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.     

Fluctuation velocity correlation closure model for dense gas-particle turbulent flow

Turbulence model of kg-εg-kp-εp-kpg-θ is proposed.In the model,the two-phase velocity correlation turbulent kinetic energy k pg is modeled by transport equation.To close this turbulence model,algebraic expressions of two-phase Reynolds stresses and two-phase velocity correlation variable are established by considering both gas-particle interaction and anisotropy.This turbulence model is used to simulate dense gas-particle flow in a riser and in a downer.The predicted results show the core-annulus flow structure observed in the riser and the skin effect of particle concentration in the downer.The present model gives simulation results in much better agreement with the experimental results than those obtained by kg-εg-kp-εp-θmodel which is simply closed using a semi-empirical dimensional analysis.     

Characteristics of flow field around an underwater projectile with natural and ventilated cavitation

The present simulation investigates the multiphase cavitating flow around an underwater projectile. Based on the homogeneous equilibrium flow assumption, a mixture model is applied to simulate the multiphase cavitating flow including ventilated cavitation caused by air injection as well as natural cavitation. The transport equation cavitating model and a local linear low-Reynolds-number turbulence model are incorporated to the equation system. The calculations are executed based on a suite of computational fluid dynamics code. The hydrodynamics characteristics of flow field under the interaction of natural cavitation and ventilated cavitation are analyzed. The results indicate that the ventilated cavitation numbers and the drag coefficient are relative to the natural cavitation numbers and gas flow rate in the multiphase cavitating flows.     

不同长宽比矩形高层建筑的分离再附流动特性

为研究边界层紊流特性、断面长宽比和空间位置等因素与矩形高层建筑分离再附流动特性之间的关联，通过不同长宽比矩形建筑模型的同步测压试验，获取不同工况下的表面风压实测数据；分析了影响矩形建筑三维分离再附流动和分离区长度演化规律的多种因素，探讨了大尺度紊流下矩形高层建筑的非定常气动力与分离再附流动特性的内在关联；定量给出了平均分离区长度在竖向的分布规律. 研究结果表明:紊流风场的积分尺度与建筑特征尺寸的比值关系会影响分离再附流动和气动力特性的试验精准度，最大偏差可达约24%；边界层紊流的干扰导致分离剪切层曲率增大，加强了对分离区的卷夹作用，稳定再附将发生在长宽比为2时；平均分离区长度在竖向方向逐渐增大，并依据试验结果给出了高层建筑侧面风压取值的修正建议.      

Pulsation characteristics of ventilated supercavitation on a 2D hydrofoil

AbstractBased on a suite of computational fluid dynamics code,the pulsation characteristics are studied for turbulent supercavitating flows over a 2D base-vented symmetric hydrofoil using a pressure-based Navier-Stokes solver coupled with a phase mass fraction transport cavitation model and local linear low-Reynolds-number k-ε turbulence model.It is found that there exists a critical air supply flow under certain inflow condition.When the gas supply flow exceeds the critical value,the cavity begins to pulsate.Pulsating cavity has a visually wavy surface,and air leakage shows as a mass of air-water mixture detaching from the rear part of the cavity periodically.     

微尺度下气体在过渡区内流动的格子Boltzmann模拟

为研究微尺度下气体在过渡区内的流动特性,基于气体动理学及Knudsen层效应理论,推导了Knudsen数与无量纲松弛时间的关系;应用Succi的边界处理方法和广义二阶滑移边界条件,推导了壁面滑移速度和反弹比例系数的计算公式,建立了适用于过渡区微尺度气体流动的格子Boltzmann模型,并应用该模型对过渡区内微尺度Poiseuille流动进行模拟.结果表明,当稀薄参数取1.64时,计算得到的无量纲速度剖面在整个过渡区与Karniadakis给出的无量纲速度剖面吻合较好,无量纲速度分布在过渡区基本上保持为抛物线形状,边界上的无量纲滑移速度随着Knudsen数的增加而增大,中心线上的无量纲速度随着Knudsen数的增加而减小.      

横向风作用下高速铁路车桥系统绕流特性分析

基于不可压缩二维雷诺平均N-S方程，采用RING（renormalization group）k-ε两方程湍流模型，对德国ICE（intercity-express）列车和标准简支梁组成的车桥系统在横向风作用下的二维绕流特性进行了分析．计算网格为由三角形非结构网格和四边形结构网格组成的混合网格，采用有限体积法对微分方程进行离散，应用SIMPLE算法解决压强-速度耦合关系．根据高速铁路桥梁带挡风墙的特点，引入空腔流动研究成果对车桥系统的绕流特性进行了理论分析．结果表明，高速铁路桥梁挡风墙的挡风机理在于充分运用了空腔流的旋涡特性．     

翼吊发动机转子系统在大气紊流下的响应分析

针对翼吊发动机高空工作时受大气紊流影响这一现象,基于机翼气动弹性力学和转子动力学理论, 采用Dryden紊流模型作为大气速度模型,建立了翼吊发动机转子系统在大气紊流下的动力学模型,运用虚拟激励法对模型进行数值计算.结果表明:转子系统中各转盘站的位移响应功率谱密度变化趋势与大气紊流速度功率谱密度变化趋势基本相似,但在固有频率处会出现位移响应强度峰值;当角频率大于100 rad/s时,大气紊流引起的位移响应强度随着角频率的增大而减小,并且不可忽视转子系统本身特性对位移响应强度的影响.      

Effects of compressibility and Knudsen number on the aero optics in hypersonic flow fields

Tsien summarized the similarity in hypersonic flows, and related Knudsen number to Mach number and Reynolds number. Recently, a path-based problem, aero-optical effect, arises in hypersonic flows, and it concerns about the compressibility and the Knudsen number of the gas flows, which differs from the Tsien’s focus to some extent. In this paper, the similarity of hypersonic aero optics is theoretically studied, and both flow fields and induced aero-optical effect after flows pass through a cylinder are predicted by a well-accepted particle-based method, direct simulation Monte-Carlo (DSMC) method. The results show that the optical distortions are inversely proportional to the Knudsen number, while the compressibility plays an important role in the optical degradations. Hence, it is confirmed that the effects of Mach number and Knudsen number on the aero-optical effect induced by hypersonic flows are of great significance. Besides, since the Knudsen number is defined straightforwardly based on the optically active region, the physics is clearer than any other similarity criteria.     

Phase Behaviors in Bi-phase Simulation of Powder Segregation in Metal Injection Molding

Powder segregation induced by mold filling is an important phenomenon that affects the final quality of metal injection molding （MIM）. The prediction of segregation in MIM requires a bi-phase flow model to describe distinctly the flows of metallic powder and polymer binder. Viscous behaviors for the flows of each phase should hence be determined. The coefficient of interaction between the flows of two phases should also be evaluated. However, only viscosity of the mixed feedstock is measurable by capillary tests. Wall sticking is supposed in the traditional model for capillary tests, while the wall slip is important to be taken into account in MIM injection. Objective of the present paper is to introduce the slip effect in bi-phase simulation, and search the suitable way to determine the viscous behaviors for each phase with the consideration of wall slip in capillary tests. Analytical and numerical methods were proposed to realize such a specific purpose. The proposed method is based on the mass conservation between the capillary flows in mono-phase model for the mixed feedstock and in bi-phase model for the flows of two phases. Examples of the bi-phase simulation in MIM were realized with the software developed by research team. The results show evident segregation, which is valuable for improving the mould designs.     

施工隧道负离子除尘效率的主要影响因素

为研究施工隧道内粉尘颗粒粒径、颗粒浓度、通风风速和负离子系统工作电压、纵向安装位置对负离子系统粉尘降除效率的影响，依据调研和实测选取隧道计算参数，建立隧道及负离子系统三维模型，采用RNG k-ε双方程湍流模型，并通过动量方程附加电场力源项的方法求解电流场，利用拉格朗日法求解粉尘颗粒的运动轨迹，用SIMPLE算法对颗粒运动与电场流场进行离散相和流体相相间耦合的数值模拟计算，并将模拟结果和隧道现场抽样试验结果对比分析. 研究结果表明:隧道粉尘粒径越大，浓度越大，风速越低，负离子系统工作电压越高，系统纵向安装位置越偏于上风口，负离子系统除尘效率越高；两组现场抽样试验与对应数值模拟所得的除尘效率分别为41.2%、56.7%和38.2%、51.1%，误差分别为15.5%和12.9%. 考虑施工隧道大空间复杂环境的影响，通过数值模拟的方法来研究负离子系统除尘效率及其与主要影响因素的关系是可行的.      

城市街道峡谷机动车排放氮氧化物扩散模拟

为了研究风速、风向、环境温度、污染源以及建筑物距离等因素对街道峡谷中汽车排放污染物扩散影响特性,本文采用RNG k-ε湍流模型方程及污染物对流扩散方程对其进行了二维数值模拟,得到了街道峡谷中机动车排放氮氧化物的流场和浓度场,从而证明了风速、风向、环境温度、污染源与建筑物距离等因素对街道峡谷中汽车排放污染物扩散确有明显影响作用。     

Revised Three-Dimensional Navier-Stokes Characteristic Boundary Conditions for Intense Reactive Turbulence

The three-dimensional Navier-Stokes characteristic boundary conditions(3D-NSCBC), although physically reasonable and popular in many applications, may encounter the instability problem in simulating complex flows, especially for large Reynolds number reactive turbulence where locally the strong reversed flow appears at the outflow boundary surfaces. In the present work, a revised 3D-NSCBC strategy is proposed based on the kinematic relation in different moving coordinate systems. Following this strategy, a systematic formulation is presented for the outflow surface with local reversed flow and can be easily extended to the coupled edge and corner boundaries. Direct numerical simulation(DNS) tests of flow with different turbulence intensities are carried out. Compared with the conventional 3D-NSCBC, the newly proposed method exhibits satisfactory performance to confine numerical instability in the strong reversed flow region. The results confirm the robustness and effectiveness of this newly proposed algorithm.     

大气湍流对飞机颠簸的影响

利用具有6个自由度的飞机的运动方程，研究了大气湍流对飞机垂直过负荷的影响，推导了飞机在湍流大气中过负荷的传递函数，并分析了飞机对非定常湍流的反应．研究表明，在亚音速下，飞机飞行速度增大。飞机颠簸增强；在同样的气象条件和大气湍流强度下，小型飞机的颠簸比大型机剧烈。     

表面有螺旋装饰条的大跨贝壳形屋盖风荷载特性

为了解装饰造型对大跨贝壳形屋盖风荷载特性的影响，研究了某表面有螺旋形装饰条高铁站房屋盖的风压分布特征. 首先，基于Reynolds时均RNG k-ε湍流模型，建立表面有装饰条大跨贝壳形屋盖的数值风洞模型，以模拟屋盖风压分布，通过与风洞试验结果的对比分析，验证了数值模型的可靠性和适用性；然后，建立表面无装饰条贝壳形屋盖的数值风洞，并进行数值模拟；最后，将表面有装饰条屋盖和表面无装饰条屋盖的计算结果进行对比，从风荷载升力系数、局部体型系数和局部区域速度矢量分布方面，分析表面有螺旋形装饰条贝壳形屋盖的风场变化特性. 研究结果表明:数值模拟得到的表面有装饰肋条大跨贝壳形屋盖风载体型系数与风洞试验的体型系数相对误差在 ± 25%以内，风荷载升力系数偏差率在?7.1%～6.1%；表面有装饰条模型的风载升力系数小于表面无装饰条模型，最大偏差率达22.4%，设置装饰条对大跨贝壳形屋盖的整体抗风有利；设置装饰条可以使屋面大部分区域的局部风压减小0～50%，但也会使得个别区域增大2～5倍，在对局部附属构件设计时应予以重点关注；装饰肋条间有一定的狭管效应，肋条对风流的阻挡效应致使风流在肋条处有明显的漩涡产生，并使得肋条自身的风压较高，表面有装饰条屋盖会在背风面产生比表面无装饰条屋盖更大范围的尾涡.      

积分尺度对矩形迎风面脉动风压特性的影响

为了研究来流紊流积分尺度对矩形断面高层建筑迎风面脉动风压及其分布特性的影响，选取2∶1和1∶2矩形为对象，通过风洞测压试验，对不同积分尺度紊流场中矩形迎风面的平均风压系数及脉动风压均方根系数、脉动风压相关函数和相干函数、脉动风压功率谱进行了对比分析. 研究结果表明:对于矩形同一高度处，脉动风压功率谱在低频区始终受准定常效应控制，而在高频区脉动风压功率谱随积分尺度的增大而增大；风压的相关性高于风的相关性，风压的相关函数与相干函数也随积分尺度的增大而增大，但相关宽度随积分尺度的增大而减小；脉动风压均方根系数随积分尺度的增大而增大；对于同一流场中矩形不同测点处，离驻点越远，风压相关函数和相干函数越小，脉动风压均方根系数越大；来流紊流积分尺度对平均风压系数的影响较小.      

考虑排队阻滞和换道的改进宏观交通流模型

考虑路段上车辆换道及交叉口处排队阻滞作用,对交通流宏观演化模型进行改进.首先在排队中考虑不同车道的速度、密度差异,建立了换道模型;然后在交叉口内部划分各流向排队车辆的累积路径,分析不同的排队溢出位置对其他车流的阻滞影响;综合考虑车辆换道及交叉口处阻滞影响,建立了改进的车道组宏观交通流模型;最后设计仿真实验对模型进行验证.结果表明,在高流量需求下,本文模型能够模拟出下游的排队溢出对其他车流的阻滞影响;且在低中高3种不同的流量需求下,改进模型计算结果与仿真输出的驶离流量基本相符,与原有模型相比精度更高;由此证实了改进模型的准确性.     

基于时变边界屋面积雪分布数值模拟

为了预测风作用下屋面积雪的分布,采用Euler-Euler两相流混合模型,结合最新改进的k-ω模型等数值方法开发分析程序,对风致屋面积雪分布进行了数值模拟.模拟中为考虑积雪对屋面绕流的影响,计算区域边界根据雪深变化采用时变边界.对一典型阶梯形屋面积雪分布进行数值模拟,获得了几种不同状态下屋面积雪分布的时间历程.模拟结果表明:随着时间的发展,屋面积雪分布对屋面绕流产生较大影响,屋面积雪沉积率随之发生变化;模拟时长22 h的计算结果与实测结果基本一致,不考虑积雪对屋面绕流的影响将产生较大误差;不同风速比下模拟的屋面雪深分布形态基本一致,但入流风速比越小,雪深分布系数相对越大,达到近似分布的模拟时间就越短.      

基于PIV技术的明渠非恒定流试验系统

建立了一套高精度的明渠非恒定流试验系统,通过计算机及变频器控制水泵转速产生非恒定流过程,由电磁流量计、超声水位计实时监测流量和水位变化过程,并通过PIV流场仪进行流场测量。该系统不但能生成连续、稳定的周期性非恒定流过程,很大程度改进了明渠非恒定流的试验手段。     

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研究先进出行者信息系统(ATIS)环境下道路网络系统流量的随机动态变化,有利于实现ATIS在网络系统中的进一步优化配置.在出行者经验积累过程基础上,引入ATIS作用下的认知更新过程,将路网系统的路径流量明确视为随机变量,提出了一个基于认知更新的随机动态分配模型.证明了该模型产生的路径流量渐近收敛于一个平稳概率分布.在算例网络中验证了模型的可行性,模拟有ATIS和无ATIS两种情形下路径流量的随机动态变化.结果表明,两种情形路径流量均收敛于平稳概率分布.前者的平均流量近似于随机用户平衡(SUE),而后者的平均流量不是平衡流量,但其网络总费用要低于前者.