过渡区微尺度流动的有效黏性多松弛系数格子Boltzmann模拟

为提高采用二维九速离散速度模型的格子Boltzmann方法 (LBM)模拟微尺度流动中非线性现象的精度和效率,引入Dongari等提出的有效平均分子自由程对黏性进行修正(Dongari N,Zhang Y H,Reese J M2011 J.Fluids Eng.133 071101);并针对以往研究微尺度流动时采用边界处理格式含有离散误差的问题,采用多松弛系数格子Boltzmann方法结合二阶滑移边界条件,对微尺度Couette流动和周期性Poiseuille流动进行模拟,并将速度分布以及质量流量等模拟结果与直接模拟蒙特卡罗方法模拟数据、线性Boltzmann方程的数值解以及现有的LBM模型模拟结果进行对比.结果表明,相对于现有的LBM模型,引入新的修正函数所建立的有效黏性多松弛系数LBM模型有效提高了LBM模拟过渡区的微尺度流动中的非线性现象的能力.     

粗糙微通道内液体流动的多尺度耦合模拟

采用分子动力学与有限容积法多尺度耦合算法对粗糙微通道内的液体Poiseuille流动进行了模拟。分析了粗糙元高度、分布以及几何形状对通道内流动速度和边界滑移长度的影响。结果表明：随着粗糙元高度的增加,流动速度和粗糙元间隙底部壁面上滑移长度均减小;粗糙元分布越密或不同几何形状粗糙元所对应的固壁原子数越多,滑移长度越小,但...     

微尺度流动的扩展二阶滑移边界条件

本文提出了求解微流动Navier-Stokes方程的扩展二阶滑移边界条件.该边界条件依赖一个有效的分子平均自由程,并考虑了气体分子与壁面碰撞的影响,能够刻画Knudsen层内的流动.基于该边界条件,对微通道内的气体流动进行了研究,计算结果与实验数据和其他理论结果相吻合.     

测量液体粘滞系数实验的研究

本文利用液体的绝对速度理论得到液体的粘滞系数随温度变化的关系式，在不同的温度下测得液体的粘滞系数，找出液体的粘滞系数随温度变化的数学表达式。     

用SPH-MD耦合算法模拟微通道中的液氩流动

本文构造了SPH(光滑粒子动力学)-MD(分子动力学)耦合程序,在不同宽度微通道内,对不同驱动力下的液氩流动进行了模拟计算。结果表明,在通道宽度较窄时,流体速度分布和黏性系数分布均表现出很强的微尺度效应,随着通道宽度增大,这种微尺度效应逐渐减弱,流动特征逐渐接近于宏观Poiseuille流。     

分子按自由程的分布与输运系数的推导

首先对分子按自由程的分布律进行重新认识．然后给出输运系数一种新的推导方法．这种方法不仅适合在普通物理中采用而且避免了一些令人费解的假设     

固体微/纳米尺度传热理论研究进展

随着半导体技术的飞速发展,器件的尺寸已进入到微/纳米尺度。由于量子效应、表面及界面效应,使得微尺度下的热物性与宏观尺度下有了明显的区别。人们针对微观传热领域的特点,发展了声子玻尔兹曼传输方程、分子动力学等方法,取得了一定的成果,但仍存在不少问题。一些基础概念问题,特别是非平衡态下的局域温度的定义,有待澄清。本文主要回顾近年来微/纳米尺度传热在理论和数值模拟方面的进展,以及目前所面临的挑战和问题。     

固体微/纳米尺度传热理论研究进展

随着半导体技术的飞速发展,器件的尺寸已进入到微/纳米尺度。由于量子效应、表面及界面效应,使得微尺度下的热物性与宏观尺度下有了明显的区别。人们针对微观传热领域的特点,发展了声子玻尔兹曼传输方程、分子动力学等方法,取得了一定的成果,但仍存在不少问题。一些基础概念问题,特别是非平衡态下的局域温度的定义,有待澄清。本文主要回顾近年来微/纳米尺度传热在理论和数值模拟方面的进展,以及目前所面临的挑战和问题。     

黏性液体中激光空泡脉动特性的理论和实验研究

通过自行研制的光偏转测试系统对黏性液体中激光空泡脉动特性进行了实验，获得了激光等离子体空泡前两次脉动全过程，从而判定了空泡在脉动过程中对应的最大和最小泡半径；并将不同黏性系数下空泡脉动结果与基于空泡溃灭理论的计算结果进行了对比.结果表明：液体黏性对空泡生存周期、空泡半径，以及泡壁的运动速度等均有影响.受液体黏性影响，空泡膨胀或收缩过程明显变缓，其相应的生存周期也越长.这一研究结果不仅可促进空化研究的深入，也可为合理利用空化现象提供参考. 关键词： 激光泡 脉动特性 黏性 光纤传感器     

微尺度水平单管内流动沸腾特性实验研究

对非共沸混合工质R134a/R32(75/25)在水平微尺度管道内的流动沸腾换热实验结果进行了分析和讨论,以探究微细通道内流动沸腾换热的主导机制。对影响其换热的多种因素(热流密度、质量流量和质量干度)进行了分析,实验得出,当质量干度较低时,热流密度和质量流量共同控制着微尺度管内的换热方式,当热流密度的影响占主导地位时,管道内的换热以核态沸腾为主;当质量流量的影响占主导地位时,管道内的换热以强制对流为主。     

金属熔体黏度与结构相关性的分子动力学模拟

运用EAM(embed atom method)作用势，采用非平衡分子动力学模拟获得Al熔体的偶分布函数与黏度数值随温度的变化曲线，偶分布函数的计算结果与实验值符合得较好.对模拟所得到的黏度数据编程实现黏度的Arrhenius公式拟合，得到激活能E.并利用模拟所得到的黏度值及激活能对Lennard-Jones(L-J)作用势进行修正，获得黏度与偶分布函数及原子间相互作用势之间的关系式，两条黏度拟合曲线与分子动力模拟结果符合得比较好，说明拟合程序的编写是比较成功的，实现了对L-J作用势的修正.该研究为金属及合金原子间相互作用势的建立提供了新的思路. 关键词： 非平衡分子动力学模拟 L-J作用势修正 Al熔体 结构与黏度相关性     

温度对液体黏度影响的实验研究

利用自主开发的多光电门可变温式液体黏度测试仪研究了液体黏度随温度变化的关系,得到了实验曲线,显示了不同温度区段的变化特性.     

Molecular field theory for nematic liquid crystal film with finite layers

The nematic liquid crystal film composed of n molecular layers is studied based upon a spatially anisotropic pair potential, which reproduces approximately the elastic free energy density. On condition that the system has perfect nematic order, as in the Lebwohl—Lasher model, the director in the film is isotropic. The effect of the temperature is investigated by means of molecular field theory. Some new results are obtained. Firstly, symmetry breaking takes place when taking account of the temperature, and the state with the director along the normal of the film has the lowest free energy. Secondly, the N—I phase transition temperature increases as an effect of finite sizes instead of decreasing as in the Lebwohl—Lasher model. Thirdly, the nematic order is induced in the layers near the surface in the isotropic phase.     

基于混合液晶分子动力学模拟比较液晶分子旋转黏度大小

提高液晶波前校正器的响应速度是增加液晶自适应光学系统校正带宽的关键, 而研究设计低旋转黏度的液晶分子是提高液晶波前校正器响应速度的根本方法. 利用原子水平上的分子动力学方法获得了目标分子的液相、向列相以及近晶相, 给出了理论计算液晶分子序参数以及旋转黏度的方法. 与此同时, 结合实验方法, 提出利用混合液晶分子动力学模拟来比较液晶分子旋转黏度的大小, 通过多次模拟、多起始点数据处理最大限度消除了因边界尺寸效应带来的数据波动, 最后给出了两种高性能液晶分子的具体比较结果. 这种分子动力学模拟方法能够探查分子结构细微差别对液晶相态以及旋转黏度的影响, 为设计低旋转黏度的液晶分子提供了理论支持, 必将为快速响应液晶材料的设计提供帮助.     

Rotational viscosity of a liquid crystal mixture: a fully atomistic molecular dynamics study

Fully atomistic molecular dynamics (MD) simulations at 293, 303 and 313~K have been performed for the four-component liquid crystal mixture, E7, using the software package Material Studio. Order parameters and orientational time correlation functions (TCFs) were calculated from MD trajectories. The rotational viscosity coefficients (RVCs) of the mixture were calculated using the Nemtsov--Zakharov and Fialkowski methods based on statistical-mechanical approaches. Temperature dependences of RVC and density were discussed in detail. Reasonable agreement between the simulated and experimental values was found.     

Molecular cluster perturbation theory. I. Formalism

We present second-order molecular cluster perturbation theory (MCPT(2)), a linear scaling methodology to calculate arbitrarily large systems with explicit calculation of individual wave functions in a coupled-cluster framework. This new MCPT(2) framework uses coupled-cluster perturbation theory and an expansion in terms of molecular dimer interactions to obtain molecular wave functions that are infinite order in both the electronic fluctuation operator and all possible dimer (and products of dimers) interactions. The MCPT(2) framework has been implemented in the new SIA/Aces4 parallel architecture, making use of the advanced dynamic memory control and fine-grained parallelism to perform very large explicit molecular cluster calculations. To illustrate the power of this method, we have computed energy shifts, lattice site dipole moments, and harmonic vibrational frequencies via explicit calculation of the bulk system for the polar and non-polar polymorphs of solid hydrogen fluoride. The explicit lattice size (without using any periodic boundary conditions) was expanded up to 1000 HF molecules, with 32,000 basis functions and 10,000 electrons. Our obtained HF lattice site dipole moments and harmonic vibrational frequencies agree well with the existing literature.     

Molecular mean field theory for liquid water

Attractive bonding interactions between molecules typically have inherent conservation laws which influence the statistical properties of such systems in terms of corresponding sum rules. We have considered lattice water as an example, and we have enunciated the consequences of the sum rule through a general computational procedure called molecular mean field theory. Fluctuations about the mean field are computed and many of the liquid properties have been deduced and compared with Monte Carlo simulation, molecular dynamics, and experimental results. Large correlation lengths are seen to be a consequence of the sum rule in the liquid phase. Long-range Coulomb interactions are shown to have minor effects on our results.     

Molecular dynamics calculations of shear viscosity time-correlation functions for hard spheres

The time-correlation function for shear viscosity is evaluated for hard spheres at volumes of 1.6 and 3 times the close-packed volume by a Monte Carlomolecular dynamics technique. At both densities, the kinetic part of the timecorrelation function is consistent, within its rather large statistical uncertainty, with the long-timet ^–3/2 tail predicted by the mode-coupling theory. However, at the higher density, the time-correlation function is dominated by the cross and potential terms out to 25 mean free times, whereas the mode-coupling theory predicts that these are asymptotically negligible compared to the kinetic part. The total time-correlation function decays roughly ast ^–3/2, with much larger than the mode-coupling value, similar to the recent observations by Evans in his nonequilibrium simulations of argon and methane. The exact value of the exponent is, however, not very precisely determined. By analogy with the case of the velocity autocorrelation function, for which results are also presented at these densities, it is argued that it is quite possible that at high density the asymptotic behavior is not established until times substantially longer than those attainable in the present work. At the lower density, the cross and potential terms are of the same magnitude as the kinetic part, and all are consistent with the mode-coupling predictions within the relatively large statistical uncertainties.Work performed under the auspices of U.S. Department of Energy.     

利用蜂鸣片受迫振动测液体黏度

用信号发生器驱动蜂鸣片在真空泵油中产生受迫振动,通过测量液体中振动物体的谐振频率求得液体阻尼系数,进而确定了液体黏度.     

毛细管法测量液体黏度实验再设计

设计了简单的毛细管法测量液体黏度的实验装置,通过测量液面高度与时间的变化,拟合实验数据得到液体的黏度,省去了传统的恒压液槽的设计.通过毛细管外加恒温管的方法,可较准确地测量不同温度下液体黏度的变化.通过实验给出泊肃叶公式中液体流速与毛细管长度、内径以及两端压强差的关系.