基于卷积神经网络的钝体尾迹识别研究

针对相同特征长度不同钝体的尾迹结构相近,肉眼难于分辨的问题,提出了一种基于卷积神经网络的钝体尾迹识别方法,并在竖直肥皂膜水洞的典型钝体模型尾迹实验中获得高准确率验证.实验平台由自建竖直肥皂膜实验装置、钝体模型(方柱、圆柱和三角柱)及图像采集系统组成,可基于光学干涉法实现对不同速度下钝体肥皂膜尾迹的高清持续拍摄.所建立卷...     

轴对称钝物体的尾涡结构

本文对于圆球和70°角斜切尾钝头旋成体的尾涡结构作了流态显示和部分热线测量,流态显示结果表明,这类钝物体尾迹中的三维尾涡结构均呈现为一列相互锁定的“涡链”状结构,每一个涡元素自身并不闭合,呈“发卡”形,并与后继的涡相连接,对于70°斜切尾钝头旋成体模型,由于其斜切尾有固定三维尾涡面卷起和脱落方位的作用,故呈现出规则的三维尾涡结构。而圆球模型,由于涡面卷起和涡脱落的方位是随机旋转的,因此“涡链”呈现为被任意扭曲的千姿百态,流态照片各不相同,圆球尾迹的这种既有周期性又有随机旋转的特性,在热线测量的结果中也有所反映。     

微型开缝钝体燃烧器燃烧特性数值分析

本文将开缝钝体稳燃技术应用于微型燃烧器中,采用详细化学反应机理模拟了不同速度下微型开缝钝体燃烧器与微型常规钝体燃烧器的燃烧情况.结果表明:开缝钝体燃烧器火焰宽度一致性较好,火焰中心温度沿轴向分布更加均匀,尤其在速度较大时,开缝钝体燃烧器优势更加明显;开缝钝体燃烧器燃烧效率高于常规钝体燃烧器,速度大于25m/s时,开缝钝体燃烧器效率高出常规钝体燃烧器5%左右;由于开缝钝体中钝体缝隙过大,濒临吹熄极限时,钝体后值班火焰被吹熄,开缝钝体燃烧器吹熄极限略有降低.     

混合层流动拟序结构的大涡模拟

采用大涡模拟方法对空间发展的二维平面混合层进行了数值模拟,动量方程采用分步投影法求解,亚格子项采用标准Smagorinsky亚格子模式模拟,压力泊松方程采用修正的循环消去法快速求解,同时求解了标志物输运方程以实现数值流场显示.模拟结果给出了混合层流动的瞬态发展过程以及流动中拟序结构的发展演变过程,成功地模拟了混合层发展中的各种瞬态细节过程,如涡的卷起、增长,涡与涡之间的配对、合并过程,以及大涡破碎为小涡的级联过程,为各种以混合层流动为原型流动的射流、尾流等工业流动的控制和优化提供了理论基础.     

钝体尾流控制机理及方法研究进展

首先从涡脱落生成理论出发对钝体尾流控制方法进行了分类,并简单介绍了国内尾流控制研究情况.之后介绍了我们用窄条或小方柱取代小圆柱后,对Strykowsky和Sreenivasan控制方法的改进及其在高雷诺数下对圆柱和方柱尾流涡脱落的有效抑制情况,并探讨了控制件钝度对抑制效果的影响.第3部分用实验数据对各个涡脱落生成模型做了分析与检验,指出控制件方法的机理与改变钝体分离位置、减小钝体背压吸力、改变流动的展向相关性、防止钝体两侧剪切层相互作用等无关,而与钝体近尾流速度剖面的局部修正及其稳定性的改变有关.最后简单介绍了控制件方法今后研究工作展望及其工程应用前景.     

入口速度比对射流拟序结构的影响

为了深入了解湍流流动机理以及湍流拟序结构发现过程的影响因素,本文采用大涡模拟方法对不同入口射流伴流速度比的平面湍射流流动进行了数值模拟。采用分步投影法求解动量方程,亚格子项采用标准Smagorinsky亚格子模式模拟,压力泊松方程采用修正的循环消去法快速求解,空间方程采用二阶精度的差分格式,在时间方向上采用二阶精度的显式差分格式。模拟结果给出了平面射流中湍流拟序结构的瞬态发展演变过程,分析了入口速度比对射流拟序结构发展演化过程及宏观流场形态的影响。为进一步研究射流拟序结构及其在湍流流动中的作用提供了基础。     

湍流边界层拟序结构的实验研究

20世纪60年代后, 先后从流动显示发现了快慢斑、猝发、上升流、下扫流和多种涡结构等湍流边界层的拟序结构. 它们对湍流边界层的摩阻、传热传质和湍动能的产生等特性有重要影响. 涡结构是上述拟序结构的核心, 它影响其它拟序结构的发展和演变. 发卡涡通常被认为是基本涡结构. 发卡涡等涡结构的再生, 是湍流边界层拟序结构能够自持续的必要的因素.壁面低速流上升产生猝发, 是湍流边界层湍能的主要来源; 条件采样是测量猝发频率和其它拟序结构出现频率的重要手段. 流动显示对湍流边界层拟序结构作了大量定性观察, 有许多减阻和增加传热率等应用性研究在此基础上发展起来. 80年代后, 出现了测量湍流边界层的瞬时流速矢量场的多热线法和PIV技术, 三维PIV技术可望将来为湍流边界层的实验研究带来重大进展. 本文评述了流动显示法、多热线法和PIV技术的优点和不足之处, 以及它们在对湍流边界层拟序结构的研究中的贡献.      

圆湍射流拟序结构研究进展

圆湍射流拟序结构的实验研究,经历了流场显示定性观测、热线、激光单点测量、数字图像等方法全流场定量测量以及当前三维演变机制的探讨。与此同时,静态激励和主动技术的拟序结构控制研究也获得了大量的成果。应用涡方法、直接数值模拟和大涡模拟等对圆湍射流拟序结构进行的数值模拟,在印证试验现象的同时,还进一步深入研究了拟序结构的输运特性、激发机制等相关内容。本文对上述进展进行了综述,以期为进一步开展湍射流的研究和应用提供依据。      

后向台阶湍流拟序结构的实验研究

在水槽中用氢气泡法观测了一个后向台阶的回流区、再附区和边界层再生区的大涡和湍流团的发展,以及近壁区的流动结构,观察到大涡结构对湍流团产生的作用。     

钝体绕流的分隔板控制技术研究进展

钝体是工程中一种常见结构,流体绕过钝体时产生的旋涡脱落易诱发结构振动,进而导致结构破坏．钝体后安装分隔板是一种典型的被动控制技术,分隔板推迟钝体尾流区剪切层之间的相互作用,进而有效改变钝体后旋涡脱落及尾迹特性,延长结构寿命,并且可以利用钝体一分隔板结构进行能量收集．本文全面回顾了利用分隔板进行流动控制和能量收集的研究及...     

A HYBRID VORTEX METHOD FOR FLOWS OVER A BLUFF BODY

A hybrid vortex method was developed to simulate the two-dimensional viscous incompressible flows over a bluff body numerically. It is based on a combination of the diffusion–vortex method and the vortex-in-cell method by dividing the flow field into two regions. In the region near the body surface the diffusion–vortex method is used to solve the Navier–Stokes equations, while the vortex-in-cell method is used in the exterior domain. Comparison with results obtained by the finite difference method, other vortex methods and experiments shows that the present method is well adapted to calculate two-dimensional external flows at high Reynolds number. It is capable of calculating not only the global characteristics of the separated flow but also the evolution of the fine structure of the flow field with time precisely. The influence of the grid system and region decomposition on the results will also be discussed. © by 1997 John Wiley & Sons, Ltd.     

Large eddy simulations of flow interference between two unequal sized square cylinders

A uniform flow past two unequal sized square cylinders arranged in a side-by-side pattern and at a Reynolds number of 50,000 has been investigated using large eddy simulation (LES) technique. The modelling of sub-grid scales of turbulence is done using the Smagorinsky model. The effect of the transverse gap ratio (T/D) on the flow characteristics has been studied. Numerical simulations are carried out for five different transverse gap ratios (T/D), namely 1.120, 1.250, 1.375, 1.750 and 2.500. Results in terms of the aerodynamic forces, Strouhal number, mean base pressure coefficient, streamlines, vorticity, surface pressure distribution, normal and shear stresses are presented. A shift in the stagnation point for the small square cylinder from the centre of its front face towards its gap side is seen at smaller T/D ratios. The presence of a jet-like flow seen in the gap side is more pronounced at T/D = 1.12. A biased gap side flow towards the near wake of the small square cylinder is seen at smaller T/D ratios. No interference effect is seen at T/D = 2.5. The flow behaviour is similar to that of the isolated square cylinder at this gap ratio.     

A Fourier–Chebyshev spectral collocation method for simulating flow past spheres and spheroids

An accurate Fourier–Chebyshev spectral collocation method has been developed for simulating flow past prolate spheroids. The incompressible Navier–Stokes equations are transformed to the prolate spheroidal co‐ordinate system and discretized on an orthogonal body fitted mesh. The infinite flow domain is truncated to a finite extent and a Chebyshev discretization is used in the wall‐normal direction. The azimuthal direction is periodic and a conventional Fourier expansion is used in this direction. The other wall‐tangential direction requires special treatment and a restricted Fourier expansion that satisfies the parity conditions across the poles is used. Issues including spatial and temporal discretization, efficient inversion of the pressure Poisson equation, outflow boundary condition and stability restriction at the pole are discussed. The solver has been validated primarily by simulating steady and unsteady flow past a sphere at various Reynolds numbers and comparing key quantities with corresponding data from experiments and other numerical simulations. Copyright © 1999 John Wiley & Sons, Ltd.     

Wake properties of a single gas bubble in three-dimensional liquid-solid fluidized bed

Experiments were performed to investigate the wake properties of a single gas bubble in a three-dimensional liquid-solid fluidized bed via a video camera moving at the same speed as the bubble. The solids holdup in the fluidized bed varied up to around 10%. The bubble size varied from 5 to 20 mm with corresponding bubble Reynolds numbers ranging from 1000 to 6500. The bubble was observed to have two types of wake configurations depending on the bubble size: the asymmetric/helical vortex wake for small bubbles and the symmetric wake for large bubbles. The bubble shape and relative rise velocity in the fluidized bed can be well-represented by correlations developed for single bubbles in liquid media, although the bubble shape in liquid-solid media is slightly more flattened compared to that in liquid media. The bubble rocking frequency was found to be independent of particle properties and to correspond in magnitude to the vortex shedding frequency in a two-dimensional liquid-solid fluidized bed. The average primary wake size in three dimensions is comparable to that in two dimensions.     

圆柱尾迹涡的三维演化及结构特征

应用无粘涡丝运动学理论和局部诱导近似(LIA)方法,以Lagrange观点跟踪涡丝在背景流场中运动,用数值方法研究了中等Re数(≈10~3)下圆柱分离尾迹中Kármán涡和涡辫区涡丝的三维演化的机制和动力学过程,及其结构特征。背景流场考虑为尾迹时间平均速度流场和Kármán涡街流场。初始展向小扰动为指数形式和谐波形式。结果指出:Kármán涡和涡辫区中的涡丝具有展向不稳定性,形成流向涡量。在尾迹的初期输运过程中,表现出有序的大、小尺度涡结构。并进一步分析了其产生的机理。