一种改进的颗粒移动床μ(I)拟流体模型及应用

颗粒移动床在工业领域应用广泛, 发展实用可靠的颗粒移动床模型具有理论和应用价值. 本文基于颗粒流μ(I)模型, 补充局部颗粒体积分数与颗粒局部压力和局部颗粒流密度的关系式, 将移动床内密集颗粒处理成可压缩拟流体, 建立了颗粒流单相可压缩流μ(I)模型, 并建立了颗粒流?壁面摩擦条件, 在计算中对颗粒流拟黏度和拟压力项进行正则化处理. 采用上述模型与方法对3种典型散料在移动床缩口料仓内的流动进行模拟, 与实验对比, 得到了玻璃珠、刚玉球和粗沙的μ(I)模型参数, 分析了3种不同散料在料仓内的颗粒速度、体积分数等分布特性, 模拟结果较好地揭示了料仓内不同物料的整体流和漏斗流特性; 进而以玻璃珠为例, 对移动床颗粒单管绕流流动进行了模拟, 所得结果合理揭示了管流附近的流动特性. 计算结果表明, 对于本文的计算工况, 颗粒体积分数变化最大范围为0.510 ~ 0.461, 绝大部分区域流动惯性数小于0.1, 改进的单相μ(I)模型能合理预测出密集颗粒流移动床内的流动特性, 方法可行且较多相流算法能明显减小计算量.      

固体颗粒在热对流下沉降的直接数值模拟研究

在任意拉格朗日-欧拉算法模拟等温惰性颗粒两相流的基础上,增加对能量方程的联立求解,在热对流条件下对固体颗粒在不同雷诺数下的沉降规律进行了直接数值模拟.结果表明:热对流引起了流场流动的变化和不对称,颗粒在热流体中沉降,热对流产生的力加速了冷颗粒的运动,尾部形成了涡脱落;颗粒在冷流体中沉降,热对流产生的力阻碍了冷颗粒的运动,尾部形成了羽流. 随着雷诺数的增大,颗粒经历了稳定沉降、周期性摆动到不稳定、无规则运动3个阶段.      

颗粒在黏弹性流体扩张和收缩流中沉降的格子Boltzmann模拟分析

对于Oldroyd-B型黏弹性流体，本文应用格子Boltzmann方法(LBM)，实现了流体在二维1:3扩张流道及3:1收缩流道中流动的数值模拟，获得了黏弹性流体在扩张和收缩流道中的流场分布．结合颗粒的受力和运动规则，基于点源颗粒模型，数值分析了颗粒在扩张流和收缩流中的沉降过程和特征，讨论了颗粒相对质量和起始位置以及雷诺数Re和威森伯格数Wi对颗粒沉降特征的影响．结果表明，颗粒相对质量和起始位置以及Re对颗粒沉降轨迹和落点影响较大，而Wi的影响则较小．     

含液颗粒材料液固耦合分析的离散颗粒模型及特征线SPH法

对含液颗粒材料流固耦合分析建议了一个基于离散颗粒模型与特征线SPH法的显式拉格朗日-欧拉无网格方案。在已有的用以模拟固体颗粒集合体的离散颗粒模型[1]基础上,将颗粒间间隙内的流体模型化为连续介质,对其提出并推导了基于特征线的SPH法。数值例题显示了所建议方案在模拟颗粒材料与间隙流相互作用的能力和性能以及间隙流体对颗粒结构承载能力及变形的影响。     

非球形颗粒两相流的数值模拟研究进展

非球形颗粒两相流是多相流的重要研究方向之一, 常见于自然界及工业生产过程中. 不同于球形颗粒, 由于非球形颗粒形状的各向异性, 除了颗粒平动行为, 还需要考虑颗粒的转动与取向行为, 颗粒的取向与转动行为会影响颗粒所受的力和力矩. 为了准确模拟非球形颗粒的运动行为, 目前非球形颗粒两相流的数值模拟研究主要基于欧拉?拉格朗日的求解框架展开, 常见的非球形颗粒两相流数值模拟方法主要包括点颗粒法与全分辨颗粒法. 本文将对这两类方法进行介绍, 同时会全面介绍非球形颗粒两相流研究的基础理论模型, 并系统总结非球形颗粒在简单基本流和复杂湍流中的研究进展, 包括对于非球形颗粒在湍流中的取向与转动行为机理, 以及颗粒对湍流减阻调制作用的研究. 最后, 本文提出了非球形颗粒两相流研究存在的问题及未来研究方向.      

纳米颗粒在人类鼻腔中沉降率的研究

借助欧拉和拉格朗日方法数值模拟了纳米颗粒在人类鼻腔中的输运和沉降. 在采用有限体积法以及k-w湍流模型求解流场的基础上, 通过单向耦合的拉格朗日方法得出了水动力、热泳力和布朗力等综合作用下的纳米颗粒的运动轨迹以及沉降率. 研究发现, 非常微小的纳米颗粒在鼻腔内的沉降率非常高; 粒径在1~10nm的纳米颗粒在鼻腔中的沉降率从80%降至18%; 粒径在10sim150nm之间的纳米颗粒在鼻腔中的沉降率变化很小,且其值介于15%~18%之间.      

颗粒毛细效应影响因素的离散元分析

颗粒毛细效应是指将一根细管插入填充有颗粒物质的容器中并对管施加竖直振动时颗粒在管内上升并最终达到一个稳定的高度的现象, 该现象为颗粒物料的逆重力输运提供了一种潜在的技术途径. 为探究颗粒毛细效应的影响因素, 采用离散元方法, 模拟再现了颗粒毛细效应过程,展示了不同管径下颗粒竖直方向速度演变特性, 考察了不同容器宽度和振动条件下颗粒最终毛细上升高度随管径的演变规律. 结果表明, 在容器宽度与粒径比为40、管振幅与粒径比为14.33、管振动频率为12 Hz情况下, 管径与粒径比$D/d = 3.33$时, 管内颗粒堵塞严重, 使得颗粒上升缓慢,并造成颗粒柱中断; $D/d = 8.33$时, 起初毛细上升高度增加迅速, 随后毛细上升高度的增大逐渐减缓, 管内颗粒在管径方向几乎不存在速度梯度; $D/d =15$时, 随着颗粒毛细上升高度的增大, 管内颗粒柱分离为速度截然不同的两层, 上层颗粒在管径方向几乎不存在速度梯度, 而下层颗粒存在明显的速度梯度.研究还发现, 在毛细效应能够发生的管径范围内, 存在一个对应于颗粒最终毛细上升高度最大值的临界管径, 当管径小于临界管径时, 颗粒最终毛细上升高度随管径的增大而增大, 当管径大于临界管径时, 颗粒最终毛细上升高度随管径的增大而趋于减小; 增大容器宽度,临界管径有所增大; 增大振幅、适当提高频率能够有效促进临界管径的增大.      

颗粒毛细效应影响因素的离散元分析

颗粒毛细效应是指将一根细管插入填充有颗粒物质的容器中并对管施加竖直振动时颗粒在管内上升并最终达到一个稳定的高度的现象,该现象为颗粒物料的逆重力输运提供了一种潜在的技术途径.为探究颗粒毛细效应的影响因素,采用离散元方法,模拟再现了颗粒毛细效应过程,展示了不同管径下颗粒竖直方向速度演变特性,考察了不同容器宽度和振动条件下颗粒最终毛细上升高度随管径的演变规律.结果表明,在容器宽度与粒径比为40、管振幅与粒径比为14.33、管振动频率为12 Hz情况下,管径与粒径比D/d=3.33时,管内颗粒堵塞严重,使得颗粒上升缓慢,并造成颗粒柱中断; D/d=8.33时,起初毛细上升高度增加迅速,随后毛细上升高度的增大逐渐减缓,管内颗粒在管径方向几乎不存在速度梯度; D/d=15时,随着颗粒毛细上升高度的增大,管内颗粒柱分离为速度截然不同的两层,上层颗粒在管径方向几乎不存在速度梯度,而下层颗粒存在明显的速度梯度.研究还发现,在毛细效应能够发生的管径范围内,存在一个对应于颗粒最终毛细上升高度最大值的临界管径,当管径小于临界管径时,颗粒最终毛细上升高度随管径的增大而增大,当管径大于临界管径时,颗粒最终毛细上升高度随管径的增大而趋于减小;增大容器宽度,临界管径有所增大;增大振幅、适当提高频率能够有效促进临界管径的增大.     

颗粒物在人体上呼吸道模型中沉降规律的数值研究

本文采用RNG k-ε湍流模型对简化的上呼吸道内的流场进行数值仿真,并结合随机涡相互作用模型对上呼吸道内颗粒的局部运动沉积特性进行了数值模拟研究.结果表明,惯性冲击是微米级颗粒物主要的沉降机理,惯性参量能很好地衡量颗粒的沉降规律.口腔中流动较平稳,颗粒受扰动较小.颗粒由于惯性冲击而运动靠近咽部后部并易于沉积在咽部后壁面...     

密度不同的颗粒在流体中的沉降特性

采用格子Boltzmann方法(LBM)对雷诺数范围为5≤Re≤12的双颗粒沉降进行了直接数值模拟研究,主要关注颗粒之间的密度差异k对其周期性振动的影响。根据雷诺数颗粒沉降特性可以分成三个阶段,即当Re较小时,颗粒的振动幅度随k的增大而减小,当Re较大时则正好相反,介于两者之间存在一个临界雷诺数,在此雷诺数附近,颗粒的沉降同时具有以上两种特征。同时还研究了两个颗粒的稳定沉降结构以及重颗粒摆脱轻颗粒的条件。     

A geometric approach for predicting vertical stationary profiles of weakly inertial advecting-diffusing particles in closed incompressible flows

Mixing of weakly inertial particles in closed flows is often addressed by considering individual particles as passive advecting-diffusing tracers, subjected to an additional settling velocity resulting from body forces (e.g. gravity). We show that the qualitative and quantitative features of the vertical particle distribution (i.e. the horizontal cross-sectional averages of particle concentration) can be predicted from the structure of the flow resulting from the superposition of the stirring field and the settling velocity. The prediction is based upon the observation that the resulting flow can be divided into two nonoverlapping regions, namely trajectories that are confined within the mixing space (recirculation loops), and trajectories that cross the mixing space. The spatial extent of these regions is exploited to define an effective vertical convective velocity entering the one-dimensional lumped model. Model two-dimensional flows possessing different flow patterns are used to illustrate the proposed estimate for effective velocity. A CFD-computed three-dimensional turbulent flow inside a baffled stirred vessel is used as a benchmark test to assess the model performance in typical industrial flows.     

谱体积方法单元分割与问题单元标记方法的改进研究

谱体积方法是一种本质上解决网格依赖性的高精度CFD计算方法,本文研究了二维Euler方程的谱体积方法,提出一种基于切比雪夫多项式的单元分割方法,建立了基于WENO的变量限制器方法,并发展了结合谱体积和控制体的问题单元标记方法.采用15°超声速压缩拐角和NACA0012跨声速流动两个典型算例进行验证,结果表明,该分区方法具有更好的计算精度,标记方法可有效识别不连续区域,在较少的网格下即可获得与密网格传统有限体积法相当的计算精度.     

An Euler solver for three-dimensional turbomachinery flows

A time-marching finite volume numerical procedure is presented for three-dimensional Euler analysis of turbomachinery flows. The proposed scheme is applied to the conservative form of the Euler equations written in general curvilinear co-ordinates. A simple but computationally efficient grid is constructed. Numerical solution results for three 3D turbine cascade flows have been presented and compared with available measurements as well as with another state-of-the-art 3D Euler analysis numerical solution in order to demonstrate the accuracy and computational efficiency of the analysis method. Also, the predicted results are compared with a 3D potential flow solver and comparison is made with the analytical solution. The proposed method is an accurate and reliable technique for solving the compressible flow equations in turbomachinery geometries.     

Computation of the stability derivatives via CFD and the sensitivity equations

The method to calculate the aerodynamic stability derivates of aircrafts by using the sensitivity equations is ex- tended to flows with shock waves in this paper. Using the newly developed second-order cell-centered finite volume scheme on the unstructured-grid, the unsteady Euler equations and sensitivity equations are solved simultaneously in a non-inertial frame of reference, so that the aerodynamic stability derivatives can be calculated for aircrafts with complex geometries. Based on the numerical results, behavior of the aerodynamic sensitivity parameters near the shock wave is discussed. Furthermore, the stability derivatives are analyzed for supersonic and hypersonic flows. The numerical results of the stability derivatives are found in good agree- ment with theoretical results for supersonic flows, and variations of the aerodynamic force and moment predicted by the stability derivatives are very close to those obtained by CFD simulation for both supersonic and hypersonic flows.     

基于摄像测量原理的船体垂向变形测量半实物仿真实验研究

针对船体垂向变形测量的迫切需求,以及现有方法对于垂向变形适用性差的实际情况,提出了一种利用摄像测量原理实现垂向变形测量的新方法,推导了设备垂向变形角与姿态欧拉角变化量之间的转换关系,在此基础上设计了船体垂向变形测量方案。通过在平台上的CCD像机对与垂向设备基座固连的合作标志的实时观测,解算由于变形引起的合作标志姿态变化量,再利用合作标志与设备基座固连关系计算设备的垂向变形角。在实验室条件下进行了半实物仿真实验,实验结果测量值与参考值变化趋势一致,测量误差均在10″以下,说明该方法在垂向变形测量中正确可行。     

Diffusion-based coarse graining in hybrid continuum–discrete solvers: Theoretical formulation and a priori tests

Coarse graining is an important ingredient in many multi-scale continuum–discrete solvers such as CFD–DEM (computational fluid dynamics–discrete element method) solvers for dense particle-laden flows. Although CFD–DEM solvers have become a mature technique that is widely used in multiphase flow research and industrial flow simulations, a flexible and easy-to-implement coarse graining algorithm that can work with CFD solvers of arbitrary meshes is still lacking. In this work, we proposed a new coarse graining algorithm for continuum–discrete solvers for dense particle-laden flows based on solving a transient diffusion equation. Via theoretical analysis we demonstrated that the proposed method is equivalent to the statistical kernel method with a Gaussian kernel, but the current method is much more straightforward to implement in CFD–DEM solvers. A priori numerical tests were performed to obtain the solid volume fraction fields based on given particle distributions, the results obtained by using the proposed algorithm were compared with those from other coarse graining methods in the literature (e.g., the particle centroid method, the divided particle volume method, and the two-grid formulation). The numerical tests demonstrated that the proposed coarse graining procedure based on solving diffusion equations is theoretically sound, easy to implement and parallelize in general CFD solvers, and has improved mesh-convergence characteristics compared with existing coarse graining methods. The diffusion-based coarse graining method has been implemented into a CFD–DEM solver, the results of which are presented in a separate work.     

Consistent inlet and outlet boundary conditions for particle methods

In this paper, simple and consistent open boundary conditions are presented for the numerical simulation of viscous incompressible laminar flows. The present approach is based on an arbitrary Lagrangian-Eulerian particle method using upwind interpolation. Three kinds of inlet/outlet boundary conditions are proposed for particle methods, a pressure specified inlet/outlet condition, a velocity profile specified inlet/outlet condition, and a fully developed flow outlet condition. These inlet/outlet conditions are realized by using boundary particles and modification to the physical value such as velocity. Poiseuille flows, flows over a backward-facing step, and flows in a T-shape branch are calculated. The results are compared with those of mesh-based methods such as the finite volume method. The method presented herein exhibits accuracy and numerical stability.     

An efficient 3D particle transport model for use in stratified flow

Random‐walk models are a versatile tool for modelling dispersion of both passive and active tracers in turbulent flow. The physical and mathematical foundations of stochastic Lagrangian models of turbulent diffusion have become more and more solid over the years. An important aspect of these types of models that has not received much attention is the behaviour of the particles near boundaries. Often, a simple stochastic, numerical scheme is used. Because turbulent mixing in the vertical direction is much more complicated than in the two horizontal directions, it is in the vertical direction that a simple numerical scheme, such as the Euler scheme, may cause problems. In this paper our main goal is the development of an efficient 3D particle transport model that can be used in stratified flow. For this type of situation the vertical direction is of special interest. First, a closer look is taken at some considerations that should be regarded when choosing a numerical scheme. Specifically schemes are investigated that can be used in the vertical direction, where the diffusion coefficient is varying in that direction. Experiments are performed regarding the accuracy of different numerical schemes in various situations. The behaviour of the particles near an impermeable layer interface is investigated. The stochastic Heun and Runge–Kutta schemes turn out to be very attractive for this type of model. For the simulation of the transport of various physical quantities, such as salinity, heat, silt, oxygen, or bacteria, different types of models are available. In this case we will take a closer look at the modelling of the transport of pollutants from point sources (either instantaneous or continuous transport). For this purpose a 3D particle transport model has been developed that is especially suited for stratified situations such as can be found in estuaries. The main idea is to use a simple numerical scheme for the horizontal directions and a higher‐order method for the vertical direction. The results play an important role in making specific choices for this type of particle transport model. Copyright © 2005 John Wiley & Sons, Ltd.     

Enhancement of the accuracy of the finite volume particle method for the simulation of incompressible flows

A finite volume particle (FVP) method for simulation of incompressible flows that provides enhanced accuracy is proposed. In this enhanced FVP method, a dummy neighbor particle is introduced for each particle in the calculation and used for the discretization of the gradient model and Laplacian model. The error‐compensating term produced by introducing the dummy neighbor particle enables higher order terms to be calculated. The proposed gradient model and Laplacian model are applied in both pressure and pressure gradient calculations. This enhanced FVP scheme provides more accurate simulations of incompressible flows. Several 2‐dimensional numerical simulations are given to confirm its enhanced performance.