不同界面计算方法对液滴在电场作用下变形数值模拟精度的影响

液滴在电场作用下的变形是电流体动力学的基础课题之一,表面张力的计算精度对液滴变形量的模拟结果有重要影响。本文以开源计算流体动力学平台OpenFOAM的VOF模型为框架,研究了MULES和isoAdvector两类界面更新算法与相分数梯度和RDF函数两类曲率算法对电场作用下液滴变形模拟精度的影响。研究表明,isoAdvector算法相比MULES算法对网格密度的要求更低,但其耦合相分数梯度算法计算表面张力的误差较高。isoAdvector算法耦合RDF函数算法计算误差较低,并且在使用轴对称网格时,只有该算法能够同时处理液滴平行于电场和垂直于电场方向的变形,得到的数值结果与解析解吻合较好。     

Oldroyd-B黏弹性液滴弹跳行为的改进SPH模拟

基于光滑粒子流体动力学SPH(Smoothed Particle Hydrodynamics)方法对Oldroyd-B黏弹性液滴撞击固壁面产生的弹跳行为进行了模拟与分析。首先,为了解决SPH模拟黏弹性自由表面流出现的张力不稳定性问题,联合粒子迁移技术提出了一种改进SPH方法。然后,对Oldroyd-B黏弹性液滴撞击固壁面产生的铺展行为进行了改进SPH模拟,与文献结果的比较验证了方法的有效性。最后,通过降低Reynolds数捕捉到了液滴的弹跳行为;并在此基础上,分析了液滴黏度比、Weissenberg数和Reynolds数对液滴弹跳行为的影响。结果表明,改进SPH方法可有效地模拟黏弹性自由表面流问题;液滴黏度比、Weissenberg数和Reynolds数对液滴最大回弹高度均有显著的影响。     

超高速碰撞产生等离子体的毁伤特性研究

以热力学为基础,结合化学反应速率方程,推导得出描述超高速碰撞产生的等离子体电子密度与系统内能关系的物理方程组. 应用自行编写的二维光滑粒子流体动力学(smoothed particle hydrodynamics, SPH) 程序求解了此方程组,在模拟超高速碰撞过程中计算产生的等离子体,实现对超高速碰撞产生等离子体的数值模拟.进行铝球超高速碰撞双层铝板的数值模拟研究,给出与实验对比的结果. 统计前后两次碰撞产生等离子体的电量,发现碰撞较薄的前板产生了较少的电荷而一次碎片云对较厚后板的碰撞产生了大量的电荷. 交换前后两板的位置. 进行相同速度碰撞的模拟,分析结果发现一次碎片云碰撞后板产生的等离子体电量远小于第一次的模拟结果,而前板碰撞产生的等离子体电量要高于第一次模拟结果,由此可见,超高速碰撞产生等离子体的总电量不仅与弹丸的质量和碰撞速度有关,与薄板的厚度也有很大的关系,通过一次碎片云与第二层板碰撞可以产生远高于弹丸碰撞单层板产生的电量,可以提高等离子体产生效率,增强对航天器的电磁毁伤.      

液滴在激波冲击下的破裂过程

对液滴在入射激波作用下的变形破碎过程进行了实验研究和数值模拟,得知数值模拟结果与实验结果基本吻合,以及在什么情况下两者出现分歧。结果显示,液滴在激波的作用下要经历从压缩变形、RM不稳定性变形、细小液雾剥离到全部雾化破碎等过程。结果还表明,不同液滴直径、入射激波马赫数和液滴介质等参数下的液滴变形破碎的发展趋势是一致的,而其发展速度明显则不同。其中Weber数的增加加速了液滴的破碎,而Ohnesorge数和黏性的增加则抑制了液滴的破碎。     

双流体雾化液滴荷电特性分析

本文通过理论分析,提出了双流体荷电雾化过程中,增大电场强度或减小液滴粒径均能提高荷电效果的论点。针对不同的电极布置情况建立了数值模拟模型,计算结果表明,双流体喷嘴接地时,喷嘴与环状电极之间的电场强度比喷嘴不接地时大很多。通过实验,验证了电场强度、液滴粒径等参数对荷电效果的影响规律,在环状电极荷电情况下,双流体喷嘴接地时,液体荷质比高于双流体喷嘴不接地时的荷质比;电场强度增大,荷质比增加;液滴粒径越小,荷质比越大;在一定的电导率范围内,电导率越大,荷质比越小。实际喷雾中,运动液滴发生二次破碎的临界荷电量小于Rayleigh极限。     

复合圆柱壳冲击压缩数值模拟及稳定性研究

针对复合圆柱壳在炸药爆轰作用下的动力学响应及在此过程中伴随的失稳问题,研究了其制造工艺中可能出现的缺陷以及圆柱壳中铜线螺旋角和直径对复合圆柱壳稳定性产生的影响。采用SPH-FEM耦合算法,建立了复合圆柱壳二维细节模型,并提出了一种基于圆柱壳内壁粒子速度历史的失稳判据,计算了在不同参数条件下复合圆柱壳的失稳时间及对应的压缩率,对影响复合圆柱壳稳定性的因素进行了评估。分析结果表明,在复合圆柱壳制备过程中存在的折返层缺陷和铜线直径对复合圆柱壳的稳定性有较大影响,而螺旋角度对其稳定性影响不大。

     

电场作用下气泡上升行为特性的数值计算研究

利用电场控制气泡形态及运动,强化气液相间传热传质是电流体动力学的重要研究内容之一.然而目前多数研究集中在非电场下的气泡动力学上,对于电场下的气泡行为特性及电场的作用机制仍需开展深入研究.本研究对电场作用下单个气泡在流体中上升过程的动力学行为进行了数值模拟研究.在建立二维模型的基础上求解电场方程与Navier-Stokes方程,并采用水平集方法捕捉了上升气泡的位置及形状.模拟结果的准确性与有效性通过与前人实验和数值结果进行对比得到了验证.通过改变雷诺数、邦德数和电邦德数等不同参数研究了电场下液体黏度、表面张力和电场力对气泡运动变形的影响.计算结果表明,电场对气泡的动态特性有显著影响.非电场情况下液体黏度和表面张力较大时气泡基本维持球状,反之气泡发生变形并逐步达到稳定状态.此外,电场作用使气泡在初始上升阶段发生剧烈形变,随着不断上升,气泡形变程度不断减小,且气泡的上升速度和长径比均出现振荡.垂直电场使气泡的上升速度有较大的提高,且随着电邦德数的增大,难以达到相对稳定的状态.     

电场作用下气泡上升行为特性的数值计算研究

利用电场控制气泡形态及运动,强化气液相间传热传质是电流体动力学的重要研究内容之一.然而目前多数研究集中在非电场下的气泡动力学上,对于电场下的气泡行为特性及电场的作用机制仍需开展深入研究.本研究对电场作用下单个气泡在流体中上升过程的动力学行为进行了数值模拟研究.在建立二维模型的基础上求解电场方程与Navier-Stokes方程,并采用水平集方法捕捉了上升气泡的位置及形状.模拟结果的准确性与有效性通过与前人实验和数值结果进行对比得到了验证.通过改变雷诺数、邦德数和电邦德数等不同参数研究了电场下液体黏度、表面张力和电场力对气泡运动变形的影响.计算结果表明,电场对气泡的动态特性有显著影响.非电场情况下液体黏度和表面张力较大时气泡基本维持球状,反之气泡发生变形并逐步达到稳定状态.此外,电场作用使气泡在初始上升阶段发生剧烈形变,随着不断上升,气泡形变程度不断减小,且气泡的上升速度和长径比均出现振荡.垂直电场使气泡的上升速度有较大的提高,且随着电邦德数的增大,难以达到相对稳定的状态.     

应用基于GPU的SPH方法模拟二维楔形体入水砰击问题

光滑粒子流体动力学(SPH)法是一种无网格的拉格朗日效值方法,广泛应用于计算流体领域模拟复杂自由表面流问题.SPH方法的主要缺点就是计算量过大,而基于GPU的并行计算方法可使SPH计算得到有效加速.本文应用基于GPU的SPH并行计算方法研究了二维楔形体的入水砰击问题.数值计算结果与文献中对应的解析解比较一致,验证了基于GPU的SPH方法的精度和可靠性.仿真结果同时显示基于GPU的并行计算方法可使SPH计算速度得到显著提高.     

SPH-FEM接触算法在冲击动力学数值计算中的应用

为了充分发挥光滑粒子流体动力学方法(Smoothed Particle Hydrodynamics,SPH)易于处理大变形以及有限元(Finite Element Method,FEM)计算精度和效率高的优势,论文基于无网格粒子接触算法,在有限元节点处设置背景粒子,通过接触力的方式计算SPH粒子和有限单元之间的接触问题.使用SPH-FEM接触算法分别对球头钢弹斜冲击钢板和平头钢弹正冲击钢板的情况进行了三维数值计算,SPH采用完全变光滑长度算法,FEM采用矩阵向量积的EBE(Element-By-Element)算法.将SPH-FEM接触算法的计算结果与LS-DYNA的计算结果以及实验结果进行了对比验证.     

Numerical analysis of pressure drop oscillations in parallel channels

In this study pressure drop oscillations in two parallel channels are analyzed taking into account the thermal capacity of the pipes. A different limit cycle than the one that takes place in a single channel system is found. During the instability one channel always follows the typical pressure drop oscillations limit cycle while the other channel oscillates always in the superheated vapor region. This behavior leads to very high wall temperatures at the outlet of the heated pipe. This undesirable situation with one channel operating in the superheated vapor region takes place also for the maldistributed stable solutions.     

Numerical analysis of secondary droplets characteristics due to drop impacting on 3D cylinders considering dynamic contact angle

This study is investigating three-dimensional numerical simulation of a Newtonian droplet impact and break on two square cylinders based on dynamic contact angle of droplet at the spatial interface between two solid–fluid phases. The droplet impact details and morphology studied in the present work could provide ideas for the spray wall impingement modeling in the simulation of many industrial applications, such as spray painting and liquid cooling of surfaces. The droplet impact is investigated on two square cylinders in 9 different modes with different droplet diameters and physical conditions such as different positions of droplet. The volume of fluid (VOF) method was used with open-source software. The results have been compared and validated quantitatively and qualitatively with the experimental results. Results represent droplet diameter into cylinder dimension and velocity profiles are affected on number of broken droplets, break times and droplet deformation. Also, mean velocities of droplet after impact on two square cylinders at first break time were 0, 0.025, 0.12, 0.47, 0.11, 0.08, 0.2, 0.012, 0.19 m/s for cases 1–9, respectively. Moreover, in case 7 that droplet diameter into cylinder dimension was 2, the maximum number of break-up into secondary droplets was 10 drops that occurred for 4 times.

     

Numerical analysis of effective elastic properties of geomaterials containing voids using 3D random fields and finite elements

The purpose of the study is to investigate the influence of porosity and void size on effective elastic geotechnical engineering properties with a 3D model of random fields and finite element. The random field theory is used to generate models of geomaterials containing spatially random voids with controlled porosity and void size. A “tied freedom” analysis is developed to evaluate the effective Young’s modulus and Poisson’s ratio in an ideal block material of finite elements. To deliver a mean and standard deviation of the elastic parameters, this approach uses Monte-Carlo simulations and finite elements, where each simulation leads to an effective value of the property under investigation. The results are extended to investigate an influence of representative volume element (RVE). A comparison of the effective elastic stiffness of 2D and 3D models is also discussed.     

Experimental analysis of crack tip fields in rubber materials under large deformation

A three-nested-deformation model is proposed to describe crack-tip fields in rubber-like materials with large deformation.The model is inspired by the distribution of the measured in-plane and out-of-plane deformation.The inplane displacement of crack-tip fields under both Mode I and mixed-mode(Mode I-II) fracture conditions is measured by using the digital Moire’ method.The deformation characteristics and experimental sector division mode are investigated by comparing the measured displacement fields under different fracture modes.The out-of-plane displacement field near the crack tip is measured using the three-dimensional digital speckle correlation method.     

Experimental investigation of viscoelastic drop deformation in Newtonian matrix at high capillary number under simple shear flow

The steady state deformation of a viscoelastic drop (Boger fluid) in a Newtonian liquid at high capillary number under simple shear flow is investigated by direct visualization using a specially designed Couette apparatus which enables visualization from two perpendicular directions. Two drop deformation modes are found: (1) Mode I – drop deformation in the flow direction and (2) Mode II – drop deformation in the vorticity direction. The drop deformation mode depends on the relative strength of the elastic contribution to viscous contribution. If the elastic contribution is weak compared to the viscous contribution, the drop elongates in the flow direction via Mode I. If the elastic contribution is strong, the drop elongates in the vorticity direction via Mode II. The drop size also affects the drop deformation. At the same capillary number, bigger drops have larger deformations than smaller drops.     

Deceleration and deformation of a liquid drop in a gas stream

An analytic method is developed for calculating the nonstationary motion and spreading of two-dimensional and axisymmetric liquid drops in a gas stream. The method is based on an expansion of the Navier-Stokes equations in a small parameter.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 58–69, March–April, 1981.     

Three‐dimensional boundary element analysis of drop deformation in confined flow for Newtonian and viscoelastic systems

An adaptive (Lagrangian) boundary element approach is proposed for the general three‐dimensional drop deformation in confined flow. The adaptive method is stable as it includes remeshing capabilities of the deforming interface between drop and suspending fluid, and thus can handle large deformations. Both drop and surrounding fluid are viscous incompressible and can be Newtonian or viscoelastic. A boundary‐only formulation is implemented for fluids obeying the linear Jeffrey's constitutive equation. Similarly to the formulation for two‐dimensional Newtonian fluids (Khayat RE, Luciani A, Utracki LA. Boundary element analysis of planar drop deformation in confined flow. Part I. Newtonian fluids. Engineering Analysis of Boundary Elements 1997; 19 : 279), the method requires the solution of two simultaneous integral equations on the interface between the two fluids and the confining solid boundary. Although the problem is formulated for any confining geometry, the method is illustrated for a deforming drop as it is driven by the ambient flow inside a cylindrical tube. The accuracy of the method is assessed by comparison with the analytical solution for two‐phase radial spherical flow, leading to good agreement. The influence of mesh refinement is examined for a drop in simple shear flow. Copyright © 2000 John Wiley & Sons, Ltd.