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This thesis attempts to elucidate the mechanism of air ventilation and pollutant removal in three-dimensional (3D) street canyons by parametric 代写留学生论文tests, with comparison to their two-dimensional (2D) counterpart. Computational fluid dynamics (CFD) based on the unsteady-Reynolds-averaged Navier-Stokes (URANS) equation equipped with the Renormalization Group (RNG) k-epsilon turbulence model was adopted. Varies of numerical settings, including the solvers, discrete methods, wall functions and so on, were examined to conclude a stable approach in 3D simulation. In 2D cases whose geometry was defined by the building-height-to-street-width (aspect) ratios (AR), the flow patterns performed by simulation followed Oke’s typical flow regimes theory of idealized street canyons firmly. While in 3D case, the flow exhibited oscillating behaviours and so did the pollutant in lateral direction of street canyons, which showed the limitation of traditional 2D research and was implying a brand new theory should be studied in 3D model.

In this study, the air ventilation and pollutant removal were estimated by normalized air (ACH) and pollutant (PCH) exchange rates for three interfaces of a street canyon (two sides and one rooftop). The ACH and PCH were then separated into their mean ("(ACH) ̅" and "(PCH) ̅") and turbulent (ACH' and PCH') components to study their respective contributions to air ventilation and pollutant removal in different building-breadth-to-street-width ratios (BR), which have the same building height and street width, but different building breadth. The wind tunnel experiments and field measurements available in previous literature were used to validate the CFD simulation results. To compare the airflow and pollutant transport properties, the ACH and PCH across the roof and ends of the street canyon are plotted against time. In the 3D street canyon, pollutant is carried away from both roof level (upward) and the street ends (sideward). Hence, pollutant is transported to the shear layer aloft and the street junctions. CFD results depicted the pollutant transport in the form of plume dispersion in the shear layer and channelling below the canopy level. Temporal averaging was performed to measure the ventilation rate and pollutant removal rate of the street canyons. The ventilation and pollutant removal through the roof and street ends are found to be (based on results).

These results imply that the influence of sideward to pollutant transport is significant. Additional CFD calculations were performed to illustrate the pollutant entrainment in the street canyons downwind of the street canyon with pollutant source. High levels of pollutant were found in the first and second rows of street canyons behind the pollutant source. Therefore, pollutant re-entrainment from roof or street levels should not be overlooked in our urban environment, and the respective contribution of this pollutant re-entrainment from side and rooftop were discussed.

1 Introduction

Background and Research intention

Air pollution is a public concerned problem in modern metropolises. The major sources of air pollutants in urban areas are stationary industrial emissions and non-stationary vehicular exhausts. Numerous systematic researches, with expertise in physics, chemistry, geography or biology, have been 本论文由英语论文网提供整理，提供论文代写，英语论文代写，代写论文，代写英语论文，代写留学生论文，代写英文论文，留学生论文代写相关核心关键词搜索。