1,720,969 research outputs found
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
Dynamic Mode Decomposition: A Tool to Extract Structures Hidden in Massive Datasets
No full textDynamic Mode Decomposition (DMD) is able to decompose flow field data into coherent modes and determine their oscillatory frequencies and growth/decay rates, allowing for the investigation of unsteady and dynamic phenomena unlike conventional statistical analyses. The decomposition can be applied for the analysis of data having a broad range of temporal and spatial scales since it identifies structures that characterize the physical phenomena independently from their energy content. In this work, a DMD algorithm specifically created for the analysis of massive databases is used to analyze three-dimensional Direct Numerical Simulation of spatially evolving turbulent planar premixed hydrogen/air jet flames at varying Karlovitz number. The focus of this investigation is the identification of the most important modes and frequencies for the physical phenomena, specifically heat release and turbulence, governing the flow field evolution.</p
Physics-based reduced-order modeling of flash-boiling sprays in the context of internal combustion engines
No full textFlash-boiling injection is one of the most effective ways to accomplish improved atomization compared to the high-pressure injection strategy. The tiny droplets formed via flash-boiling lead to fast fuel-air mixing and can subsequently improve combustion performance in engines. Most of the previous studies related to the topic focused on modeling flash-boiling sprays using three-dimensional (3D) computational fluid dynamics (CFD) techniques such as direct numerical simulations (DNS), large-eddy simulations (LES), and Reynolds-averaged Navier-Stokes (RANS) simulations. However, reduced order models can have significant advantages for applications such as the design of experiments, screening novel fuel candidates, and creating digital twins, for instance, because of the lower computational cost. In this study, the previously developed cross-sectionally averaged spray (CAS) model is thus extended for use in simulations of flash-boiling sprays. The present CAS model incorporates several physical submodels in flash-boiling sprays such as those for air entrainment, drag, superheated droplet evaporation, flash-boiling induced breakup, and aerodynamic breakup models. The CAS model is then applied to different fuels to investigate macroscopic spray characteristics such as liquid and vapor penetration lengths under flash-boiling conditions. It is found that the newly developed CAS model captures the trends in global flash-boiling spray characteristics reasonably well for different operating conditions and fuels. Moreover, the CAS model is shown to be faster by up to four orders of magnitude compared with simulations of 3D flash-boiling sprays. The model can be useful for many practical applications as a reduced-order flash-boiling model to perform low-cost computational representations of higher-order complex phenomena
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
The importance of Soret effect, preferential diffusion, and conjugate heat transfer for flashback limits of hydrogen-fueled perforated burners
Full text linkAvoiding flashback is a primary challenge in the development of modern burners, which should be capable of substituting natural gas with hydrogen in domestic end-user devices. The heat exchange between the burner plate and the burned and unburned gases, as well as the effects of preferential diffusion, significantly impact the flashback of such burners fueled with hydrogen. For these effects, the design of the burner plate plays a pivotal role. In this study, three-dimensional simulations with detailed chemistry have been performed to investigate the effect of three competing physical mechanisms, namely, preheating of fresh gases, preferential diffusion, and Soret effect, which drive the flame flashback dependence on the holes/slits size. Two different geometries are considered: circular holes with varying diameters and slits with fixed lengths but different widths. Steady-state simulations with decreasing inlet velocities are employed to estimate the critical inlet velocity for flashback. Conjugate heat transfer (CHT) is considered for the heat exchange between the burner plate and the gases. For circular holes, the enclosed geometry promotes more effective heat transfer, leading to a higher influence of preheating effects for small diameters. This results in a non-monotonic dependence on hole size, with a non-trivial optimum diameter to avoid flashback. This behavior is specific to circular holes and differs from that observed in previously studied infinitely long slits, where a linear dependence on the slit width was found. Additionally, the individual influence of non-unity Lewis numbers and Soret diffusion is analyzed. Notably, the Soret effect, in combination with CHT, is found to instaurate a strong, non-linear, self-accelerating mechanism that has a leading-order effect on the flashback propensity of larger holes. This finding underscores the necessity of including both effects in numerical simulations for accurate estimations of the flashback limits in domestic burners
Tabulation-based sample-partitioning adaptive reduced chemistry and cell agglomeration
Full text linkIn this study, we combine the SPARC (Sample-Partitioning Adaptive Reduced Chemistry) and the Cell Agglomeration (CA) techniques, to accelerate the simulation of laminar and turbulent reactive flows with detailed kinetics. The reduced mechanisms adopted by SPARC are generated on the basis of representative thermo-chemical states corresponding to laminar, steady-state flamelets parameterized by the mixture fraction and a progress variable, in line with the TRAC (Tabulated Reactions for Adaptive Chemistry) method, recently proposed by Surapaneni and Mira (Comb and Flame, 2023). To further speed-up the calculation, CA (consisting in grouping the cells having similar thermo-chemical states) is carried out before identifying the local reduced mechanism by means of SPARC. To demonstrate the effectiveness of the approach, we considered two benchmark cases: (i) a laminar, pulsating laminar coflow diffusion flame fueled by a mixture of C
2H
4 and N
2 burning in air; (ii) a 2D, turbulent, non-premixed flame burning n-C
7H
16 in air subject to decaying isotropic turbulence. In both cases, a detailed kinetic mechanism accounting for the formation of PAHs and soot particles and aggregates was considered. The results are promising, showing both accuracy and computational efficiency. While this study uses non-premixed flamelets with mixture fraction and progress variable as an illustrative example, the proposed methodology has the potential to be applied to various combustion modes, including premixed and partially premixed scenarios.</p
Influence of adversarial training on super-resolution turbulence models
No full textSupervised super-resolution deep convolutional neural networks (CNNs) have gained significant attention for large eddy simulation subfilter-scale (SFS) modeling due to their ability to reconstruct statistically meaningful flow fields on fine meshes. Despite their popularity, CNNs lack the ability to accurately reconstruct high-frequency features and generalization performance on out-of-sample flows. Generative adversarial networks (GANs) are a potential alternative, allowing for both semi-supervised and fully unsupervised training, though they have not been thoroughly investigated as turbulence closures, and a comprehensive understanding of the discriminator's role has not been developed. This study assesses the effectiveness of GANs for a priori SFS stress modeling in forced homogeneous isotropic turbulence. It is found that GAN-based architectures outperform supervised CNN models for SFS reconstruction for in-sample cases. The reconstruction accuracy of both models decreases for out-of-sample data, though the GAN discriminator applied as a "feature extractor" narrows the model's solution space and enhances the generator's out-of-sample robustness. The extrapolation ability of the GAN-based model for higher-Reynolds-number flows is also demonstrated. This highlights the effectiveness of the GAN discriminator in optimizing robust and accurate SFS models for out-of-sample flows. Based on these findings, training with a discriminator is recommended before integrating super-resolution CNN closures into numerical solvers
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
- …
