Cellular automaton fluids — A review

This paper is basically a review of cellular automaton fluids, which are the class of cellular automata used for describing fluids. Cellular automaton fluids are discrete analogues of molecular dynamics in which the particles have discrete velocities and move on the sites of a lattice according to some rule of evolution. We restrict ourselves mainly to two-dimensional fluids, but make some comments regarding models for fluids in three dimensions. Analytical as well as numerical simulation results, including ours on the wake behind a cylinder, are discussed for the two-dimensional cellular automaton (ca) models. We also discuss briefly some issues which need resolution before theca models can be used for practical simulations.     

ER流体特性及其研究现状

本文简要介绍了ER流体现象及其力学和电学性能,给出了描述ER流体行为的一般理论,提出了ER流体应用中目前存在的主要问题。     

Corresponding-States Analysis of the Surface Tension of Simple, Polar, and Ionic Fluids

The liquid–vapor interfacial tension of various simple, polar, and ionic fluids is studied in a corresponding-states analysis that was originally suggested by Guggenheim. Data for real fluids are compared to results of simulations and theoretical predictions for model fluids of each of the three types (namely, the Yukawa fluid, the square-well fluid, a fluid consisting of dipolar hard spheres, and the restricted primitive model of ionic fluids). As already demonstrated by Guggenheim, the data for simple and weakly polar fluids map onto a master curve. Strongly dipolar, associating fluids, which may also exhibit hydrogen-bonding (e.g., water), show deviations from this master curve at low temperatures. In addition, the surface tension of these fluids shows a characteristic sigmoid behavior as a function of temperature. A similar behavior is found from simulations of the ionic model fluid, but not from the electrolyte theories available up to now, for which we present new results here. Exceptionally low values of the reduced surface tension are obtained for hydrogen fluoride and for the Onsager model of dipolar fluids, which, however, agree remarkably well with each other in a corresponding-states plot.     

Flows of inelastic non-Newtonian fluids through arrays of aligned cylinders. Part 2. Inertial effects for square arrays

Numerical simulations are presented for flows of inelastic non-Newtonian fluids through periodic arrays of aligned cylinders. The truncated power-law fluid model is used for the relationship between the viscous stress and the rate-of-strain tensor. Results for the drag coefficient for creeping flows of such fluids have been presented in a companion paper [1]. In this second part the effects of finite fluid inertia are investigated for flows through square arrays. It is shown that the Reynolds-number dependence of the drag coefficient of a cylinder in the array is of the form C _d≡ F/(ηU) = k ₀ + k ₂ Re²+ .. for small values of the Reynolds number Re ≡ ρaU/η, where F is the drag force, U is the averaged velocity in the array, η = K (U/a)^n-1 is a viscosity scale with K and n the power-law coefficient and index and a the cylinder radius, and k ₀ is the drag coefficient for creeping flows. The proportionality constant k ₂ depends on the way the drag coefficient and the Reynolds number are defined. It is shown that the observed strong dependence of k ₂ on n can almost be eliminated by using length scales different from a in the viscosity scales η used in the definition of Re and in the definition of the drag coefficient. Numerical simulation results are also presented for the velocity variance components. Results for flows at moderate Reynolds number, of order 100, are also presented; these are qualitatively similar to those for Newtonian fluids. The value of the Reynolds number beyond which the flow becomes unsteady was related to the Newtonian fluid case by rescaling. These results for moderate-Reynolds-number flow are compared against previously published experimental data.     

Prediction of enthalpy and entropy departures using a two-fluid corresponding-states principle

The generalized corresponding-states principle (GCSP), based on the properties of two nonspherical reference fluids, has been shown to be a powerful technique for the correlation and prediction of thermodynamic properties. In this work we show GCSP calculations of enthalpy and enropy departures for pure fluids and fluid mixtures. The mixtures studied include those conforming well to traditional corresponding states theory (e.g., n-pentane + n-octane), as well as those that have not hitherto been amenable to such treatments (e.g., n-pentane + ethanol). It is shown that the GCSP method works well for all classes of mixtures and compares favorably with other methods of prediction. The use of cubic equations of state to represent the reference fluids gives the GCSP method flexibility while maintaining accuracy in the prediction. No adjustable parameters are required in the GCSP calculations of enthalpy and entropy departures.     

A four-parameter corresponding-states method for prediction of Newtonian,pure-component viscosity

The extended Lee-Kesler (ELK) method, introduced for calculating thermodynamic properties of polar as well as nonpolar fluids and their mixtures, has been adapted to the calculation of Newtonian, pure-fluid viscosity. The method is a four-parameter, corresponding-states technique requiring as input the critical temperature, critical pressure, a size/shape parameter , and a polar interaction parameter . Because and have been previously tabulated for many fluids (for calculation of thermodynamic properties) and may also be obtained directly from the radius of gyration and a single liquid density, respectively, the method contains no adjustable parameters and is predictive in nature. ELK viscosity predictions were compared to experimental data for nonpolar and polar fluids. For 36 different nonpolar fluids and a total of 5748 different points, the comparison yielded an absolute average deviation (AAD) of 7.88% with a bias of –4.45%. Similarly, the AAD was 10.62% with a bias of –5.34% for a comparison of 15 different polar fluids involving 1500 different points. With this method, viscosities can be calculated within the range 0.55 T _r2.00 and 0<P _r10.     

Supercritical Fluids as Experimental Models for Geophysical Flows

A pure component is supercritical when its temperature and pressure are above the temperature and pressure of the critical point (CP). In the supercritical domain of the phase diagram, there no longer exists a difference between gaseous and liquid states, and fluids are in an intermediate and somewhat paradoxical state where their thermophysical properties are similar to those of gases for some of them and those of liquids for others. Taking into account the gravity, the diverging compressibility at the CP induces a stable stratification of the fluid density. The stratification is significant, sometimes as much as 10% in a one cm layer. For example, a one cm high cell containing He³ at 3 mK above its critical temperature is equivalent to a 180 m high column of air or a 7 km high column of water in terms of stratification. Therefore, supercritical fluids (SCFs) at the scale of the laboratory share several features with large geophysical systems. This observation has led several authors into suggesting their use as laboratory models for geophysical flows. However, the peculiarity of near-critical systems could make this analogy fail. In the present work, we have investigated the analogy between both kinds of systems (SCFs at small scale and geophysical flows at large ones) through the study of two examples: the onset of convection in a SCF layer subjected to an adverse temperature gradient, and the generation of internal gravity waves in an isothermal SCF layer. In both cases, the use of asymptotic techniques and of stability analysis has shown that the role of the initial stratification was dominant. At the same time, the fluid flow has been shown to be very similar to that of weakly compressible fluids, the peculiar phenomena specific to SCFs being in this case of second order.     

Equations of State for Technical Applications. I. Simultaneously Optimized Functional Forms for Nonpolar and Polar Fluids

New functional forms for multiparameter equations of state have been developed for non- and weakly polar fluids and for polar fluids. The resulting functional forms, which were established with an optimization algorithm which considers data sets for different fluids simultaneously, are suitable as a basis for equations of state for a broad variety of fluids. With regard to the achieved accuracy, the functional forms were designed to fulfill typical demands of advanced technical application. They are numerically very stable, and their substance-specific coefficients can easily be fitted to restricted data sets. In this way, a fast extension of the group of fluids for which accurate empirical equations of state are available becomes possible. This article deals with characteristic features of the new class of simultaneously optimized equations of state. Shortcomings of existing multiparameter equations of state widely used in technical applications are briefly discussed, and demands on the new class of equations of state are formulated. Substance specific parameters and detailed comparisons are given in subsequent articles for the non- and weakly polar fluids (methane, ethane, propane, isobutane, n-butane, n-pentane, n-hexane, n-heptane, n-octane, argon, oxygen, nitrogen, ethylene, cyclohexane, and sulfur hexafluoride) and for the polar fluids (trichlorofluoromethane (CFC-11), dichlorodifluoromethane (CFC-12), chlorodifluoromethane (HCFC-22), difluoromethane (HFC-32), 1,1,2-trichlorotrifluoroethane (CFC-113), 2,2-dichloro-1,1,1-trifluoroethane (HCFC-123), pentafluoroethane (HFC-125), 1,1,1,2-tetrafluoroethane (HFC-134a), 1,1,1-trifluoroethane (HFC-143a), 1,1-difluoroethane (HFC-152a), carbon dioxide, and ammonia) considered to date.     

稀土铈改性二氧化钛微粉的制备及其介电性质

用Sol-gel法制备了掺杂稀土元素铈的二氧化钛微粉,测试了它的介电常数和电导率,由该材料作分散相与甲基硅油配制了无水电流变液,对其电常数和电导率增加,材料的电流变性能也发生很大的改变,存在一个稀土含是珠最佳浓度范围,在此区域介电常数取得最大值,电流变液的剪切强度也取得极大值,比纯二氧化钛电流变液的流变性能提高5倍以上。     

Universal Crossover Approach to Equation of State for Fluids

It is well known that any classical equation of state fails to describe the properties of fluids in the critical region, where the behavior of fluids is strongly affected by density fluctuations. In the present work, a universal approach to incorporate the effects of density fluctuations in the global behavior of one-component fluids is proposed. As an illustration of our general approach, a crossover generalization of a four-parametric cubic equation of state, which can be useful for engineering applications, is demonstrated. The obtained crossover equation reproduces Ising-like singular scaling behavior in the critical region and reduces to the original cubic equation of state far away from the critical point. In addition, the crossover equation of state is applied to describe thermodynamic properties of methane, ethane, carbon dioxide, and water. It is shown that incorporation of critical fluctuations leads to a significant improvement in the ability of the cubic equation to represent thermodynamic properties and liquid–vapor equilibrium of one-component fluids.     

稀土改性二氧化钛电流变液的剪切强度随温度变化关系

用溶胶-凝胶法制备了系列锐钛矿型掺杂稀土元素二氧化钛粉粒,在干态下对其所配制的电流变液进行了力学性能测试及其温度效应研究。结果表明,掺杂后的电流变液性能远优于同条件下所制纯二氧化钛电流变液。温度效应明显优化,在10 ℃~100 ℃均有较强的电流变活性,使用温度范围比纯二氧化钛电流变液大幅度加宽,80℃剪切应力达到最大。材料中的RE/Ti摩尔比对电流变效应影响显著,不同温度下RE/Ti=0.07~0.11之间电流变液呈现最佳的电流变效应,RE/Ti摩尔比引起电流变液介电性质的改变是电流变效应增强和不同的起因。     

Phase equilibria of associating fluids of spherical and chain molecules

The development of equations of state for strongly associating fluids and fluid mixtures has proved over the years to be a difficult problem. The first-order perturbation theory solution of a resummed cluster expansion has been used to investigate the effect of molecular associations on the critical and phase coexistence properties of fluids with one and two off-center attractive sites. The individual molecules are represented by hard-sphere repulsive cores with square-well attractive sites. Model systems comprising chains of hard spheres have also been examined. Isothermal-isobaric Monte Carlo simulations of hard-sphere fluids with one and two attractive sites are shown to be in good agreement with the results of the theory. A simple van der Waals mean-field term is also added to account for the dispersion forces. The critical points and phase equilibria of the associating fluids are determined for various values of the strength and range of the attractive site, as well as for different chain lengths. The theory can treat fluids with strong hydrogen-bonding associations such as the carboxylic acids the aliphatic alcohols, hydrogen fluoride, water, etc.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

聚苯胺链段含量对PAn-g-PEG/LiClO4电流变性能的影响

用化学氧化共聚法制备了聚乙二醇-接枝-聚苯胺(PAn-g-PEG)纳米粒子,并将其与LiClO4形成的络合物粒子分散到硅油中得到了无水电流变(ER)液.研究了PAn-g-PEG中的聚苯胺链段含量(y/x)对ER液性能的影响.结果表明PAn-g-PEG在水溶液中能自组装形成核壳结构,PAn-g-PEG/LiClO4络合物ER液的流变性能对电场能产生显著和快速的响应.随着y/x的增加,ER液的电致剪切应力在y/x=64时出现最大值,漏电流单调降低.     

Fluids for high temperature heat pumps

The theoretical performances of some 250 potential work fluids in vapour compression heat pumps condensing at 150°C and evaporating at 100°C have been predicted, using expression for coefficient of performance (COP) and minimum superheat that involve only easily accessible physical properties. Expected correlations were found between COP and critical temperature, between specific compressor displacement and normal boiling point, T_bp, and between condensing pressure and T_bp. Correlations were also found between minimum superheat and both molecular weight and critical pressure. From these correlations, the desirable basic properties of a high temperature heat pump fluid are deduced. The principle of corresponding states is invoked to explain the connection between minimum superheat and critical pressure, and hence the reason why perfluorinated compounds tend to make poor work fluids.     

Thermal Conductivity Modeling of Pure Refrigerants in a Three-Parameter Corresponding States Format

The potential of the corresponding states (CS) principle for modeling a pure fluid thermal conductivity surface is studied here. While for thermodynamic properties and for viscosity, successful results have been previously obtained by directly applying an improved three-parameter CS method, significant difficulties were encountered while trying to extend this method to thermal conductivity and, in particular, it fails if applied without separately dealing with the dilute-gas term, and the residual and critical enhancement contributions. These last two parts are also combined in the excess term. It is shown that the dilute-gas term cannot be expressed in such a format, and it has necessarily to be individually modeled for each target fluid. On the contrary, the excess contribution can be described through a specific conductivity scaling factor that can be individually determined from a single saturated liquid conductivity experimental value. The model for the excess part is set up in a three-parameter CS format on two reference fluids, in the present case, methane and R134a, for which dedicated thermal conductivity equations are available, and it has a predictive character. The models for the dilute-gas and for the excess contributions are then combined to give the final TC model. The model has been successfully validated for two homologous families of refrigerant fluids obtaining an AAD of 3.67% for 3332 points for haloalkanes and an AAD of 2.87% for 354 points for alkanes.Paper presented at the Sixteenth European Conference on Thermophysical Properties, September 1–4, 2002, London, United Kingdom.     

A theory-based method for correlation and prediction of dense-fluid transport properties

Dense-fluid transport property data for a wide range of compounds have been successfully correlated on the basis of universal curves for the reduced diffusion coefficient,D ^*, the reduced viscosity, η^* and the reduced thermal conductivity, λ^*, against the reduced volumeV/V _o , whereV is the molar volume, andV _o is a characteristic volume equal to the volume of close packing for a system of hard spheres. The reduced transport properties,X ^*, are defined in terms of the low-density hard-sphere values by (X/X _o )(V/V _o )^2–3, whereX is ν, λ, or the product of the number density and the diffusion coefficient. To provide a theoretical justification for this approach, extensive computer simulation results for these transport properties, given in the literature for a system of Lennard-Jones (12–6) molecules, have been considered. It is found that the reduced transport properties for different temperatures are superimposable upon the results for any reference isotherm when plotted versus logV, as found previously for real fluids. However, to reproduce this density dependence at any given temperature on the basis of the universal curves, the characteristic volume for self-diffusion must be greater than that for viscosity or thermal conductivity. Invited paper presented at the Fourth Asian Thermophysical Properties Conference, September 5–8, 1995, Tokyo, Japan.     

Viscosity Modeling and Prediction of Reservoir Fluids: From Natural Gas to Heavy Oils

Viscosity and density are key properties for the evaluation, simulation, and development of petroleum reservoirs. In previous work, the friction theory (f-theory) models have already been shown capable of providing simple but accurate viscosity modeling results of petroleum reservoir fluids with molar masses up to around 200 g · mol^–1. As a base, the f-theory approach requires a compositional characterization procedure to be used in conjunction with a van der Waals type of equation of state (EOS). This is achieved using simple cubic EOS, which are widely used within the oil industry. In this work, the f-theory approach is further extended to the viscosity modeling of heavy reservoir fluids with viscosities up to thousands of mPa · s. Essential to the extended approach presented here is the achievement of accurate pvT results for the EOS characterized fluid. In particular, it has been found that for accurate viscosity modeling of heavy oils, a compressibility correction in the way the EOS is coupled to the viscosity model is required. With the approach presented in this work, the potential of the f-theory for viscosity modeling of reservoir fluids is extended to practically all kind of reservoir fluids, from light ones to heavy ones. Additionally, the approach has been completed with an accurate density modeling scheme.     

一种新型电流变体材料——酮醛树脂盐类电流变体

决定电流变体材料性能的主要因素是其分散相的选择。文中首次提出了以酮醛树脂盐类作为电流变体的分散相，研究了反应温度、反应时间及催化剂用量等各种因素对其分子量的影响，测试了以酮醛树脂盐与硅油组成的电流变体材料的稳定特性、温度特性、流变特性、介电特性，结果表明该流体具有良好的抗高剪切速率的特性。     

The Effect of Model Internal Flexibility Upon NEMD Simulations of Viscosity

The influence of model flexibility upon simulated viscosity was investigated. Nonequilibrium molecular dynamics (NEMD) simulations of viscosity were performed on seven pure fluids using three models for each: one with rigid bonds and angles, one with flexible angles and rigid bonds, and one with flexible bonds and angles. Three nonpolar fluids (propane, n-butane, and isobutane), two moderately polar fluids (propyl chloride and acetone), and two strongly polar fluids (methanol and water) were studied. Internal flexibility had little effect upon the simulated viscosity of nonpolar fluids. While model flexibility did affect the simulated viscosity of the polar fluids, it did so principally by allowing a density-dependent change in the dipole moment of the fluid. By using a rigid model with the same geometry and dipole moment as the average flexible molecule at the same density, it was shown that the direct effect of flexibility is small even in polar fluids. It was concluded that internal model flexibility does not enhance the accuracy of viscosities obtained from NEMD simulations as long as the appropriate model geometry is used in the rigid model for the desired simulation density.     

Accurate Density and Viscosity Modeling of Nonpolar Fluids Based on the “f-Theory” and a Noncubic Equation of State

Recently, the f-theory, a theory for viscosity modeling based on friction concepts of classical mechanics, has been introduced. This new theory allows accurate viscosity–pressure–temperature (–p–T) modeling based on a van der Waals type of equation of state, one with a repulsive pressure term and an attractive pressure term. Thus, popular cubic equations of state (CEOS), such as the SRK and the PR, have been successfully applied to obtain accurate –p–T models (even close to the critical region) of fluids such as n-alkanes, N₂, CO₂, etc., and some of their mixtures. However, even though it has been shown that a CEOS f-theory-based model can accurately reproduce the viscosity behavior of, at least, nonpolar fluids, the accuracy of the density predictions is still limited by the algebraic structure of the CEOS. In this work, a noncubic van der Waals type of equation of state is introduced for the accurate modeling of both the density and the viscosity behavior of selected nonpolar fluids. The achieved accuracy, for both the density and the viscosity fluid properties, is close to, or within, experimental uncertainty and applies to wide temperature and pressure ranges.