1,720,959 research outputs found
Time dependent fluid dynamics and Lagrangian tracking to model mixing in Rushton-impeller baffled-tank reactor
No full textThe object of this work is to investigate the role of large-scale convective structures in promoting mixing in a stirred tank. We focus on a standard geometry (uid particles. The three-dimensional, time-dependent, fully developed flow field is calculated with a computationally effcient procedure using a RANS solver with k–epsilon turbulence modeling and the $ow 8eld is assessed precisely against experimental data. Then, fluid parcels are tracked in the calculated flow field. Analyzing the trajectory of fluid parcels, the segregated regions within the flow are identified and mixing indicators are calculated (mixing time, circulation length and sojour
time distribution). A physical explanation is thus proposed to establish a link between large-scale mixing and complex fluid dynamics generated by the interactions of radial-discharge jet, ring vortices, and upper counter rotating vortex
Appraisal of 3D Numerical Simulation for sub-micron particle behavior in a microporous ceramic filter
No full textA computational, three-dimensional approach to investigate the behavior of diesel soot particles in the micro-channels of a wall-flow, porous-ceramic particulate filter is presented. Particle size examined is in the PM2.5 range. The flow field is simulated with a finite-volume Navier-Stokes solver and the Ergun equation is used to model the porous material. The permeability coefficients were obtained by fitting experimental data. Particle flow, dispersion, deposition and wall-particle interactions are investigated tracking large swarms of 2 and 0.2 mu m diameter particles in a Lagrangian frame of reference. Particle dynamics included rarefied gas hypotheses (the Knudsen number being larger than unity) and bounce/capture models based on impact kinetic energy loss. The influence of gas molecules-particle interaction on overall particle behavior is also examined by including Brownian motion and partial slip in particle equation of motion. Simulations help to highlight three-dimensional non-uniform particle deposition, mainly due to flow distribution in the micro-channel. All particles deposit onto the porous filter wall following the distribution of the through-wall velocity. The larger, 2 mu m, particles show a larger tendency to deposit at the end of the filter. Due to the flow contraction at the inlet, virtually no particle deposit in the inlet section of the filter. Reasons for the scarce influence on particle deposition due to particle-flow slip and Brownian motion are given
Turbulent Flow and Dispersion of Inertial Particles in a Confined Jet Issued by a Long Cylindrical Pipe
No full textIn this work we examine first the flow field of a confined jet produced by a turbulent flow in a long cylindrical pipe issuing in an abrupt angle diffuser. Second, we examine the dispersion of inertial micro-particles entrained by the turbulent flow. Specifically, we examine how the particle dispersion field evolves in the multiscale flow generated by the interactions between the large-scale structures, which are geometry dependent, with the smaller turbulent scales issued by the pipe which are advected downstream. We use Large-Eddy-Simulation (LES) for the flow field and Lagrangian tracking for particle dispersion. The complex shape of the domain is modelled using the immersed-boundaries method. Fully developed turbulence inlet conditions are derived from an independent LES of a spatially periodic cylindrical pipe flow. The flow field is analyzed in terms of local velocity signals to determine spatial coherence and decay rate of the coherent KH vortices and to make quantitative comparisons with experimental data on free jets. Particle dispersion is analyzed in terms of statistical quantities and also with reference to the dynamics of the coherent structures. Results show that the particle dynamics is initially dominated by the KelvinHelmholtz (KH) rolls which form at the expansion and only eventually by the advected smaller turbulence scales
Mechanisms for Selective Radial Dispersion of Microparticles in the Transitional Region of a Confined Turbulent Round Jet
No full textThe dispersion of particles of different diameters in a confined turbulent round jet is dominated by the local interactions between dispersed phase and large-scale, time dependent flow structures which populate the near-field of the jet. In this work, we address first the problem of identifying the flow structures which form in a three-dimensional, turbulent confined round jet, considering also the influence of the solid walls. Second, we examine particle dispersion specifically focusing on their preferential distribution. The three-dimensional, time dependent flow field is calculated using a finite-difference LES solver of the Navier-Stokes equations in a cylindrical reference frame, whereas the dispersion of particles is computed using a one-way coupling Lagrangian approach. The flow field is characterized by mutually interacting, transitional structures of different temporal and spatial scales. We investigate the behavior of particles, in turn characterized by different size and time-scales, with the object of examining their selective response to the various scales of the flow. Our analyses confirm that multiple organized structures grow up following different types of instability, and successively interact, creating the conditions for the developing of a three-dimensional vorticity field downstream the near-field of the jet. We show that the vortical structures interact selectively with the different size particles, producing different distribution patterns and dispersion rates qualitatively depending on the particle-to-fluid Stokes number. Following the model proposed previously, we complete the description of the particle dispersion mechanism based on the action of the large spanwise vortices, evidencing the effect of the secondary structures (ribs) on the smaller particles. We characterize the different distribution pattern of particle swarms and we analyze from a qualitative viewpoint particle dispersion behavior with the dynamics of the different transitional structures providing guidelines for dispersion control
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
Direct Numerical Simulation of turbulent particle dispersion in an unbaffled Stirred-Tank Reactor,
No full textTurbulent dispersion of inertial particles in a flat-bottom stirred-tank reactor equipped with an eight-blade Rushton impeller is investigated using accurate numerical techniques (Verzicco et al., 2004, Flow in an impeller-stirred tank using an immersed-boundary method. A.I.Ch.E. Journal, 50(6), 1109–1118.). Direct Numerical Simulation of the turbulent flow field in the vessel is obtained using a second-order finite-difference scheme coded in a cylindrical reference frame, and an immersed-boundary approach is used to simulate the motion of the impeller. The flow scales are resolved explicitly down to the Kolmogorov scale. To give a comprehensive picture of the turbulence structure in the vessel, angle-resolved averages of turbulent kinetic energy, turbulent energy dissipation rate and Kolmogorov time-scales are evaluated in vertical planes aligned with the blade and mid-way between two blades. The dispersion of heavy particles of different diameter is then investigated by Lagrangian tracking. The particle-to-fluid mass loading ratio is low enough to assume one-way coupling momentum transfer between continuous and dispersed phase. Three sets of particles, characterized by different response time, are investigated and, for each set, two equal, randomly distributed swarms are initially released above and below the impeller, which is placed mid-way between top and bottom of the tank. Statistics calculated after 3 impeller revolutions are used to evaluate the evolution of particle dispersion in the flow and to quantify their preferential accumulation into specific regions of the tank
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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