壁面约束对柱状粒子在牛顿流体中沉降影响的研究

格子Boltzmann方法(LBM)直接数值模拟了在有壁面约束的流场中柱状粒子的沉降。结果说明粒子在壁面的影响下，会离开管壁向管中心移动，移动速度先是增大，然后随着与管壁距离的增加，逐渐减小，粒子中心最后并不一定停留在管中心。随着粒子与壁面距离的增加，粒子的沉降速度将增大，可见壁面对粒子沉降有阻碍作用，平行于粒子轴线的壁面对粒子沉降的影响比垂直于粒子轴线壁面的影响大。     

不同润湿性纳米通道内库埃特流动的模拟

利用非平衡分子动力学模拟方法, 模拟了两无限大平行平板组成的纳米通道内的库埃特流动, 并给出了壁面润湿性和速度对流场密度、速度分布及壁面滑移的影响规律.数值模拟中, 统计系综采用微正则系综, 势能函数选用LJ/126模型, 壁面设为刚性原子壁面, 温度校正使用速度定标法, 牛顿运动方程的求解则采用文莱特算法.结果表明, 纳米通道内流体密度呈对称的衰减振荡分布, 且随壁面润湿性的降低, 振荡幅度减小, 振荡周期保持不变;滑移量随壁面润湿性的提高而降低, 甚至在亲水壁面时出现负滑移现象;随壁面速度的增加滑移速度逐渐增大, 且在流体呈现非线性流动阶段其增幅显著加大.另外, 还发现当壁面设置为超疏水性时, 壁面滑移呈现出随润湿性降低而减小的反常现象, 并基于杨氏方程对其进行了解释.     

砂岩球体法向恢复系数实验研究

法向恢复系数是岩崩块石运动分析的关键参数，其取值直接决定了块石的运动轨迹。本文中采用自行设计的碰撞实验装置和声频采样技术，测定了砂岩球体碰撞的法向恢复系数，研究了粒径、碰撞速度、含水状态和板的弹性特性4个因素对恢复系数的影响。结果表明:砂岩球体法向恢复系数存在复杂的尺寸效应，恢复系数随粒径的增大先增大后减小；碰撞过程中存在的黏弹性耗能机理和弹塑性损伤耗能机理共同作用产生了复杂的尺寸效应；受砂岩非均质特性的作用，粒径较小时，恢复系数的速度效应较明显(随速度增大而增大)，粒径较大时速度对恢复系数的影响消失；砂岩饱和使黏弹性耗能和弹塑性损伤耗能增加，使恢复系数比风干时低；等效弹性模量对恢复系数的影响较大，等效弹性模量越大，法向恢复系数越小。     

水平流动边界层内气固相间作用的试验研究

应用三维粒子动态分析仪（threedimensionalparticledynamicsanalyzer）,测量了含有230μm颗粒的气固两相水平流的特性,特别是壁面边界层内的两相流动特性．结果表明颗粒载荷比（质量流率）对相间作用有较大影响,随颗粒流率的增加颗粒对气流平均速度和湍流的影响增大,颗粒使气流速度边界展变薄．颗粒和气流相互作用在不同方向上呈各向异性,颗粒对气流垂直方向的脉动影响较大．颗粒与湍流边界层气流的作用行为大致可以分成三个区域：贴壁区、中间区和外流区．     

切应力协同下受热过冷层流液膜的破断特性

针对界面切应力协同下受热过冷层流液膜流动的破断过程, 建立了不同气液 流向下的临界液膜厚度和最小润湿量的理论模型, 分析了不同驱动力作用下, 接触角、流体 温度、界面切应力和壁面热流密度对液膜破断特性的影响. 研究表明: 临界液膜厚度和最小 润湿量均随壁面热流密度的增加而增大; 重力驱动下的接触角影响在不同热流密度下有所不 同, 流体温度在不同驱动力下对最小润湿量的影响截然相反; 同向切应力驱动下临界液膜厚 度和最小润湿量随切应力增加而减小; 在重力和切应力协同驱动下, 同向切应力对最小润湿 量的影响与重力和切应力所起作用的相对大小有关, 反向切应力使得临界液膜厚度和最小润 湿量有所增大.     

具有壁面射流激励的圆管内气相流动大涡模拟

采用大涡模拟方法和单方程亚格子模式对小尺度量进行模拟。研究了不同强度壁面射流激励对圆管内气相流动的影响,模拟结果给出了射流对瞬态拟序结构发展、时平均流向速度分布的影响。随着射流强度的增加,射流入口附近流体的回流现象增强。射流强度足够大时可以减小管壁处的切应力值,同时会减小壁面附近流动速度,这种速度分布会导致气体夹带颗粒的能力下降,从而在实际两相流动中容易造成壁面附近的气粒返混现象。     

大涡模拟研究展向旋转槽道湍流流动

本文用大涡模拟预测了以不同转速做展向旋转的槽道湍流流动,统计平均的流向速度型在壁面附近与已有实验数据符合很好,在通道中部的预测差异也能给出合理解释,对比不同转速的计算结果,表明展向旋转通道的湍流应力和壁面摩擦力在压力面附近提高、在吸力面附近降低,这些高阶湍流统计量的变化规律可以结合湍流应力输运方程加以解释,漩涡识别技术显示了近壁条带结构,其形态和猝发率受旋转附加力的影响发生改变,进而影响壁面摩擦速度的数值和分布,进一步考察垂直流动方向的截面内速度分布,发现旋转引起了垂直壁面方向的流动,形成正负相间排列的流向涡对,并随着转速的增加向压力面靠近。     

圆柱绕流局部冲刷机制的实验研究

采用粒子图像测速技术和粒子图像识别技术研究了冲刷坑的演化过程及局部冲刷的流体动力机制。首先研究了动床条件下圆柱周围的床面演化过程，给出垂直对称面输沙率随时间变化以及圆柱下游垂直对称面的沙颗粒浓度和泥沙分布规律；其次针对典型冲刷坑形状固定底床，测量了圆柱下游的瞬时流场，并给出平均流场。结果表明尾流区平均剪应力较低，但垂向涡量和湍流强度较高。分析指出平均剪应力较低的尾流区域，局部冲刷的主要动力机制是尾涡和湍流的综合作用。     

超薄油膜润滑的分子动力学模拟Ⅰ．刚性分子模型

分子量级超薄油膜的润滑特性与流体润滑及边界润滑的都有所不同，而且在超薄油膜中也同样存在着润滑剂的流动。因此，利用分子动力学方法模拟了超薄油膜中的压力流动，模拟中采用了刚性分子流体模型，重点研究固体壁面对流动的影响，结果表明，当油膜厚度远比流体分子“直径”大时，模拟所得速度剖面和流量均与流体力学的理论值基本一致；随着油膜厚度逐渐变薄，压力流动或动压流因受到固体壁面的阻碍作用而不断减小；当油膜厚度减小到几个分子直径量级时，两固体壁面之间的流体分子发生固化或晶体化，由此而导致压力流动消失。这可能是流体动力润滑向边界润滑转化的机理之一。由这些研究结果可见，分子动力学模拟在薄膜润滑研究中具有广阔的应用前景。     

固体颗粒对沟槽湍流边界层影响的实验研究

本文采用粒子图像测速技术(particles image velocimetry, PIV)研究固体颗粒对放置在平板湍流边界层中的平壁和沟槽壁面减阻效果的影响. 实验对清水和加入粒径为155 μm聚苯乙烯颗粒的流法向二维速度场信息进行采集, 对不同工况下的平均速度剖面、雷诺应力和湍流度等统计量进行对比, 分析流体在边界层中的行为. 运用空间局部平均结构函数提取了不同工况湍流边界层喷射?扫掠行为的空间拓扑结构并进行比较. 结果发现, 在不同的壁面条件下, 粒子加入后的对数律区中无量纲速度均略大于清水组, 雷诺切应力有所降低, 湍流度有所减弱. 对于不同流场速度下的沟槽而言, 颗粒的加入均降低了壁面附近的阻力, 而颗粒单独作用于光滑壁面的减阻效果并不明显. 加入粒子后的相干结构数目有所增加, 法向脉动速度下降. 沟槽壁面附近的相干结构数目有所增加, 法向脉动速度在自由来流速度较大时有所上升, 在速度较小时有所下降. 这表明不同减阻状况下的沟槽均能将大涡破碎成更多的涡, 并且粒子的加入强化了这种破碎作用.      

DEM simulation of particle percolation in a packed bed

The phenomenon of spontaneous particle percolation under gravity is investigated by means of the discrete element method. Percolation behaviors such as percolation velocity,residence time distribution and radial dispersion are examined under various conditions. It is shown that the vertical velocity of a percolating particle moving down through a packing of larger particles decreases with increasing the restitution coefficient between particles and diameter ratio of the percolating to packing particles. With the increase of the restitution coefficient,the residence time and radial dispersion of the percolating particles increase. The packing height affects the residence time and radial dispersion. But,the effect can be eliminated in the analysis of the residence time and radial dispersion when they are normalized by the average residence time and the product of the packing height and packing particle diameter,respectively.In addition,the percolation velocity is shown to be related to the vertical acceleration of the percolating particle when an extra constant vertical force is applied. Increasing the feeding rate of percolating particles decreases the dispersion coefficient.     

DEM simulation of particle percolation in a packed bed

The phenomenon of spontaneous particle percolation under gravity is investigated by means of the discrete element method. Percolation behaviors such as percolation velocity, residence time distribution and radial dispersion are examined under various conditions. It is shown that the vertical velocity of a percolating particle moving down through a packing of larger particles decreases with increasing the restitution coefficient between particles and diameter ratio of the percolating to packing particles. With the increase of the restitution coefficient, the residence time and radial dispersion of the percolating particles increase. The packing height affects the residence time and radial dispersion. But, the effect can be eliminated in the analysis of the residence time and radial dispersion when they are normalized by the average residence time and the product of the packing height and packing particle diameter, respectively. In addition, the percolation velocity is shown to be related to the vertical acceleration of the percolating particle when an extra constant vertical force is applied. Increasing the feeding rate of percolating particles decreases the dispersion coefficient.     

Application of a screw conveyor with axial tilt blades on a shearer drum and investigation of conveying performance based on DEM

Screw conveyors are widely employed in industrial fields for conveying bulk materials. The shearer drum which uses the screw conveying principle is responsible for excavation and conveying coal particles onto the chain conveyor. Screw conveyor performance is affected by potential factors, such as the blade axial tilt angle and style, core shaft form and diameter. The effect of blade axial tilt angle on the conveying performance was investigated with the help of DEM. In the case of the screw conveyor, the mass flow rate, and particle axial velocity increased with increasing positive axial tilt angle, and declined with increasing negative axial tilt angle. In the case of the drum, the mass flow rate, particle axial velocity, and loading rate first increased and then decreased with increasing positive axial tilt angle, and decreased with increasing negative axial tilt angle. These results can be considered as a benchmark for screw conveyor and drum structural designs with axial tilt screw blades.     

单自由度冲击扰动下模拟磁头／磁盘系统接触回复特性的实验研究

采用自制的试验装置研究了单自由度冲击扰动下磁头 /磁盘系统的接触回复特性 ,分析了不同载荷下球形模拟磁头跳跃振动回复系数和接触时间与输入速度之间的关系 .结果表明 :当载荷较小时 ,回复系数随着输入速度的减小而增加 ,而当载荷较大时 ,回复系数随着输入速度的减小而减小 .在不同载荷条件下 ,接触时间均随输入速度的减小而缩短 ,但小载荷下接触时间缩短的幅度较大 .球形模拟磁头的回复特性和接触时间与载荷、输入速度及润滑膜特性有关 ,增大载荷和选择合适的润滑剂均可抑制跳跃振动     

Hydrodynamics of gas-solid fluidization of a homogeneous ternary mixture in a conical bed： Prediction of bed expansion and bed fluctuation ratios

Hydrodynamic characteristics of fluidization in a conical or tapered bed differ from those in a columnar bed because the superficial velocity in the bed varies in the axial direction. Fixed and fluidized regions could coexist and sharp variations in pressure drop could occur, thereby giving rise to a noticeable pressure drop-flow rate hysteresis loop under incipient fluidization conditions. To explore these unique properties, several experiments were carried out using homogeneous, well-mixed, ternary mixtures with three dif- ferent particle sizes at varying composition in gas-solid conical fluidized beds with varying cone angles. The hydrodynamic characteristics determined include the minimum fluidization velocity, bed fluctuation, and bed expansion ratios. The dependence of these quantities on average particle diameter, mass fraction of the fines in the mixture, initial static bed height, and cone angle is discussed. Based on dimensional analysis and factorial design, correlations are developed using the system parameters, i.e. geometry of the bed （cone angle）, particle diameter, initial static bed height, density of the solid, and superficial velocity of the fluidizing medium. Experimental values of minimum fluidization velocity, bed fluctuation, and bed expansion ratios were found to agree well with the developed correlations.     

Dynamic Deformation Behavior of a Golf Ball during Normal Impact

The normal impact between a golf ball and a rigid steel target was studied to examine ball deformation and the contact force during the impact. Using high-speed video images, the normal and tangential compression ratios of the ball were measured to analyze the ball deformation quantitatively. In addition, the inbound and rebound ball velocities, contact time, and coefficient of restitution were determined as basic parameters of the impact. As the inbound ball velocity increased, the maximum normal compression ratio increased while the maximum tangential compression ratio, contact time and coefficient of restitution decreased. The ball center displacements during the impact were measured to determine the ball center velocity and acceleration, and the contact force was calculated by the product of the mass and acceleration. The contact force increased almost linearly with the inbound ball velocity, and its relationship agreed well quantitatively with the results from a load-cell, and also agreed well qualitatively with Hertz contact theory.     

基于速度梯度张量特征值的陷窝内旋涡分析

作为一种新型的涡流发生器,陷窝具有流动阻力小、综合传热性能高的特点,是现代高性能涡轮叶片内部冷却新技术. 旋涡的定量分析是陷窝强化传热优化设计的重要依据. 针对在不同陷窝模型下的旋涡结构、分离方式和背景压力变化引起的旋涡强度无法定量分析的问题,本文提出采用涡核速度和 涡核速度梯度张量特征值来定量分析旋涡的方法. 通过采用涡核处局部坐标系表示的速度矢量和速度梯度张量,得到了涡核的轴 向速度、径向速度、旋转角速度、轴向加速度和径向加速度,并在此基础上简化出了用最大轴向速度、最大轴向加速度和最大旋 转角速度综合表示的旋涡强度的定量分析方法. 用该方法分析了不同深宽比陷窝诱导的旋涡结构,随着深宽比的增大,最大轴向 速度、最大轴向加速度和最大旋转角速度均呈现明显的增大趋势,旋涡强度增大. 研究表明此方法具有数据处理简单、通用性强、 不受分离方式限制、不受背景压力影响的特点,且提取到的数据具有明确的物理意义,适用于各类旋涡定量分析.      

A study of the flow characteristics in micro-abrasive jets

An experimental study of particle velocities in micro-abrasive jets by using the particle image velocimetry (PIV) technique is presented. It has been found that the particle jet flow has a nearly linear expansion downstream. The particle velocities increase with air pressure, and the increasing rate increases with nozzle diameter within the range considered. The instantaneous velocity profile of the particle flow field in terms of the particle velocity distribution along the axial and radial directions of the jets is discussed. For the axial profile in the jet centerline downstream, there exists an extended acceleration stage, a transition stage, and a deceleration stage. For the radial velocity profiles, a relatively flat shape is observed at a jet cross-section near the nozzle exit. Mathematical models for the particle velocities in the air jet are then developed. It is shown that the results from the models agree well with experimental data in both the variation trend and magnitude.     

Particle Measurement Sensor for in situ determination of phase structure of fluidized bed

Based on three-dimensional (3D) acceleration sensing, an intelligent particle spy capable of detecting, transferring, and storing data, is proposed under the name of Particle Measurement Sensor (PMS). A prototype 60-mm-dia PMS was tested to track its freefall in terms of velocity and displacement, and served as a particle spy in a fluidized bed delivering the in situ acceleration information it detects. With increasing superficial gas velocity in the fluidized bed, the acceleration felt by PMS was observed to increase. The variance of the signals, which reflect the fluctuation, increased at first, reaching a maximum at the gas velocity (Uc) which marks the transition from bubbling to turbulent fluidization. Through probability density distribution (PDD) analysis, the PDD peak can be divided into the emulsion phase peak and the bubble phase peak. The average acceleration of emulsion and bubble phase increased, while the variance of both phases reached a maximum at Oc, at the same time. However, the difference between the variances of two phases reached the maximum at Oc. Findings of this study indicate that PMS can record independent in situ information. Further, it can provide other in situ measurements when equipped with additional multi-functional sensors.     

粗糙颗粒动理学及稠密气固两相流动的数值模拟

考虑颗粒碰撞过程中摩擦作用, 给出了粗糙颗粒碰撞动力学. 引入颗粒相拟总温来表征颗粒平动和转动脉动能量的特征. 基于气体分子运动论, 建立颗粒碰撞中平动和旋转共同作用的粗糙颗粒动理学, 给出了颗粒相压力和黏度等输运参数计算模型. 运用基于颗粒动理学的欧拉-欧拉气固两相流模型, 数值模拟了流化床内气体颗粒两相流动特性, 分析了颗粒旋转流动对颗粒碰撞能量交换和耗散的影响. 模拟得到的流化床内径向颗粒浓度和提升管内颗粒轴向速度与他人实验结果相吻合. 模拟结果表明随着颗粒浓度的增加, 颗粒相压力和能量耗散逐渐增加, 而颗粒拟总温先增加后下降. 随着颗粒粗糙度系数的增加, 床内平均颗粒相拟总温和能量耗散增加, 表明颗粒旋转产生的摩擦将导致颗粒旋转脉动能量的改变, 影响床内气体-颗粒两相宏观流动特性.