1,720,971 research outputs found
Large-eddy simulation of turbulent flow over a rough surface
Full text linkA family of wall models is proposed that exhibits more satisfactory performance than previous models for the large-eddy simulation (LES) of the turbulent boundary layer over a rough surface.The time and horizontally averaged statistics such as mean vertical profiles of wind velocity, Reynolds stress, turbulent intensities, turbulent kinetic energy and also spectra are compared with wind-tunnel experimental data. The purpose of the present study is to obtain simulated turbulent flows that are comparable with wind-tunnel measurements for use as the wind environment for the numerical prediction by LES of source dispersion in theneutral atmospheric boundary layer
Large-eddy simulation of dispersion: comparison between elevated source and ground level source
No full textLarge-eddy simulation (LES) is used to calculate the concentration fluctuations of passive plumes from an elevated source (ES) and a ground-level source (GLS) in a turbulent boundary layer over a rough wall. The mean concentration, relative fluctuations and spectra are found to be in good agreement with the wind-tunnel measurements for both ES and GLS. In particular, the calculated relative fluctuation level for GLS is quite satisfactory, suggesting that the LES is reliable and the calculated instantaneous data can be used for further post-processing. Animations are shown of the meandering of the plumes, which is one of the main features to the numerical simulations. Extreme value theory (EVT), in the form of the generalized Pareto distribution (GPD), is applied to model the upper tail of the probability density function of the concentration time series collected at many typical locations for GLS and ES from both LES and experiments. The relative maxima (defined as maximum concentration normalized by the local mean concentration) and return levels estimated from the numerical data are in good agreement with those from the experimental data. The relative maxima can be larger than 50. The success of the comparisons suggests that we can achieve significant insight into the physics of dispersion in turbulent flows by combining LES and EVT
Modelling extreme concentration from a source in a turbulent flow over rough wall
Full text linkThe concentration fluctuations in passive plumes from an elevated and a groundlevel
source in a turbulent boundary layer over a rough wall were studied using
large eddy simulation and wind tunnel experiment. The predictions of statistics
up to second order moments were thereby validated. In addition, the trend of relative
fluctuations far downstream for a ground level source was estimated using
dimensional analysis. The techniques of extreme value theory were then applied
to predict extreme concentrations by modelling the upper tail of the probability
density function of the concentration time series by the Generalised Pareto Distribution.
Data obtained from both the simulations and experiments were analysed in
this manner. The predicted maximum concentration (?0) normalized by the local
mean concentration (Cm) or by the local r.m.s of concentration fluctuation (crms),
was extensively investigated. Values for ?0/Cm and ?0/crms as large as 50 and 20
respectively were found for the elevated source and 10 and 15 respectively for the
ground-level source
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
Balancing Errors in LES
No full textErrors in LES are easily made and hard to control. We identify the main local sources of error involved in LES. The non-commutation terms arising in complex flows when using nonuniform filter-widths are analysed in some detail. The magnitudes of discretisation and modelling errors arising in a turbulent mixing layer are determined and their interaction is shown to lead to partial cancellation of errors for various spatial discretisation methods
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