

通过直接数值模拟,计算了空间模式下,来流马赫数为2.5, 半锥角为$5^{\circ}$, 零攻角的绝热钝锥湍流边界层,研究了湍流的统计特性,并把结果与超音速平板湍流边界层和马赫数为6的高超音速钝锥湍流边界层的结果进行了比较,重点定量地考察了锥体效应对边界层湍流统计特性的影响. 研究发现,锥体效应对平均温度剖面以及压缩性的影响是显著的;而其它统计量,比如速度壁面律、雷诺应力的分布和湍动能各项的贡献等,受锥体效应的影响很小.      

绕振荡水翼流动及其转捩特性的数值计算研究

通过对比标准k-ω SST 湍流模型和基于标准k-ω SST 湍流模型修正的γ-Re_θ 转捩湍流模型对绕振荡NACA66 水翼流动的数值计算结果与实验结果,对水翼振荡过程的水动力特性和流场结构变化进行了分析研究. 结果表明:与标准k-ω SST 湍流模型的数值计算结果相比,基于标准k-ω SST 湍流模型修正的γ-Re_θ 转捩湍流模型能有效预测绕振荡翼型流场结构和水动力特性,捕捉流场边界层发生的流动分离和转捩现象;绕振荡水翼的流动过程可分为5 个特征阶段,当来流攻角较小时,在水翼前缘发生层流向湍流的转捩现象,水翼动力特征曲线出现变化拐点;随着来流攻角的增大,顺时针尾缘涡逐渐形成并向水翼前缘发展;当攻角较大时,前缘涡分离导致动力失速,水翼的动力特征曲线出现大幅波动;水翼处于顺时针向下旋转阶段,绕水翼的流动状态逐渐由湍流过渡为层流.      

固体颗粒对沟槽湍流边界层影响的实验研究

本文采用粒子图像测速技术(particles image velocimetry, PIV)研究固体颗粒对放置在平板湍流边界层中的平壁和沟槽壁面减阻效果的影响. 实验对清水和加入粒径为155 μm聚苯乙烯颗粒的流法向二维速度场信息进行采集, 对不同工况下的平均速度剖面、雷诺应力和湍流度等统计量进行对比, 分析流体在边界层中的行为. 运用空间局部平均结构函数提取了不同工况湍流边界层喷射?扫掠行为的空间拓扑结构并进行比较. 结果发现, 在不同的壁面条件下, 粒子加入后的对数律区中无量纲速度均略大于清水组, 雷诺切应力有所降低, 湍流度有所减弱. 对于不同流场速度下的沟槽而言, 颗粒的加入均降低了壁面附近的阻力, 而颗粒单独作用于光滑壁面的减阻效果并不明显. 加入粒子后的相干结构数目有所增加, 法向脉动速度下降. 沟槽壁面附近的相干结构数目有所增加, 法向脉动速度在自由来流速度较大时有所上升, 在速度较小时有所下降. 这表明不同减阻状况下的沟槽均能将大涡破碎成更多的涡, 并且粒子的加入强化了这种破碎作用.      

激波/湍流边界层干扰压力脉动特性数值研究

激波/湍流边界层干扰问题广泛存在于高速飞行器内外流动中, 激波干扰会导致局部流场出现强压力脉动, 严重影响飞行器气动性能和飞行安全. 为了考察干扰区内脉动压力的统计特性, 对来流马赫数2.25, 激波角33.2°的入射激波与平板湍流边界层相互作用问题进行了直接数值模拟研究. 在对计算结果进行细致验证的基础上, 分析比较了干扰区外层和物面脉动压力的典型统计特征, 如脉动强度、功率谱密度、两点相关和时空关联特性等, 着重探讨了两者的差异及其原因. 研究发现, 激波干扰对外层和物面压力脉动的影响差异显著. 分离区内脉动以低频特征为主, 随后再附区外层压力脉动的峰值频率往高频区偏移, 而物面压力脉动的低频能量仍相对较高. 两点相关结果表明, 外层和物面脉动压力的展向关联性均明显强于其流向, 前者积分尺度过激波急剧增长随后缓慢衰减, 而后者积分尺度整体上呈现逐步增大趋势. 此外, 时空关联分析结果指出, 脉动压力关联系数等值线仍符合经典的椭圆形分布, 干扰区下游压力脉动对流速度将减小, 外层对流速度仍明显高于物面.      

主动射流控制水翼空化的数值模拟与分析

为了改善高速流动工况下水翼吸力面上流场的空化特性,提出了水翼表面主动射流对绕水翼周围流动加以控制的方法.基于密度分域滤波的FBDCM混合湍流模型联合Zwart-Gerber-Belamri空化模型,分析了来流空化数为0.83,来流攻角为8°,射流位置距水翼前缘为x=0.19c时,主动射流对于水翼吸力面上流动的空化特性和水动力特性影响.对回射流的强度进行了量化分析,以探究回射流与流场空化特性的关系.数值分析结果表明,在射流水翼吸力面上的时均空泡体积为原始水翼的1/15,使得流场内空化流动由云空化状态转变为较为稳定的片空化状态,显著地削弱了云空化的发展.此外,射流极大地改善了水翼的水动力性能,使得水翼的升阻比较原始水翼提高了22.9%,空泡的脱落频率减少了26.2%,空泡脱落所引起的振幅减小了9.1%.射流大幅降低了水翼吸力面上低压区面积,水翼吸力面上流体的逆向压力减小,回射流强度降低;同时,射流使水翼吸力面上的边界层减薄,增强了流动的抗逆压梯度能力,一定程度上阻挡了回射流向水翼前缘的流动,这也从机理上分析了主动射流抑制空化的原因.     

主动射流控制水翼空化的数值模拟与分析

为了改善高速流动工况下水翼吸力面上流场的空化特性,提出了水翼表面主动射流对绕水翼周围流动加以控制的方法.基于密度分域滤波的FBDCM混合湍流模型联合Zwart-Gerber-Belamri空化模型,分析了来流空化数为0.83,来流攻角为8°,射流位置距水翼前缘为x=0.19c时,主动射流对于水翼吸力面上流动的空化特性和水动力特性影响.对回射流的强度进行了量化分析,以探究回射流与流场空化特性的关系.数值分析结果表明,在射流水翼吸力面上的时均空泡体积为原始水翼的1/15,使得流场内空化流动由云空化状态转变为较为稳定的片空化状态,显著地削弱了云空化的发展.此外,射流极大地改善了水翼的水动力性能,使得水翼的升阻比较原始水翼提高了22.9%,空泡的脱落频率减少了26.2%,空泡脱落所引起的振幅减小了9.1%.射流大幅降低了水翼吸力面上低压区面积,水翼吸力面上流体的逆向压力减小,回射流强度降低;同时,射流使水翼吸力面上的边界层减薄,增强了流动的抗逆压梯度能力,一定程度上阻挡了回射流向水翼前缘的流动,这也从机理上分析了主动射流抑制空化的原因.      

Effect of the Streamwise Component of the Vorticity Formed in a Turbulent Jet Source on the Acoustic Characteristics of the Jet

The results of an experimental study of the effects of different nozzle heads on turbulent jet noise are analyzed. A configuration of four cylindrical heads, tabbed heads, and chevron nozzles are considered and the decreases in the acoustic-mechanical efficiency of the jet (acoustic power reduction) for jets exposed to different modes of action are compared.It is shown that the effects of tabbed and cylindrical heads, as well as of chevrons, share a common property which is associated with the occurrence of vorticity in the jet source and can be described on the basis of a unified criterion characterizing the action on both the jet flow structure and the jet noise.     

Noise Field in the Human Chest Due to Turbulent Flow in a Larger Blood Vessel

An acoustic model of a larger human blood vessel is developed. This model takes into account the main features of the acoustic generation and propagation of noise in the human chest from the source (turbulent pressure fluctuations in blood flow) to a receiver resting on the skin, and allows the consideration of a simple stenotic narrowing in the vessel. The low Mach number turbulent wall pressure models of Corcos, Chase, Ffowcs Williams, and Smol'yakov and Tkachenko are used to describe random sources in the vessel. The relationships obtained permit one to analyse the dependence of the resultant acoustic field in the thorax on the parameters of the blood flow and the vessel, and show the possibility of finding characteristic signs of the presence of a stenosis by comparison of noise fields from intact and diseased arteries. This revised version was published online in July 2006 with corrections to the Cover Date.     

A note on a new theory of turbulence

In this paper,the creation and annihilaton of turbulent eddies are described aselementary particles in the quantum field theory.An elementary particle may be consideredas a solid entity as it exists in quantum theory,but a turbulent eddy is often changed in sizeand shape with time due to its energy dissipation in a turbulent field.Therefore,in order toapply the method of the quantum field theory to the turbulent field by analogy,the entity ofthe same eddy should be defined firstly.According to the linearized theory,the turbulenteddies with the similarity character in lime duration may be considered as the entity of thesame eddy,and the creation and annihilation of turbulent eddies without the similarcharacters are related to the interaction term φ_2 in equation(2.6).Then,the creationoperator and annihilation operator similar to those in the quantum field theory are used todescribe the state of turbulent eddy field.Finally,a“Schrǒdinger”equation of turbulenteddies is formulated based upon the nonline     

Cloud dispersion models

A new technique has been developed to compute mean and fluctuating concentrations in complex turbulent flows. An initial distribution of material is discretised into any small clouds which are advected by a combination of the mean flow and large scale turbulence. The turbulence can be simulated either by Kinematic Simulation or by a stochastic model for the motion of each cloud centroid. The clouds also diffuse relative to their centroids; the statistics for this are obtained from a separate calculation of the growth of individual clouds in small scale turbulence, generated by Kinematic Simulation. The ensemble of discrete clouds is periodically rediscretised, to limit the size of the small clouds and prevent overlapping. The model is illustrated with simulations of dispersion in uniform flow and in a coastal flow, and the results are compared with analytic, steady state solutions where available.     

Numerical prediction of turbulent boundary layer noise from a sharp-edged flat plate

An efficient hybrid uncorrelated wall plane waves–boundary element method (UWPW-BEM) technique is proposed to predict the flow-induced noise from a structure in low Mach number turbulent flow. Reynolds-averaged Navier-Stokes equations are used to estimate the turbulent boundary layer parameters such as convective velocity, boundary layer thickness, and wall shear stress over the surface of the structure. The spectrum of the wall pressure fluctuations is evaluated from the turbulent boundary layer parameters and by using semi-empirical models from literature. The wall pressure field underneath the turbulent boundary layer is synthesized by realizations of uncorrelated wall plane waves (UWPW). An acoustic BEM solver is then employed to compute the acoustic pressure scattered by the structure from the synthesized wall pressure field. Finally, the acoustic response of the structure in turbulent flow is obtained as an ensemble average of the acoustic pressures due to all realizations of uncorrelated plane waves. To demonstrate the hybrid UWPW-BEM approach, the self-noise generated by a flat plate in turbulent flow with Reynolds number based on chord Re_c = 4.9 × 10⁵ is predicted. The results are compared with those obtained from a large eddy simulation (LES)-BEM technique as well as with experimental data from literature.     

DNS of Mixing and Reaction of Two Species in a Turbulent Channel Flow: A Validation of the Conditional Moment Closure

We consider the chemical reaction in a turbulent flow for the case that the time scale of turbulence and the time scale of the reaction are comparable. This process is complicated by the fact that the reaction takes place intermittently at those locations where the species are adequately mixed. This is known as spatial segregation. Several turbulence models have been proposed to take the effect of spatial segregation into account. Examples are the probability density function (PDF) and the conditional moment closure (CMC) models. The main advantage of these models is that they are able to parameterize the effects of turbulent mixing on the chemical reaction rate. As a price several new unknown terms appear in these models for which closure hypothesis must be supplied. Examples are the conditional dissipation 〈 χ ∣ φ 〉, the conditional diffusion 〈 κ ∇² φ ∣ u, φ 〉 and the conditional velocity 〈 u ∣ φ 〉. In the present study we investigate these unknown terms that appear in the PDF and CMC model by means of a direct numerical simulation (DNS) of a fully developed turbulent flow in a channel geometry. We present the results of two simulations in which a scalar is released from a continuous line source. In the first we consider turbulent mixing without chemical reaction and in the second we add a binary reaction. The results of our simulations agree very well with experimental data for the quantities on which information is available. Several closure hypotheses that have been proposed in the literature, are considered and validated with help of our simulation results. This revised version was published online in July 2006 with corrections to the Cover Date.     

关于太阳湍流发电机理论

本文介绍了太阳湍流发电机问题的双尺度平均场电动力学分析方法以及有关太阳发电机问题的研究结果:α,β效应及各种发电机模型。总结分析了现阶段太阳湍流发电机问题在观测和理论方面的主要困难。对其他的太阳发电机理论也进行了简要介绍。      

Large eddy simulation of turbulent statistical and transport properties in stably stratified flows

Three dimensional large eddy simulation （LES） is performed in the investigation of stably stratified turbulence with a sharp thermal interface. Main results are focused on the turbulent characteristic scale, statistical properties, transport properties, and temporal and spatial evolution of the scalar field. Results show that the buoyancy scale increases first, and then goes to a certain constant value. The stronger the mean shear, the larger the buoyancy scale. The overturning scale increases with the flow, and the mean shear improves the overturning scale. The flatness factor of temperature departs from the Gaussian distribution in a fairly large region, and its statistical properties are clearly different from those of the velocity fluctuations in strong stratified cases. Turbulent mixing starts from small scale motions, and then extends to large scale motions.     

表面活性剂减阻溶液湍流流动研究进展

与聚合物添加剂相比,表面活性剂具有寿命长,不受机械力和高温影响发生降解的特点,目前被认为是最具有适用价值的减阻添加剂．针对表面活性剂湍流减阻机理,尽管已经开展了大量的实验研究、理论分析和数值模拟,但仍处于探索阶段,尚未定论．文中对目前国内外有关表面活性剂减阻溶液湍流流动的研究如平均速度、湍流强度、雷诺应力和相关系数等湍流统计量以及取得的成果进行了归纳和总结．目前普遍认为阻力减小是由流动垂直方向的湍流强度受抑和脉动速度分量解耦导致雷诺应力极大降低引起的．分析了目前研究存在的问题,对今后继续开展研究提出了自己的观点．      

EXPERIMENTAL STUDY ON PROPERTIES OF TURBULENT/NON-TURBULENT INTERFACE OVER RIBLETS SURFACES AT LOW REYNOLDS NUMBERS1)

湍流/非湍流界面是流动中湍流和无旋流的边界,其相关研究在加深对湍流与无旋流之间的物质、动量和能量交换的理解有重要意义.本文采用时间解析的二维粒子图像测速技术,分别对零压梯度光滑、顺流向锯齿形沟槽表面平板在不同雷诺数下对湍流/非湍流界面的几何特征及动力学特性进行了实验研究.实验雷诺数为$Re_{\tau } =400\sim1000$.本文采用了湍动能准则对湍流/非湍流界面进行了识别,并分析界面高度分布、分形特征及界面附近的条件平均速度和涡量.结果表明在不同雷诺数下, 无论是光滑壁面还是沟槽壁面,界面平均高度在0.8 $\sim$ 0.9$\delta_{99} $附近. 对于沟槽壁面而言,减阻时对应的界面高度的概率密度分布与光滑壁面基本一致, 均遵循正态分布,而当阻力增大时, 界面高度分布偏离正态分布出现正的偏度. 在本实验情况下,界面分形维度、跨界面速度跳变均会随着雷诺数增大而增大. 此外,不同壁面情况下无量纲条件平均涡量在界面附近的分布相近,而界面附近无量纲速度梯度最大值近似为常数.