对流边界层湍流特性的数值研究

采用大涡模拟方法研究了存在逆温层的情况下大气对流边界层的湍流特性。实际大气边界层中出现逆温层是较常见的，逆温层会导致大气边界层湍流结构的变化，从而影响大气的湍流扩散和输运特性。本文比较了不同逆温梯度的工况，着重分析了逆温层对边界层中热量逆梯度输运（counter gradient heat transportation，CGHT）的影响。计算结果表明：逆温梯度越大，对流边界层的发展越受到抑制；逆温层高度降低会影响整个对流边界层的温度抬升；逆温梯度越大，垂直速度方差越小；在逆温梯度较大的情况下，其逆梯度输运区域要略微低一些，初步分析认为是由于逆温层对热对流的抑制造成的；对于逆温层高度不同的情况，高度越低的逆温层对逆梯度输运的抑制作用更明显。     

Numerical Study of Turbulent Jet Ignition in a Lean Premixed Configuration

Direct numerical simulations (DNS) of a hot combustion product jet interacting with a lean premixed hydrogen-air coflow are conducted to fundamentally investigate turbulent jet ignition (TJI) in a three-dimensional configuration. TJI is an efficient method for initiating and controlling combustion in ultra-lean combustion systems. Fully compressible gas dynamics and species equations are solved with high order finite difference methods. The hydrogen-air reaction is simulated with a reliable detailed chemical kinetics mechanism. The physical processes involved in the TJI-assisted combustion are investigated by considering the flame heat release, temperature, species concentrations, vorticity, and Baroclinc torque. The complex turbulent flame and flow structures are delineated in three main: i) hot product jet, ii) burned-mixed, and iii) flame zones. In the TJI-assisted combustion, the flow structures and the flame features such as flame speed, temperature, and species distribution are found to be quite different than those in “standard” turbulent premixed combustion due to the existence of a high energy turbulent hot product jet. The flow structures and statistics are also found to be different than those normally seen in non-isothermal non-reacting jets.     

Passive Scalar Transport in a Turbulent Mixing Layer

This paper describes a combined experimental and numerical study of scalar transport in spatially developing, two-stream, turbulent mixing layers with velocity ratios of approximately 2:1. The experimental mixing layer was created by an S-shaped splitter plate mounted in a wind tunnel, and the concentration field was realized by releasing incense smoke into the high-speed side boundary layer above the splitter plate. Simultaneous measurements of the velocity and concentration fields were performed. A 12-sensor hot-wire probe was used to measure the velocity field and its gradients, while the concentration field was recorded with digital photographs of the laser-illuminated smoke. In parallel, a large-eddy simulation (LES) of the spatially developing mixing layer was carried out. Auxiliary turbulent boundary layer LES were used to provide high quality inflow boundary conditions for the velocity and concentration fields. By synchronizing the velocity and concentration measurements, concentration fluxes were also determined. Octant analysis based on the sign combinations of the velocity and concentration fuctuations was performed on the flux data to investigate the scalar transport processes. It was found that octants compatible with mean gradient transport of the scalar contribute most to the scalar fluxes. Conditional planar averages of scalar and momentum fluxes were obtained to determine their spatial distribution with respect to the organized roller and rib vortices of the mixing layer, and distinct patterns were observed. The simulation provided additional insight about the flow and scalar flux distribution topology. This topology was found to be partially compatible with simple models of roller and rib vortices that transport the scalar in a mean gradient sense.     

Direct Numerical Simulations of Localised Forced Ignition in Turbulent Mixing Layers: The Effects of Mixture Fraction and Its Gradient

The effects of mixture fraction value ξ and the magnitude of its gradient |∇ξ| at the ignitor location on the localised forced ignition of turbulent mixing layers under decaying turbulence is studied based on three-dimensional compressible Direct Numerical Simulations (DNS) with simplified chemistry. The localised ignition is accounted for by a spatial Gaussian power distribution in the energy transport equation, which deposits energy over a prescribed period of time. In successful ignitions, it is observed that the flame shows a tribrachial structure. The reaction rate is found to be greater in the fuel rich side than in stoichiometric and fuel-lean mixtures. Placing the ignitor at a fuel-lean region may initiate ignition, but extinction may eventually occur if the diffusion of heat from the hot gas kernel overcomes the heat release due to combustion. It is demonstrated that ignition in the fuel lean region may fail for an energy input for which self-sustained combustion has been achieved in the cases of igniting at stoichiometric and fuel-rich locations. It is also found that the fuel reaction rate magnitude is negatively correlated with density-weighted scalar dissipation rate in the most reactive region. An increase in the initial mixture fraction gradient at the ignition centre for the ignitor placed at stoichiometric mixture decreases the spreading of the burned region along the stoichiometric mixture fraction isosurface. By contrast, the mass of the burned region increases with an increase in the initial mixture fraction gradient at the ignition location, as for a given ignition kernel size the thinner mixing layer includes more fuel-rich mixture, which eventually makes the overall burning rate greater than that compared to a thicker mixing layer where relatively a smaller amount of fuel-rich mixture is engulfed within the hot gas kernel. Submitted as a full-length article to Flow Turbulence and Combustion.     

Effects of Fuel Lewis Number on Localised Forced Ignition of Turbulent Mixing Layers

The influences of fuel Lewis number Le _F on localised forced ignition of inhomogeneous mixtures are analysed using three-dimensional compressible Direct Numerical Simulations (DNS) of turbulent mixing layers for Le _F  = 0.8, 1.0 and 1.2 and a range of different root-mean-square turbulent velocity fluctuation u′ values. For all Le _F cases a tribrachial flame has been observed in case of successful ignition. However, the lean premixed branch tends to merge with the diffusion flame on the stoichiometric mixture fraction isosurface at later stages of the flame evolution. It has been observed that the maximum values of temperature and reaction rate increase with decreasing Le _F during the period of external energy addition. Moreover, Le _F is found to have a significant effect on the behaviours of mean temperature and fuel reaction rate magnitude conditional on mixture fraction values. It is also found that reaction rate and mixture fraction gradient magnitude \(\vert \nabla \xi \vert \) are negatively correlated at the most reactive region for all values of Le _F explored. The probability of finding high values of \(\vert \nabla \xi \vert \) increases with increasing Le _F . For a given value of u′, the extent of burning decreases with increasing Le _F . A moderate increase in u′ gives rise to an increase in the extent of burning for Le _F  = 0.8 and 1.0, which starts to decrease with further increases in u′. For Le _F  = 1.2, the extent of burning decreases monotonically with increasing u′. The extent of edge flame propagation on the stoichiometric mixture fraction ξ = ξ _st isosurface is characterised by the probability of finding burned gas on this isosurface, which decreases with increasing u′ and Le _F . It has been found that it is easier to obtain self-sustained combustion following localised forced ignition in case of inhomogeneous mixtures than that in the case of homogeneous mixtures with the same energy input, energy deposition duration when the ignition centre is placed at the stoichiometric mixture. The difficultly to sustain combustion unaided by external energy addition in homogeneous mixture is particularly prevalent in the case of Le _F  = 1.2.     

The Numerical Decomposition of Turbulent Fluctuations in a Compressible Boundary Layer

In many flows the turbulence is weakly compressible even at large Mach number. For example, in a compressible boundary layer Ma<5, the differences relative to an incompressible boundary layer understood as being caused by density variations that accompany variations temperature across the layer. Turbulent fluctuations in a boundary layer are therefore expected to be dominated by the effects nonconstant temperature, and low Mach number theories in which fluctuations are not dominant should be applicable to the fluctuating field. However, the analysis of compressible boundary layer DNS data reveals presence of significant acoustic fluctuations. To distinguish acoustic and thermal effects, a numerical decomposition procedure compressible boundary layer fluctuations is applied to determine the and nonacoustic fluctuations. Except for very near the wall, where decomposition procedure is not valid, it is found that the fluctuations are only weakly coupled to the acoustic fluctuations at numbers as high as 6. Received 13 March 2000 and accepted 21 May 2001     

Numerical Simulation of Particle Dispersion in a Spatially Developing Mixing Layer

Although there have been several numerical studies on particle dispersion in mixing layers, most of them have been conducted for temporally evolving mixing layers. In this study, numerical simulations of a spatially developing mixing layer are performed to investigate particle dispersion under various conditions. The full compressible Navier--Stokes equations are solved with a high-order compact finite difference scheme, along with high-order time-integration. Accurate non-reflecting boundary conditions for the fluid flow are used, and several methods for introducing particles into the computational domain are tested. The particles are traced using a Lagrangian approach assuming one-way coupling between the continuous and the dispersed phases. The study focuses on the roles of the large-scale vortex structures in particle dispersion at low, medium and high Stokes numbers, which highlights the important effects of interacting vortex structures in nearby regions in the spatially developing mixing layer. The effects of particles with randomly distributed sizes (or Stokes numbers) are also investigated. Both instantaneous flow fields and statistical quantities are analyzed, which reveals essential features of particle dispersion in spatially developing free shear flows, which are different from those observed in temporally developing flows. The inclusion of the gravity not only modifies the overall dispersion patterns, but also enhances stream-crossing by particles. Received 7 June 2001 and accepted 19 February 2002     

Experimental and Numerical Study of the Scalar Turbulent/Non-Turbulent Interface Layer in a Jet Flow

Based on two large-eddy simulations (LES) of a non-reacting turbulent round jet with a nozzle based Reynolds number of 8,610 with the same configuration as the one that has recently been investigated experimentally (Gampert et al., 2012; J Fluid Mech, 2012; J Fluid Mech 724:337, 2013), we examine the scalar turbulent/non-turbulent (T/NT) interface layer in the mixture fraction field of the jet flow between ten and thirty nozzle diameters downstream. To this end, the LES—one with a coarse grid and one with a fine grid—are in a first step validated against the experimental data using the axial decay of the mean velocity and the mean mixture fraction as well as based on radial self-similar profiles of mean and root mean square values of these two quantities. Then, probability density functions (pdf) of the mixture fraction at various axial and radial positions are compared and the quality of the LES is discussed. In general, the LES results are consistent with the experimental data. However, in the flow region where the imprint of the T/NT interface layer is dominant in the mixture fraction pdf, discrepancies are observed. In a next step, statistics of the T/NT interface layer are studied, where a satisfactory agreement for the pdf of the location of the interface layer from the higher resolved LES with the experimental data is observed, while the one with the coarse grid exhibits considerable deviations. Finally, the mixture fraction profile across the interface is investigated where the same trend as for the pdf of the location is present. In particular, it is found that the sharp interface that is present in experimental studies (Gampert et al., J Fluid Mech, 2013; Westerweel et al., J Fluid Mech 631:199, 2009) is less distinct in the LES results and rather diffused in radial direction outside of the T/NT interface layer.     

Experimental 3D Analysis of the Large Scale Behaviour of a Plane Turbulent Mixing Layer

A new structure education method is proposed for representing the three dimensional large scale structures from hot wires measurement data. Due to the limited number of hot wire probes that can be used simultaneously, we firstly have to optimise the spatial location of sensors in order to detect large scale coherent structures. For such a purpose a detailed analysis of the 3D space-time flow organisation is given with particular emphasis on the full two point correlation tensor of the velocity field. Secondly, several procedures are developed in order to reconstruct instantaneous 3D velocity field on any finer and larger mesh than the experimental one from the limited available experimental information. The particular case of a spatially developing turbulent plane mixing layer is investigated. We then show that Stochastic Estimation and Proper Orthogonal Decomposition lead to a satisfactory reconstruction preserving the 3D coherent character of the flow. Finally, this reconstructed velocity field provides a 3D space-time representation of the large scale structures in a streamwise section of the flow and can then be used as realistic turbulent inflow condition for Large Eddy Simulations.     

Pre-Chamber Ignition Mechanism: Simulations of Transient Autoignition in a Mixing Layer Between Reactants and Partially-Burnt Products

The structure of autoignition in a mixing layer between fully-burnt or partially-burnt combustion products from a methane-air flame at ? = 0.85 and a methane-air mixture of a leaner equivalence ratio has been studied with transient diffusion flamelet calculations. This configuration is relevant to scavenged pre-chamber natural-gas engines, where the turbulent jet ejected from the pre-chamber may be quenched or may be composed of fully-burnt products. The degree of reaction in the jet fluid is described by a progress variable c (c = taking values 0.5, 0.8, and 1.0) and the mixing by a mixture fraction ξ (ξ = 1 in the jet fluid and 0 in the CH₄-air mixture to be ignited). At high scalar dissipation rates, N₀, ignition does not occur and a chemically-frozen steady-state condition emerges at long times. At scalar dissipation rates below a critical value, ignition occurs at a time that increases with N₀. The flame reaches the ξ = 0 boundary at a finite time that decreases with N₀. The results help identify overall timescales of the jet-ignition problem and suggest a methodology by which estimates of ignition times in real engines may be made.     

气固两相混合层二维涡配对的数值研究

采用双向耦合模型中的速度耦合模型,数值模拟了气固两相混合层中涡的配对、合并过程,文中采用拟谱方法对流场进行了直接数值模拟,用Lagrange模型跟踪固粒,结果发现,在双向耦合过程中Stokes数仍然是重要的参数,但不是唯一影响流场的参数,流场的发展还与固粒的体积浓度、固粒的相对密度以及固粒大小等因素有关。固粒抑制流场的发展,阻碍涡的配对、合并,加快了涡量的扩散;小St数的固粒仍然跟随流体运动,大St数的固粒趋向于低涡量区的趋势减弱。     

Effects of the Local Flow Topologies Upon the Structure of a Premixed Methane-air Turbulent Jet Flame

Local flow topologies have been identified and their interactions with the iso-scalar surfaces geometries have been investigated using the results of a three-dimensional direct numerical simulation (DNS) of a turbulent premixed methane-air flame in a piloted Bunsen burner configuration with tabulated chemistry. The universal teardrop shape of the joint probability density function (jpdf) of the second and third invariants of the velocity-gradient tensor disappears in the different flame regions under study. A ‘canonical’ vortex, which affects the fine-scale structure of the turbulent premixed flame, has been identified and analyzed at three times, differing by increments of the order of the Kolmogorov time micro-scale.     

滚筒干燥器中颗粒混合运动的三维数值模拟

滚筒干燥器中的颗粒混合运动是关系干燥效率的重要因素.为了研究滚筒干燥器中颗粒的混合特性,基于力学原理,采用离散元方法直接跟踪滚筒中的每一个颗粒.建立了三维数值实验平台,对滚筒中颗粒碰撞、摩擦的混合运动全过程进行了数值模拟,并着重探讨了滚筒转速、抄板半径及抄板数目等关键参数对滚筒中颗粒混合特性的影响.结果表明:随着转速的增加,颗粒混合均匀的速度加快;滚筒内壁抄板半径和数目对颗粒的混合特性也有重要影响;并发现了近壁区域颗粒的混合均匀程度比内部区域好.     

湍流能量耗散率标度律实验研究

应用粒子图像测速(PIV)系统对平板湍流边界层内流向和法向的瞬时速度进行了测量。湍流的能量耗散率由轴对称假设得到,同时在研究湍流动能耗散率标度律的过程中采用传统的统计学方法。实验结果显示,对于不同尺度上和不同法向位置湍流耗散率标度律来说,湍流耗散主要发生在小尺度上,也就是说湍动能耗散率标度律在小尺度上具有普适性。另外,根据层次结构理论假设,通过PIV实验数据对最高激发态的标度指数进行了研究,结果发现,最高激发态存在绝对标度指数,并且绝对标度律是由信号中最强耗散涨落的局部结构产生的。     

壁湍流边界层奇异标度律的实验研究

采用热线风速仪对平板湍流边界层的流向速度进行测量,用速度结构函数研究不同尺度结构标度律的变化规律,结果显示小尺度区的概率密度曲线尾部明显偏离高斯型,说明高幅值间歇性事件占的份额较多;惯性子区的曲线向高斯型靠近,间歇性事件所占份额减少;大尺度结构的曲线趋于高斯型,间歇性事件所占份额最小。在耗散区、惯性子区和较大的尺度结构区存在大小不同的绝对标度指数,越靠近壁面这些区域的标度指数均越偏离p/3而逐渐变小。绝对标度指数与边界层位置有关,在缓冲层各阶标度指数与线性标度律偏差很大,显示较强的奇异性,当过渡到对数层及外区,标度指数逐渐增大,接近均匀各向同性湍流的状态。缓冲层、对数层及外区具有各异的绝对标度指数增长率,与各层的不同湍流结构特征和运动形式有关。     

Numerical Study and Lagrangian Modelling of Turbulent Heat Transport

A stochastic Lagrangian model for both fluid velocities and temperature fluctuations is evaluated from Direct Numerical Simulation of heat transport in homogeneous isotropic turbulence submitted to a linear mean temperature gradient. The first stage lies on the study of the Lagrangian fluid turbulence statistics (Lagrangian correlations functions) computed from predictions of DNS. They are crucial for the analysis and the modelling of the fluid turbulent properties along discrete particle trajectories. In the second stage, a velocity-scalar Lagrangian stochastic model is proposed and evaluated from the DNS data. The coefficients of the drift and diffusion terms of the model are determined by only Lagrangian timescales, temperature variance and turbulent flux. The shapes of correlation functions present a good agreement between DNS results and stochastic modelling approach.     

离心泵叶轮内宾汉流体湍流流场的数值计算

考虑宾汉流体本构关系特点,建立了任意曲线坐标系（ζ,η）下宾汉流体湍流流动的基本方程,应用压力加权校正算法,实现了速度场和压力场的关联,采用交错网格技术,解决了非物理压力振荡问题,在此基础上,对离心泵叶轮内回转面上宾汉流体湍流流动进行了数值模拟,并分析探讨了离心泵叶轮内宾汉流体湍流流动机理。     

壁面湍流发卡涡包空间模态的TRPIV实验研究

运用高时间分辨率粒子图像测速(Time-resolved PIV简称TRPIV),测量得到平板湍流边界层流向/法向平面内瞬时速度矢量空间分布的时间序列;采用空间局部平均速度结构函数的概念,识别和提取湍流边界层中大尺度发卡涡包结构的空间特征。发现在湍流边界层中不同法向位置多个正负发卡涡包结构同时交替存在。这些分布在不同法向高度的发卡涡包结构之间通过倾斜的涡量剪切层相联系,构成了湍流边界层中内、外区紧密相连、相互作用的一种稳态的分布方式。     

Transport, Mixing and Agglomeration of Particles in Turbulent Flows

This paper describes methods and approaches that have been used to simulate and model the transport, mixing and agglomeration of small particles in a flowing turbulent gas. The transported particles because of their inertia are assumed not to follow the motion of the large scales of the turbulence and or the motion of the small dissipating scales of the turbulence. We show how both these behaviours can be represented by a PDF approach analogous to that used in classical kinetic theory. For large scale dispersion the focus is on transport in simple generic flows like statistically stationary homogeneous and isotropic turbulence and simple shear flows. Special consideration is given to the transport and deposition of particles in turbulent boundary layers. For small scale transport the focus is on how the small scales of turbulence together with the particle inertial response enhance collision processes like particle agglomeration. In this case the importance of segregation and the formation of caustics, singularities and random uncorrelated motion is highlighted and discussed.