Simulated pulsed flow of gas and particles in a horizontal oppose-pulsed gas jets of bubbling fluidized bed

Flow behavior of gas and particles with a horizontal oppose-pulsed gas jets are simulated by means of a three dimensional Computational Fluid Dynamics (CFD) model with the kinetic theory of granular flow in a gas-particles bubbling fluidized bed. The effects of amplitudes and frequencies on the hydrodynamics of gas and particles are analyzed. The simulation results are presented in terms of phase velocity vector plot, volume fraction of phases, granular temperature, power spectrum and Reynolds stresses in the bed. Results show that the impingement caused by the oppose-pulsed gas jets oscillates with the variation of pulsed gas velocity. The impingement zone with the high solid volume fraction reciprocates from the left side to the right side through the bed center with the variation of pulsed jet gas velocities. The lateral velocity and gas turbulent kinetic energy, granular temperature and Reynolds stresses of gas and particles are larger near the pulsed gas jets than that at the center of the bed. The large dispersion coefficients of particles using the horizontal oppose-pulsed gas jets enhance the mixing of particles in gas-solid fluidized bed.     

Simulation of a premixed turbulent flame with the discrete vortex method

The behaviour of a premixed turbulent flame is numerically studied in this paper. The numerical model is based on solving turbulent flow field by the discrete vortex method. The flame is considered to be of zero thickness boundary which separates burnt and unburnt regions with different constant density and propagates into the fresh mixture at a local curvature‐dependent flame speed. The flame front is located by means of level‐set algorithm. The flow turbulence is simulated through the unsteady vortex‐shedding mechanism. The computed velocity fields, turbulence scalar statistics as well as flame brush thickness for the turbulent V‐flame are well comparable to experimental results. The computed Reynolds stresses in the flame brush region based on unconditioned velocities are substantial, but the two conditioned Reynolds stresses are negligible. These results show that the intermittency effect is a major influence on turbulent statistics in premixed flame and should require careful consideration in numerical models. Copyright © 2000 John Wiley & Sons, Ltd.     

直径30cm圆柱的气动力参数和绕流特性研究

为研究光滑圆柱的气动力系数和绕流特性,在均匀流中进行不同风速下的测压风洞试验,试验获得了阻力系数、升力系数、表面风压分布、风压相关性系数、斯托罗哈数等随雷诺数的变化特征,并将试验结果与以往结果进行比较。研究表明:升力系数的脉动值大于阻力系数的脉动值,说明涡脱造成的横风向激励比顺风向紊流激励剧烈;雷诺数位于临界区域时,圆柱表面风压分布呈现出对称-不对称-对称的变化过程,反映了由层流分离转化为湍流分离的全过程;在雷诺数为352000时呈现一侧为层流分离、另一侧为湍流分离的临界流态,风压呈现出左右不对称的单边泡形式;获得层流分离和湍流分离时的表面风压相关性分布特征,层流分离时圆柱同一侧的风压测点均呈较强的正相关,而湍流分离时在分离点前的区域相关性较强,分离点之后的区域相关性较弱;层流分离的升力系数谱有显著的峰值,表明尾流是规则的漩涡脱落,而湍流分离的升力系数谱没有明显峰值,表明尾流是随机的漩涡脱落。     

Turbulence characteristics of open channel flow over non-equilibrium 3-D mobile dunes

This paper reports velocity measurements over mobile dunes using an acoustic Doppler velocimetry (ADV). Experiments were conducted with two different flow conditions resulting in the formation of two different size mobile dunes. Dunes height, wavelength and velocity of dunes found to be increasing with increase in average flow velocity for a constant flow depth. The quasi-stationary bed condition was assumed while measuring the velocity distribution along the depth. The effect of the non-equilibrium mobile dunes on the flow characteristics and turbulence is examined by computing turbulent intensities, turbulent kinetic energy and Reynolds shear stresses using time averaged and time–space averaged velocity measurements. The magnitudes of transverse velocities are approximately 1/10 of streamwise velocities and vertical velocities are approximately half of the transverse velocities. The considerable magnitudes of transverse velocities over mobile bedforms necessitate measurement of 3-D velocity components to analyze the flow field. Computed turbulence intensities are found to be maximum in the region consisting of the trough and the reattachment point of the dunes. It is observed that streamwise turbulence intensities near the bed are twice the transverse turbulence intensities, and transverse turbulence intensities are twice the vertical turbulence intensities. Reynolds stresses (transverse fluxes of streamwise and vertical momentum) are observed to be high on mobile bedforms which shows mobile dunes reinforce the secondary currents. Peak values of turbulent kinetic energy (TKE) and Reynolds stresses are also found in the region consisting of the trough and the reattachment point. It is visually observed in the present experiments that maximum erosion takes place at the reattachment point and eroded sediment is carried as total load and dropped on the lee slope of the subsequent downstream dune. This phenomenon is caused by flow expansion in the separation zone, and which is also the main reason for mobility of dunes and associated bedload transport. Most importantly, it is found that turbulence anisotropy increases with increase in size of mobile bedforms and anisotropy is extended up to the free surface in the flows over mobile bedforms, which proves the entire depth of flow is being disturbed by the mobile dunes.     

圆柱绕流的流场特性及涡脱落规律研究

采用粒子图像测速技术对630、800及950三种雷诺数条件下的圆柱绕流场进行了实验,给出了圆柱下游沿流动方向4倍圆柱直径和垂直方向3倍圆柱直径区域内的速度场、涡量场以及涡脱落现象的时空演化规律.结果表明:圆柱尾流区域位于垂直方向约1.5~2.5倍圆柱直径范围内,随着雷诺数增大,这一范围呈现缩小趋势,而主流对涡的拉伸和输运能力有所增强;涡脱落频率随雷诺数增大而增大,小雷诺数时能够较为完整地捕捉到涡生成、脱落、发展和耗散过程,由于PIV采集频率的限制,大雷诺数条件下涡脱落整个过程不易被完整捕捉到.     

微管道内湍流转捩的实验研究

研究旨在确定微管道内流动从层流到湍流转捩的临界雷诺数。利用微观粒子图像测速技术(Micro-PIV)研究了去离子水在内径为230μm的圆形截面玻璃微管道内的流场结构,得到了从层流到充分发展湍流各流动状态下的轴向平均速度分布和湍流度分布,实验雷诺数为1020~3145,同时研究了微管道内的流动阻力特性。平均速度场和脉动速度场的实验结果表明微管道内从层流到湍流的转捩发生在Re=1800~1900左右,与流动阻力的测量结果一致,与宏观流动比较,并未发现微管道内的流动转捩有明显提前。实验结果还显示,当Re>2700时,微管道内的平均流速分布和相对湍流度分布呈现典型的充分发展湍流状态特征。     

旋风分离器减阻杆减阻的PIV实验研究

采用先进的PIV实验技术对Stairmand型旋风分离器中安装减阻杆前后的强湍流场进行了测量。通过速度场、湍流强度、Reynolds应力等物理量的对比分析,表明减阻杆降低了中心涡核区的旋转动能和湍流强度。对减阻杆减阻机理进行了更深入的探讨。     

含气泡液体流动的剪切雷诺应力研究

含气泡液体流动的雷诺应力主要是由气泡存在及液体湍动而引起的,它们分别在层流和湍流流动中占主导地位。通过对粘性流体绕球形颗粒的流场进行研究,运用单元系综平均方法推导出由气泡引起的剪切雷诺应力关系式,并根据Sato(1981)的方法获得最佳剪切雷诺应力系数。此关系式与雷诺数的平方根成正比,且仅适用于空泡率较小的含气泡液体流动。通过模型预测及与Sato(1981)所得关系式进行比较,显示同意所得关系式更具有理论性,更合理。     

Numerical simulation of gas-solid flow in a novel spouted bed: Influence of row number of longitudinal vortex generators

The flow characteristics in a novel cylindrical spouted bed with spherical longitudinal vortex generators is numerically investigated by two-fluid model (TFM) with kinetic theory for granular flow, the longitudinal vortex technology is adopted in the spouted bed so as to strengthen the particles radial mixing between spout and annulus zones, the row number effect (1–3 rows) of longitudinal vortex generators (LVGs) on gas–solid flow behavior in three dimensional spouted beds was numerically simulated. The CFD results show that, longitudinal vortices can effectively increase particle volume fraction near annulus zone in the spouted bed, the maximum increase of particle volume fraction near annulus region is 183％, and the pressure drop in spouted beds increases with increasing of LVGs’ row number. There exists an optimal row number (equal 2) of LVGs, at witch the radial velocity of particle phase reaches maximum in the limited spouted bed space, the value of turbulent kinetic energy of gas phase in spouted bed can be significantly promoted by longitudinal vortex, espeically in the spout zone and near the annulus region. Also, the enhancement effect of multi-row LVGs on turbulent kinetic energy of gas phase decreases when the cross section height of spouted beds increases.     

Incompressible micromorphic fluid model for turbulence

A theory of incompressible micromorphic fluids is introduced as a rational model for turbulence studies. Balance laws and constitutive equations are given. The theory is then applied to obtain the solution of the turbulent channel flow problem. Turbulent velocity, gyrations, Reynolds stresses, root-mean squares of longitudinal and transverse turbulent velocities, and turbulent shear stress are given.     

Velocity and temperature fluctuations and their correlations for turbulent air flow in a tube

The average fields and statistical characteristics of velocity and temperature fluctuations in a turbulent air flow have been measured at Reynolds numbers of 260,000 and 32,500. The correlation coefficients between the fluctuation components are determined, along with the distribution of the turbulent Prandtl number over the flow cross section.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 19, No. 6, pp. 1060–1069, December, 1970.     

Typical flow between enclosed corotating disks and its dependence on Reynolds number

Abstract

The flow in an enclosed co‐rotating disk pair is investigated by Laser Doppler Velocimetry (LDV) measurements and flow visualizations. First, the typical flow structure at Re = 5.25 × 10⁵ and S = 0.09 is clarified. The flow fields in the r – θ and the r – z planes are both investigated and then divided into several flow regions based on the distinct flow types observed. The flow regions found in the two different planes are also compared and integrated. Second, with S fixed, the dependence of the flow field structure upon the Reynolds number is discussed. Three regimes of the r – θ plane flow with different Reynolds numbers are identified based on the measured mean velocity and spectral intensity. When Re < 1.6 × 10⁵, no solid body region is found and the flow is in a laminar regime. In the range 1.6 × 10⁵ ≤ Re ≤ 2.0 × 10⁶, the solid body region and the outer region vortices coexist, and an empirical equation is developed to estimate the number of vortices. When Re > 2.0 × 10⁶, the flow becomes turbulent. As Re increases from 9.3 × 104 to 5.25 × 10⁵, the spectral intensity initially increases and then decreases before increasing again to an even higher level, resulting in an increasing sawtooth pattern.     

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.     

激光粉末床熔融成形腔内非稳态流场特性研究

目的 研究激光粉末床熔融(LPBF)中成形腔内保护气的流动规律,获得气流速度脉动和旋涡等流场非稳态特征及其变化规律。方法 利用热线测速计测量腔内的瞬时速度,研究保护气的速度分布及其脉动特性;基于数值模拟方法探究腔内气流形成的旋涡情况,分析涡的分布及其旋转速度;利用烟雾示踪方法对保护气流场进行可视化处理,分析气流的运动过程。结果 腔内气流经历了射流扩散、上下波动、大涡流、汇流等复杂运动过程,气流速度随时间的变化呈明显脉动特征,且气流脉动幅度受位置影响较大,进出风口的平面流速最大可达2.4 m/s,最小为0.25 m/s。同时,气速随平面高度的增大而逐渐减小;腔内存在以纵向大尺度旋涡为主、若干小尺度旋涡共同作用的涡流,由腔内边壁至中心,涡流切向速度呈先上升后下降的趋势,且随入口气速的增大而增大,在切向速度急剧降低的腔体角落、透镜等区域,易形成流动“死区”,导致烟尘颗粒聚集且难以排出,影响构件的高质量成形制造。结论 保护气在LPBF成形腔内形成了复杂的非稳态流动,并以剧烈的速度脉动和多尺度的涡流为典型特征,而针对非恒定的层流、成形腔结构的优化设计仍需进行更深入的研究。     

Numerical experiments on turbulent flow using the random vortex method

Chorin's random vortex method is used to predict the growth of a large-scale coherent vortex structure in the early stages of the development of turbulence in a two-dimensional co-flowing shear layer. The numerical algorithm has been simplified to such an extent that the numerical analysis can be performed on a microcomputer. The numerical solution exhibits the same early turbulent instabilities and vorticity pairings as found in recent flow-visualization experiments. In addition the results are in reasonable agreement with experimental measurements of mean velocity, root mean square fluctuations and Reynolds stresses. One could thus test the shear layer sensitivity to initial conditions and the upsteam boundary conditions.     

Hydrodynamic stability of viscous flow between curved porous channel with radial flow

A linear stability analysis has been presented for the flow between long concentric stationary porous cylinders driven by constant azimuthal pressure gradient, when a radial flow through the permeable walls of the cylinders is present. The radial Reynolds number, based on the radial velocity at the inner cylinder and the inner radius is varied from −100 to 30. The linearized stability equations form an eigenvalue problem which are solved using a numerical technique based on classical Runge-Kutta scheme combined with a shooting method, termed as unit disturbance method. It is observed that radially outward flow and strong inward flow have a stabilizing effect, while weak inward flow has a destabilizing effect on the stability. Profiles of the relative amplitude of the perturbed radial velocities show that radially outward flow shifts the vortices toward the outer cylinder, while radially inward flow shifts the vortices toward the inner cylinder.     

Identification of self-organized vortexlike structures in numerically simulated turbulent flow of a viscous incompressible liquid streaming around a well on a plane

A system of Reynolds equations closed by adding a low-Reynolds-number Menter’s dissipative model of turbulence are solved with the use of a factorized finite-volume method. The generation of vortexlike structures in the case of stalled turbulent flow streaming around a deep well on a plane is analyzed. The effect of vortex intensification due to asymmetry of the well is discussed.     

自激振动圆柱体湍流场及发展变化的研究

采用雷诺平均法模拟低质量比弹性支撑的刚性圆柱体在均匀来流中的涡激振动问题,是近年来人们研究的热点。本文采用有限体积法结合SST k-ω湍流模型求解时间平均的纳维尔——斯托克斯方程(RANS),对低质量比弹性支撑的刚性圆柱体在均匀来流中横向振动的三个响应分支（初始分支、上端分支和下端分支）湍流场及其发展变化进行了研究,并与同等雷诺数下固定圆柱体的湍流场进行了对比。结果表明：三个响应分支的流体力系数、壁面压力系数、壁面切应力、流向平均速度、流向速度的脉动、横向速度的脉动、雷诺应力、湍流动能和涡量的分布均与固定圆柱体有明显差异。     

The flow structure of a puff

From time-resolved stereoscopic particle image velocimetry measurements over the entire circular cross section of a pipe, a first-of-its-kind quasi-instantaneous three-dimensional velocity field of a turbulent puff at a low Reynolds number is reconstructed. At the trailing edge of the puff, where the laminar flow undergoes transition to turbulence, pairs of counterrotating streamwise vortices are observed that form the legs of large hairpin vortices. At the upstream end of the puff, a quasi-periodic regeneration of streamwise vortices takes place. Initially, the vortex structure resembles a travelling wave solution, but as the vortices propagate into the turbulent region of the puff, they continue to develop into strong hairpin vortices. These hairpin vortices extract so much energy from the mean flow that they cannot be sustained. This structure provides a possible explanation for the intermittent character of the puffs in pipe flow at low Reynolds numbers.