Dispersion In Urban Areas Research Articles

Dispersion modelling of traffic particulate matter in city-scale area

This paper introduces utilisation of the CFD technique for modelling of PM10 (particulate matter 10 µm) dispersion in urban areas, with focus on detailed description of processes in the ground surface boundary layer. The description of deposition and re-suspension processes is introduced and applied on the city-scale model of the city of Brno (pop. 400 000). The air threshold velocity of re-suspension was used as basic parameter for quantification of re-suspension. The particular value of the air threshold velocity of re-suspension was obtained from analysis of the long-term measurements.

Measurements and CFD simulations of flow and dispersion in urban geometries

Dispersion in urban areas is determined by transport and mixing processes within the street network and ventilation into the external atmosphere. In order to improve the understanding of the exchange mechanisms within the urban canopy, wind tunnel and numerical investigations have been performed on two idealised typical urban configurations, the street intersection and the square. The numerical simulations were compared to the experimental values, in order to evaluate the reliability of the numerical approach in simulating local flow patterns and study the influence of the geometrical layout on mass and momentum exchange mechanisms.

Using Temperature Fluctuation Measurements to Estimate Meteorological Inputs for Modelling Dispersion During Convective Conditions in Urban Areas

We examine the performance of several methods to estimate meteorological inputs for modelling dispersion in urban areas during convective conditions. Sensible heat flux, surface friction velocity and turbulent velocities are estimated from measurements of mean wind speed and the standard deviation of temperature fluctuations at a single level on a tower at two suburban sites and at one urban site in Riverside, California. These estimates are compared with observations made at these sites during a field study conducted in 2007. The sensible heat flux is overestimated in the urban area, while it is underestimated at a suburban site when temperature fluctuations are used in the free convection formulation to estimate heat flux. The bias in heat flux estimates can be reduced through a correction that depends on stability. It turns out that the bias in heat flux estimates has a minor effect on the prediction of surface friction velocity and turbulent velocities. Estimates of sensible heat flux, surface friction velocity and turbulent velocities are sensitive to estimates of aerodynamic roughness length, and we suggest estimating the aerodynamic roughness length through detailed micrometeorological measurements made during a limited field study. An examination of the impact of the uncertainty in estimating surface micrometeorology on concentrations indicates that, at small distances from a surface release, ground-level concentrations computed using estimates of heat flux and surface friction compare well with the those based on observed values: the bias is small and the 95% confidence interval of the ratio of the two concentrations is 1.7. However, at distances much larger than the Obukhov length, this confidence interval is close to 2.3 because errors in both friction velocity and heat flux affect plume spread. Finally, we show that using measurements of temperature fluctuations in estimating heat flux is an improvement on that based on the surface energy balance, even when net radiation measurements are available.

Tracer Flux Balance at an Urban Canyon Intersection

Despite their importance for pollutant dispersion in urban areas, the special features of dispersion at street intersections are rarely taken into account by operational air quality models. Several previous studies have demonstrated the complex flow patterns that occur at street intersections, even with simple geometry. This study presents results from wind-tunnel experiments on a reduced scale model of a complex but realistic urban intersection, located in central London. Tracer concentration measurements were used to derive three-dimensional maps of the concentration field within the intersection. In combination with a previous study (Carpentieri et al., Boundary-Layer Meteorol 133:277-296, 2009) where the velocity field was measured in the same model, a methodology for the calculation of the mean tracer flux balance at the intersection was developed and applied. The calculation highlighted several limitations of current state-of-the-art canyon dispersion models, arising mainly from the complex geometry of the intersection. Despite its limitations, the proposed methodology could be further developed in order to derive, assess and implement street intersection dispersion models for complex urban areas.

Crimes and Crime Dispersion in Urban Areas in Turkey

Crimes and Crime Dispersion in Urban Areas in Turkey Crime is a phenomenon that arises from the interaction between social, political, economic, physical, psychological circumstances and geographical factors. Since crime occurs in geographical locations, the relationship between crime and location is worth discussing characteristics of a location occur in differents ways. Not only natural geographical factors but also human geographical factors may lead to crime. In this study, the situation, distribution of crimes in the cities which are in the police service zone are dealt with. In addition to crime dispersion in the cities and crimes committed to people and property, crimes committed by month is also studied. In this context, it is seen that since city centres are composed of dense population and urban functions, they feed crime, social control weakens in those areas and they create opportunities for criminals. This causes weakness in society and social capital apart from economic losses in cities. The number of crime incidents, which was 229,513 in 1995, in Turkey, has risen to 785,510 in 2006 with a 3.4 fold increase. 41% of the crimes which were committed in the year of 2006 were against people, 59% were against property.

Street canyon ventilation and atmospheric turbulence

Operational models for pollutant dispersion in urban areas require an estimate of the turbulent transfer between the street canyons and the overlying atmospheric flow. To date, the mechanisms that govern this process remain poorly understood. We have studied the mass exchange between a street canyon and the atmospheric flow above it by means of wind tunnel experiments. Fluid velocities were measured with a Particle Image Velocimetry system and passive scalar concentrations were measured using a Flame Ionisation Detector. The mass-transfer velocity between the canyon and the external flow has been estimated by measuring the cavity wash-out time. A two-box model, used to estimate the transfer velocity for varying dynamical conditions of the external flow, has been used to interpret the experimental data. This study sheds new light on the mechanisms which drive the ventilation of a street canyon and illustrates the influence of the external turbulence on the transfer process.

Using measurements in urban areas to estimate turbulent velocities for modeling dispersion

This study extends a study [Princevac, M., Venkatram, A., 2007. Estimating micrometeorological inputs for modeling dispersion in urban areas during stable conditions. Atmospheric Environment, doi:10.1016/j.atmosenv.2007.02.029.] in which mean winds and temperatures measured at one or two levels on towers located in urban areas were fitted to Monin–Obukhov similarity equations to obtain estimates of micrometeorological variables required in modeling dispersion in the stable boundary layer. This study shows that such methods are also useful in unstable conditions: measurements of the mean wind speed and the standard deviation of temperature fluctuations, σT, at one level on a tower yield estimates of surface heat flux, surface friction velocity, and standard deviations of turbulent velocities that are within a factor of two of values observed at two urban sites over 80% of the time.

Estimating micrometeorological inputs for modeling dispersion in urban areas during stable conditions

We examine the performance of three methods to estimate the surface friction velocity and the Monin–Obukhov (MO) length in stable conditions. Estimates from these methods are compared with measurements made at two urban sites: the Wilmington site located in the middle of an urban area, and the VTMX site located on a sloping, smooth area in Salt Lake City. The first method uses the mean wind at a single height (Single U or SU), the second uses the wind speed at a single level and the temperature difference between two levels (U delta T or UDT), and the third method uses two levels of wind speed and temperature (delta U delta T or DUDT). The performance of the SU and UDT methods in estimating u * are comparable. The SU method yields better estimates of the MO length than the UDT method does. The DUDT method performs poorly in estimating both u * and L. The major conclusions of this study are that (1) measurements of mean winds and temperatures at one or two levels at an urban location can provide adequate estimates of micrometeorological variables required in modeling dispersion in the stable boundary layer, and (2) methods based on using differences in temperatures and velocities between two levels can provide unreliable estimates of these variables because these differences can be overwhelmed by inevitable uncertainties in the measurement of mean variables.

CFD Modelling Of Traffic-related Air PollutantsAround An Urban Street-canyon In Suzhou

The wind flow field around urban street-building configurations has an important influence on the microscale pollutant dispersion from road traffic, affecting overall dilution and creating localized spatial variations of pollutant concentration. It is important to understand these small-scale effects not only for assessing personal exposure to pollutants in the vicinity of roads, but also because air quality in a city as a whole is assessed on the basis of data from a few urban monitoring sites. This paper reports some outcomes of a study aimed at exploring the transfer and diffusion characteristics of vehicular exhaust emissions and the constraints associated with significant monitoring locations such as street canyons, courtyards and enclosed spaces, and the conditions propitious to pollutant diffusion and air dispersion in urban areas. The wind field around a street-canyon in the city of Suzhou (China) has been modelled by using the Computational Fluid Dynamic technique. The results have been analysed to better understand the values of gaseous pollutants, meteorological parameters and traffic flows measured by means of automatic analyzers along the same street-canyon at two different heights. It has been found that the model correctly describes the vortex recirculation pattern around the building geometry studied and it is very useful in assessing the micro-scale pollution trends.

Albanian immigration to Greece: Diffusion and dispersion in urban area: The case of Thessaloniki

Albanian immigration in Greece could be qualified as singular, given its ample repartition throughout the national territory, which is not the case for other migrant groups that tend to be concentrated in specific areas, in particular, in the Athenian agglomeration. This "exemplary" migration seems to generate comparable patterns even within the urban space. The object of this paper will be to relate these two portrayals of Albanian migration: within both the national Greek territory and the urban space, through the example of Thessaloniki. Our objective will thus be to illustrate Albanian households? repartition in Thessaloniki and to compare their spatial distribution with the distribution of other migrant groups. In this way, we will be able to demonstrate the spatial pattern that Albanian migration takes on in Thessaloniki, as well as to reveal the Albanian immigrants? mode of territorial insertion.

Mixing and transport in urban areas

Concern over terrorist releases of harmful material has generated interest in short-range air-borne dispersion in urban areas. Here, we review the important fluid dynamical processes that control dispersion in the first kilometre, the neighbourhood scale, when much of the material remains within the urban canopy. Dispersion is then controlled by turbulent mixing and mean flow transport through the network of streets. We consider mixing and transport in a long straight street, street intersections and then a network of streets connected by intersections. The mixing and transport in these systems are illustrated with results from recent fine-resolution numerical simulations and laboratory models, which then inform simpler scaling estimates and modelling schemes. Finally, we make some tentative steps to pull the process studies together to begin to understand results from full-scale observations. In particular, it is shown that the positions of 'shear layers' and 'dividing streamlines' largely control the patterns of mixing and transport. It is also shown that neighbourhood-scale dispersion follows one scaling in the near field and another in the far field after passage through many intersections. The challenge for the future is to bring these threads together into a coherent mathematical model.

Application of GIS to modified models of vehicle emission dispersion

This paper reports on a preliminary study of the forecast and evaluation of transport-related air pollution dispersion in urban areas. Some modifications of the traditional Gauss dispersion models are provided, and especially a crossroad model is built, which considers the great variation of vehicle emission attributed to different driving patterns at the crossroad. The above models are combined with a self-developed geographic information system (GIS) platform, and a simulative system with graphical interfaces is built. The system aims at visually describing the influences on the urban environment by urban traffic characteristics and therefore gives a reference to the improvement of urban air quality. Due to the introduction of a self-developed GIS platform and a creative crossroad model, the system is more effective, flexible and accurate. Finally, a comparison of the simulated (predicted) and observed hourly concentration is given, which indicates a good simulation.

FLOW ANDDISPERSION INURBANAREAS

▪ Abstract Increasing urbanization and concern about sustainability and quality of life issues have produced considerable interest in flow and dispersion in urban areas. We address this subject at four scales: regional, city, neighborhood, and street. The flow is one over and through a complex array of structures. Most of the local fluid mechanical processes are understood; how these combine and what is the most appropriate framework to study and quantify the result is less clear. Extensive and structured experimental databases have been compiled recently in several laboratories. A number of major field experiments in urban areas have been completed very recently and more are planned. These have aided understanding as well as model development and evaluation.

Coupling between air flow in streets and the well-developed boundary layer aloft

A field experiment was conducted to study the mean and turbulent characteristics of air flow within, and above, a street under neutral stratification. Profiles of the mean wind and turbulent statistics were obtained and compared with flow over smoother terrain. Our results modify the conventional picture of a persistent re-circulation within the street with small-scale turbulent fluctuations about this mean ( Oke, 1987). Instead, the mean re-circulation within the street was found to be much weaker than the unsteady turbulent fluctuations. Hence, the mean flow is merely a residual of an unsteady turbulent re-circulation. The re-circulation in the street is coupled to the wind aloft through a shear layer that develops at the roof-level. The shear layer is unstable, through Kelvin–Helmholtz instability, which leads to intermittency in the re-circulation in the street, and thence to ventilation of the air in the street. This finding is likely to be important for dispersion in urban areas.

Modelling of vehicle-induced turbulence in air pollution studies for streets

Vehicle-induced turbulence can be an important factor of pollutant dispersion in urban areas, especially under conditions of low wind speeds which are typical for street canyons. An experimental concept (Plate, 1982) for modelling the effects of vehicle-induced turbulence was applied in the present study. The movement of vehicles was simulated in a boundary-layer wind tunnel by small metal plates mounted on two belts moving along a modelled street canyon. The scaling factor was based on the ratio of turbulence production by cars to that by wind flow. The traffic was represented by the velocity, density, frontal area and drag coefficients of the vehicles. The velocity and traffic density were varied, and the influence of the vehicle-induced turbulence on concentration patterns at the canyon walls was studied. It was found that concentration decreases with an increasing ratio of vehicle to wind velocity and with an increase of traffic density. A dimensionless combination of vehicle to wind-velocity ratio and density factor was proved to be a universal parameter describing the dependence of the concentration on vehicle-induced turbulence. The wind tunnel measurements were compared with predictions by the numerical Operational Street Pollution Model (OSPM; Hertel and Berkowicz, 1989a). Differences between the wind tunnel and numerical results regarding effects of vehicle-induced turbulence are discussed. The comparison revealed general agreement between wind tunnel and numerical data. Turbulence and concentration measurements in a street canyon in Copenhagen have been additionally employed for analysis of the model results.