气体介质中海水液滴碰撞过程数值模拟

为揭示对喷式海水无填料冷却塔内海水液滴碰撞的规律及影响因素,采用VOF方法开展了相同尺寸的二元海水液滴碰撞数值模拟.首先利用Qian等的实验结果对计算模型进行校验,验证了模拟的准确性.开展了常温常压下不同Weber(韦伯)数以及不同碰撞参数下的海水液滴碰撞过程数值模拟,计算Weber数范围为0. 5~200,碰撞参数范围为0~1.得到了聚合、自反分离以及拉伸分离3种碰撞结果,对心碰撞聚合与自反分离的临界Weber数为22;然后对结果类型进行统计,得到了海水液滴碰撞结果分布We-x图.     

三氧化二铝液滴对心碰撞直接数值模拟

为研究固体火箭发动机内三氧化二铝液滴碰撞的物理规律及结果预测模型,针对两个相同尺寸的三氧化二铝液滴对心碰撞,开展了直接数值模拟.首先进行了正十四烷液滴在氮气环境下的对心碰撞数值研究,数值与实验结果基本一致,验证了计算方法的可行性及准确性.针对三氧化二铝液滴开展了6 MPa压强下不同Weber数的对心碰撞数值研究,计算Weber数范围为10~200,Ohnesorge数为0.036 4;获得了反弹、大变形后聚合和自反分离3种结果类型,反弹与大变形后聚合的临界分离Weber数为26,大变形后聚合与自反分离的临界分离Weber数为44.根据临界Weber数对其他流体液滴碰撞模型进行修正,可以获得三氧化二铝液滴的碰撞模型.     

基于格子Boltzmann方法的液滴撞击具有不同润湿性孔板的研究

基于格子Boltzmann方法,对液滴撞击不同湿润性节流孔板表面进行了数值模拟.主要研究了在液滴撞击过程中,Weber数(We)、孔板表面湿润性和孔板尺寸对液滴通过孔板时不同状态的影响.数值模拟结果表明:孔板为亲水特性时,在较低We下,液滴不会与孔板表面脱离,而是附着在孔板下表面,并且在毛细作用下液滴会在孔道中上升一段...     

相场方法模拟液滴的动态润湿行为

液滴的动态湿润现象广泛存在于自然界和工业生产中,该现象数值研究的建模需要解决接触线附近的奇异性并引入合理的接触角描述.基于相场方法,结合Yokoi动态接触角模型,建立了考虑动态润湿效应的两相流数值模型,并在OpenFOAM开源平台上实现相应程序.针对液滴撞击壁面的动态湿润过程,数值模拟和对比研究了不同的接触角模型.结果表明:接触角模型的选择对液滴动态润湿过程的模拟结果具有较大的影响,其中基于改进动态接触角模型的结果与文献中的实验结果具有很好的吻合度,反映了提出的数值模型在液滴的动态润湿行为模拟的有效性.     

液滴碰撞和聚合模型研究

建立了描述液滴碰撞和聚合过程的数学模型,设计了一种高效的计算液滴碰撞对搜索算法,并在已有研究成果的基础上对液滴碰撞的结果做了进一步的发展．借助平滑粒子流体动力学方法两种实现方式各自的优点,将液滴间的相互作用局限在其周围一定数目的液滴之间,并采用积分核函数定义了液滴间碰撞的概率,通过数值模拟探讨了模型的特性．结果表明,所建模型对所采用的计算网格没有明显的依赖性,具有较高的计算精度和计算效率,不但能很好地维持系统动量的守恒性,而且对液滴初始尺寸分布没有明显的依赖性．     

液滴在固体平表面上均匀蒸发过程的格子Boltzmann模拟

通过格子玻尔兹曼(lattice Boltzmann method,LBM)数值模拟,研究了液滴在固体平表面上蒸发过程形状变化的机理.揭示了不同静态接触角下液滴蒸发过程中重力对其形状变化的影响规律.结果表明,重力的影响随着液滴尺度的减小而减弱,达到某一临界点后,重力对蒸发过程的影响可以忽略.模拟定量确定了液滴尺寸的这一临界值,并分析了蒸发过程中几个典型时刻液滴内部的流场分布,进一步研究了重力的影响.     

三维PTT黏弹性液滴撞击固壁面问题的改进SPH模拟

基于光滑粒子动力学(smoothed particle hydrodynamics, SPH)方法,对三维Phan-Thien Tanner(PTT)黏弹性液滴撞击固壁面问题进行了数值模拟.为了有效地防止粒子穿透固壁,且缩减三维数值模拟所消耗的计算时间,提出了一种适合三维数值模拟的改进固壁边界处理方法.为了消除张力不稳定性问题,采用一种简化的人工应力技术.应用改进SPH方法对三维PTT黏弹性液滴撞击固壁面问题进行了数值模拟,精细地捕捉了液滴在不同时刻的自由面,讨论了PTT黏弹性液滴不同于Newton(牛顿)液滴的流动特征,分析了PTT拉伸参数对液滴宽度、高度和弹性收缩比等的影响.模拟结果表明,改进SPH方法能够有效而准确地描述三维PTT黏弹性液滴撞击固壁面问题的复杂流变特性和自由面变化特征.     

乙醇液滴撞击高温壁面蒸发过程的模拟预测研究北大核心CSCD

采用CLSVOF方法,引入描述壁面润湿特性的动态接触角,建立了乙醇液滴撞击高温壁面的数值模型,对乙醇液滴撞击高温壁面后的沸腾蒸发过程展开了研究,并与实验数据进行了对比验证.研究表明:在相同液滴温度下,壁面温度越高,亲水性越强,乙醇液滴的撞击速度越快,液滴的沸腾时间越早,蒸发完成所用时间也越短.在此研究基础上,基于机器学习算法,建立了液滴蒸发预测模型,对乙醇液滴撞击高温壁面后蒸发剩余量随时间的变化进行了预测研究,并通过将不同机器学习算法的预测结果与模拟结果对比,选出最优预测模型.     

Lattice Boltzmann Study on the Motion of Dual Droplets in Microchannels With Contact Angle Hysteresis北大核心CSCD

接触角滞后表现为流体在非理想固体表面上运动时前进接触角和后退接触角不同,是两相流体在润湿表面上流动的重要现象.该文采用改进的伪势格子Boltzmann(LB)多组分模型,并与几何润湿边界条件相结合,研究了两个液滴在具有接触角滞后性微通道表面上的运动行为,主要研究了通道内特征数、通道表面性质以及液滴初始参数的影响.研究结果表明：毛细数的增大有助于液滴的移动,然而并不利于液滴的排出,且毛细数的增加对上游液滴的影响大于其对下游液滴的影响;另一方面,接触角滞后性窗口越大,液滴运动和形变更迟缓,但形变程度更明显,两液滴更早地发生合并,但更晚地排出管道;液滴间距的增加使液滴的运动行为在不同阶段表现为不同的模式,但都导致通道中残留小液滴,使得液滴排出通道的时间增加.研究结果还表明：上游液滴和下游液滴的相对尺寸差距越大,越不利于液滴排出管道.     

室内细颗粒物碰撞模型及碰撞结果讨论

为了研究室内细颗粒物凝并机理,基于统计平均及颗粒物随机运动的思想对室内细颗粒物碰壁数和细颗粒物之间发生对心碰撞的概率进行了总结计算,并对碰撞之后的结果进行了定量和定性的分析,得到了0.3μm的DOP颗粒的最终压缩变形与碰撞发生初始速度以及碰撞效率随粒径变化曲线.颗粒物碰撞后凝并的可能性会随着颗粒碰撞初速度的增加而降低,小粒径颗粒发生碰撞后凝并的可能性一般会大于大粒径颗粒,碰撞效率随粒径的增大而减小.     

On the characteristics of the head-on collision between two ion thermal waves in isothermal pair-ion plasmas containing charged dust grains

The head-on collisions between two ion thermal solitary waves in isothermal pair-ion plasmas containing charged dust grains is investigated. Using the extended Poincaré-Lighthill-Kuo (PLK) method, the Korteweg-de Vries (KdV) equations and the analytical phase shifts after the head-on collision of two- solitary waves are derived. The results show that the number densities of positive and negative dust grains to the density of positive ion ratios and the negative-to-positive ion temperature ratio have strong effects on the phase shifts.     

Dark solitonic excitations and collisions from a fourth-order dispersive nonlinear Schrödinger model for the alpha helical protein

Recent protein observations motivate the dark-soliton study to explain the energy transfer in the proteins. In this paper we will investigate a fourth-order dispersive nonlinear Schrödinger equation, which governs the Davydov solitons in the alpha helical protein with higher-order effects. Painlevé analysis is performed to prove the equation is integrable. Through the introduction of an auxiliary function, bilinear forms and dark N-soliton solutions are constructed with the Hirota method and symbolic computation. Asymptotic analysis on the two-soliton solutions indicates that the soliton collisions are elastic. Decrease of the coefficient of higher-order effects can increase the soliton velocities. Graphical analysis on the two-soliton solutions indicates that the head-on collision between the two solitons, overtaking collision between the two solitons and collision between a moving soliton and a stationary one are all elastic. Collisions among the three solitons are all pairwise elastic.     

Lax pair, Bäcklund transformation and multi-soliton solutions for the Boussinesq-Burgers equations from shallow water waves

Under investigation in this paper is the set of the Boussinesq-Burgers (BB) equations, which can be used to describe the propagation of shallow water waves. Based on the binary Bell polynomials, Hirota method and symbolic computation, the bilinear form and soliton solutions for the BB equations are derived. Bäcklund transformations (BTs) in both the binary-Bell-polynomial and bilinear forms are obtained. Through the BT in the binary-Bell-polynomial form, a type of solutions and Lax pair for the BB equations are presented as well. Propagation characteristics and interaction behaviors of the solitons are discussed through the graphical analysis. Shock wave and bell-shape solitons are respectively obtained for the horizontal velocity field u and height v of the water surface. In both the head-on and overtaking collisions, the shock waves for the u profile change their shapes, which denotes that the collisions for the u profile are inelastic. However, the collisions for the v profile are proved to be elastic through the asymptotic analysis. Our results might have some potential applications for the harbor and coastal design.     

Co-ordinated movements to avoid collisions of a twin-robot system

A method of modelling collision avoidance between separate coordinated arms in the process of changing hands is presented. The shortest distance between two forearms is chosen as the criterion for kinematic modelling. Trajectories of various geometries are classified with respect to different collision situations. Parameters to be adjusted to avoid collisions are also discussed. To illustrate the modelling technique, two redundant anthropomorphic robots are considered to cooperate in rotating a wheel.     

Binary quantum collision operators conserving mass momentum and energy

In this Note, we generalize the Boltzmann collision operator modeling binary particle–particle collisions to a quantum framework using nonlocal quantum entropy principles. To cite this article: P. Degond, C. Ringhofer, C. R. Acad. Sci. Paris, Ser. I 336 (2003).     

一个两流体系统中mKdV孤立波的迎撞*

本文从文[2]的基本方程出发,采用约化摄动方法和PLK方法,讨论了三阶非线性和色散效应相平衡的修正的KdV(mKdV)孤立波迎撞问题.这些波在流体密度比等于流体深度比平方的两流体系统界面上传播.我们求得了二阶摄动解,发现在不考虑非均匀相移的情况下,碰撞后孤立波保持原有的形状,这与Fornberg和whitham[6]的追撞数值分析结果一致,但当考虑波的非均匀相移后,碰撞后波形将变化.     

On the inequality of partial temperatures of a homogeneous gas mixture in the thermodynamic equilibrium state

A modified definition of the state of thermodynamic equilibrium of a homogeneous multicomponent gas mixture is given. It is based on the discrete character of collisions but not on the smoothed-by-multiple-collisions dynamic structure with the physical determination of the kinetic time dt. Application of rigorous mathematical methods which adequately describe the discrete character of collisions leads to a qualitatively new structure of the distribution functions and unexpected physical consequences. It is shown that owing to the inequality of head-on and head-tail collisions of particles, partial temperatures of a homogeneous multicomponent gas mixture in thermodynamic equilibrium cannot be equal to each other. It is found that relations of the partial temperatures to the mixture temperature are dependent on the molecular weights and concentrations of the gas components.     

Simulations of collision of ice particles

The objective of this paper is to develop a realistic model for ice–structure interaction. To this end, the experiments made by Bridges et al. [Bridges FG, Hatzes A, Liu DNC. Structure, stability and evolution of Saturn’s rings. Nature 1984;309:333–5] in order to measure the coefficient of restitution for ice particles are thoroughly analyzed. One particularly troublesome aspect of the aforementioned experiments is fracture of the ice particles during a collision. In the present effort, the collisional properties of the ice particles are investigated using a Finite Element approach. It is found that a major challenge in modeling collision of the ice balls is the prediction of the onset of fracture and crack propagation in them. In simulations of a block of ice collision to a structure, it is crucial that fracture is determined correctly, as it will influence the collisional properties of the ice particles. The results of the simulation, considering fracture criterion implemented into the Finite Element Model [Zamankhan P, Bordbar M-H. Complex flow dynamics in dense granular flows. Part I: experimentation. J Appl Mech (T-ASME) 2006;73:648–57; Zamankhan P, Huang J. Complex flow dynamics in dense granular flows. Part II: simulations. J Appl Mech (T-ASME) 2007;74:691–702] together with a material model for the ice, imply that most of the kinetic energy dissipation occurs as a result of fracturing at the contact surface of the ice particles. The results obtained in the present study suggest that constitutive models such as those proposed by Brilliantov et al. [Brilliantov NV, Spahn F, Hertzsch JM, Poschel T. Model for collisions in granular gases. Phys Rev E;1996;53:5382–92] for collisions of ice particles are highly questionable.