Evaluation of reduced combustion kinetic mechanisms using global sensitivity-based similarity analysis (GSSA)

Reduced combustion kinetic mechanisms, instead of detailed ones, are often used in computational fluid dynamics (CFD) simulations for reduced and frequently even affordable computational cost. The criterion for the evaluation of a reduced mechanism usually focuses on its prediction error for the global properties such as the ignition delay time, while ignoring the detailed features of reaction kinetics such as reaction pathways. In our opinion, good reduced mechanisms should have similar predicting behaviors as the detailed ones, and these behaviors include model predictions for specific targets, prediction error bars, and uncertainty sources for the errors. In this work, a new approach using global sensitivity-based similarity analysis (GSSA) is proposed to compare reduced mechanisms with detailed ones. The similarity coefficient for the reduced mechanism is calculated by similarity method based on Euclidean distance between sensitivity indices of the reduced mechanism and those of the detailed mechanism. The larger the similarity coefficient, the higher the degree of similarity between the reduced and detailed mechanisms. To demonstrate this similarity method, directed relation graph with error propagation (DRGEP) is employed to simplify both the GRI 3.0 mechanism without the NOx chemistry and the JetSurF mechanism consisting of 1459 reactions, resulting in reduced mechanisms with different sizes which can accurately predict the ignition delay times for corresponding fuel mixtures. Similarity analysis is then employed to evaluate these reduced mechanisms. The result shows that the actual reaction kinetic features cannot be replicated by some of the reduced mechanisms. First, the rankings of the important reactions obtained by reduced mechanisms are not consistent with those obtained by the detailed mechanism. Second, by investigating the sensitive reactions, the actual impact of uncertainties in reaction rates on the ignition delay times cannot be presented by reduced mechanisms. The similarity analysis on reduced mechanisms can be used to select a reduced mechanism which shows much better performance to replicate the actual combustion reaction kinetics. GSSA can provide information on the uncertainty sources induced by the reactions parameters of reduced mechanisms for target predictions, which is important for further reduced model optimization and for the sensitivity analysis of CFD simulations.     

压电驱动器布置对光栅拼接误差的影响

分析了两种光栅拼接调整机构（三角形和L形）,利用有限元分析软件对这两种光栅调整机构进行了模态分析和优化分析设计。相同光栅尺寸下（以口径为450 mm420 mm60 mm光栅为例）,驱动器三角形分布的固有频率与L形分布的固有频率相差无几,一阶频率分别为58.816 Hz和58.864 Hz。对两种驱动器布置方式引入的误差进行了分析比较,计算结果表明:三角形调整机构的最大误差比L形的大,同时三角形分布控制算法较L形的复杂。在主动控制中三角形模型误差迭代次数多,不利于控制。因此L形的压电驱动器布置方式优于三角形。     

Classification of backscattering-enhancement mechanisms

A classification is provided for the principal backscattering-enhancement mechanisms, which are divided into singleparticle and collective mechanisms, which, in turn, consist of geometrical mechanisms and mechanisms associated with phase coherence. A distinction is made between dynamic mechanisms, which appear in nearly all realizations, and statistical mechanisms, which are observed only after averaging.     

Physiological mechanisms of vocal frequency control: The role of tension

Control of fundamental frequency by the human voice may be accomplished using a variety of physiological mechanisms. Most of these mechanisms are myoelastic and include the length, mass, and longitudinal tension of the vocal folds. In this paper, a review of some of the data concerning the mechanisms for the creation of tension in the vocal folds is presented. The physiological mechanisms for the development of passive tension are reviewed and the potential contribution of passive tension in the control of fundamental frequency is discussed. Some of the mechanisms for the development of active muscle tension are described and the biomechanical characteristics of muscle are considered in the effort to explain the physiological control of fundamental frequency in the human voice     

Conditions of stochasticity of two-dimensional billiards

There are two known mechanisms that produce chaos in billiard systems. The first one, discovered by Ya. G. Sinai, is called dispersing, the second, found by the author, is called defocusing. The same mechanisms produce chaos for geodesic flows. Some results on two-dimensional billiards, which indicate that only these two mechanisms can produce chaos in Hamiltonian systems, are discussed.     

Contributions of different mechanisms of self-diffusion in face-centered cubic metals under equilibrium conditions

The contributions of different mechanisms of self-diffusion in face-centered cubic metals Ni, Cu, and Al at thermodynamic equilibrium have been analyzed using the molecular dynamics method. The vacancy, divacancy, and cyclic mechanisms of self-diffusion, as well as the mechanisms involving the vacancy migration to the second coordination sphere and the formation and recombination of Frenkel pairs, have been considered. It has been shown that the second in contribution to the self-diffusion after the vacancy mechanism in the metals under consideration is the migration of divacancies. The third is the mechanism involving the formation and recombination of dynamic Frenkel pairs. The cyclic mechanisms (with simultaneous atomic displacements) and the vacancy migration directly to the second coordination sphere in face-centered cubic metals are unlikely.     

Mechanisms of the strain-induced dissolution of phases in nanostructured metals

The possible mechanisms of the strain-induced dissolution of phases in metals subjected to severe plastic deformation have been investigated. The mechanisms of impurity absorption by new interphase and grain boundaries formed under severe plastic deformation and the high-temperature phase at its strain-induced nucleation are shown to be the most effective. The possible dislocation mechanisms of phase dissolution are low-efficient. In carbon steels, the strain-induced dissolution of cementite continues until the limiting nanostructure absorbs about 10–12 at % carbon from the mixture volume.     

重复频率脉冲流注放电演变现象与机制研究进展

重复频率脉冲流注放电是低温等离子体前沿应用的关键使能因子,然而,高重复频率脉冲作用下流注放电呈现复杂的不稳定和记忆效应现象,放电基础演变机理和调控方法尚不完善,极大影响应用的安全性和放电特性调控的有效性。综述了重复频率脉冲流注放电演变现象与机制的研究进展。首先归纳了重复频率脉冲流注放电的强非线性和渐进式演变特征,然后分析不同类型放电记忆效应因子对后续流注起始和传播的作用机制,最后总结了脉冲波形参数对重复频率脉冲流注放电的影响规律。凝练了重复频率脉冲流注放电演变机制研究的若干挑战,对脉冲放电等离子体机理研究具有一定的借鉴作用。     

Texture evolution and operative mechanisms during large-strain deformation of nanocrystalline nickel

The large-strain deformation of nanocrystalline nickel was investigated at room temperature and cryogenic (liquid N₂) temperature. Deformation mechanisms ranging from grain boundary sliding to slip, operate due to a wide distribution of grain sizes. These mechanisms leave their finger print in the deformation texture evolution during rolling of nanocrystalline nickel. The occurrence and severance of different mechanisms is understood by a thorough characterization of the deformed samples using X-ray diffraction, X-ray texture measurements, electron back-scattered diffraction and transmission electron microscopy. Crystal plasticity-based viscoplastic self-consistent simulations were used to further substantiate the experimental observations. Thus, a comprehensive understanding of deformation behavior of nanocrystalline nickel, which is characterized by simultaneous operation of dislocation-dominated and grain boundary-mediated mechanisms, has been developed.     

Analysis and modeling of resistive switching mechanisms oriented to resistive random-access memory

With the progress of the semiconductor industry,the resistive random-access memory(RAM) has drawn increasing attention.The discovery of the memristor has brought much attention to this study.Research has focused on the resistive switching characteristics of different materials and the analysis of resistive switching mechanisms.We discuss the resistive switching mechanisms of different materials in this paper and analyze the differences of those mechanisms from the view point of circuitry to establish their respective circuit models.Finally,simulations are presented.We give the prospect of using different materials in resistive RAM on account of their resistive switching mechanisms,which are applied to explain their resistive switchings.     

Column-growth mechanisms during KrF laser micromachining of Al2O3–TiC ceramics

This paper aims to contribute to the understanding of column-formation mechanisms in Al₂O₃–TiC ceramic composites due to processing with excimer laser radiation. The mechanisms proposed in the literature to explain the formation of such columns can be grouped into four categories: hydrodynamic mechanisms, vapour phase deposition mechanisms, spatial modulation of absorbed energy mechanisms, and shadowing mechanisms. In the case of Al₂O₃–TiC ceramics, the first two types of mechanisms can be excluded because experimental results show that the column core is composed of material in a pristine condition. A theoretical simulation of the spatial modulation of absorbed energy due to radiation reflected from pre-existing topographic artefacts reveals that this mechanism does not explain the fact that columns only grow during the first 100–200 laser pulses and can, therefore, be ruled out. By contrast, predictions of the shadowing mechanism with TiC globules formed during the first laser pulses shielding the substrate and favouring column growth are in semiquantitative agreement with experimental observations. PACS 42.62.Cf; 81.05.Je; 81.65.Cf     

Comparative analysis of the mechanisms of the heterogeneous stages of hydrogen and methane oxidation

A comparative analysis of the possible mechanisms of heterogeneous hydrogen and methane oxidation at mixture pressures below the third ignition limit and during the induction period of ignition was performed. The suggested reaction mechanisms are based on the properties of the well-defined active centers on the surface of quartz. These mechanisms explain the available experimental data on the heterogeneous reactions of the initial stages of hydrogen and methane oxidation at a qualitative level.     

Structural mechanisms of fracture of nanocrystalline materials

Structural mechanisms and features of brittle and quasi-brittle fracture of nanocrystalline materials are theoretically analyzed. The role of size effects and internal stresses caused by a nonequilibrium structure during brittle trans-and intercrystallite fracture is studied. The dependence of the nanocrystalline material durability on the working stress and grain size is calculated. The conditions for certain mechanisms of plastic deformation to be operative in nanocrystalline materials are analyzed. The influence of the grain-boundary and dislocation mechanisms of plastic deformation on the conditions of nanocrack formation is studied. The dependence of the fracture toughness of nanomaterials on structure parameters is calculated.     

Anharmonic processes of scattering and relaxation of slow quasi-transverse phonons in cubic crystals

Relaxation of slow quasi-transverse phonons in anharmonic processes of scattering in cubic crystals with positive (Ge, Si, diamond) and negative (KCl, NaCl) anisotropies of the second-order elastic moduli has been considered. The dependences of the relaxation rates on the direction of the wave vector of phonons in scattering processes with the participation of three quasi-transverse phonons (the TTT relaxation mechanisms) are analyzed within the anisotropic continuum model. It is shown that the TTT relaxation mechanisms in crystals are associated with their cubic anisotropy, which is responsible for the interaction between noncollinear phonons. The dominant contribution to the phonon relaxation comes from large-angle scattering. For crystals with significant anisotropy of the elastic energy (Ge, Si, KCl, NaCl), the total contribution of the TTT relaxation mechanisms to the total relaxation rate exceeds the contribution of the Landau-Rumer mechanism either by several factors or by one to two orders of magnitude depending on the direction. The dominant role of the TTT relaxation mechanisms as compared to the Landau-Rumer mechanism is governed, to a considerable extent, by the second-order elastic moduli. The total relaxation rates of slow quasi-transverse phonons are determined. It is demonstrated that, when the anharmonic processes of scattering play the dominant role, the inclusion of one of the relaxation mechanisms (the Landau-Rumer mechanism or the mechanisms of relaxation of the slow quasi-transverse mode by two slow or two fast modes) is insufficient for describing the anisotropy of the total relaxation rates in cubic crystals.     

Numerical simulation of laminar co-flow methane–oxygen diffusion flames: effect of chemical kinetic mechanisms

Laminar co-flow methane–oxygen flames issuing into the unconfined atmosphere have been studied. A numerical model, which employs different chemical kinetics sub-models, including a skeletal mechanism with 43 reaction steps and 18 species and four global reaction mechanisms (two 2-steps and two 4-steps mechanisms), and an optically thin radiation sub-model, has been employed in the simulations. Numerical model has been validated against the experimental results available in literature. The numerical predictions from the global kinetic mechanisms have been compared with the 43-steps mechanism predictions. At all oxygen flow rates, the predictions of the distributions of temperature, mass fractions of CH₄, O₂ and CO₂ by the 2-steps mechanisms are closer to 43-steps mechanism. The overall distribution of H₂O predicted by 2-steps mechanisms is close to that of 43-steps except for the maximum value. Especially at higher oxygen flow rates, the modified 2-steps mechanism predicts these quantities much closer to those predicted by the 43-steps mechanism. Further, the 2-steps mechanisms predict location of the reaction zone accurately. However, they can just give an idea of overall CO distribution in terms of the axial and radial locations within which CO will almost be consumed, but not its maximum value in the domain. The 4-steps mechanisms predict the trend of variation of these quantities quite reasonably. However, they under-predict the location of the reaction zone. At higher oxygen flow rates, the predictions by 4-steps mechanisms becomes better, especially in the prediction of maximum CO and H₂O. Over all, the modified 2-steps mechanism can be recommended for reasonable and economical predictions of oxy-rich methane flames.     

Current–time characteristic of the transient regime of electrohydrodynamic flow formation

The paper presents the results of computer simulation of the current–time characteristic (CTC) of a cell with low-conducting liquid. The basis of simulation is the complete set of electrohydrodynamic (EHD) equations. The injection and dissociation mechanisms of charge formation as well as field-enhanced dissociation are considered. The simulation is carried out in the needle–plane electrode system. The relation between sections of CTC and stages of EHD flow formation is revealed. The shape of CTC is shown to be dependent on mechanisms of charge formation, the ratio of the initial and steady-state ion concentrations and the mechanisms of charge transport.     

Carrier-envelope offset dynamics of mode-locked lasers

We investigate coupling mechanisms between the amplitude and the carrier-envelope offset phase in mode-locked lasers. We find that nonlinear beam steering in combination with the intracavity prism compressor is the predominant mechanism that causes amplitude-to-phase conversion in our laser. A second mechanism, induced by self-steepening, is also identified. These mechanisms are important for stabilizing the carrier-envelope offset phase and also explain the extremely low pulse-to-pulse energy fluctuations observed in some lasers with carrier-envelope lock. The coupling mechanisms described have important implications for applications of few-cycle optical pulses.     

Computer simulation of fast recoil ejection from single crystal surfaces

The computer simulation program MARLOWE is used to analyze the most probable surface recoil processes leading to ejection of atoms from (001) gold surfaces subsequent to the irradiation with 20 keV argon atoms. The occurence of two-and threedimensional mechanisms resulting from high-energy recoils involving one and two atomic layers is discussed for atoms ejected in a direction parallel to the plane of incidence. Generally, a close correspondence is found between the mechanisms involved and the features in the energy distributions. The occurence of direct and deflected recoils is confirmed, as well as mechanisms involving the generation of displacements in the two first atomic layers. The dependence of these mechanisms on the conditions of incidence and the ejection direction is investigated. It is suggested that three-dimensional effects, although dominating, mainly contribute to the background in the energy distributions. The intensities in the features in the energy distributions were found to be strongly influenced by shadowing; affecting both the one-and two-layer processes. The influence of thermal vibrations, surface defects and impurities is briefly examined.     

Microstructural evolution of nickel under high-pressure torsion

The evolution peculiarities of grain and defect structures in nickel under high-pressure torsion were studied by transmission electron microscopy and X-ray diffraction analysis. Lattice reorientation mechanisms characteristic of different stages of plastic deformation were disclosed. The conditions and features of cooperative realization of various structure formation mechanisms under severe deformation were discussed.