1,720,994 research outputs found
A simple and fast model to compute concentration moments in a convective boundary layer.
No full textRecently, a modified meandering plume model for concentration fluctuations in a convective boundary layer has been formulated (Atmos. Environ. 34 (2000) 3599). This model is based on a hybrid Eulerian-Lagrangian approach and it accounts for the skewed and inhomogeneous turbulence characteristics of the convective flow. Using the same hypotheses, but eliminating the need for Lagrangian particle model, we propose a generalised approach, that only requires the knowledge of mean concentration field. The proposed model is independent from the method used to obtain the mean concentration field. The evaluation of the concentration fluctuation field needs a computational time of only few seconds on a standard PC. Therefore, the model is suitable for practical applications
A semi-analytical model for mean concentration in a convective boundary layer.
No full textA model to predict the mean concentration field in convective conditions is proposed. This model is inspired by the probability density function (pdf) models, retaining their assumption of splitting a plume into updraft and a downdraft components. However, the proposed model is more flexible than the pdf models since it includes the effects of turbulence inhomogeneity and it can be adapted to non-stationary conditions. The formulation of the model ensures that in the limit of zero skewness a traditional Gaussian model is obtained. Thus, the model is applicable to conditions ranging from neutral to unstable. The model is solved numerically, but time consumption is negligible. Despite its simplicity, the model shows good performance compared to classical experiments and to more complex and physically consistent numerical model
Gli hedge fund: caratteristiche, impatto sui mercati e ruolo nelle crisi finanziarie
No full textGli hedge fund (HF) sono sempre pi`u diffusamente considerati responsabili
delle crisi finanziarie che si sono susseguite nel corso degli ultimi decenni. La loro natura
di segretezza e la fama di particolare propensione al rischio, unite ad un ampio ricorso alla leva
finanziaria, fanno si che questi operatori siano spesso indicati come principale causa dellaumento
della volatilit`a ed instabilit`a dei mercati o quantomeno in grado di accentuarne fortemente
le turbolenze. Per comprenderne correttamente il ruolo nelle situazioni di stress dei mercati, il
lavoro ne analizza dapprima la natura e le caratteristiche distintive: principali strategie di investimento
adottate, modalit`a di misurazione delle performance, struttura organizzativa tipica. Si
esaminano poi i vantaggi che tipicamente la dottrina e la prassi riconoscono alla loro operativit`
a: efficienza dei prezzi, liquidit`a dei mercati, innovazione innanzitutto. Si analizzano inoltre
gli aspetti critici connessi allattivit degli HF, soprattutto in relazione alle situazioni di turbolenza
dei mercati finanziari. Questa analisi `e essenziale per comprenderne il ruolo nellattuale crisi
globale. Lo studio si conclude con alcune riflessioni sul dibattito che divide la comunit finanziaria
circa lopportunit`a di introdurre misure specifiche di regolamentazione anche per lindustria
degli HF
On the anomalous behaviour of the Lagrangian structure function similarity constant inside dense canopies
No full textAN ADVANCED PUFF MODEL BASED ON A MIXED EULERIAN/LAGRANGIAN APPROACH FOR TURBULENT DISPERSION IN THE CONVECTIVE BOUNDARY LAYER
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Concentration Fluctuations from Localized Atmospheric Releases
Full text linkWe review the efforts made by the scientific community in more than seventy years to elucidate the behaviour of concentration fluctuations arising from localized atmospheric releases of dynamically passive and non-reactive scalars. Concentration fluctuations are relevant in many fields including the evaluation of toxicity, flammability, and odour nuisance. Characterizing concentration fluctuations requires not just the mean concentration but also at least the variance of the concentration in the location of interest. However, for most purposes the characterization of the concentration fluctuations requires knowledge of the concentration probability density function (PDF) in the point of interest and even the time evolution of the concentration. We firstly review the experimental works made both in the field and in the laboratory, and cover both point sources and line sources. Regarding modelling approaches, we cover analytical, semi-analytical, and numerical methods. For clarity of presentation we subdivide the models in two groups, models linked to a transport equation, which usually require a numerical resolution, and models mainly based on phenomenological aspects of dispersion, often providing analytical or semi-analytical relations. The former group includes: large-eddy simulations, Reynolds-averaged Navier–Stokes methods, two-particle Lagrangian stochastic models, PDF transport equation methods, and heuristic Lagrangian single-particle methods. The latter group includes: fluctuating plume models, semi-empirical models for the concentration moments, analytical models for the concentration PDF, and concentration time-series models. We close the review with a brief discussion highlighting possible useful additions to experiments and improvements to models
An efficient algorithm for scalar PDF modelling in incompressible turbulent flow; numerical analysis with evaluation of IEM and IECM micro-mixing models.
No full textIn this paper, we investigate the application of probability density function (PDF) Monte Carlo methods to scalar release from small sources in a turbulent flow spanning a large physical domain. This is a typical situation encountered when modeling the dispersion of a gaseous substance in the atmosphere. Monte Carlo PDF methods have recently been applied to atmospheric modeling responding to the need for predicting the higher statistics and the concentration PDF generated by the continuous release of reactive and non-reactive substances. In this work we introduce some optimized numerical techniques based on the paradigm that the main field of interest is the scalar field and not the fluid dynamic field; the scalar are considered dynamically passive and the statistical characteristics of the turbulence velocity field are assumed known. These techniques are a block-structured grid coupled with a particle splitting/erasing algorithm and a localized time stepping. The proposed technique is different from others presented before since the particle splitting and erasing is done in a more straightforward and consistent manner. This method has been applied to the study of scalar dispersion from localized line sources in a canopy generated boundary layer. The line source has been treated as a point source in a two-dimensional space but the extension to three dimensions is straightforward. Our framework allows for an evaluation of the effects induced by different levels of discretization in the velocity space of the involved micro-mixing model, starting from the interaction by the exchange with the mean (IEM) toward the more physically consistent interaction by exchange with the conditional mean (IECM). Therefore, aside from the algorithm description and a complete numerical analysis of the code, a comparison between the IEM and IECM micro-mixing models has been investigated
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
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