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  • 1.
    Buccoliere, Riccardo
    et al.
    Univ Salento, Dipartimento Sci Mat, Lecce, Italy, and Univ Ca Foscari Venezia, Dipartimento Informat, Venice, Italy.
    Sandberg, Mats
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering, Building science - installation technology.
    Di Sabatino, Silvana
    Univ Salento, Dipartimento Sci Mat, Lecce, Italy .
    City breathability and its link to pollutant concentration distribution within urban-like geometries2010In: Atmospheric Environment, ISSN 1352-2310, E-ISSN 1873-2844, Vol. 44, no 15, p. 1894-1903Article in journal (Refereed)
    Abstract [en]

    This paper is devoted to the study of pollutant concentration distribution within urban-like geometries. By applying efficiency concepts originally developed for indoor environments, the term ventilation is used as a measure of city “breathability”. It can be applied to analyse pollutant removal within a city in operational contexts. This implies the evaluation of the bulk flow balance over the city and of the mean age of air. The influence of building packing density on flow and pollutant removal is, therefore, evaluated using those quantities. Idealized cities of regular cubical buildings were created with packing density ranging from 6.25% to 69% to represent configurations from urban sprawl to compact cities. The relative simplicity of these arrangements allowed us to apply the Computational Fluid Dynamics (CFD) flow and dispersion simulations using the standard k turbulence model. Results show that city breathability within the urban canopy layer is strongly dependent from the building packing density. At the lower packing densities, the city responds to the wind as an agglomeration of obstacles, at larger densities (from about 44%) the city itself responds as a single obstacle. With the exception of the lowest packing density, airflow enters the array through lateral sides and leaves throughout the street top and flow out downstream. The air entering through lateral sides increases with increasing packing density.

    At the street top of the windward side of compact building configurations, a large upward flow is observed. This vertical transport reduces over short distance to turn into a downward flow further downstream of the building array. These findings suggest a practical way of identifying city breathability. Even though the application of these results to real scenarios require further analyses the paper illustrates a practical framework to be adopted in the assessment of the optimum neighbourhood building layout to minimize pollution levels.

  • 2.
    Hang, Jian
    et al.
    Department of Mechanical Engineering, TheUniversity of Hong Kong, Hong Kong SAR, Hong Kong.
    Sandberg, Mats
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för inomhusmiljö.
    Li, Yuguo
    University of Hong Kong, Hong Kong SAR, Hong Kong.
    Effect of urban morphology on wind condition in idealized city models2009In: Atmospheric Environment, ISSN 1352-2310, E-ISSN 1873-2844, Vol. 43, no 4, p. 869-878Article in journal (Refereed)
    Abstract [en]

    Wind conditions in urban environments are important for a number of reasons. They can serve to transport air pollutants out of the urban environment and to moderate urban microclimatic conditions if satisfactory, yet can compromise pedestrian comfort and safety if not. We aim to study experimentally and numerically the effects of urban morphology (e.g., overall city form (skyline), street orientation, and street configuration) on wind conditions in cities. This report considers our initial investigations of two idealized city forms that are coincidentally similar to ancient Roman cities that were organized on one or two primary streets - a main north-south street, the cardus maximus, and a secondary east-west street, the decumanus maximus - and contained within a well-defined perimeter. We first consider round and square city models with one main street set parallel to the approaching wind and a secondary street producing an intersection at city centre. Not surprisingly, wind conditions in the two city models are dissimilar due to their shape differences. We then consider a long rectangular city model with a fully developed steady flow region along the main street. If the main street of the round city model is narrow, the parallel approaching wind cannot blow through the entire street and a penetrating inflow exists at the leeward opening. For the round city model with two crossing streets, a slightly non-parallel wind to the main street generates a stronger wind level in the entire street volume.

  • 3. Hang, Jian
    et al.
    Sandberg, Mats
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för inomhusmiljö.
    Li, Yuguo Li
    Claesson, Leif
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för inomhusmiljö.
    Pollutant dispersion in idealized city models with different urban morphologies2009In: Atmospheric Environment, ISSN 1352-2310, E-ISSN 1873-2844, Vol. 43, no 38, p. 6011-6025Article in journal (Refereed)
    Abstract [en]

    The mechanism of pollutant dispersion in idealized city models is investigated numerically by the introduction of a uniformly distributed pollutant source at street pedestrian level. We first study three short city forms with a single main street or two crossing streets, characterized by street length/street height ratios of L/H = 6 or 7 and a street height/street width ratio of H/W = 1, including a sharp-edged round city model, a smooth-edged round city model, and a sharp-edged square city model. For short city models with a single street and a parallel approaching wind, pollutant dilution mainly depends on the horizontal flow rate which decreases along the street. This decreasing rate is smallest for the smooth-edged round city model, which results in the lowest street concentrations. For city models with two crossing streets and the approaching wind parallel to the main street, the differences in overall city form result in different dispersion processes. For a sharp-edged round city model with two crossing streets, an approaching wind slightly non-parallel to the main street generates a lower pollutant concentration in the entire street volume. We also studied a sharp-edged round city model with one narrow street (L/H = 6; H/W = 6.7), finding that the uniformly distributed pollutants are transported from two street entries to the city centre, and are then removed out across the street roof. In contrast to the short city models we studied a single-street sharp-edged long rectangular city model (L/H = 21.7; H/W = 1) in which the horizontal flow rate remained nearly constant in a region far from the two entries. Within this region the turbulence across the street roof contributed more to the pollutant removal than vertical mean flows.

  • 4.
    Skote, M
    et al.
    University of Gävle, Department of Technology and Built Environment.
    Sandberg, Mats
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för inomhusmiljö.
    Westerberg, Ulla
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för byggnadskvalitet.
    Claesson, Leif
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för inomhusmiljö.
    Johansson, Arne
    Numerical and experimental studies of wind environment in an urban morphology2005In: Atmospheric Environment, ISSN 1352-2310, E-ISSN 1873-2844, Vol. 39, no 33, p. 6147-6158Article in journal (Refereed)
    Abstract [en]

    The purpose of the paper is to examine the relation between urban morphology (three-dimensional structure) and windiness. We regard a city as a porous obstacle, which is open at the top. We consider the interaction between the atmospheric boundary layer and a city to be both a function of the overall shape (silhouette or skyline) and the internal resistance to the flow caused by the friction when the wind flows over the urban surfaces. We regard the street pattern as an interconnected flow network with the crossings as nodes. Flow, along the streets is generated by pressure differences.

    We here use a highly idealized city model consisting of a circular block divided into two or four equally large sectors. Two types of cases are studied, the first with only one street through the city model with different angles between the street and the oncoming wind. The second case also contains a perpendicularly crossing street (through the center). Both wind tunnel experiments and numerical flow computations (computational fluid dynamics, CFD) are used and compared. The general agreement between the two is good and the CFD method offers new possibilities for quantifying the urban wind environment. (c) 2005 Elsevier Ltd. All rights reserved.

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