The influence of sub-grid scale motions on particle collision in homogeneous isotropic turbulence

The absence of sub-grid scale(SGS) motions leads to severe errors in particle pair dynamics, which represents a great challenge to the large eddy simulation of particle-laden turbulent flow. In order to address this issue,data from direct numerical simulation(DNS) of homogenous isotropic turbulence coupled with Lagrangian particle tracking are used as a benchmark to evaluate the corresponding results of filtered DNS(FDNS). It is found that the filtering process in FDNS will lead to a non-monotonic variation of the particle collision statistics, including radial distribution function, radial relative velocity, and the collision kernel. The peak of radial distribution function shifts to the large-inertia region due to the lack of SGS motions, and the analysis of the local flowstructure characteristic variable at particle position indicates that the most effective interaction scale between particles and fluid eddies is increased in FDNS. Moreover,this scale shifting has an obvious effect on the odd-order moments of the probability density function of radial relative velocity, i.e. the skewness, which exhibits a strong correlation to the variance of radial distribution function in FDNS.As a whole, the radial distribution function, together with radial relative velocity, can compensate the SGS effects for the collision kernel in FDNS when the Stokes number based on the Kolmogorov time scale is greater than 3.0. However,it still leaves considerable errors for St< 3.0.

Studies on gas turbulence and particle fluctuation in dense gas-particle flows

Particle fluctuation and gas turbulence in dense gas-particle flows are less studied due to complexity of the phenomena. In the present study, simulations of gas turbulent flows passing over a single particle are carried out first by using RANS modeling with a Reynolds stress equation turbulence model and sufficiently fine grids, and then by using LES. The turbulence enhancement by the particle wake effect is studied under various particle sizes and relative gas velocities, and the turbulence enhancement is found proportional to the particle diameter and the square of velocity. Based on the above results, a turbulence enhancement model for the particle-wake effect is proposed and is incorporated as a sub-model into a comprehensive two-phase flow model, which is then used to simulate dilute gas-particle flows in a horizontal channel. The simulation results show that the predicted gas turbulence by using the present model accounting for the particle wake effect is obviously in better agreement with the experimental results than the prediction given by the model not accounting for the wake effect. Finally, the proposed model is incorporated into another two-phase flow model to simulate dense gasparticle flows in a downer. The results show that the particle wake effect not only enhances the gas turbulence, but also amplifies the particle fluctuation.

Solution of general dynamic equation for nanoparticles in turbulent flow considering fluctuating coagulation

A new averaged general dynamic equation （GDE） for nanoparticles in the turbulent flow is derived by considering the combined effect of convection, Brownian diffusion, turbulent diffusion, turbulent coagulation, and fluctuating coagulation. The equation is solved with the Taylor-series expansion moment method in a turbulent pipe flow. The experiments are performed. The numerical results of particle size distribu- tion correlate well with the experimental data. The results show that, for a turbulent nanoparticulate flow, a fluctuating coagulation term should be included in the averaged particle GDE. The larger the Schmidt number is and the lower the Reynolds number is, the smaller the value of ratio of particle diameter at the outlet to that at the inlet is. At the outlet, the particle number concentration increases from the near-wall region to the near-center region. The larger the Schmidt number is and the higher the Reynolds num- ber is, the larger the difference in particle number concentration between the near-wall region and near-center region is. Particle polydispersity increases from the near-center region to the near-wall region. The particles with a smaller Schmidt number and the flow with a higher Reynolds number show a higher polydispersity. The degree of particle polydispersity is higher considering fluctuating coagulation than that without considering fluctuating coagulation.

LARGE EDDY SIMULATION OF PARTICLE TRANSPORT IN FULLY DEVELOPED VERTICAL TURBULENT CHANNEL FLOW

Fully developed vertical turbulent channel flow with particle transport wasinvestigated by use of Large Eddy Simulation (LES) approach coupled with dynamic the Sub-Grid Scale(SGS) model. It was assumed that the motion of each particle is followed in a Lagrangian frame ofreference driven by the forces exerted by fluid motion and gravity under the condition of one-waycoupling. The goal of this study is to investigate the effectiveness of the LES technique forpredicting particle transport in turbulent flow and the behavior of particle-laden turbulent channelflow for three kinds of particles at different Stokes numbers. To depict the behavior ofparticle-laden turbulent channel flow, statistical quantities including particle fluctuation andfluid-particle velocity correlation, and visualization of the particle number density field wereanalyzed.

The dispersion of particles in turbulent semi-circular duct flows

The flow field in a semi-circular duct is simulated by Large Eddy Simulation(LES)and its particle field is simulated by Lagrange particle tracking method.Reynolds number Reb(based on bulk velocity and hydraulic diameter)is 80,000 and Ret(based on friction velocity and hydraulic diameter)is 3528.Particle diameter dpis chosen as 10,50,100,500 mm corresponding to St as 0.10,2.43,9.72,243.05.The results show that the intensity of the secondary flow near the ceiling is less than that near the floor because the ceiling is curved and able to inhibit the secondary flow.It is found that the difference between the semicircular duct and the square duct is that the secondary flow in a corner of the semi-circular duct is not symmetrical along the diagonal although they have the same generation mechanism.Regarding the particles,small particles(dp≤10 mm)are found to uniformly distribute in the duct,while large particles(dp≥50 mm)preferentially distribute in the corner and floor center.The maximum particles(dp=500mm)fall on the floor quickly and their dispersion mainly depends on the secondary flow near the floor.Particle deposition in the corner depends on particle size due to the effect of secondary flow and gravity.The effect of lift force on particles becomes more significant for 50 and 100 mm particles in comparison with other smaller particles.In the end,the effect of secondary flow is found to be more significant to dominate particle behavior than that of flow fluctuation.

Transport of particles in an atmospheric turbulent boundary layer

A program incorporating the parallel code of large eddy simulation （LES） and particle transportation model is developed to simulate the motion of particles in an atmospheric turbulent boundary layer （ATBL）. A model of particles of 100-micrometer order coupling with large scale ATBL is proposed. Two typical cases are studied, one focuses on the evolution of particle profile in the ATBL and the landing displacement of particles, whereas the other on the motion of particle stream.

Estimation of the turbulent viscous shear stress in a centrifugal rotary blood pump by the large eddy particle image velocimetry method

The non-physiologic turbulent flows in centrifugal rotary blood pumps (RBPs) may result in complications such as the hemolysis and the platelet activation. Recent researches suggest that the turbulent viscous dissipation in the smallest eddies is the main factor of the blood trauma caused by the turbulent flow. The turbulent viscous shear stress (TVSS) was taken as the realistic physical force acting on the cells. However, limited by the temporal and spatial resolutions of the instrumentation currently available, very limited studies are available for the TVSS in the RBPs. In this paper, the large eddy particle image velocimetry (PIV) method is used to estimate the turbulent dissipation rate in the sub-grid scale, to investigate the effect of the TVSS on the blood trauma. Detailed flow characteristics, such as the relative velocity vectors, the estimated TVSS levels and the Kolmogorov length scales, are analyzed in three impeller phases at three constant flow rates (3 L/min, 5 L/min and 7 L/min). Over the measures range in this study, the maximum TVSS in the investigated RBP is lower than the reported critical value of stress. This study demonstrates that the large eddy PIV method is effective to evaluate the flow-dependent force on the cells. On the other hand, it is found that the TVSS is highly dependent on the flow behavior. Under severe off-design conditions, the complex flow characteristics, such as the flow separation and the vortical structures, will increase the TVSS. Thus, in order to reduce the hemolysis in the RBPs, the flow disturbance, induced by the departure of the incidence angle, should be avoided during the design of the RBPs.

Comparison of Gas Particle Flow Prediction from Large Eddy Simulation and Reynolds-Averaging Navier-Stokes Modeling

In order to account for the effect of particle existence on gas-particle turbulence flow in large-eddy simulation (LES),a new gas-particle turbulent kinetic energy subgrid-scale (SGS) turbulence model is established,and the effect of particle wake is also considered in gas turbulent kinetic energy SGS turbulence model.Simulation of gas-particle turbulence flow in backward-facing step is carried out by LES using present model and by unified second-order moment (USM) model.The prediction statistical results including mean velocity and fluctuation velocity by LES using present model are in reasonable agreement with the experimental results.It is shown that present model is with higher calculating accuracy than USM model,which indicates that the turbulent kinetic energy SGS turbulence model is suitable.

A numerical method for spherical particle motion in turbulent flow with large Reynolds number

A new mathematical model, fluctuation spectrum random trajectory model (FSRTM) for the particle motion in environmental fluid was developed using Lagrangian method, in which the time mean velocity of the fluid was calculated by a time mean velocity formula for two dimensional homogeneous shear turbulent flows in open channel, the velocity fluctuation of the fluid was determined by Fourier expansion and fluctuation spectrum, and the particle motion equation was solved using Ronge Kutta method. For comparison, the spherical cation exchange resins with a density of 1 44 g/cm\+3 and diameters ranging from 0 50—0 60 mm, 0 60—0 70 mm and 0 80—0 90 mm were selected as the experimental solid particles, and their moving velocities and trajectories in shear turbulent flows with the flow Reynolds number of 4710, 10240, 11900 and 20760 were investigated. The comparing analyses of the modeled results with the measured results have shown that the model developed in this paper can describe the motions of the particles in shear turbulent flow.

Investigation of Wall Pressure Fluctuations in a Turbulent Boundary Layer by Large Eddy Simulation

Large eddy simulation (LES) was used to investigate the space-time field of the low Mach number, fully developed turbulent boundary layer on a smooth, rigid flat plate. The wall-pressure field simulated by LES was analyzed to obtain the pressure statistics, including the wall-pressure root-mean square, skewness and flatness factors, which show the wall pressure distribution was not Gaussian. The profile of the auto-power spectral density and the contour of the streamwise wavenumber-frequency spectral density of wall-pressure were plotted. The "convection ridge" can be observed clearly and the convection velocity can be calculated from the location of the convection peak.

Sufficient noise and turbulence can induce phytoplankton patchiness

Phytoplankton patchiness ubiquitously obser- ved in marine ecosystems is a simple phy- sical phenomenon. Only two factors are required for its formation: one is persistent variations of inhomogeneous distributions in the phytopl- ankton population and the other is turbulent stirring by eddies. It is not necessary to assume continuous oscillations such as limit cycles for realization of the first factor. Instead, a certain amount of noise is enough. Random fluctua-tions by environmental noise and turbulent ad-vection by eddies seem to be common in open oceans. Based on these hypotheses, we pro-pose seemingly the simplest method to simulate patchiness formation that can create realistic images. Sufficient noise and turbulence can induce patchiness formation even though the system lies on the stable equilibrium conditions. We tentatively adopt the two-component model with nutrients and phytoplankton, however, the choice of the mathematical model is not essen-tial. The simulation method proposed in this study can be applied to whatever model with stable equilibrium states including one-com-ponent ones.

Large-eddy Simulation of Near-field Dynamics in a Particle-laden Round Turbulent Jet

This article investigates the near-field dynamics in a particle-laden round turbulent jet in a large-eddy simulation （LES）. A point-force two-way coupling model is adopted in the simulation to reveal the particle modulation of turbulence. The particles mainly excite the initial instability of the jet and bring about the earlier breakup of vortex rings in the near-field. The flow fluc- tuating intensity either in the axial or in the radial directions is hence increased by particles. The article also describes the mean velocity modulated by particles. The changing statistical velocity induced by particle modulation implies the effects of modulation of the local flow structures. This study is expected to be useful to the control of two-phase turbulent jets.

Effect of turbulent fluctuation on the ignition of millimeter particle:Experimental studies and numerical modelling with a new correlation of nusselt number Author links open overlay panel

Understanding the influencing mechanism of turbulent fluctuation on the ignition characteristics of millimeter coal particles is essential.In this work,to study the effect of turbulent fluctuation on ignition time,millimeter coal particles are subjected to a specific flow field,generated in a furnace with symmetric fans.A one-dimensional model with the new proposed correlation and the Ranz-Marshall(R-M)correlation for Nu(Nusselt number)is established to simulate the coal ignition process.In addition,the effects of fan speed,temperature,particle diameter,particle distance and coal type on the ignition time are investigated.It is found that an increase in fan speed from 0 to 3000 rpm leads to a particle Reynolds number Re_(p)increase from 0 to 22.5,and a turbulent particle Reynolds number Re_(t)*increase from 0 to 71.5.With a consideration of the fluctuation effect,the new correlation of Nu gives a better prediction of ignition time compared to the R-M correlation.Moreover,the ignition time is revealed to decrease with an increasing fan speed and an elevating temperature.While the ignition time shows merely an initial boost with enlarging particle distance,it exhibits a linearity with the term of particle diameter dp1.3-1.7 and Reynolds numbers(Nu*/Nu)-0.6(Nu*is turbulent Nusselt number).Based on this relationship,the difference of predicted ignition time is calculated at different Re_(p)and Re_(t)*.It is shown that at low Re_(p)or high Re_(t)*values,the new correlation should substitute for the R-M correlation.

Numerical analysis of turbulent fluctuations around an axisymmetric body of revolution based on wall-modeled large eddy simulations

Wall-modeled large eddy simulation(WMLES)is used to investigate turbulent fluctuations around an axisymmetric body of revolution.This study focuses on evaluating the ability of WMLES to predict the fluctuating flow over the axisymmetric hull and analyzing the evolution of turbulent fluctuations around the body.The geometry is the DARPA SUBOFF bare model and the Reynolds number is 1.2×10^(7),based on the free-stream velocity and the length of the body.Near-wall flow structures and complex turbulent fluctuation fields are successfully captured.Time-averaged flow quantities,such as time-averaged pressure and skin-friction coefficients,and time-averaged velocity profiles on the stern,achieved great agreements between WMLES results and experimental data.Self-similarity of time-averaged velocity defects within a self-similar coordinate up to twelve diameters from the tail.A comprehensive analysis of second-order statistics in the mid-body,stern,and wake regions is condutced.Numerical results agree well with experimental data and previous wall-resolved large eddy simulation(WRLES)results about root mean square(rms)of radial and axial fluctuating velocities at the stern.Turbulent fluctuations including turbulent kinetic energy(TKE)and second-order velocity statistics are identified as dual peak behavior and non-self-similar over the wake length,consistent with previous findings in the literature.This assessment enhances the understanding of WMLES capabilities in capturing complex fluctuating flow around axisymmetric geometries.

湍流热环境中毫米级木质生物质颗粒燃烧特性

燃煤机组耦合生物质甚至全燃生物质是高效低成本降碳的可行技术。生物质燃料破碎能耗高导致入炉粒径相对较大,部分大粒径颗粒在炉内高温湍流环境中的燃尽问题值得重视。采用四风扇对冲高温湍流实验装置,构造近均匀各向同性湍流流场。以两种粒径的木质生物质颗粒(d_(p,0)=2.5、6.0 mm)为研究对象,通过改变炉温(T_(gas)=500、700、900℃)和湍流脉动速度(u_(rms)=0~1.8 m/s),研究湍流脉动速度urms对毫米级生物质颗粒燃烧特性的影响。试验通过颗粒表面-中心温度测量系统记录了颗粒温度,通过彩色图像测量系统捕捉燃烧全过程,确定不同工况下生物质颗粒的燃烧时间、着火模式、火焰形态及粒径变化。结果表明,生物质颗粒通常倾向于发生均相着火,仅在T_(gas)=500℃时增大u_(rms)使着火模式由均相着火转变为异相着火。u_(rms)增至1.8 m/s,着火前颗粒升温速率提高了近30%,挥发分燃烧阶段颗粒表面温度升高约300℃。u_(rms)增加引起挥发分火焰锋面褶皱变形,均相燃烧强度增加,tvol略微缩短;生物质焦炭的孔隙发展更加迅速,大量氧气扩散进颗粒内发生反应,使焦炭燃尽时间大幅缩短40%以上,焦炭燃烧温度亦随之增加。颗粒湍流雷诺数Re_(p,t)越大,受湍流脉动影响越显著。升高炉温,增大u_(rms)对颗粒温度的影响将减弱,对缩短燃烧时间的影响越强烈。

搅拌槽内湍流动能耗散率的估算进展

在搅拌槽内,化学反应效率取决于微观混合程度,而微观混合程度又取决于湍流动能耗散率的大小。本文介绍了5种估算湍流动能耗散率的方法———直接测量法、量纲分析法、湍流动能平衡法、大涡PIV法和湍流能谱法,并且从流体力学理论和测量技术两方面分析了这些方法的优劣。

PROBABILITY DISTRIBUTION FUNCTION OF NEAR-WALL TURBULENT VELOCITY FLUCTUATIONS

By large eddy simulation （LES）, turbulent databases of channel flows at different Reynolds numbers were established. Then, the probability distribution functions of the streamwise and wall-normal velocity fluctuations were obtained and compared with the corresponding normal distributions. By hypothesis test, the deviation from the normal distribution was analyzed quantitatively. The skewness and flatness factors were also calculated. And the variations of these two factors in the viscous sublayer, buffer layer and log-law layer were discussed. Still illustrated were the relations between the probability distribution functions and the burst events-sweep of high-speed fluids and ejection of low-speed fluidsIin the viscous sub-layer, buffer layer and loglaw layer. Finally the variations of the probability distribution functions with Reynolds number were examined.

采用大涡PIV方法研究搅拌槽内湍流动能耗散率

在槽径为0.476m的六直叶涡轮桨搅拌槽内,采用粒子图像测速仪(PIV)对桨叶区的流场进行了实验研究,得到了桨叶区的平均流速和湍流动能(k)分布,采用大涡PIV方法对湍流动能耗散率(ε)分布进行了估算,计算了ε与k的相关系数.结果表明大涡PIV方法能有效地估算ε分布;桨叶区的射流向上倾斜,两尾涡分布于射流两侧,射流的倾角和两尾涡中心间距随射流向壁面运动而变化,射流倾角先增大再减小,相位角θ=40o时达到最大值13.2o,两尾涡中心间距先减小再增大,θ=20o时达到最小值0.0387(用槽径T无因次化);湍流动能和湍流动能耗散率峰值均位于尾涡靠近射流的区域;湍流动能和湍流动能耗散率的平均相关系数为0.363,射流核心区相关系数小于周边区域.

基于湍流涡调控的煤气化渣炭-灰浮选分离过程强化

煤气化渣因炭、灰包裹夹杂严重、嵌布粒度细,导致浮选分离困难,制约了其资源化利用。浮选大多发生在湍流环境中,调控湍流是强化微细颗粒矿物浮选回收的有效途径,湍流小尺度涡直接作用于微细颗粒运动,研究借助涡流发生器实施湍流涡调控以进行煤气化渣中的炭-灰浮选分离过程强化。利用计算流体力学数值模拟对涡流矿化管内部流场进行数值计算,分析涡流发生器结构对湍流特征参量及煤气化渣浮选指标的影响,在此基础上设计了与矿物可浮性相适配的梯级涡流浮选过程。结果表明:管内矩形涡流发生器可诱导出发卡涡、流向涡及旋转方向相反的二次流向涡对,涡-涡、涡-主流之间的交互作用显著提高了湍流动能、降低了涡尺度,有利于微细颗粒与气泡间的碰撞。涡流发生器的倾斜角度从25°增至55°时,湍流动能均值由0.041 m^(2)/s^(2)增到0.142 m^(2)/s^(2),最小涡尺度均值由16.10μm减至10.34μm。采用内置结构相同涡流发生器的均衡涡流浮选装置对煤气化渣进行炭-灰浮选分离试验,不同粒级浮选回收率表明,粒度越细,需要的湍流动能越大、涡尺度越小,诱发的湍流特性不当时颗粒可能从气泡表面脱附。在研究范围内,与-45,45~75μm煤气化渣颗粒相适配的最小涡尺度均值分别为12.74μm和14.71μm,相应湍流动能均值分别不宜超过0.080 m^(2)/s^(2)及0.056 m^(2)/s^(2);将不同倾斜角度的涡流发生器在矿化管内沿着流动方向有序排列,形成与矿物可浮性相适配的梯级涡流浮选过程,实现不同可浮性颗粒的逐步回收,浮选试验表明利用梯级涡流浮选装置进行煤气化渣炭-灰浮选分离时,可燃体回收率为89.99%,尾矿烧失量低至4.66%,优于相同条件下均衡涡流浮选装置和机械搅拌式浮选机的浮选指标。通过对流体环境的物理调控,可为煤气化渣的炭-灰浮选分离提供新的过程强化方式。

近壁湍流脉动的概率分布函数

采用大涡模拟方法,模拟槽道湍流,获得了不同雷诺数情形下的槽道流大涡模拟数据库.在此基础上,获得了流向和垂向脉动速度的概率分布函数,并运用假设检验,分析了其与正态分布的定量差别.进一步计算了流向和垂向脉动速度的偏斜度、平坦度,讨论了二者在粘性子层、过渡区和对数律区的变化.同时,讨论了粘性子层、过渡区和对数律区流向和垂向脉动速度概率分布函数的特点及其与湍流猝发的高速流下扫和低速流喷发事件的关系.最后,分析了雷诺数对流向、垂向脉动速度分布的影响.