圆柱尾流温度标量场的小波分析*

通过使用一排16根冷线探头排在多个空间点同时测量微加热圆柱的尾流温度场,用小波分析技术对瞬时温度场的时间序列信号进行多尺度分析,目的是研究不同尺度脉动温度对总体温度场的贡献.直径为d=12.7 mm的圆柱产生了被测的尾流,对应的雷诺数为5500,测量区域位于下游距离为2d和20d之间.基于小波多尺度分辨技术,尾流温度场被分解为不同温度脉动特征尺度的小波分量.通过分析这些小波分量的瞬时温度等值线图,能够直接观测到不同特征尺度的涡结构运动特征和湍流间歇过程.特别地,我们在近场区从原始信号分解获得的高频区域中发现了K-H涡的存在.不同尺度的温度方差沿流向的变化表明,在下游距离为x=3d和20d之间,中等尺度的结构比大尺度和小尺度结构对总的温度均方根的贡献更大.不同尺度的自相关函数表明,大尺度和中等尺度的结构显示出较大的相关性,而高频的小波分量则更快地失去了原有的拟序性.     

圆柱热尾流中温度的概率密度函数

本文通过实验研究圆柱热尾流中温度的概率密度函数在不同雷诺数下沿中心线的演变以及其与湍流混合程度的关系.实验中雷诺数的取值范围是1200-8600,温度是由直径为0.63 μm的冷线探针测量的.实验结果表明,温度概率密度函数在尾流中场区随空间位置变化显著.雷诺数的增加加快了这个变化过程,特别加速了尾流中心线温度的概率密度函数从非高斯向近似高斯分布的演变.     

圆柱尾流温度标量场的小波分析$lt;font color=\"#ff0000\"$gt;$lt;font color=\"#ff0000\"$gt;（已撤稿）$lt;/font$gt;$lt;/font$gt;

通过使用一排16根冷线探头排在多个空间点同时测量微加热圆柱的尾流温度场, 用小波分析技术对瞬时温度场的时间序列信号进行多尺度分析, 目的是研究不同尺度脉动温度对总体温度场的贡献.直径为d = 12.7 mm 的圆柱产生了被测的尾流, 对应的雷诺数为5500, 测量区域位于下游距离为2d 和 20d 之间. 基于小波多尺度分辨技术, 尾流温度场被分解为不同温度脉动特征尺度的小波分量. 通过分析这些小波分量的瞬时温度等值线图, 能够直接观测到不同特征尺度的涡结构运动特征和湍流间歇过程. 特别地, 我们在近场区从原始信号分解获得的高频区域中发现了K-H涡的存在. 不同尺度的温度方差沿流向的变化表明, 在下游距离为x=3d和 20d之间, 中等尺度的结构比大尺度和小尺度结构对总的温度均方根的贡献更大. 不同尺度的自相关函数表明, 大尺度和中等尺度的结构显示出较大的相关性, 而高频的小波分量则更快地失去了原有的拟序性.关键词：湍流尾流被动标量小波分析     

对具有平均标量梯度的被动标量的直接模拟

采用直接数值模拟方法,对具有平均标量梯度的被动标量场在稳定、均匀各向同性湍流中的统计特性进行了研究(计算Reλ为25和48,Pr数从0.3到4.0)。模拟结果表明:标量的概率密度函数是高斯分布的;标量耗散率的概率分布是不对称性的,在它的左枝还存在明显的拖尾现象,并非对数正态分布;在Re数相同的情况下,随着Pr数的增加, 沿着平均标量梯度方向上的标量梯度其概率分布的偏斜度是减少的,反映出回归各向同性的趋势,而且在以涡量为条件的标量梯度的偏斜度变化曲线中发现涡量影响的饱和现象,我们认为这是由于涡量通过对标量场小尺度结构进行旋转作用来影响标量场的结果。     

湍流场中被动标量的细微结构

本文采用直接数值模拟方法,在具有平均标量梯度的各向同性湍流中,研究被动标量的小尺度结构特性及其与湍流场中应变与涡量的关系.对欧拉统计量及拉格朗日统计量的统计表明:标量耗散的形成主要是由于标量梯度同流场的应变张量压缩主轴耦合的结果,而涡量对标量梯度的形成只有较弱的影响,然而它可以间接影响大强度标量耗散的产生.强标量耗散的细微片状结构的形成时间尺度大约为10倍Kolmogrov时间尺度;在形成强标量梯度的细微片状结构过程中,应变强度随标量梯度同步增大,而涡量则先减小后增大,并在5倍Kolmogorov时间尺度时达到最大.     

超音速平板混合层被动标量的数值模拟

采用空间五阶精度紧致-WENO混合格式,数值模拟了Mc=0.2和1.0的二维空间发展超音速平板混合层被动标量混合过程.并与Clemens的实验结果进行比较,结果表明,被动标量时均值与实验结果符合很好,而被动标量脉动均方根值与实验值相比较小.还研究了可压缩性对标量混合过程的影响,随着对流马赫数的增加,被动标量时均值剖面变化不大,而脉动均方根峰值增大.     

颗粒惯性对标量混合的影响

本文采用直接数值模拟的方法,研究了颗粒惯性对均匀各向同性湍流场中被动标量混合特性的影响.结果表明:与无颗粒算例相比,Tp/Tk<1的小颗粒,增大了标量脉动在高波数上的能量,增强了标量耗散率,标量耗散率最大高出了25%;Tp/Tk>1的大颗粒,在初期显著削弱了标量脉动强度,同时也加快了标量耗散率的衰减,标量脉动强度降低了约15%,标量耗散减少了约5%;而Tp/Tk=1的临界颗粒,对标量场混合的影响较弱,标量脉动强度和标量耗散偏差在5%以内;此外,随着颗粒惯性的增大,标量耗散率的概率密度函数向左平移,但分布特征的改变并不显著.     

旋转槽道湍流标量场输运特性数值模拟研究

展向旋转槽道湍流中的标量场输运过程是与众多工程流动直接相关的模型问题。基于直接数值模拟工作对该问题开展系统性的研究。由壁面摩擦速度定义的流动Reynolds数固定在180, 重点考察Schmidt数和旋转数的影响。结果表明, 较弱旋转强度即可对主导流动结构形态产生明显的影响: 此时槽道不稳定侧产生流向大尺度结构, 由此导致标量场出现条带状结构。强旋转时不稳定侧出现被湍流充分混合的区域, 而在稳定侧流动层流化并出现近似传导区。平均标量剖面在湍流区和层流区呈现斜率不同的线性分布。Schmidt数小于1时, 湍流区标量场脉动和湍流输运随旋转数出现非单调变化, 而Schmidt数大于等于1时两者都随旋转数单调下降。由此导致总标量传输率在Schmidt数小于1时随旋转数先上升后下降, 而当Schmidt数大于1时单调下降且在弱旋转区域下降趋势最快。     

湍流预混火焰LES/FDF模拟中标量混合时间尺度建模

在实际应用中,精确模拟高湍流度预混火焰十分关键但极具挑战性。本文将被动标量混合时间尺度动态封闭方法与hybrid混合时间尺度模型结合,在湍流预混火焰LES/FDF模拟中提出一种新的标量混合时间尺度建模方法。该方法不需要人为选取混合模型参数C_M,且动态地考虑了由湍流和火焰结构分别引起的亚网格标量混合。对悉尼值班预混射流燃烧器高湍流度PM1-150火焰进行了LES/FDF模拟。结果表明,新模型显著改善了对整体燃烧进程的预测结果,正确地预测了局部熄火/再燃现象。进一步的结果显示,模型参数C_M明显大于在RANS框架下量级为1的常用取值,这表明该参数在不同框架下具有不同的物理内涵。     

绕圆柱流体的LB模拟

介绍一个流体动力学的格子Boltzmann并行模拟模型。用该模型模拟了流体动力学中的卡门涡街现象。利用计算机可视化技术该模型可动态显示流体流动状态的演化过程,便于分析各种流体动力学现象。     

Mixing of a passive scalar across a thin shearless layer: concentration of intermittency on the sides of the turbulent interface

The advection of a passive scalar through an initial flat interface separating two different isotropic decaying turbulent fields is investigated in two and three dimensions. Simulations have been performed for a range of Taylor’s microscale Reynolds numbers from 45 to 250 and for a Schmidt number equal to 1. Different to the case where the transport involves the momentum and kinetic energy only and one intermittency layer is formed in the low-turbulent energy side of the system, in the passive scalar concentration field two intermittent layers are observed to develop at the sides of the interface. The layers move normally to the interface in opposite directions. The dimensionality produces different time scaling of the passive scalar diffusion, which is much faster in the two-dimensional case. In two dimensions, the propagation of the intermittent layers exhibits a significant asymmetry with respect to the initial position of the interface and is deeper for the layer which moves towards the high kinetic energy side of the system. In three dimensions, the two intermittent layers propagate nearly symmetrically with respect the centre of the mixing region. During the temporal decay, inside the mixing, which is both inhomogeneous and anisotropic but devoid of a mean velocity shear, the passive scalar spectra are computed. In three dimensions, the exponent in the scaling range gets in time a value close to that of the kinetic energy spectrum of isotropic turbulence (?5/3). In two dimensions, instead the exponent settles down to a value that is about one-half of the corresponding isotropic case. By means of an analysis based on simple wavy perturbations of the interface we show that the formation of the double layer of intermittency is a dynamic general feature not specific to the turbulent transport. These results of our numerical study are discussed in the context of experimental results and numerical simulations.     

Reassessment of the classical closures for scalar turbulence

In deducing the consequences of the Direct Interaction Approximation, Kraichnan was sometimes led to consider the properties of special classes of nonlinear interactions in degenerate triads in which one wavevector is very small. Such interactions can be described by simplified models closely related to elementary closures for homogeneous isotropic turbulence such as the Heisenberg and Leith models. These connections can be exploited to derive considerably improved versions of the Heisenberg and Leith models that are only slightly more complicated analytically. This paper applies this approach to derive some new simplified closure models for passive scalar advection and investigates the consistency of these models with fundamental properties of scalar turbulence. Whereas some properties, such as the existence of the Kolmogorov–Obukhov range and the existence of thermal equilibrium ensembles, follow the velocity case closely, phenomena special to the scalar case arise when the diffusive and viscous effects become important at different scales of motion. These include the Batchelor and Batchelor–Howells–Townsend ranges pertaining, respectively, to high and low molecular Schmidt number. We also consider the spectrum in the diffusive range that follows the Batchelor range. We conclude that improved elementary models can be made consistent with many nontrivial properties of scalar turbulence, but that such models have unavoidable limitations.     

压气机转子尾迹三维紊流特性分析

根据单转子压气机和单级压气机在设计状态和近失速状态转子出口流场的详细测量数据，分析了这两种试件在不同状态、不同叶高情况下转子尾迹的三维素流特性。设计状态除端壁区外；转子各叶高尾迹的速度型很相似，尾迹宽度沿叶高基本相同．近失速状态转子尾迹明显变宽，且沿叶高变化不均匀，这主要决定于吸力面附面层的增长，另一方面尾迹内紊流强度增强，吸力面一侧的紊流强度高于压力面一侧。本文对转子尾迹的目相似定律进行了检验和改进。     

Conditional velocity statistics in the double scalar mixing layer – A mapping closure approach

In this work we use 3D direct numerical simulations (DNS) to investigate the average velocity conditioned on a conserved scalar in a double scalar mixing layer (DSML). The DSML is a canonical multistream flow designed as a model problem for the extensively studied piloted diffusion flames. The conditional mean velocity appears as an unclosed term in advanced Eulerian models of turbulent non-premixed combustion, like the conditional moment closure and transported probability density function (PDF) methods. Here it accounts for inhomogeneous effects that have been found significant in flames with relatively low Damköhler numbers. Today there are only a few simple models available for the conditional mean velocity and these are discussed with reference to the DNS results. We find that both the linear model of Kutznetzov and the Li and Bilger model are unsuitable for multi stream flows, whereas the gradient diffusion model of Pope shows very close agreement with DNS over the whole range of the DSML. The gradient diffusion model relies on a model for the conserved scalar PDF and here we have used a presumed mapping function PDF, that is known to give an excellent representation of the DNS. A new model for the conditional mean velocity is suggested by arguing that the Gaussian reference field represents the velocity field, a statement that is evidenced by a near perfect agreement with DNS. The model still suffers from an inconsistency with the unconditional flux of conserved scalar variance, though, and a strategy for developing fully consistent models is suggested.     

Passive Scalar Advection in Burgers Turbulence: Mapping-Closure Model

Passive scalar advection in Burgers turbulence is considered. Mapping-closure model based on the amplitudes is used. Advective stretching of scales is included. The simpler nature of this problem affords a convenient framework to clarify some aspects of mapping-closure analysis of coupled stochastic fields. The ability (or lack of it) of the joint action of advective stretching and molecular diffusion to generate non-Gaussian scalar statistics in Burgers turbulence in the absence of a non-Gaussian forcing is investigated.