1,721,020 research outputs found
Wind fluctuations affect the mean behaviour of naturally ventilated systems
Full text linkWe study the dynamics of a naturally ventilated room in which a point source provides a steady source of buoyancy and which is affected by an opposing unsteady wind. The wind is modelled as a stochastic forcing, which aims at simulating realistic velocity fluctuations as observed in the lower atmosphere. Our main finding is the occurrence of a "noise-induced transition", namely a structural change of the mean behaviour of the system: the warm-cold air interface does not fluctuate around the elevation exhibited when wind is constant, but oscillations occur around a new (significantly lower) interface elevation. We provide the physical explanation for such a counter-intuitive behaviour and show its dependence on (i) wind characteristics (intensity and timescale of fluctuations) and (ii) relative strength of wind over thermal loads. A realistic example case shows that the behaviour highlighted here has potentially major implications in the design and management of naturally ventilated buildings
A combined citizen science-modelling approach for NO2 assessment in Torino urban agglomeration
Full text linkThe #CHEARIATIRA citizen science campaign was developed in February 2019 in Torino (western part of the Po Valley megacity region). The aim of the campaign was public engagement with measuringNO2 concentrations in an urban area that often exceeds air quality standards. NO2 diffusion tubes were employed by citizens under our supervision. In this paper, we present the main outcomes of a combined approach between the #CHEARIATIRA campaign and the urban dispersion model SIRANE. The results were validated against the available public Air Quality Monitoring Stations (AQMS). The citizens' passive samplers and the modelled data show a good response in central districts both during the campaign interval and by annual projection. Traffic hotspots and sensitive receptors (schools, hospital) have high concentrations of NO2. Most of the study area (83% of the tubes) is subject to an increased risk of premature death according to epidemiological literature
Influence of meteorological input parameters on urban dispersion modelling for traffic scenario analysis
No full textEvaluation of Photostationary and Non-Photostationary Operational Models for NOX Pollution in a Street Canyon
Full text linkTo predict pollutant concentration in urban areas, it is crucial to take into account the chemical transfor-mations of reactive pollutants in operational dispersion models. In this work, we derive and discuss two photostationary (with constant or varying transformation rates) and one non-photostationary chemical models for NO - NO2 - O3 pollution in a street canyon. In the analytical derivation, we focus on the chemical and transport time scales to evaluate the applicability of the models in different urban contexts. We then assess their performance in predicting NO2, NO and O3 concentration at three locations within an urban district by comparing the model predictions with measurements acquired in a field campaign. The results are in line with analytical speculations and highlight in which street types non-photostationary models can bring substantial advantages. In courtyards with limited ventilation and without direct emissions, the performance of the photostationary model with meteorology-based transformation rates is satisfactory. On the other hand, the application of a non-photostationary model significantly improves the predictions in urban canyons with direct vehicular emissions. The applicability of the proposed models in operational tools at the city scale is finally discussed
Complex network analysis of wind tunnel experiments on the passive scalar dispersion in a turbulent boundary layer
Full text linkConcentration 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
Physical and Numerical Models of Atmospheric Urban Dispersion of Pollutants
No full textIn this paper the application of numerical and physical models for the simulation of airborne pollutants in urban areas are presented. The assessment of the impact of cruise ships during the hoteling phase in the port of Naples is considered as case study. A physical model of the urban area of Naples has been realized (scale 1:500) and tested in the wind tunnel facility of the Ecole Central de Lyon. Results of wind tunnel tests are compared with CALPUFF and CFD simulations with the aim to validate the performances of the models. The results obtained give useful information for an optimized use of dispersion models
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
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