1,721,079 research outputs found
COMPARISON BETWEEN DIFFERENT METHODS FOR TURBULENT GAS-LIQUID SYSTEMS BY USING MULTIVARIATE POPULATION BALANCES
No full textEquilibrium-Eulerian LES Model for Poly-disperse Particle-laden Channel Flow
No full textAn efficient Eulerian method for poly-disperse particles in turbulent flows is implemented, verified and validated for a channel flow. The approach couples a mixture model with a quadrature-based moment method for the particle size distribution in a LES framework, augmented by an approximate deconvolution method to reconstructs the unfiltered velocity. The particle velocity conditioned on particle size is calculated with an equilibrium model, valid for low Stokes numbers. A population balance equation is solved with the direct quadrature method of moments, that efficiently represents the continuous particle size distribution. In this first study particulate processes are not considered and the capability of the model to properly describe particle transport is investigated for a turbulent channel flow. First, singlephase LES are validated through comparison with DNS. Then predictions for the two-phase system, with particles characterised by Stokes numbers ranging from 0.2 to 5, are compared with Lagrangian DNS in terms of particle velocity and accumulation at the walls. Since this phenomenon (turbophoresis) is driven by turbulent fluctuations and depends strongly on the particle Stokes number, the approximation of the particle size distribution, the choice of the sub-grid scale model and the use of an approximate deconvolution method are important to obtain good results. Our method can be considered as a fast and efficient alternative to classical Lagrangian methods or Eulerian multi-fluid models in which poly-dispersity is usually neglecte
QBMMlib: A library of quadrature-based moment methods
Full text linkQBMMlib is an open source package of quadrature-based moment methods and their algorithms. Such methods are commonly used to solve fully-coupled disperse flow and combustion problems, though formulating and closing the corresponding governing equations can be complex. QBMMlib aims to make analyzing these techniques simple and more accessible. Its routines use symbolic manipulation to formulate the moment transport equations for a population balance equation and a prescribed dynamical system. The resulting moment transport equations are closed by first trading the moments for a set of quadrature points and weights via an inversion algorithm, of which several are available. Quadratures then compute the moments required for closure. Embedded code snippets show how to use QBMMlib, with the algorithm initialization and solution spanning just 13 total lines of code. Examples are shown and analyzed for a linear harmonic oscillator and a bubble dynamics problem.This article is published as Bryngelson, Spencer H., Tim Colonius, and Rodney O. Fox. "QBMMlib: A library of quadrature-based moment methods." SoftwareX 12 (2020): 100615. DOI: 10.1016/j.softx.2020.100615. Posted with permission.</p
Computational fluid dynamics for design and optimization of tubular low-density polyethylene reactors
No full textPolymer reactor models often assume that the reactants are mixed rapidly and thus the concentrations can be considered to be uniform at small scales. However, for fast reactions or for viscous systems, poor mixing of chemical species significantly affects the reactor performance while adversely affecting product quality. The purpose of this research is to formulate a generalized algorithm based on state-of-the-art computational fluid dynamics (CFD) techniques such as full or presumed probability density function (PDF) methods to better understand the role of micromixing in mixing-sensitive chemical processes. The impressive capabilities of the algorithm are investigated using an industrial test-case of tubular low-density polyethylene (LDPE) reactors. The precise control and optimization of these reactors are of primary industrial concern due to tight coupling between fluid dynamics and complex LDPE chemistry under extreme operating conditions. CFD simulations are carried out by combining the CFD model and detailed LDPE chemistry into a single FORTRAN code as well as into a commercial CFD software--Fluent(c). The technique of in situ adaptive tabulation enables the computationally efficient inclusion of the stiff and non-linear LDPE chemistry. Results include temperature profiles, various species profiles and prediction of polymer quality with and without chain transfer mechanisms under various inlet and operating conditions, along with comparisons against pilot-plant scale data and/or comparison of various CFD techniques for accurate and efficient predictions of micromixing effects. Interesting features such as a bimodal temperature distribution and local hot-spots as well as global decomposition after an induction time or due to pulsating initiator feed are also observed under certain conditions using the full PDF simulations near critical points where instabilities occur. Considering the advantages of the two CFD methods, efforts are also directed towards efficient combination of the two techniques in order to obtain reactor stability plots and catalyst efficiency profiles, which are extremely helpful in operational decisions as well as design of control strategies. Thus the study not only illustrates the importance of mixing effects on LDPE polymerization in tubular reactors, but also yields insight into choosing appropriate operating conditions for maximizing catalyst efficiency while controlling reactor dynamics and product quality in plant-scale tubular LDPE reactors.</p
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
Simulation methods for polydisperse, multiphase flows using moment methods
Full text linkWhile the ability to solve for multiphase flows that contain of a distribution of properties is crucial to the accurate prediction of physical system, there is currently a lack of numerical solution methods to solve for these types of flows. In this work, three new numerical procedures are developed in order to accurately solve for systems containing polydisperse multiphase flows, and flows with velocity distributions. The ability to correctly solve these flows allows for the local segregation of size that is generally not possibly due to the limitations of the standard solution techniques. First, a numerical algorithm is presented to solve bubbly flows using the standard two fluid model coupled to the moment transport equations of a monokinetic number density function (NDF). This provides the stability of a two-fluid solver, while adding additional accuracy that comes from the inclusions of a range of sizes, and corresponding velocities. The algorithm is first tested to ensure numerical stability, and then validated against against experimental data, as well as the two-fluid and multi-fluid models. Next, a semi-implicit solution method for the handling of the particle pressure flux for polydisperse granular systems is presented in the multifluid framework, and is based on the work of Syamlal et al. (1993). The method is first verified by examining the segregation of sizes in a settling bed, then is validated against existing implementations of polydisperse kinetic theory, as well as experimental results in a bidisperse fluidised bed and a cyclic vertical riser. Finally, a solution method to the transport of the joint size-velocity NDF is presented using QBMM. The presented method makes no assumptions on the size or velocity distribution. Additionally, the relevant source terms to describe change in size are presented using a volume fraction formulation which is important for numerical stability when small particles are under consideration. The solution procedure is first validated using simple 0-D cases for both population balance equations and collision models, then using an axisymmetric 1-D spray cases in which both the size and velocity evolution are important, and finally using 2-D crossing jet cases. All work has been implemented in the open-source framework OpenFOAM.</p
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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