Variational Multiscale Large Eddy Simulation of Turbulent Flow in a Diffuser

In the present study, variational multiscale large eddy simulation (LES) is applied to turbulent flow in a diffuser, which represents a challenging test case due to the appearance of flow separation and subsequent reattachment. Two different scale-separating operators based on multigrid operators are used for separating large and small resolved scales. Dynamic as well as constant-coefficient-based subgrid-scale modeling are employed in the variational multiscale LES. The results show that the variational multiscale LES with a simple constant-coefficient Smagorinsky model using a projective scale-separating operator yields a very good prediction of most of the crucial flow features. Furthermore, it turns out to be by far the most computationally efficient approach, resulting in a computing time reduction of about 18% compared to the traditional LES with a dynamic Smagorinsky model. The variational multiscale LES with a dynamic Smagorinsky model does not prove to be a superior approach.     

Fully Stratified Particle-Laden Flow in Horizontal Circular Pipe

This article describes the results of an experimental measurement and visualization of coarse-grained fully stratified particle-water mixtures. This article focuses on the study of the carrier liquid velocity field and behavior and local velocities of conveyed particles. Glass balls and graded pebble gravel of 6 mm mean diameter were conveyed by water in a horizontal smooth pipe loop with a transparent pipe viewing section of inner diameter 40 mm. The measurements were performed in the smooth pipe and the pipe with rough stationary bed created by two layers of spherical particles of the same mean diameter as the conveyed particles. Particle movement along the pipe bottom was studied and the effect of the stationary bed on local velocity values of the carrier liquid and conveyed particles were evaluated. It was concluded that in the flow with stationary bed the maximum liquid velocity is markedly shifted from the pipe center to its top. The coarse-grained particles moved principally in a layer close to the pipe invert, for higher flow velocities particle saltation becomes the dominant mode of transport.     

HL-1M装置边缘扰动和流速的实验研究

利用一组马赫探针研究HL-1M装置刮离层和边缘等离子体流在欧姆放电、壁硼化、偏压抽气孔栏、偏压电极、低混杂波电流驱动、电子回旋共振加热、弹丸注入、分子束注入、激光吹气和补充送气等情况下的平行流马赫数、离子饱和电流扰动、平行流速度剪切和极向流速度的分布。实验中发现局部等离子体电位快速变化，改变了电场分布，改变了边缘等离子体的流速和方向。从而改善了等离子体约束性能。     

Symmetry-Preserving Discretization of Heat Transfer in a Complex Turbulent Flow

Convective and diffusive operators are discretized such that their symmetries are preserved. The resulting discretization inherits all symmetry-related properties of the continuous formulation. It is shown that a symmetry-preserving discretization is unconditionally stable and conservative. A fourth-order, symmetry-preserving discretization method is developed and tested for the numerical simulation of turbulent (flow and) heat transfer in a channel with surface-mounted cubes, where the temperature is treated as a passive scalar. The Reynolds number (based on the channel width and the mean bulk velocity) is Re=13,000. The results of the numerical simulation agree well with available experimental data.     

速度流型对微混合器混合效果的影响

微流控芯片是“芯片实验室”发展的重点,微混合器是微流控芯片的重要作用形式,在微尺寸下获得高效混合是重要的研究方向．采用Fluent软件对T型管两液流混合过程进行了数值模拟,对三种典型速度流型,即抛物线型、塞状型和凹面型速度流型条件下的混合效果进行了比较分析、数值模拟和理论分析表明：流速以及流型对混合效果具有很大的影响,抛物线型的混合效果最好;利用泵驱动系统比电渗驱动系统更有利于混合．     

气液两相流管道噪声测试

气液两相流管道内气流扰动、气体与液体的扰动产生噪声。利用双通道数据采集器/频谱分析仪对实验管道沿程噪声进行数据采集与处理,得到管道噪声频谱图。改变实验条件,对不同气体流量,不同含液量的管道进行噪声测试。通过对噪声频谱图的数据统计与分析,对管道噪声特性进行总结。通过分析可知,管道噪声检测是确定输气管道是否含液及是否出现液塞的有效方法。     

竖直通道内降膜流动数值模拟

降膜蒸发是一种高效的传热技术,平均液膜厚度是考察降膜蒸发传热性能的一个重要影响因素。本文基于VOF算法,建立了水和空气沿二维竖直通道降膜流动的CFD模型,模拟研究了液膜速度、工质种类、同向和逆向气流对平均液膜厚度的影响。结果表明：提高液膜速度会增大平均液膜厚度;气相工质对液膜厚度影响不大,而液相工质对液膜厚度影响较大,液膜厚度随液相黏度增大而增大;同向气流对入口段和发展段的液膜厚度影响不大,稳定段液膜厚度会随着同向气流速度的增大而减小;平均液膜厚度随逆向气流速度增大而降低,当逆向气流速度达到2.5 m/s后,气流速度对液膜厚度的影响减小。     

经验模态分解和空间滤波在两相流速度测量中的应用

在利用空间滤波和电容传感器测量两相流速度时,需要准确测量电容传感器输出信号的带宽.针对此问题提出一种利用经验模态分解算法来测量传感器带宽的方法.文章首先介绍电容传感器的空间滤波效应和经验模态分解的基本原理,并给出固体速度和电容传感器输出信号带宽之间的关系.然后将经验模态分解和平滑滤波器结合对测量信号进行平滑处理,测量处...     

Methodic Errors in Laser Measurements of Euler Statistical Moments of a Nonuniform Turbulent Velocity Field

Probability distributions of the velocity obtained by means of laser Doppler spectroscopy with different ratios between the dimensions of the probe volume and the scales of the nonuniformity are studied using a model of a locally normal, spatially nonuniform turbulent field. Sample calculations of the measured probability distributions of the velocity and the statistical moments of these distributions up to the fourth order are presented for model examples that take into account the nonuniformity of the mean flow velocity and the intensity of turbulent fluctuations of the velocity. It is shown that the substantial deviations in the velocity distributions from the normal law which are observed in a number of experiments may be related to noncompliance with the locality condition.     

Conjugate Turbulent Heat Transfer in the Zone of Incidence of a Pressure Shock on a Wall with a Porous Insert

The reduction of heat loads under conditions of transpiration cooling is analyzed using numerical simulation of convective heat transfer in the zone of pressure shock incidence on a solid surface subjected to a turbulent flow. The Darcy equation is used to preassign the boundary conditions in the Reynolds equations for the momentum components on a porous insert.     

Control of Velocity Profiles in a Boundary Layer

The control of average velocity in the turbulent boundary layer of a flat plate is investigated theoretically and experimentally. The control is exercised by injecting jets of air into the incident flow. Interpolation expressions are derived for the distribution of average velocities, dependent on the control parameters.     

A Higher-Order Asymptotic Theory for Fully Developed Turbulent Flow in Smooth Pipes

A complete second-order asymptotic theory for fully developed turbulent flow in smooth pipes at high turbulent Reynolds numbers is presented in the paper. The theory is based on Prandtl's mixing-length hypothesis involving a fourth-order polynomial representation for the mixing length and taking into account its dependence on the Reynolds number. Two main contributions with respect to the existing literature have been achieved:(a) the friction law is obtained by asymptotic evaluation of an integral, completely independently of the velocity field, and(b) an axis layer (in addition to the wall layer and the outer layer) has to be included in the analysis in order to remove a nonuniformity appearing in the second-order solution for the velocity fieldClosed-form analytic expressions for all constants and wake functions appearing up to the second-order solution in both the friction law and the velocity field are obtained. The results are in a very good agreement with experiments.     

Particle dispersion in turbulent sedimentary duct flows

A simplified geometric model of a circular tube with a sedimentary layer is proposed and named as sedimentary duct. Based on h = 1/4d (h is the thickness of sedimentary layer and d is the pipe diameter), the flow field (Re = 40000) and particle distribution (5, 10, 50 μm and St = 0.6, 2.5, 63) in the sedimentary duct are simulated using Large Eddy Simulation (LES) coupled with Lagrange Particle Tracking (LPT) method. As a result, four streamwise eddies are found in the sedimentary duct as distributed in pairs near the corner. The eddy center near ceiling is found to be farther from the corner than that from the floor. Small particles (5 μm, St = 0.6) tend to move with the secondary flow as their upward movements distribute in both sides. Their centripetal movement is near the floor and preferential distribution near the top. For large particles (50 μm, St = 63), it is the drag force that dominates their motion while for medium particles (10 μm, St = 2.5) lift force may have significant influence on their motion. This study is the first work to investigate the characteristics of particle behavior in turbulent sedimentary duct flows.     

横板型稳压罐气液两相流场数值模拟及流量稳定性研究

依据湍流模式理论中的标准k-ε湍流模型及流体体积函数多相流模型,实现了水流量标准装置中横板型稳压罐多相湍流流场的数值研究,采用PISO方法求解离散控制方程,并使用UDF编写入口速度脉动信号。获得稳压罐压力场、速度场、流线等关键信息,分析了入口脉动频率、气液比、竖隔板位置因素对流量稳定性的影响,并通过实验验证了计算结果的有效性。结果表明:低频脉动对稳定性影响较大,当泵出口脉动5Hz时,气液比1:3,竖板位于罐体D/3处,稳定性最好,流量波动系数为0.058%。     

The Use of a Laser Doppler Anemometer to Investigate Vortex Structures in Highly Turbulent Swirl Flow When There Are Predominant Fluctuations

The modeling and experimental investigation of large-scale vortex structures at the exit of a model burner of a low-emission combustion chamber when there is artificial generation of air flow-rate fluctuations are considered. A method for the diagnostics of vortex structures is developed using a two-component laser Doppler anemometer, which enables vortex structures in the jet mixing zone to be distinguished. The intensity of the swirling of the flow is calculated. A simplified rapid method of vortex-structure diagnostics is proposed, which significantly reduces the time of the experiment and the number of measurements required.     

Simulation and numerical investigations of the kinetics of atmospheric aerosol droplets in the wake behind a flat plate

Within the framework of the physicomathematical model of evolution of a polydisperse condensate, numerical investigations of the kinetics of atmospheric aerosol droplets in a supersonic two-phase flow past a flat plate were carried out. The gas flow was described by the Reynolds equations with the use of the two-parameter turbulence model. In view of the smallness of the condensate mass fraction in the incoming flow, the inverse effect of the dispersed phase on the gas was not considered. For various regimes of exposure to a flow, the characteristic features of the spatial distribution of the main parameters of the condensate fractions have been studied: the number densities, radii, temperatures, and averaged velocities of microdrops. The dependence of the dispersed phase dynamics on the Mach number and the incoming flow angle of attack has been investigated and the influence of the allowance for the processes of coagulation/fragmentation on the mass spectrum of droplets is shown. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 82, No. 2, pp. 331–341, March–April, 2009.     

新型超音速压气机内气固两相流场CFD模拟分析

为预测和预防压气机叶轮磨损，基于多相流理论对超音速压气机内的三维气固两相流场进行数值模拟。在分析中将压气机内的流场按可压缩气体处理，同时考虑温度对颗粒运动轨迹的影响。模拟结果表明，叶片出口端部前缘，温度达到最大，颗粒的运动速度也达到最大值。由此判明叶轮磨损最严重处位于叶片端部。     

流量测量中超声多普勒信号的频谱特点及其与管道中流速的关系

研究了工业测流的超声多普勒信号的频谱特点，并以此为依据分析了不同安装方式对多普勒频谱的影响，提出了不同测量环境下，应用多普勒频谱计算流速的公式及必要的修正，为提高检测准确度提供了理论依据。     

Three-dimensional numerical simulations of barchan dune interactions in unidirectional flow

ABSTRACT

In this study, a constitutive model based on fluvial hydrodynamics is coupled with a large eddy simulation to better understand barchan dune interactions from the perspective of morphological dynamics. The results show that the developed model can reproduce crescent-shaped dunes, as well as variations in turbulent flow structures above the dune surface. The interactions between two barchan dunes arranged in coaxial and staggered alignments are simulated by changes in their initial mass ratios. A critical moment of fast erosion is observed for the larger of the two coaxial dunes, and the value of this moment converges to a constant with a linear increase in the mass ratio. The interaction pattern of “coalescence” agrees with that of the experiments. These results provide deep insights into the transition of the barchan dune interaction patterns from “coalescence” to “ejection”, which corresponds to a dynamic mass equilibrium state in large dunes. In addition, one elongated horn created from merged barchans after a staggered collision reaffirms that dune collision and influx asymmetry are two potential mechanisms in barchan asymmetry.