射流抛光多相紊流流场的数值模拟

 理论分析了射流抛光的紊动冲击射流特点,构建了射流抛光的垂直冲击射流模型和斜冲击射流模型。根据射流抛光冲击射流的特点,比较各种流体模型后,采用RNG k-e 模型应用于射流抛光模型的计算。利用计算流体力学理论的二阶迎风格式对抛光模型方程离散,用SIMPLEC数值计算方法对射流抛光过程的紊动冲击射流和离散相磨粒分布进行数值模拟,得到了射流抛光过程的连续流场和离散相磨粒与水溶液的耦合流场,同时计算出了抛光液射流在工件壁面上的压力、速度、紊动强度、剪切力分布和磨粒体积质量分布,分析了垂直射流抛光模型和斜冲击射流抛光模型紊流流场的特点。     

Characterisation of the Eulerian and Lagrangian accelerations in the intermediate field of turbulent circular jets

A rich data-set of Lagrangian trajectories from 3D particle tracking velocimetry is used to study the structure of various acceleration components, vorticity, and strain in the intermediate field of a circular jet at Reynolds number Re = 6000. The total acceleration is decomposed into three distinctive sets: (1) streamwise–radial; (2) tangential–normal; and (3) local–convective components. Probability density function (PDF) and joint distributions of each set are characterised at various radial locations from the jet core within a streamwise band 16 ≤ x/d_h ≤ 17, where d_h is the diameter of the pipe. The PDF of the relative angle between the acceleration components and the velocity vector is also included to aid the characterisation. Results show that the acceleration components are described by two distinctive distributions: one of them exhibits symmetry and heavy tails, while the other is best fitted by a power-law type. The tails of acceleration PDFs are heavier with larger radial distance from the core. The increased departure from the Gaussian distribution with the distance from the core is a result of the increasing turbulence levels promoted by the mean shear. The variation of the third and fourth moments between the streamwise–tangential and the radial–normal accelerations indicate the anisotropy of the jet. Joint PDF of each acceleration decomposition exhibits distinctive distribution that appears to depend from the distance from the jet core. However, the vorticity and strain show similar PDF across radial distances. Finally, complementary analysis of a jet from a semicircular pipe shows the footprint of the nozzle geometry in the acceleration structure of jets.     

射流抛光多相紊流流场的数值模拟

理论分析了射流抛光的紊动冲击射流特点,构建了射流抛光的垂直冲击射流模型和斜冲击射流模型。根据射流抛光冲击射流的特点,比较各种流体模型后,采用RNG k-e 模型应用于射流抛光模型的计算。利用计算流体力学理论的二阶迎风格式对抛光模型方程离散,用SIMPLEC数值计算方法对射流抛光过程的紊动冲击射流和离散相磨粒分布进行数值模拟,得到了射流抛光过程的连续流场和离散相磨粒与水溶液的耦合流场,同时计算出了抛光液射流在工件壁面上的压力、速度、紊动强度、剪切力分布和磨粒体积质量分布,分析了垂直射流抛光模型和斜冲击射流抛光模型紊流流场的特点。     

Stabilization mechanism of lifted flame edge in the near field of coflow jets for diluted methane

The stabilization mechanism of lifted flames in the near field of coflow jets has been investigated experimentally and numerically for methane fuel diluted with nitrogen. The lifted flames were observed only in the near field of coflow jets until blowout occurred in the normal gravity condition. To elucidate the stabilization mechanism for the stationary lifted flames of methane having the Schmidt number smaller than unity, the behavior of the flame in the buoyancy-free condition, and unsteady propagation characteristics after ignition were investigated numerically at various conditions of jet velocity. It has been found that buoyancy plays an important role for flame stabilization of lifted flames under normal gravity, such that the flame becomes attached to the nozzle in microgravity. The stabilization mechanism is found to be due to the variation of the propagation speed of the lifted flame edge with axial distance from the nozzle in the near field of the coflow as compared to the local flow velocity variation at the edge.     

An experimental study of impulsively started turbulent axisymmetric jets

An impulsively started turbulent jet injected into quiescent surroundings with a constant inlet velocity has been studied experimentally. Results show that the jet length increases linearly with the square-root of time, over a wide range of Reynolds number calculated with respect to the jet diameter. The celerity factor, x_f/t U, has been found to be nearly constant at 2.47 throughout with a 5% variance. Here, x_f is the jet length, t is the time and U is the jet exit velocity. These results compare favourably with earlier results reported at lower Reynolds numbers. Finally, we present a simple model based on the integral energy balance of the turbulent boundary layer equation for an impulsively started turbulent axisymmetric jet. The model predicts a jet length that scales as, where d is the nozzle diameter and B(≈6.0) is the velocity-decay constant. This gives a celerity factor, in close agreement with the experiments.     

Topological evolution near the turbulent/non-turbulent interface in turbulent mixing layer

The topological evolution near the turbulent/non-turbulent interface (TNTI) in turbulent mixing layer is studied by means of statistical analysis of the invariants of velocity gradient tensor (VGT) based on direct numerical simulation data. The dynamics of topological evolution is investigated in terms of the source terms of the evolution equations for the invariants, including the pressure effect term, viscous effect term and interaction term among the invariants. It is found that the local topology of fluid particles at the TNTI evolves from non-focal region to focal region in the plane of the second (Q) and the third (R) invariants of the VGT. The topological evolution is mainly associated with the pressure effect term in the TNTI region. According to the analysis of the evolution of enstrophy and dissipation, the enstrophy increase and the dissipation decrease are revealed in the TNTI region, which are caused by viscous vorticity diffusion near the TNTI. A weak correlation between the strain rate and the rotation rate is found in the TNTI region which is related to the reduction of enstrophy production. The alignments between vorticity and strain near the TNTI are investigated and a strong alignment of the vorticity with the extensive eigenvector direction is identified in the TNTI region.     

平面射流沿轴线的特征尺度及其对测量信号过滤程度的依赖

对出口雷诺数约为13000的湍流平面射流瞬时速度(u)测量信号(u_m)进行高频过滤研究. 在分析了高频噪声对u测量精度的影响后,使用Mi等人 (2005, Phys. Rev. E 71, 066304)提出的快速收敛迭代法对u_m进行了数值过滤,并考察过滤前后统计湍流量的变化. 结果发现,噪声的存在不仅对湍流小尺度统计量的估算影响显著,也对与大尺度关联的量产生一定的影响. 在有效去除噪声造     

波浪破碎湍流混合研究综述

波浪破碎及其湍流混合研究一直是物理海洋学具有挑战性的问题. 文章系统综述了自20世纪90年代以来, 波浪破碎混合观测及其模式化研究方面取得的主要进展: 1)波浪破碎湍流混合不能用经典的海气边界层Wall-layer相似性理论进行描述. 波浪破碎能够在近海面几米的深度范围内形成湍流混合的增强层, 所产生的湍流动能耗散率ε_dis是Wall-layer相似性理论预测结果的10-1000倍. 2)波浪破碎湍流动能耗散率在波峰区域ε_dis∝z^-2.3, 而在波谷以下区域ε_dis∝exp(-αz)或ε_dis∝z^-2. 3)湍流混合长度是刻画波浪破碎混合非常重要的物理量, 但迄今为止对波浪破碎湍流混合长度l的量化研究存在较大差异, l从0.1 H_s到H_s(H_s为有效波高). 如何确定波浪破碎湍流混合长度的定量表示是今后需要亟待解决的重要问题. 关键词： 波浪破碎 湍流动能耗散率 湍流混合长度     

Analysis of free space optical link in turbulent atmosphere

Free space optical (FSO) communication is an upgraded supplement to the existing wireless technologies. FSO technology provides vast modulation bandwidth, unlicensed spectrum, cost effective deployment, low power consumption and less mass requirement. Today, researchers are preliminary focused to use the free space communication systems for inter satellites links. In this paper, the performance analysis of FSO communication link in weak atmospheric turbulence has been analyzed for different atmospheric transmission windows using OOK modulation. The analysis has been done using bit error rate as the performance metric. The effect of attenuation on the link performance has been investigated by varying distance between transmitter and receiver for a given power and data rate. Further, BER performance analysis has been carried out for varying data rate and transmitted power. Also, the effect of attenuation on received optical power has been studied. The work has been performed in OptSim environment.     

Studies of the flow and turbulence fields in a turbulent pulsed jet flame using LES/PDF

A turbulent piloted jet flame subject to a rapid velocity pulse in its fuel jet inflow is proposed as a new benchmark case for the study of turbulent combustion models. In this work, we perform modelling studies of this turbulent pulsed jet flame and focus on the predictions of its flow and turbulence fields. An advanced modelling strategy combining the large eddy simulation (LES) and the probability density function (PDF) methods is employed to model the turbulent pulsed jet flame. Characteristics of the velocity measurements are analysed to produce a time-dependent inflow condition that can be fed into the simulations. The effect of the uncertainty in the inflow turbulence intensity is investigated and is found to be very small. A method of specifying the inflow turbulence boundary condition for the simulations of the pulsed jet flame is assessed. The strategies for validating LES of statistically transient flames are discussed, and a new framework is developed consisting of different averaging strategies and a bootstrap method for constructing confidence intervals. Parametric studies are performed to examine the sensitivity of the predictions of the flow and turbulence fields to model and numerical parameters. A direct comparison of the predicted and measured time series of the axial velocity demonstrates a satisfactory prediction of the flow and turbulence fields of the pulsed jet flame by the employed modelling methods.     

近自由面水下爆炸气泡的运动规律研究

基于边界积分法建立水下爆炸气泡与自由面耦合数值模型;并开发相应的计算程序,计算值与实验值符合较好.从气泡与自由面相互作用的基本现象入手,运用开发的程序系统地研究了近自由面水下爆炸气泡的动态特性,包括近自由面气泡的环状回弹及自由面的水冢现象;研究距离参数、浮力参数以及强度参数等特征参数与自由面Bjerknes效应之间的关系,总结相关规律,近自由面气泡的动态特性与这些特征参数有密切的关系.并在研究过程中,讨论了基于Kelvin-impulse理论的Blake准则的适用范围,并解释了Blake准则的失效的原因. 关键词： 气泡 自由面 环状 水冢     

近自由面水下爆炸气泡的运动规律研究

基于边界积分法建立水下爆炸气泡与自由面耦合数值模型；并开发相应的计算程序，计算值与实验值符合较好.从气泡与自由面相互作用的基本现象入手，运用开发的程序系统地研究了近自由面水下爆炸气泡的动态特性，包括近自由面气泡的环状回弹及自由面的水冢现象；研究距离参数、浮力参数以及强度参数等特征参数与自由面Bjerknes效应之间的关系，总结相关规律，近自由面气泡的动态特性与这些特征参数有密切的关系.并在研究过程中，讨论了基于Kelvin-impulse理论的Blake准则的适用范围，并解释了Blake准则的失效的原因.     

高聚物减阻溶液对壁湍流输运过程的影响

基于相同雷诺数下清水和高分子聚合物溶液壁湍流的高时间分辨率粒子图像测速技术(time-resolved particle image velocimetry, TRPIV)的对比实验, 从高聚物溶液对湍流边界层动量能量输运影响的角度分析其减阻的机理. 对比两者的雷诺应力发现高聚物的存在抑制了湍流输运过程. 这一影响与高聚物对壁湍流中占主导地位的涡旋运动和低速条带等相干结构的作用密切相关. 运用条件相位平均、相关函数和线性随机估计(linear stochastic estimation, LSE)等方法, 分析提取了高聚物溶液流场中的发卡涡和发卡涡包等典型相干结构的空间拓扑形态. 相比于清水, 高聚物溶液中相干结构的流向尺度增大, 涡旋运动的发展及低速流体喷射的强度受到削弱, 表明了添加的高聚物阻碍了湍流原有的能量传递和自维持的机理. 正是通过影响相干结构, 高聚物抑制了湍流边界层中近壁区与外区之间的动量和能量输运, 使得湍流的无序性降低, 从而减小了湍流流动的阻力.     

非均匀磁场中自由表面液态射流的MHD稳定性分析

基于在液态金属实验回路上的实验,对非均匀磁场中液态金属射流的MHD稳定性进行了研究,建立了一个描述射流性能的简化模型。由此简化模型所得的结果与从实验获得的结果相比较表明,它们相当吻合,并发现在此液态金属射流中存在一个固有稳定性区域。     

湍流大气传输中跟踪抖动对远场影响的仿真研究

分析了跟踪抖动对湍流大气传输远场光斑的影响。基于麦克斯韦电磁场理论,采用大气相干长度对大气湍流进行描述,推导了发射光束因跟踪抖动导致光轴偏离的远场表达式。在此基础上,利用相位屏法模拟抖动引起的倾斜相位和大气折射率起伏引起的相位调制,并采用低频补偿的功率谱反演法对传输过程进行了数值仿真。分析了不同跟踪抖动、湍流强度条件下远场光斑质心脱靶量的变化,以及不同尺寸模拟目标的回波概率。分析结果表明,在传输距离为10 km时,强湍流造成的远场光斑脱靶量可达几十μrad;当跟踪抖动较大时,湍流强弱对脱靶量影响差别很小。最后,对一定尺寸的模拟目标,从探测回波概率的角度给出了发射系统跟踪抖动量的控制范围。     

一种简化非Gauss场模型在随机介质中光传播问题中的应用

为了研究大气湍流间歇性的光传播效应，构造出一种比较简单的非Gauss场模型(Poission场 )用于描述大气介电常数(或折射率)随机起伏.模型特征泛函含有四个待定函数，根据大气湍 流的统计均匀性，介电起伏的单点概率分布函数，以及介电起伏能谱可以选择或确定它们. 对在这种简化湍流中传播的光波平均场及二阶统计矩性质进行了理论分析，并给出数值模拟 的一个简单例子. 关键词： 光波传播 大气湍流 间歇性     

雷诺数对圆形渐缩喷嘴湍流射流的影响

本文研究雷诺数(Re)对圆形渐缩喷嘴湍流射流的影响.实验在射流出口雷诺数为 Re = 4050—20100 的范围内进行,分别测量了射流出口、中心线的平均及湍流流场以及部分径向剖面速度分布.所有测量均采用单热线恒温热线风速仪进行高频采样,所测流场范围在轴向上为 0—30d(这里d为射流出口直径).虽然出口速度分布均为"平顶帽"形,但测量结果依然反映出Re对射流出口以及下游流场有强烈的影响.当Re小于临界值(～10000)时 关键词： 雷诺数 圆形射流 热线风速仪     

Inverse radiation problem of axisymmetric turbulent sooting free flame

An iteration method is extended to reconstruct the time-averaged temperature distribution in turbulent axisymmetric sooting free flame by the multi-wavelength measured data of low time-resolution outgoing emission and transmission radiation intensities. A Gaussian probability density function is used to simulate the turbulent fluctuation of temperature. The reconstruction of time-averaged temperature profile consists of three steps. First, the time-averaged spectral absorption coefficient is retrieved from the time-averaged transmissivity data by iteration method. Then the time-averaged blackbody spectral radiation intensity is estimated from the low time-resolution outgoing spectral emission radiation intensities. Finally, the time-averaged temperature and its standard deviation are approximately reconstructed from the multi-wavelength time-averaged spectral emission radiation data by the least-square method. Both exact and noisy input data have been used to test the performance of the proposed inversion method. The results show that the time-averaged temperature profiles can be estimated with good accuracy by the presented inversion method, even with noisy input data, and the standard deviation of temperature is more sensitive to the measurement errors. In the case of large temperature fluctuation, the errors of estimation for time-averaged temperature profile are large if the turbulent fluctuation is not taken into account.