1,721,017 research outputs found
The appropriate context for the analysis of the explosive mode in reactive systems
No full textThe explosive mode that arises in the dynamics of the chemical kinetics term attracts particular attention because it is a mode that tends to drive a reacting system away from equilibrium. This is a slow mode that coexists with (1) a number of fast dissipative modes that tend to drive rapidly the system to equilibrium; and (2) a number of other slow dissipative modes, which compete with the explosive mode for the control of the system evolution after the fast modes become exhausted. Here, the proper context is established for the analysis of the explosive mode by accounting for the influence of all other modes present, both exhausted and active. It is shown that the analysis of an explosive mode is meaningful only when the fast dissipative modes become exhausted and this mode is among the slow modes that drive the system. The conclusions are highlighted through the analysis of a simple model
Explicit time integration of the stiff chemical Langevin equations using computational singular perturbation
No full textA stable explicit time-scale splitting algorithm for stiff chemical Langevin equations (CLEs) is developed, based on the concept of computational singular perturbation. The drift term of the CLE is projected onto basis vectors that span the fast and slow subdomains. The corresponding fast modes exhaust quickly, in the mean sense, and the system state then evolves, with a mean drift controlled by slow modes, on a random manifold. The drift-driven time evolution of the state due to fast exhausted modes is modeled algebraically as an exponential decay process, while that due to slow drift modes and diffusional processes is integrated explicitly. This allows time integration step sizes much larger than those required by typical explicit numerical methods for stiff stochastic differential equations. The algorithm is motivated and discussed, and extensive numerical experiments are conducted to illustrate its accuracy and stability with a number of model systems
Classification of ignition regimes in HCCI combustion using computational singular perturbation
No full textThe Computational Singular Perturbation (CSP) technique is applied as an automated diagnostic tool to classify various ignition regimes encountered in auto-ignition processes in HCCI combustion. Various model problems representing HCCI combustion are simulated using high-fidelity computation with detailed chemistry for hydrogen-air system. The simulation data are then analyzed by CSP. In spatially homogeneous systems ignition, the occurrence of two branches of positive eigenvalues characterizes chain-branching and thermal ignition. Their merging point serves as a good indicator of the completion of the explosive stage of ignition. In spatially non homogeneous systems, this merging point can also be used to differentiate front propagation from homogeneously igniting kernels. Furthermore, to classify the front propagation as deflagration or spontaneous ignition, first the reaction zone is identified as the locus of minimum number of fast exhausted time scales (based on user-specified error thresholds). Next, the relative importance of transport and chemistry is determined in the region ahead of the reaction zone. Importance index I (Quantitative) and characteristic Damköhler number (Qualitative) are employed as criteria to discriminate the spontaneous ignition front from the deflagration front. These diagnostic tools applied to 1D laminar and 2D turbulent ignition problems allow automated detection of different ignition regimes at different times and location during the ignition events. The implication of the results in the context of modeling auto-ignition of nearly homogeneous turbulent mixtures is discussed
G-scheme-based simplification and analysis methodology for hydrocarbon ignition
No full textFor the simulation of reactive flows, the use of a simplified model is a common simplification to reduce the computational cost, although the reduced model may introduce significant errors in the simulation. Using the G-Scheme, the local structure of the local tangent space is characterized through the subspaces associated with the slow, active, and fast reactive scales. This specific feature can be of great significance in the analysis of the dynamics with the aim of achieving a low-dimensional description and allowing a time-scale-aware sensitivity analysis of the problem. Such analysis can be exploited to simplify/reduce/understand the reaction dynamics of interest. We have developed specific procedures to generate simplified mechanisms with an a priori known error for chemical kinetics processes, to analyze them in order to understand the role of the most important reactions, and to identify the most important reactions paths of the processes. The procedure is based on a G-Scheme Participation Index that makes use of a G-Scheme generated database. The effectiveness of the procedures to produce simplified mechanisms is demonstrated by applying them to the auto-ignition problems for homogeneous hydrogen/air and hydrocarbon/air mixtures
Detailed numerical simulation of a n-heptane edge flame
No full textResults of a detailed numerical simulation of an n-heptane/air edge flame are presented. The equations of the low-Mach number approximation are solved in a two-dimensional domain using detailed models for species transport and chemical reactions. The reaction mechanism involves 560 species and 2536 reversible reactions. We consider an edge flame that is established in a mixing layer for a uniform velocity field. The mixing layer spans the equivalence ratios between pure air and 3.5. The detailed model enables us to analyze the chemical structure of the edge flame. We identify major species profiles, identify reactions causing the heat release, and discuss the main fuel-consumption pathways. This analysis is performed for several regions in the edge flame to discuss the different processes at work in the premixed branches and the trailing diffusion flame. Finally, we analyze the accuracy of two skeletal mechanisms which were previously developed from homogeneous ignition calculations and show that a significant reduction in size of the mechanism can be achieved without a significant decrease in accuracy
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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