Microscopic selection of fluid fingering patterns

We study the issue of the selection of viscous fingering patterns in the limit of small surface tension. Through detailed simulations of anisotropic fingering, we demonstrate conclusively that no selection independent of the small-scale cutoff (macroscopic selection) occurs in this system. Rather, the small-scale cutoff completely controls the pattern, even on short time scales, in accordance with the theory of microscopic solvability. We demonstrate that ordered patterns are dynamically selected only for not too small surface tensions. For extremely small surface tensions, the system exhibits chaotic behavior and no regular pattern is realized.     

Fingering instabilities in dewetting nanofluids

The growth of fingering patterns in dewetting nanofluids (colloidal solutions of thiol-passivated gold nanoparticles) has been followed in real time using contrast-enhanced video microscopy. The fingering instability on which we focus here arises from evaporatively driven nucleation and growth in a nanoscopically thin precursor solvent film behind the macroscopic contact line. We find that well-developed isotropic fingering structures only form for a narrow range of experimental parameters. Numerical simulations, based on a modification of the Monte Carlo approach introduced by Rabani et al. [Nature (London) 426, 271 (2003)10.1038/nature02087], reproduce the patterns we observe experimentally.     

Pattern formation in flame spread over thin solid fuels

The fingering char pattern emerging on the surface of thin cellulosic sheets burning against an oxidizing wind is discussed. Employing collocation-based averaging, the assumption of diffusive-thermal equilibrium, the strong temperature dependence of the reaction rate, and the strong disparity between the densities of the solid and gaseous phases, an elementary two-dimensional free-interface model for the flame spread is formulated. It is shown that the pattern-forming dynamics is functionally akin to the well-studied cellular instability occurring in low Lewis number premixed gas flames.     

Continuum modeling for two-lane traffic flow with consideration of the traffic interruption probability

Considering the effects that the probability of traffic interruption and the friction between two lanes have on the car-following behaviour,this paper establishes a new two-lane microscopic car-following model.Based on this microscopic model,a new macroscopic model was deduced by the relevance relation of microscopic and macroscopic scale parameters for the two-lane traffic flow.Terms related to lane change are added into the continuity equations and velocity dynamic equations to investigate the lane change rate.Numerical results verify that the proposed model can be efficiently used to reflect the effect of the probability of traffic interruption on the shock,rarefaction wave and lane change behaviour on two-lane freeways.The model has also been applied in reproducing some complex traffic phenomena caused by traffic accident interruption.     

Laser theory for semiconductor quantum dots in microcavities

Quantum dots (QDs) used as active material in microresonators are currently of strong topical interest due to breakthroughs in growth and device structuring. From the theory side, however, atomic models are still used to analyse the emission from these semiconductor systems, despite known differences between QDs and atoms. We introduce a semiconductor laser theory based on a microscopic approach with the goal of better describing the characteristic behaviour of QD-based laser devices and to show differences from predictions based on atomic models.     

Spin models as microfoundation of macroscopic market models

Macroscopic price evolution models are commonly used for investment strategies. There are first promising achievements in defining microscopic agent based models for the same purpose. Microscopic models allow a deeper understanding of mechanisms in the market than the purely phenomenological macroscopic models, and thus bear the chance for better models for market regulation. However microscopic models and macroscopic models are commonly studied separately. Here, we exemplify a unified view of a microscopic and a macroscopic market model in a case study, deducing a macroscopic Langevin equation from a microscopic spin market model closely related to the Ising model. The interplay of the microscopic and the macroscopic view allows for a better understanding and adjustment of the microscopic model, as well, and may guide the construction of agent based market models as basis of macroscopic models.     

Turbulent scalar fluxes in H2-air premixed flames at low and high Karlovitz numbers

The statistical behaviour and closure of sub-grid scalar fluxes in the context of turbulent premixed combustion have been assessed based on an a priori analysis of a detailed chemistry Direct Numerical Simulation (DNS) database consisting of three hydrogen-air flames spanning the corrugated flamelets (CF), thin reaction zones (TRZ) and broken reaction zones (BRZ) regimes of premixed turbulent combustion. The sub-grid scalar fluxes have been extracted by explicit filtering of DNS data. It has been found that the conventional gradient hypothesis model is not appropriate for the closure of sub-grid scalar flux for any scalar in the context of a multispecies system. However, the predictions of the conventional gradient hypothesis exhibit a greater level of qualitative agreement with DNS data for the flame representing the BRZ regime. The aforementioned behaviour has been analysed in terms of the properties of the invariants of the anisotropy tensor in the Lumley triangle. The flames in the CF and TRZ regimes are characterised by a pronounced two-dimensional anisotropy due to strong heat release whereas a three-dimensional and more isotropic behaviour is observed for the flame located in the BRZ regime. Two sub-grid scalar flux models which are capable of predicting counter-gradient transport have been considered for a priori DNS assessment of multispecies systems and have been analysed in terms of both qualitative and quantitative agreements. By combining the latter two sub-grid scalar flux closures, a new modelling strategy is suggested where one model is responsible for properly predicting the conditional mean accurately and the other model is responsible for the correlations between model and unclosed term. Detailed physical explanations for the observed behaviour and an assessment of existing modelling assumptions have been provided. Finally, the classical Bray–Moss–Libby theory for the scalar flux closure has been extended to address multispecies transport in the context of large eddy simulations.     

Metal particle combustion and nanotechnology

Metal combustion has received renewed interest largely as a result of the ability to produce and characterize metallic nanoparticles. Much of the highly desirable traits of nanosized metal powders in combustion systems have been attributed to their high specific surface area (high reactivity) and potential ability to store energy in surfaces. In addition, nanosized powders are known to display increased catalytic activity, superparamagnetic behavior, superplasticity, lower melting temperatures, lower sintering temperatures, and higher theoretical densities compared to micron and larger sized materials. The lower melting temperatures can result in lower ignition temperatures of metals. The combustion rates of materials with nanopowders have been observed to increase significantly over similar materials with micron sized particles. A lower limit in size of nanoenergetic metallic powders in some cases may result from the presence of their passivating oxide coating. Consequently, coatings, self-assembled monolayers (SAMs), and the development of composite materials that limit the volume of non-energetic material in the powders have been under development in recent years. After a brief review of the classifications of metal combustion based on thermodynamic considerations and the different types of combustion regimes of metal particles (diffusion vs. kinetic control), an overview of the combustion of aluminum nanoparticles, their applications, and their synthesis and assembly is presented.     

Effects of subgrid scale and combustion modelling on flame structure of a turbulent premixed flame within LES and tabulated chemistry framework

The effects of combustion and SubGrid Scale (SGS) modelling on the overall flame characteristics of a turbulent premixed flame are investigated. This is achieved in terms of mean flow statistics, variances and flame surfaces. In particular, the chemical flame structure is analysed and compared. The Artificially Thickened Flame (ATF) approach coupled with the Flamelet Generated Manifolds (FGMs) and Filtered TAbulated Chemistry for LES (F-TACLES) approaches are used for this investigation. A Germano like procedure for dynamical calculation of SGS wrinkling is used which ensures the conservation of the total flame surface for both models. It turns out that using the dynamic SGS wrinkling model improves the results. Although the results of both combustion models in terms of statistics, mean and variances show very good agreement, the resolved flame surfaces hide different dynamic behaviour.     

Mathematical model of oil shale particle combustion

At first this paper simply introduces the ignition mechanism and combustion characteristics of Huadian oil shale. Combustion behaviour was found to be homogeneous at the beginning of combustion, shifting to heterogeneous combustion in the high-temperature stage; combustible matter was noted to be volatile in the low-temperature stage, but in the high-temperature stage combustible matter included fixed carbon and residual volatile. On the basis of the combustion characteristics of Huadian oil shale, homogeneous and heterogeneous combustion processes of Huadian oil shale are modelled. In the mathematical models, conductive, convective and radiative heat transfer between particles and the surrounding atmosphere, pyrolytic heat and also the heat value of the volatiles are all included in the energy equations; inference of volatile release to particle density is also considered in the models. Thermogravimetric experimental data are used to validate the described models.     

Fingering instability in forward smolder combustion

We use numerical strategies to examine the linear and nonlinear stability of forward smolder waves in the framework of a simplified thermal–diffusive model, with the hydrodynamic effects completely filtered out. The configuration consists of a horizontal thin solid fuel, over which air blows in the same direction as the smolder front propagation. It is found that, in the absence of convective heat losses, the whole one-dimensional adiabatic solution branch is linearly stable; in contrast, when the convective heat loss effect is taken into account, fingering instability emerges provided the incoming air flow rate is within a narrow range near the one-dimensional extinction limit, a manifestation that is reminiscent of the familiar cellular instability occurring in the context of low-Lewis-number diffusion flames. Accordingly, the fingering instability herein identified in forward smolder combustion is purely thermal–diffusive in nature. Furthermore, a heuristic analysis by drawing an analogy with premixed flame suggests that the occurrence of such fingering instability is the joint consequence of the Lewis number effects and convective heat losses. It is proposed that a Hele–Shaw-type combustion channel may be adopted to experimentally reveal the fingering patterns predicted by current numerical simulations.     

煤多相燃烧分形增长模型的初步研究

本文运用分形理论对煤的多相燃烧过程进行了探索性研究。根据分形动力学的概念认为；煤的多相燃烧中，反应面积的分形增长有DLC模式和KLC模式，一般情况是这两种模式的迭加。模型还引入了孔洞的合并及煤种的影响因素，初步提出了较有通用意义的燃烧模型。该模型把对碳焦的表面分形与燃烧速率的计算结合起来初步揭示了碳焦结构对燃烧过程的影响。     

Simulation studies of soft matter: generic statistical properties and chemical details

The relation between atomistic structure, architecture, molecular weight and material properties is a basic concern of modern soft material science. This by now goes far beyond standard properties of bulk materials. A typical additional focus is on surface or interface aspects or on the relation between structure and function in nanoscopic molecular assemblies. This all implies a thorough understanding on many length and correspondingly time scales ranging from (sub)-atomic to macroscopic. At this point computer simulations are playing an increasingly important, if not the central role. Traditionally simulations have been separated in two main groups, namely simplified models to deal with generic or universal aspects of polymers, i.e. critical exponents, and those employing classical force field simulations with (almost) all atomistic detail, i.e. for the diffusion of small additives in a small “sample”. Still characteristic problems, which require huge systems and/or long times in combination with a chemistry specific model, cannot be tackled by these methods alone. More recently with the development of scale bridging or multi scale simulation techniques, these different approaches have been combined into an emerging rather powerful tool. It is the purpose of this contribution to give a few examples of how such an approach can be used to understand specific material properties.     

Stochastic simulation of variations in the autoignition delay time of premixed methane and air

A mesoscopic stochastic particle model for homogeneous combustion is introduced. The model can be used to investigate the physical fluctuations in a system of coupled chemical reactions with energy (heat) release/consumption. In the mesoscopic model, the size of the homogeneous gas volume is an additional variable, which is eliminated in macroscopic continuum models by the thermodynamic limit N→∞. Thus, continuous homogeneous models are macroscopic models wherein fluctuations are excluded by definition. Fluctuations are known to be of particular importance for systems close to the autoignition limits. The new model is used to investigate the stochastic properties of the autoignition delay time in a homogeneous system with stoichiometric premixed methane and air. Temperature and species concentrations during autoignition of sub-macroscopic volumes, including physically meaningful fluctuations, are presented. It is found that different realizations mainly differ in the time when ignition occurs; besides this the development is similar. The mesoscopic range and the macroscopic limit are identified. Which range a specific system is assigned to is not only a question of the length scale or particle number, but also depends on the complete thermodynamic state. The stochastic algorithm yields the correct results for the macroscopic limit compared to the continuous balance equations. The sensitivity of the results to two different detailed reaction mechanisms (for the same system) is studied and found to be low. We show that when approaching the autoignition limit by decreasing the temperature, the fluctuations in the autoignition delay time increase and an increasing number of realizations will have exceedingly long ignition delay times, meaning they are in practice not autoignitable. With this result the mesoscopic simulations offer an explanation of the transition between autoignitable and non-autoignitable conditions. The calculated distributions were compared with ten repetitions of the same experiment. A mesoscopic distribution that matches the experimental results was found.     

旋流喷雾燃烧的直接数值模拟

对三维旋流喷雾燃烧进行了初步的直接数值模拟,其中液滴蒸发采用无限热传导蒸发模型描述,气相燃烧采用自适应单步反应机理,液滴的跟踪在拉格朗日框架中进行。模拟结果表明,喷雾燃烧的火焰结构十分复杂,仅采用传统的非预混燃烧模型是不够的。在蒸发和燃烧共存并存在强烈相互作用的区域,组分与混合物份额之间存在着复杂的关联,这给发展精确的湍流喷雾燃烧模型带来了挑战。     

密封界面微孔结构的三维分形重构技术与泄漏特性

基于分形粗糙表面的三维数值重构技术,对界面微孔结构的分形表征进行详细研究,并应用分形多孔介质输运理论构建界面泄漏机理模型,建立微观形貌与宏观泄漏率之间的理论关系式.用有限元分析法对单粗糙峰的接触变形进行模拟,获得最大孔径的变形位移,实现微观接触力学与微观泄漏模型的有效耦合.应用所提出的预测模型对金属垫片泄漏率进行计算,结果与已有实验结果相吻合.由预测模型可知,粗糙表面分形维数、特征尺度系数以及变形位移量对微孔几何结构影响显著,是影响界面泄漏率的关键因素.     

Supramolecular motif and macroscopic pattern in alpha-methylferrocenemethanol films cast from organic solution

The macroscopic patterns were formed in alpha-methylferrocenemethanol films cast from organic solutions. The macroscopic pattern was composed of concentric rings in the solid film. The concentric rings consist of convex ridges and concave valleys; the ordered phase constitutes the convex ridges, while the concave valleys barely contain anything. It has been found that, as for the solvent which can form hydrogen bonding with the solute and has suitable evaporation rate, macroscopic pattern could be observed in the solid film; while as for the solvent that cannot form hydrogen bonding with the solute, no macroscopic pattern would appear. It was suggested that, intermolecular hydrogen bonding and aromatic π stacking interactions of the solute is responsible for the formation of the microscopic crystalline structure; while the hydrogen bonding between the solute and the solvent, and the solvent-evaporation-induced crystallization process, as well as the solvent-evaporation-induced convections are responsible for the formation of the macroscopic pattern. The results could offer a facile way to the electronic material films with well-defined spatial alignment.     

Characteristic analysis of low-velocity gas filtration combustion in an inert packed bed

This paper investigates the low-velocity filtration combustion of lean methane–air mixtures occurring in inert packed beds by using a modified one-temperature model, considering the axial thermal diffusion owing to the convective gas–solid heat transfer. Based on the scaling analysis of various transport terms in different conservation equations, a high-activation energy asymptotic method is applied in the flame zone and results in a set of powerful analytical solutions for combustion macrocharacteristics under the fully developed conditions. These are then combined with the eigenvalue method of the modified one-temperature model in the whole flow region to study the flame behaviour analytically and numerically. Our results have shown that the combustion wave velocity is a key characteristic parameter in the filtration combustion process. Compared with other existing theoretical results, the present analytical solutions demonstrate the intricate relationships among the combustion wave velocity, the flame speed, the peak flame temperature and the effects of the variable thermo-physical properties, and show better prediction performance for the combustion wave velocity, the flame speed and the peak flame temperature. Excellent agreements with experimental results have been observed, especially for very lean filtration combustion with stream-wise propagating combustion fronts.     

Multiscale analysis of a spatially heterogeneous microscopic traffic model

The microscopic Optimal Velocity (OV) model is posed on an inhomogeneous ring-road, consisting of two spatial regimes which differ by a scaled OV function. Parameters are chosen throughout for which all uniform flows are linearly stable. The large time behaviour of this discrete system is stationary and exhibits three types of macroscopic traffic pattern, each consisting of plateaus joined together by sharp interfaces. At a coarse level, these patterns are determined by simple flow and density balances, which in some cases have non-unique solutions. The theory of characteristics for the classical Lighthill-Whitham PDE model is then applied to explain which pattern the OV model selects. A global analysis of a second-order PDE model is then performed in an attempt to explain some qualitative details of interface structure. Finally, the full microscopic model is analysed at the linear level to explain features which cannot be described by the present macroscopic approaches.