三圆形颗粒在通道中沉降运动的数值研究

主要应用浸没边界的格子玻尔兹曼方法（immersed boundary-lattice Boltzmann method, IB–LBM） 对处于不同倾斜角度通道内的三个刚体圆形颗粒在重力作用下下落的动力学特性进行了计算研究. 首先分析通道倾斜角度的影响, 结果显示当通道倾斜角处于59°90°的范围时会发生后一个颗粒超越前一个颗粒的现象. 其次, 研究了Re对颗粒沉降特性的影响, 结果表明Re 越大, 颗粒间发生聚集的时间越早. 研究还发现当3 个颗粒的直径大小不均匀时, 颗粒由大到小纵向依次排列, 或者出现中间小球直径较相邻两个小球直径大的排列情况, 均能促使颗粒加快聚集. 本文的研究结果可为环境工程及地质学中的颗粒沉降问题提供有价值的参考.      

细长锥边界层绊线转捩风洞自由飞试验

通过在半锥角θ_c=10°细长锥面上布置一定数量的人工绊线,促使细长锥表面边界层在相应轴向位置上发生层流向湍流转变的固定转捩,采用运动自由度不受约束的风洞自由飞试验技术研究边界层转捩对高超声速细长锥再入体无控自由飞行下的运动特性和气动特性影响规律,并与以往无人工绊线的细长锥风洞自由飞试验结果作对比.试验马赫数Ma=5.0,通过改变风洞前室总压P_0实现两个雷诺数的模拟,以模型长为特征尺寸自由流雷诺数分别为0.84×10~6和1.68×10~6.结果表明:当自由流雷诺数Re=0.84×10~6时,人工绊线尚不足以促使边界层发生转捩,有绊线的细长锥气动特性与无绊线基本一致,动稳定导数大于零;当自由流雷诺数Re=1.68×10~6时,人工绊线促使边界层发生固定转捩,细长锥的动稳定导数小于零,细长锥自由飞行动稳定.     

SIMULATION OF SEDIMENTATION OF TWO CIRCULAR PARTICLES WITH COLLISION CONSIDERED IN VERTICAL CHANNEL

D2Q9 model of lattice Boltzmann equation method was used to simulate the sedimentation of two circular particles in a bounded two dimension channel. The characteristics of the sedimentation shows some periodicity for the Reynolds number Re chosen, 0.1-20. The larger the Reynolds number, the stronger the interaction between the two particles and the larger the transversal displacements. For large Re, the two particles leading alternately; for small Re, the initially leading particle will keep its leading position and for moderate Re, the initially upper particle will get leading position and keep it. The influence of the initially relative position of the two particles on sedimentation is small. The width of the channel won＇t change the characteristics of the sedimentation as a whole, but will change the period of the sedimentation. The wider the channel, the longer the period will be.     

椭圆颗粒在剪切流中旋转特性的数值研究

研究颗粒在流体剪切作用下的运动特性是理解和预测颗粒悬浮流流动行为的关键.当流体的惯性不能忽略时,颗粒的运动往往变得非常复杂.本文采用格子Boltzmann方法对中等雷诺数下椭圆颗粒在剪切流中的旋转运动进行了模拟.首先,研究了雷诺数(0Re 170)的影响,结果表明当雷诺数低于临界值时,颗粒以周期性的方式旋转,角速度最小时对应的长轴方向随着雷诺数的增大而逐渐远离水平方向,而且这一倾角与雷诺数呈分段线性关系;当雷诺数大于临界值时,椭圆形颗粒最终保持静止状态,且静止时的转角与雷诺数呈幂函数关系,雷诺数越大,转角越小,椭圆的长轴越远离水平位置.其次,研究了椭圆颗粒的长短轴之比α(1α10)的影响,结果表明颗粒旋转的周期与α呈幂函数关系,α越大,颗粒旋转周期越小.此外,当α超过临界值时,颗粒也在水平位置附近保持静止状态,此时的转角与α也呈幂函数关系,α越大,转角越小.研究还发现,当雷诺数较大时椭圆颗粒在旋转过程中会产生过冲现象.     

竖冷设备中烧结矿石偏析行为的GPU高性能模拟

随着钢铁工业流程和技术的不断优化及节能减排要求的不断升级,如何高效回收利用各生产工序中的工业余热成为钢铁工业面临的重要课题.竖冷设备既可以有效回收烧结矿石的显热,大幅提高能源的利用率,又可以有效减少烧结工序中的粉尘,降低烧结工艺的环境污染,因此得到了高度重视.但现有竖冷设备容易出现烧结矿石的偏析问题,严重影响了显热回收的效率和设备运行的稳定性.为此,采用离散元法(discrete elementmethod,DEM)研究竖冷设备中烧结矿石的偏析,重点针对不同结构进料管对烧结矿石偏析的影响.研究发现,虽然缓冲仓内每次加入的烧结矿石皆混合均匀,但由于不同粒径的烧结矿石在进料过程中存在动量差异,导致了竖冷设备内明显的颗粒偏析,在料堆堆尖处呈现小颗粒多大颗粒少的现象,并且料层主体的偏析与进料管的结构密切相关.由于进料管的结构是进料过程的决定性因素,因此提出了通过改变进料管的结构抑制竖冷设备内烧结矿石偏析的可能途径,对提高烧结矿石的显热回收效率具有参考意义.      

两串列扑翼的相位差对平均推力影响机理的实验研究

在一个低雷诺数的循环水洞中,实验研究了前后翅翼之间的相位差对两串列扑翼平均推力的影响.利用一个三分量的Kistler 压力传感器来测量扑翼的瞬时力;利用一个数字粒子测速仪系统(TSI DPIV) 来测量扑翼的前缘涡以及其周围的流场. 当相位差从0° 增加到360°,前翅的平均推力随着相位差正弦变化;前翅平均推力的增加是由于后翅的前缘涡和滞止区域增加了前翅的有效攻角. 后翅平均推力曲线有一个明显的V 字形低谷.低谷处较小的平均推力是由于前翅的脱落涡抑制了后翅前缘涡的形成并且减小了其有效攻角.当间距为0.5倍弦长相位差约为290°时,前后翅翼平均推力系数的合值能达到最大值0.667,明显大于两倍的单翼平均推力系数(2×0.255).      

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．     

Effect of particle shape on micro- and mesostructure evolution of granular assemblies under biaxial loading conditions

Discrete element method (DEM) numerical biaxial tests on samples with different particle shapes are performed to investigate how the multiscale evolves with varying particle shape. The samples used in such simulations are composed of circular, square, and elongated particles, respectively. For the numerical results, analyses are conducted in terms of microscopic evolution, i.e. particle rotation and evolution of fabric, and mesoscopic evolution, i.e. the evolution of loops and improved clustering coefficient. At the microscale, the mean particle rotation of circular particles is remarkably larger than those of square and elongated particles, and the shear band localization phenomenon is more obvious when the aspect ratio (AR) decreases. Considering the fabric evolving with particle shape, the value of anisotropy gradually increases when particle shape becomes irregular, and contacts of circular particles are pronouncedly less than those of irregular particles from the coordination number and curves of degree distribution. At the mesoscale, when the particle relationship is considered, the isotropic particles (i.e. circular and square particles) have similar evolutions of loops and modified clustering coefficient, whereas the elongated particles have remarkable three loops and modified clustering coefficient, which are both larger than those of isotropic particles.     

明渠中悬移质的弥散-对流方程及悬浮机理

悬移质泥沙通常构成冲积河流总输沙量的主体, 研究悬移质的悬浮机理具有重要的意义. 以双流体模型为基础, 通过引入弥散速度的概念, 建立了悬移质泥沙的输沙方程以及泥沙扩散系数的本构关系. 应用该方程分析了二维明渠均匀流中悬移质泥沙浓度垂向分布规律, 并与Einstein 和Chien 的泥沙浓度实验资料及经典扩散理论进行了对比. 以此为基础, 分析了紊动扩散、颗粒自身的紊动、颗粒碰撞应力对泥沙悬浮的影响在垂向上的变化, 以及浓度、粒径等对这些因素的影响. 结果表明, 泥沙颗粒在明渠紊流中的扩散是浑水的紊动扩散、颗粒自身的紊动、颗粒碰撞应力3 部分不同机制共同作用的结果, 把泥沙颗粒的悬浮简单归因于水流的紊动是不全面的.     

并列双圆柱流致振动的不对称振动和对称性迟滞研究

对雷诺数Re = 100 间距比s/D = 2.5 和5.0 的并列双圆柱流致振动进行了数值模拟研究, 其中圆柱质量比m = 2.0, 折合流速U_r 在2.0~10.0 之间, 两圆柱仅能做横流向振动. 研究发现, 当间距比s/D = 2.5 时, 在折合流速4.4 < U_r< 4.8区间内, 两圆柱流致振动响应出现不对称振动现象, 在折合流速4.4 < U_r< 4.8 区间内, 两圆柱流致振动响应出现对称性迟滞现象; 而当间距比s/D = 2.5时, 圆柱流致振动响应与单圆柱涡激振动响应相似, 没有出现不对称振动和对称性迟滞现象. 在不对称振动区间内, 两圆柱的升、阻力参数也出现了不相等的情况. 此外, 当两圆柱不对称振动时, 圆柱间隙流稳定地偏斜向其中的一个圆柱; 相应地, 尾涡也出现了宽窄不等的模式. 窄尾流圆柱的振幅和升、阻力均较宽尾流圆柱的大. 通过对比不对称振动现象发生前后的尾涡模式, 对新现象的产生机制进行了阐述.      

New formula for drag coefficient of cylindrical particles

The drag force on a cylindrical particle is calculated using lattice Boltzmann method.The results show that the drag coefficient of a particle with different orientation angles decreases with increasing Reynolds number.When the principal axis of the particle is parallel to flow,the drag coefficient is much larger than that of others and decreases fastest with increasing Reynolds number,which becomes more obvious with increasing particle aspect ratio.When the principal axis of the particle is inclined to flo...     

粘接-映射混合算法及其对颗粒材料中板的振动特性分析

结构与颗粒材料相互作用广泛存在于各工程领域,其研究过程中涉及的连续-离散耦合计算方法面对诸多挑战.本文提出了粘接-映射混合算法来研究连续体与离散介质耦合动力学问题.将连续体模型划分为内部区域及与颗粒接触的边界区域.边界区域采用粘接算法模拟连续体外部形状并使用高效的球形接触判断准则;提出一种包含Rayleigh阻尼映射的有限元映射质点弹簧算法来精确计算连续体内部区域内力和变形.二者相结合构成粘接-映射混合算法,并引入计算机集群和GPU(图形处理器)并行技术,对埋没于颗粒材料中受激振动固支方板的连续-离散耦合动力学问题进行了数值仿真研究.结果表明,粘接-映射混合算法有利于双层级并行算法的程序实现及优化,并在连续-离散耦合界面进行快速接触判断的同时实现对颗粒材料中方板位移、变形、振动形态等参数的研究.通过定幅扫频和定频变幅方式考察激振力频率和幅值对振动板非线性动力学行为的影响并观察到二倍周期现象,同时给出了该连续-离散耦合系统中颗粒体系的能量耗散特性.      

Discrete element-embedded finite element model for simulation of soft particle motion and deformation

The motion and deformation of soft particles are commonly encountered and important in many applications. A discrete element-embedded finite element model (DEFEM) is proposed to solve soft particle motion and deformation, which combines discrete element and finite element methods. The collisional surface of soft particles is covered by several dynamical embedded discrete elements (EDEs) to model the collisional external forces of the particles. The particle deformation, motion, and rotation are independent of each other in the DEFEM. The deformation and internal forces are simulated using the finite element model, whereas the particle rotation and motion calculations are based on the discrete element model. By inheriting the advantages of existing coupling methods, the contact force and contact search between soft particles are improved with the aid of the EDE. Soft particle packing is simulated using the DEFEM for two cases: particle accumulation along a rectangular straight wall and a wall with an inclined angle. The large particle deformation in the lower layers can be simulated using current methods, where the deformed particle shape is either irregular in the marginal region or nearly hexagonal in the tightly packed central region. This method can also be used to simulate the deformation, motion, and heat transfer of non-spherical soft particles.     

基于PIV技术分析颗粒在湍流边界层中的行为

采用粒子图像测速技术(particle image velocimetry,PIV)在平板湍流边界层内开展实验研究,对比颗粒相及单相液体的平均速度剖面、湍流强度、雷诺应力等湍流统计量,分析颗粒在湍流边界层中的行为.利用空间多尺度局部平均涡量的概念提取壁湍流发卡涡展向涡头(顺向涡)并统计其数量规律,得到不同法向位置处顺向涡周围流向脉动速度及流线的空间拓扑结构,比较分析顺向涡发展程度及周围的湍流相干结构.结果发现:与清水工况相比,颗粒相湍流边界层的缓冲层变薄、对数律区下移,湍流强度得到增强,雷诺应力在对数律区有所增大;颗粒的流向脉动速度在展向涡周围的分布与清水工况不同,颗粒能够被流体展向涡周围的猝发过程有效传递;颗粒相的顺向涡涡核较大,且随着法向位置的升高逐渐发展完整,涡和条带在流向上拉伸得更长;同时发现在两种工况下,顺向涡的左下方始终存在一个逆向涡,颗粒相逆向涡的形成弱于单相流体;两种工况下的顺向涡数量均随着法向位置的升高而减少,最后逐渐趋于稳定.      

EXPERIMENTAL INVESTIGATION ON LIFT INCREMENT OF A PLASMA CIRCULATION CONTROL AIRFOIL

利用等离子体激励器发展了新型的环量增升技术,并对二维NACA0012翼型绕流实施控制。由于NACA0012翼型为尖后缘构型,环量增升装置由2个非对称型介质阻挡放电等离子体激励器构成。一个等离子体激励器贴附于翼型吸力面靠近后缘处,其诱导的壁面射流沿来流方向指向下游;另一个等离子体激励器贴附于翼型压力面靠近后缘处,其诱导的壁面射流与来流方向相反指向上游。在风洞中通过时间解析二维PIV系统对翼型绕流流场进行了测量,基于翼型弦长的雷诺数Re=20 000。结果表明在等离子体激励器的控制下,翼型压力面靠近后缘处可以形成一个定常回流区,从而起到虚拟气动外形的作用,因此翼型吸力面的流场得到加速,压力面的流场得到减速,使得翼型压力面的吸力以及压力面的压力都得到增加,进而增加了翼型的环量。风洞天平测力实验进一步验证了该环量增升技术的有效性。在整个攻角范围内,施加控制的翼型的升力系数相比没有控制的工况有明显的提高。     

NUMERICAL STUDY ON THE GROWTH PROCESS OF SECONDARY AEROSOL IN THE FOG

雾环境气溶胶之间的碰撞和凝并是气溶胶迁移、生长的动力学基础. 基于颗粒群平衡方程和多重蒙特卡洛方法,针对雾环境中二次液态气溶胶生长过程开展数值研究,着重分析了湍流和布朗作用机制对单分散和指数分布的二次气溶胶碰撞凝并的影响. 结果表明:雾环境气溶胶之间的碰撞凝并使得颗粒的总数目减少,颗粒平均体积逐渐增大. 对于初始尺度为自由分子区、单分散的气溶胶,布朗运动时间为600 s时,二次气溶胶平均尺度增大至初始的202.7倍左右,数目降至初始的0.006. 对于湍流作用下有布朗运动的二次气溶胶,在较短的时间内(如100 s)气溶胶尺度增至初始的163倍,颗粒数目降至初始的0.025倍, 说明雾环境流场的湍流运动引起二次气溶胶较强的输运和聚集,导致颗粒碰撞凝并概率增加,颗粒尺度增大.     

Mean and fluctuating components of drag and lift forces on an isolated finite-sized particle in turbulence

We perform fully resolved direct numerical simulations of an isolated particle subjected to free-stream turbulence in order to investigate the effect of turbulence on the drag and lift forces at the level of a single particle, following Bagchi and Balachandar’s work (Bagchi and Balachandar in Phys Fluids 15:3496–3513, 2003). The particle Reynolds numbers based on the mean relative particle velocity and the particle diameter are Re?=?100, 250 and 350, which covers three different regimes of wake evolution in a uniform flow: steady axisymmetric wake, steady planar symmetric wake, and unsteady planar symmetric vortex shedding. At each particle Reynolds number, the turbulent intensity is 5–10% of the mean relative particle velocity, and the corresponding diameter of the particle is comparable to or larger than the Kolmogorov scale. The simulation results show that standard drag values determined from uniform flow simulations can accurately predict the drag force if the turbulence intensity is sufficiently weak (5% or less compared to the mean relative velocity). However, it is shown that for finite-sized particles, flow non-uniformity, which is usually neglected in the case of the small particles, can play an important role in determining the forces as the relative turbulence intensity becomes large. The influence of flow non-uniformity on drag force could be qualitatively similar to the Faxen correction. In addition, finite-sized particles at sufficient Reynolds number are inherently subjected to stochastic forces arising from their self-induced vortex shedding in addition to lift force arising from the local ambient flow properties (vorticity and strain rate). The effect of rotational and strain rate of the ambient turbulence seen by the particle on the lift force is explored based on the conditional averaging using the generalized representation of the quasi-steady force proposed by Bagchi and Balachandar (J Fluid Mech 481:105–148, 2003). From the present study, it is shown that at Re?=?100, the lift force is mainly influenced by the surrounding turbulence, but at Re = 250 and 350, the lift force is affected by the wake structure as well as the surrounding turbulence. Thus, for a finite-sized particle of sufficient Reynolds number supporting self-induced vortex shedding, the lift force will not be completely correlated with the ambient flow. Therefore, it appears that in order to reliably predict the motion of a finite-sized particle in turbulence, it is important to incorporate both a deterministic component and a stochastic component in the force model. The best deterministic contribution is given by the conditional average. The influence of ambient turbulence at the scale of the particle, which are not accounted for in the deterministic contribution, can be considered in stochastic manner. In the modeling of lift force, additional stochastic contribution arising from self-induced vortex shedding must also be included.     

Sand particle dislodgement in windblown sand

The incipient motion of sand particle from sand bed plays a very important role in the prediction of windblown sand.In this paper,we proposed a new method for predicting the incipient motion of sand particle based on wind speed fluctuation as follows,when the wind speed is larger than the critical wind speed,if the total impulse on sand particle is larger than the critical impulse,incipient motion of sand particle would take place,otherwise if not.Furthermore,from the analysis of entrainment in the rolling and lifting modes,we come to the following conclusion.When the average wind speed is smaller than the critical wind speed,if the average wind speed is used to judge the incipient motion of sand particle,one will underestimate the number of sand particles jumping from the bed,if the instantaneous wind speed is used to judge incipient motion of sand particle,one will overestimate the number of sand particles jumping from the bed;When the average wind speed is larger than the critical wind speed,either the average or the instantaneous wind speeds is used to judge the incipientmotion of sand particles,one will overestimate the number of sand particles jumping from the bed.     

湍流边界层中固体颗粒运动的数值模拟

根据Ｌａｇｒａｎｇｅ颗粒运动微分方程及不可压缩湍流边界层中流体的壁面速度分布规律，数值求解了颗粒在湍流边界层中的运动，考虑了Ｓａｆｆｍａｎ升为对颗粒运动的影响，壁面对运动阻力的影响，给出了固体颗粒沉积边壁，在边界层外缘上所需的最小速度和最小入射角，计算结果还表明边界层对固体颗粒撞击边壁的速度和入射角有较大影响，从数值结果可可以发现一个重要现象。     

INVESTIGATION ON THE ATTENUATION OF A ROTATING LIQUID MOTION IN A CYLINDER WITH BOUNDARY RESISTANCE

为了分析圆柱形容器内同步旋转液面在边界阻力作用下的衰减机理，采用层流摩阻与湍流摩阻的理论模型和实验测试的方法对同步旋转液面在边壁阻力作用下的衰减规律进行了研究. 通过分析壁面阻力的特性，采用当量湿周对圆柱形容器中旋转液体运动的能量损失进行模拟，建立了壁面糙率、旋转半径，静水深度和液体旋转速度之间的本构方程. 通过对圆柱容器中的同步旋转液面在边界阻力作用下的层流和湍流形态的衰减过程进行实验测试，验证了理论模型的正确性. 基于验证后的理论方法，获得了圆柱容器中旋转液体的角速度、高度等随时间的变化过程，分析了摩阻形态、糙率和直径对旋转液面衰减过程的影响. 结果表明，边壁糙率和容器半径是决定旋转液面的衰减过程主要因素，边壁糙率越大液体旋转速度衰减越快，圆柱形容器半径越大液体旋转的衰减越慢.