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。     

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。     

流线型轴对称钝体尾迹特性及其声激励控制

在Ｒｅ＝３．０×１０３～１．０×１０５的范围内实验研究了流线型轴对称钝体尾迹特性，并采用声激励手段对尾迹进行控制．研究表明，自然状态下流线型轴对称钝体尾涡无量纲脱落频率在Ｒｅ＝５．０×１０４～１．０×１０５的范围内具有普适性，适当频率的声激励可以减小回流区，该结果对工程减阻具有实际应用价值．实验中脉冲热线的成功应用为复杂流场的测量提供了新的技术手段．     

二维混合层拟序结构的直接数值模拟

本文是在文[1]的基础上,引用Cain等人(1981)提出的映射函数将无穷远的边界变换到有限距离处,用伪谱方法对N-S方程组进行直接数值模拟,研究时间发展的二维混合层的不稳定性,再现了大涡的卷起,涡对的合并与撕裂以及三个涡、四个涡之间的相互作用过程,并将过程进行了动态显示。     

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．     

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。     

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。     

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.     

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。     

Absolute and convective instabilities of two-dimensional bluff body wakes in ground effect

Local and global instabilities are investigated of wakes of general two-dimensional bluff bodies placed near and parallel to a plane boundary or ground. A spatio-temporal linear stability analysis is first applied to a four-parameter family of local wake profiles to investigate the fundamental local stability characteristics of the wake in ground effect. The analysis shows significant dependencies of the stability characteristics of the wake on the distance from the wake centreline to the ground (normalised by the wake width), and also on the velocity ratio of the near- and far-ground sides of the wake. The analysis is then compared with earlier experiments on a circular cylinder to examine, according to the transition scenario of the steep global modes, the streamwise variation of the local stability characteristics of the wake in ground effect. The comparison indicates that the near wake region of the cylinder changes from being absolutely unstable to being convectively unstable when the cylinder comes down into the near-ground range in which the von Kármán-type vortex shedding from the cylinder is suppressed, being qualitatively consistent with the transition scenario for general wake-type flows. A possible explanation is also given for the counter-intuitive relation between the thickness of the boundary layer on the ground and the critical gap distance for the cessation of the von Kármán-type vortex shedding in ground effect.     

Three-dimensional evolution of vortices and early features of coherent structure in the turbulent wake behind a 2-D circular cylinder

3-D evolution of Kármán vortex filaments and vortex filaments in braid regions in the turbulent wake of a 2-D circular cylinder is investigated numerically based on inviscid vortex dynamics by analyzing the response of the initially 2-D spanwise vortex filaments to periodic spanwise disturbance of varying magnitude, wavelength and initial phase angles. Our results reveal a kind of 3-D vortex system in the wake which consists of large scale horseshoe-shaped vortices and small scale γ-shaped vortex filaments as well as vortex loops. The mechanism and the dynamic process about the generation of streamwise vortical structure and the 3-D coherent structure are reported. currently published in the Chinese Edition of Acta Mechanica Sinica, Vol.25, No.3, 1993 The project supported by National Natural Science Foundation of China and the National Basic Research Project “Nonlinear Science”     

Underlay mechanism in lift-drag phase diagrams for shear flow over cylinder

The characteristics of a uniform-shear flow over a circular cylinder are in- vestigated numerically by using the alternative-direction implicit （ADI） algorithm and a fast Fourier transform （FFT） one in the exponential-polar coordinates for Re = 150 and 0 ≤ K ≤ 0.46. The diagram of lift-drag phase, implying the detail information about the fluctuations of drag and lift as well as the flow patterns in the wake and fluctuating pres- sure on the cylinder surface, is used to describe the effects of the shear rate on the flow. Results show that the upper （or lower） closed curve of a phase diagram corresponds to the first （or second） half shedding cycle. The lift-drag phase diagram will move down-left with the increase of shear rate K such that the lift is exerted from the upper side to the lower side, and the drag on the first half shedding cycle is smaller than that on the second half.     

Structure of wake of a sharp-edged bluff body in a shallow channel flow

The flow field downstream of a bluff body in a typical open channel flow was explored by two-dimensional particle image velocimetry. Measurements are obtained in horizontal planes at the near-bed, mid-depth and near-surface locations downstream of the body up to a streamwise distance of 10D, where D is the width of the body. The dimensionless streamwise defect velocity profile of the wake flow matches well with the data of a previous investigation and does not reflect any dependency on the distance from the bed. However, the nature of development of the recirculation region is found to be different at the three vertical locations. The time-averaged streamline pattern indicates the existence of a unique nodal pattern close to the bed. The variation of the half-width is also found to be affected by the presence of the bed and the free surface. The bed friction arrests the transverse growth of the shear layer, and the free-surface helps to redistribute the turbulent kinetic energy in the streamwise and transverse directions. Swirling strength analysis is carried out to compare the behavior and statistics of the vortex population in the vertical direction. The prevailing magnitude of the swirling strength is found to be different at the three vertical locations. Bed friction assists to dissipate vorticity rapidly, and therefore reduces the probability of appearance of strong vortices close to the bed.     

Active decoupling of the axisymmetric body wake response to a pitching motion

Controlled interactions between fluidic actuators and the cross flow over the aft end of a wire-mounted axisymmetric wind tunnel bluff body model (Re_D=2.3·10⁵) are exploited for modification of the near wake dynamics, and the consequent global aerodynamic loads. Actuation is effected using an array of four aft-facing synthetic jet modules through narrow, azimuthally-segmented slots that are equally distributed around the perimeter of the tail end. The model is supported by eight wires, each including a miniature inline force transducer for measurements of the time-resolved tension. The model’s position is varied in a prescribed trajectory by synchronous activation of shape memory alloy (SMA) segments in each of the mounting wires, and the aerodynamic forces and moments are manipulated over a range of pitch attitude. The effectiveness of the flow control approach is demonstrated by decoupling of the wake response from the body’s pitch motion at a low pitch frequency (k=0.013). It is shown that, under the active control, the wake symmetry can be restored or its asymmetry can be amplified.     

超声速钝体逆向喷流减阻的数值模拟研究

为研究逆向喷流技术对超声速钝体减阻的影响,采用标准k-ε湍流模型,通过求解二维Navier-Stokes方程对超声速球头体逆向冷喷流流场进行了数值模拟,并着重分析了喷口总压、喷口尺寸对流场模态和减阻效果的影响。计算结果显示:随着喷流总压的变化,流场可出现两种流动模态,即长射流穿透模态和短射流穿透模态;喷流能使球头体受到的阻力明显减小;存在最大减阻临界喷流总压值(在所研究参数范围内最大减阻可达51.1%);在其它喷流物理参数不变时,随着喷口尺寸的增大,同一流动模态下的减阻效果下降。本文的研究对超声速钝体减阻技术在工程上的应用具有一定的参考价值。     

End effects of nominally two-dimensional thin flat plates

Differences in the structure and dynamics of nominally two-dimensional turbulent wakes are investigated experimentally for a thin flat plate, normal to a uniform flow, with two different end conditions: with and without end plates. Both cases are characterized by Karman-like vortex shedding with broadband low frequency unsteadiness. Both wakes evidence a low frequency flapping motion in addition to the slowly drifting base flow common to cylinder wakes. For the case without end plates, an interaction between the drift motion and the vortex formation process is associated with a much stronger modulation of the quasiperiodic vortex shedding amplitude when compared to the case with end plates where a flapping motion is more strongly expressed. These dynamics underlie structural differences in the mean wake and Reynolds stress fields.