对基于恢复时间的剪稀流变模型的进一步探讨及其对squalane油品流变特性的模拟

本文作者前期基于球-杆变形后恢复到原状态的时间,建立了新的形式较简单的流变模型,并模拟了黏度较高的聚合油PAO 650的摩擦系数曲线.本文中将该模型的应用范围进行了推广,模拟了黏度较低的squalane油品的流变特性.把该流变公式应用到点接触热流变弹流润滑的数学模型中,通过与试验测得的摩擦系数的比较确定了使用该模型时squalane油品的待定参数值,进而得到了点接触热流变弹流润滑的完全数值解.结果表明:解得的压力、膜厚和温度的变化规律均符合预期,且摩擦系数曲线与试验结果整体吻合性较好.新流变模型对高、低黏度的油品均能得到合理的流变特性曲线,说明作者的基于恢复时间的流变模型具有一定的正确性和可应用性.另外,由新模型计算得到的squalane油品的剪应力曲线呈现出一近似水平段,这也在一定程度上解释了流变试验文献中多次提到的极限剪应力现象.     

基于粗粒化水分子模型的Cu-H_2O纳米流体黏度模拟

分子动力学模拟是研究纳米流体的黏度特性的重要手段,但计算量庞大.文章通过对基液水分子粗粒化,使得计算量大幅度减小,且计算精度与全原子模拟相当.基于平衡态分子动力学,模拟研究了Cu-H2O纳米体系的微观运动特性,通过格林-库博(Green--Kubo)公式对Cu-H2O纳米流体的黏度进行了模拟计算,并考察了温度、体积分数、粒径和颗粒形状对于Cu-H2O纳米流体黏度的影响,对已有的悬浮液黏度经验公式进行了修正.     

SIMULATION STUDIES OF VISCOSITIES OF Cu-H2O NANOFLUIDS BASED ON COARSE GRAINING WATER MOLECULES

分子动力学模拟是研究纳米流体的黏度特性的重要手段,但计算量庞大. 文章通过对基液水分子粗粒化,使得计算量大幅度减小,且计算精度与全原子模拟相当. 基于平衡态分子动力学,模拟研究了Cu-H₂O 纳米体系的微观运动特性,通过格林- 库博(Green-Kubo) 公式对Cu-H₂O 纳米流体的黏度进行了模拟计算,并考察了温度、体积分数、粒径和颗粒形状对于Cu-H₂O 纳米流体黏度的影响,对已有的悬浮液黏度经验公式进行了修正.     

非牛顿流体经波纹管流动的阻力特性

本文对Carreau流体经波纹管的蠕动流的阻力特性用差分方法作了数值求解。先施行坐标变换,提出一种保证周期性条件的混合迭代法,能较快得到问题的收敛解。流体的物质参数以及流动区域的几何参数对流动阻力特性的影响作了讨论,同时指出了直径均匀的毛管模型作为渗流模型的不足之处。     

活性流体流变行为的布朗动力学模拟研究

活性流体在用于开发新材料方面具有巨大潜力, 满足这一需求就要定量掌握活性流体所表现的特殊力学行为, 特别是流变行为. 扩展布朗运动方程, 建立自驱动活性粒子的运动模型, 基于反向非平衡法确定活性流体的黏度, 考察活性粒子体积分数、直行速度和转向扩散系数对活性流体流变行为的影响规律, 确定活性流体特殊流变行为的形成机理. 结果表明, 活性流体的流变曲线可被划分为黏度下降区、过渡区和牛顿区; 活性粒子体积分数越高, 活性流体的非牛顿特性越显著, 活性粒子的直行运动引起活性流体在低剪切速率区域黏度下降, 直行运动和转向运动的耦合作用导致中剪切速率区域流变曲线非单调变化, 活性粒子频繁发生转向运动会导致活性流体非牛顿特性受到抑制; 活性流体的宏观流变学特性和粒子的涨落直接相关, 活性粒子体积分数越高、直行速度越快和转向扩散系数越小, 活性流体中活性粒子越容易产生显著的涨落; 低剪切速率区域内活性粒子涨落明显, 随着剪切速率增大, 活性粒子的涨落逐渐被削弱, 粒子的聚集结构不断被破坏, 最终体系的流变行为类似一般被动流体.      

混凝土HJC本构模型参数的研究

运用非线性有限元动力分析软件LS-DYNA模拟了混凝土SHPB试验.将试验采集的入射波数据作为压杆的端面载荷,模拟条件尽量符合试验条件,确保模拟精度.将数值模拟与试验结果相结合,研究了混凝土HJC模型参数的确定方法,得到了C60混凝土的相关计算参数,模拟结果得到的应力-应变曲线与不同应变率下混凝土SHPB试验应力-应变曲线吻合较好.分析了HJC模型的本构特性,比较了与金属JC模型的差异与共性,讨论了HJC模型存在的问题.研究结果表明,未考虑混凝土弹性响应的应变率效应,横向效应的影响与采用的失效方式密切相关.     

磨合过程中液体润滑实现超低摩擦的混合模型研究

针对球-盘滑动试验,在磨合过程中获得超低摩擦的液体润滑状态,建立耦合流体润滑、粗糙接触力学、Archard磨损方程和相关物理参数(液体黏度、表面粗糙度和磨损系数)时变函数的混合模型,研究磨合过程中液体润滑的摩擦系数演化. 通过数值模拟结果可知:在磨合过程中,润滑介质等效黏度增大,形成流体动压润滑薄膜,有效隔开粗糙表面;其次在磨合过程中,新生成的表面粗糙度降低,减少粗糙峰承载比,实现超低摩擦润滑状态;最后在适当的液体黏度和提高表界面效应减少边界摩擦系数,可进一步实现液体超低摩擦润滑状态. 为磨合过程宏观液体润滑性能演化所建立的混合数值模型对提高液体润滑超低摩擦设计效率具有重要价值意义.      

非等温黏弹性复杂流动的改进SPH方法模拟

非等温黏弹性流体广泛存在于自然界和工业生产中,准确预测黏弹性流体的非等温流动机理和复杂流变特性有着重要的应用价值.文章提出一种改进的光滑粒子流体动力学(smoothed particle hydrodynamics,SPH)方法对非等温黏弹性复杂流动进行了数值模拟,其中流体的黏弹特性通过eXtended Pom-Pom本构模型来表征.为了提高模拟结果的精度,采用了一种核函数梯度的修正算法;为了灵活地施加边界条件,发展了边界粒子和虚拟粒子相联合的边界处理方法;为了消除流动过程中的拉伸不稳定性,施加了粒子迁移技术.运用改进SPH方法数值模拟了液滴撞击固壁和F型腔注塑成型问题,通过与Basilisk软件得到的结果进行比较验证了改进SPH方法求解非等温黏弹性流体的有效性.通过利用不同粒子初始间距进行计算,评价了改进SPH方法的数值收敛性.研究了非等温流动相较于等温流动的不同流动特征,深入分析了不同热流变参数对流动过程的影响.数值结果表明,文章提出的改进SPH方法可稳定、准确地描述非等温黏弹性复杂流动的传热机理、复杂流变特性和自由面变化特性.     

不同滑滚比下润滑油膜流变特性试验观察与数值模拟

通过在原有的球-盘接触光干涉润滑油膜测量装置上增设摩擦力测量单元，实现了任意滑滚比下油膜厚度和摩擦系数的同步测量与润滑状态的直观识别. 采用FVA3参考油，分析了不同滑滚比、速度和载荷下的摩擦系数变化规律，并结合油膜干涉图明确了润滑状态与热效应机制，推断出摩擦系数曲面在较低速工况存在混合润滑区域；通过采用基于恢复时间的流变模型对FVA3油品的流变润滑进行数值模拟，并与同等工况下的试验结果进行定量对比，两者取得了良好的吻合性，验证了试验测量的准确性和流变模型的适用性.      

点接触润滑粗糙表面滑动摩擦力的预测研究

在整个润滑区域内基于统一Reynolds方程的混合润滑模型,根据流变模型计算流体摩擦力,根据边界膜极限剪应力模型计算微突体接触摩擦力,二者相加得到混合润滑摩擦力.分析了粗糙度幅值和纹理对摩擦系数的影响以及非牛顿流变模型对流体摩擦系数的影响.模拟跨越整个润滑区,即弹流润滑、混合润滑和边界润滑,得到完整的Stribeck曲线.结果表明,表面越粗糙,混合润滑的摩擦系数越大,弹流润滑和边界润滑时粗糙度幅值影响很小.交叉斜纹的润滑效果优于横向纹理.不同极限剪应力流变模型计算的摩擦系数相差不大.     

基于载荷分担理论的渐开线斜齿轮热混合弹流润滑分析

沿接触线把斜齿轮分成许多小薄片,每一薄片看成具有当量角速度的直齿轮,根据欧拉方程得到任一接触点处的曲率半径和表面速度.然后基于载荷分担、弹流润滑和粗糙线接触理论,建立了考虑表面粗糙度的斜齿轮传动混合热弹流润滑模型.研究了斜齿轮传动稳态载荷分布下牛顿流体和Carreau流体时的润滑特性.结果表明:牛顿流体和Carreau非牛顿流体模型下,中心油膜厚度、油膜承载比例、油膜温升随时间和接触线的变化规律相同.牛顿流体下的摩擦系数较工程实际偏大.Carreau非牛顿流体模型下摩擦系数和工程实际相符,其随接触线啮合位置的变化规律与油膜厚度正好相反.     

润滑油五参数流变模型的研究

基于Evans-Johnson模型提出了润滑油五参数流变模型,利用Evans-Johnson流变模型和五参数流变模型对润滑油的拖动系数进行计算,并与试验数据进行对比.结果表明,利用五参数流变模型预测的拖动曲线与试验曲线形状一致,预测精度较高.在热效应不显著的情况下,采用Evans-Johnson流变模型和五参数流变模型预测的拖动系数基本重合,与试验值接近;在热效应比较显著的区域,利用五参数模型预测的油膜温度值高于Evans-Johnson模型的预测值,对拖动系数的预测精度远大于Evans-Johnson模型的预测精度,解决了在热效应较显著的情况下流变模型对拖动力预测精度较低的难题.     

Sensitivity analysis of the Bagley correction to shear and extensional rheology

The excess pressure losses due to end effects in the capillary flow of two linear low-density polyethylene resins (LLDPE) were studied. These losses were first determined experimentally by using two methods: 1) by extrapolating experimental data of pressure drop versus length-to-radius ratios (L/R) to zero capillary length and 2) by means of using orifice dies (L/R≅0). Both methods resulted in practically the same end corrections. Numerical simulation was also used to model this important aspect of experimental rheology. The constitutive equations used in the simulations are a multimode K-BKZ equation, a multimode Phan-Thien/Tanner, and finally a purely viscous Carreau equation. It was found that the numerical predictions agreed qualitatively but underestimated the experimental data for the various geometries used to determine the end effects. Furthermore, it is demonstrated that the entrance pressure loss is also insensitive to extensional rheology, while it depends more strongly on the shear rheology. This finding raises doubts as to the usefulness of end pressure (known also as Bagley correction) as a method of determining the extensional viscosity of polymer melts at high rates. Received: 18 December 1997 Accepted: 21 May 1998     

A rheological theory for liquid crystal thin films

A macroscopic rheological theory for compressible isothermal nematic liquid crystal films is developed and used to characterize the interfacial elastic, viscous, and viscoelastic material properties. The derived expression for the film stress tensor includes elastic and viscous components. The asymmetric film viscous stress tensor takes into account the nematic ordering and is given in terms of the film rate of deformation and the surface Jaumann derivative. The material function that describes the anisotropic viscoelasticity is the dynamic film tension, which includes the film tension and dilational viscosities. Viscous dissipation due to film compressibility is described by the anisotropic dilational viscosity. Three characteristic film shear viscosities are defined according to whether the nematic orientation is along the velocity direction, the velocity gradient, or the unit normal. In addition the dependence of the rheological functions on curvature and film thickness has been identified. The rheological theory provides a theoretical framework to future studies of thin liquid crystal film stability and hydrodynamics, and liquid crystal foam rheology. Received: 9 October 2000 Accepted: 6 April 2001     

Shear behaviour of biopolymer suspensions with spheroidal and cylindrical particles

The rheological behaviour of suspensions is influenced by many parameters, one of which is the particle shape. For rigid particle suspensions a number of studies demonstrate the effects of the particle aspect ratio. Indeed, fibres are widely used as rheology modifiers in different materials such as synthetic polymers. This work is concerned with testing the hypothesis that regularly shaped particles with aspect ratios larger than one that are made of gelled biopolymers could be used as rheology modifiers for biopolymer solutions. Biopolymers, and mixtures thereof are a widely used ingredient in foods and other products with structure functionality. Tailoring rheology modifiers by morphology offers an alternative to using different biopolymers. It is demonstrated how biopolymer suspensions with regular spheroidal, or cylindrical particle shapes can be produced by gelling the droplet phase of a liquid two phase biopolymer mixture in a shear field. Biopolymers were chosen such that gelation is initiated by cooling. Shear-cooling at constant stresses leads to the formation of ellipsoidal particles. Cylindrical particles can be generated by stepping up the shear stress prior to gelation, i.e., stretching the droplet phase into fibrils, and trapping the shape prior to break-up through gelation. Morphologies and steady shear rheological data for suspensions of the two biopolymers gellan and κ-carrageenan with an internal phase volume of 0.2 are reported. The influence of particle shape on relative viscosity is pronounced. At high shear stresses particle orientation leads to decreased viscosity with increasing particle aspect ratio. In the low shear region, higher aspect ratio suspensions show higher viscosities. Additionally, the material properties, including the interfacial tension, which influence the suspension morphology are reported. Received: 3 March 2000 Accepted: 22 August 2000     

Rheology of viscoelastic thixotropic fluids such as oil and polymer solutions and melts

In the practical transporting of anomalous oils in pipelines and in the chemical industry it is necessary to take into account simultaneously various rheological effects inherent in liquids containing high molecular components in their composition. These effects are manifested in non-linearity of the effective viscosity, the appearance of normal stresses in shear flows (Weissenberg effect), and viscoelastic and thixotropic relaxation of stresses. There are models that describe various manifestations of such rheology [1, 2]. There are also models of a complex nature [3, 4], but these require experimental determination of various functions. The present paper presents a phenomenological tensor determining relation that reflects all the listed rheological anomalies observed in oils, solutions, and melts of polymers. It can be characterized as a model of a viscoelastic medium with two-step thixotropic relaxation. Heuristic arguments are presented that lead to this model, the results are given of calculation of the flow in a rotational viscosimeter of cylinder-cylinder type, the manifestation of the Weissenberg effect in the case of slow rotation of the drum of the viscosimeter is discussed, and experimental steady rheological curves are compared with theoretical curves.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 21–26, May–June, 1984.     

CFD study on rise and deformation characteristics of buoyancy-driven spheroid bubbles in stagnant Carreau model non-Newtonian fluids

The bubbles are almost ubiquitous in many chemical and processing industries; and many of the polymeric solutions obey non-Newtonian rheological characteristics. Therefore, in this work the rise and deformation characteristics of spheroid bubbles in Carreau model non-Newtonian fluids are numerically investigated using a level set method. To demonstrate the validity of the moving bubble interface, the present simulations are compared with existing numerical and experimental results available in the literature; and for these comparisons, the computational geometries are considered same as reported in corresponding literatures. The present bubble deformation characteristics are satisfactorily agreeing with their literature counterparts. After establishing the validity of the numerical solution procedure, the same method is applied to obtain the deformation characteristics of an air bubble in Carreau model non-Newtonian fluids. Further, the results in terms of the volume fraction images, streamlines, and viscosity profiles around the deforming bubbles are presented as function of the bubble rise time.     

A generalized equation for rheology of emulsions and suspensions of deformable particles subjected to simple shear at low Reynolds number

We present analyses to provide a generalized rheological equation for suspensions and emulsions of non-Brownian particles. These multiparticle systems are subjected to a steady straining flow at low Reynolds number. We first consider the effect of a single deformable fluid particle on the ambient velocity and stress fields to constrain the rheological behavior of dilute mixtures. In the homogenization process, we introduce a first volume correction by considering a finite domain for the incompressible matrix. We then extend the solution for the rheology of concentrated system using an incremental differential method operating in a fixed and finite volume, where we account for the effective volume of particles through a crowding factor. This approach provides a self-consistent method to approximate hydrodynamic interactions between bubbles, droplets, or solid particles in concentrated systems. The resultant non-linear model predicts the relative viscosity over particle volume fractions ranging from dilute to the the random close packing in the limit of small deformation (capillary or Weissenberg numbers) for any viscosity ratio between the dispersed and continuous phases. The predictions from our model are tested against published datasets and other constitutive equations over different ranges of viscosity ratio, volume fraction, and shear rate. These comparisons show that our model, is in excellent agreement with published datasets. Moreover, comparisons with experimental data show that the model performs very well when extrapolated to high capillary numbers (C a?1). We also predict the existence of two dimensionless numbers; a critical viscosity ratio and critical capillary numbers that characterize transitions in the macroscopic rheological behavior of emulsions. Finally, we present a regime diagram in terms of the viscosity ratio and capillary number that constrains conditions where emulsions behave like Newtonian or Non-Newtonian fluids.     

Breakup of a capillary bridge of suspensions

The breakup of liquid bridges under the action of capillary forces is used for studying the rheology of suspensions under stretching. The experiments were performed with suspensions of finegrained (3–30 μm) sand in glycerin for sand volume fractions up to 0.465. The bridge thinning process was registered using an electro-optical measuring device and videofilming. The results were analyzed on the basis of a theory developed earlier for the thinning of a liquid bridge under the action of capillary forces. It is found that, for fairly slow stretching realized in the initial stage of the thinning, the rheological behavior of the suspensions considered agrees with the model of a Newtonian viscous fluid. Along with this, the measured effective viscosity of the suspension turned out to be approximately two-fold greater than the suspension viscosity under shear. The origin of this discrepancy is analyzed. With increase in the stretching rate, in the final stage of the thinning, the weakening of the suspension occurs, which is manifested in the formation of a local rapidly thinning neck in the bridge, similar to that observed in the breakup of plastic materials.