1,721,370 research outputs found
Smoke plume from fire Lagrangian simulation: dependence on drag coefficient and resolution
No full textPlume rising from wildfire due to the buoyancy generated by the heat released from the fire is a crucial phenomenon to model for a correct description of smoke dispersion within the atmosphere. During the rise, the plume experiences the drag of external air which limits the rising itself. In this work, we investigate the dependence of the hybrid Eulerian-Lagrangian plume rise scheme embedded in the Lagrangian stochastic particle model SPRAY-WEB on the drag coefficient and on the horizontal resolution of the plume rise grid. We test four different drag coefficient models depending on the Reynolds number of the cells as well as a constant drag coefficient. As for the horizontal resolution, we use three different horizontal cell sizes: 200 m, 400 m, and 600 m, namely roughly a quarter, a half, and three-quarters of the source size. We compare the simulation results with the observations taken during a field experiment performed in Idaho organized by the US Environmental Protection Agency, where they collected lidar data and aircraft CO concentration measurements. We found that the drag coefficient influences mainly the plume near the source, where the drag role is more important due to the higher vertical velocities. It has also turned out that the best cell-to-source size ratio for our purpose is one to two
Evaluation of Turbulence Depending Drag Coefficient in Plume Rise Model for Fire Smoke Dispersion
No full textIt is well-known that the correct modeling of the plume rise is fundamental for a proper description of pollutants dispersion, especially for highly buoyant plumes like those emitted by wildfires. In this work, we want to investigate the role of the drag coefficient (CD) in our plume rise scheme. We evaluate different formulations taken from the literature and suggest the best option for the plume rise scheme embedded in the Lagrangian Stochastic model SPRAY-WEB. As a matter of fact, previous works on plume rise models are based on the drag coefficient which is a quantity that should be determined. There is not a general consensus on the best values for CD. Some values suggested in the literature are simply constants that do not directly depend on the meteorological and turbulent variables. In order to evaluate the model performances obtained using different CD formulations we simulate a field experiment, carried out in August 2013 in Idaho (USA), in which prescribed fires were observed and the physical and chemical parameters were measured. As a matter of fact, plumes emitted from fires are affected by strong buoyancy and very low or even negligible initial vertical momentum. These conditions are very different from traditional plumes from stacks for which the classical plume rise models are built. For this reason, the case studies chosen for the tests are very challenging and they allow us to assess the scheme here proposed in the best way. Comparison among the results obtained with different CD parameterizations are presented and discussed. Three of the four CD models tested derive from an extension of the Stokes’ law; the fourth one is a more refined model derived from the Shanks transformation of the Goldstein series. This last model seems to give better results as regards the maximum height of the plume, but is the one that underestimates most CO concentrations
Analysis of the Influence of the Length Scales in a Boundary-Layer Model
No full textWe consider the Janjic (NCEP Office Note 437:61, 2001) boundary-layer model, which is one of the most widely used in numerical weather prediction models. This boundary-layer model is based on a number of length scales that are, in turn, obtained from a master length multiplied by constants. We analyze the simulation results obtained using different sets of constants with respect to measurements using sonic anemometers, and interpret these results in terms of the turbulence processes in the atmosphere and of the role played by the different length scales. The simulations are run on a virtual machine on the Chameleon cloud for low-wind-speed, unstable, and stable conditions
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
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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