This dissertation, "Large-eddy Simulation of Wind Flow and Air Pollutant Transport Inside Urban Street Canyons of Different Aspect Ratios" by Xianxiang, Li, 李顯祥, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. DOI: 10.5353/th_b4068732 Subjects: Air - Pollution - Mathematical models Atmospheric diffusion - Mathematical models

This dissertation, "Large Eddy Simulations of Wind Flow and Pollution Dispersion in an Urban Street Canyon" by Shuk-pan, Ellen, So, 蘇淑彬, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled "Large Eddy Simulations of Wind Flow and Pollution Dispersion in an Urban Street Canyon" submitted by So, Shuk Pan Ellen for the degree of Master of Philosophy at the University of Hong Kong in November 2003 This study provides elementary investigation of geometrical and inflow property effect on canyon flow, which is a matter of concern in urban planning. Investigations of the geometries of wind flow and pollution concentration in urban street canyons are carried out for various Reynolds numbers (Re) in this study using large-eddy simulations (LES). The application of large-eddy simulations allows computations of flow for higher Reynolds numbers in a micro-computer. Numerical simulations are performed based on the filtered Navier-Stokes Equations in large-eddy simulations for the wind flow and the filtered scalar transport equation for pollution dispersion. Eddies larger than the filtering width are resolved by these equations, while smaller eddies are modelled with Smagorinsky subgrid scale modelling (SGS). Continuity constraints are addressed by using the artificial compressibility method and a staggered marker-and-cell (MAC) mesh. A third-order upwind scheme is applied for the convective terms, and pressure terms are obtained from a Poisson pressure equation. A two-dimensional model perpendicular to the canyon is composed using composite domain technique. Buildings of different canyon aspect ratio (h/w) and canyon height ratio (h /h ) are 2 1 investigated for Reynolds numbers 400, 800, 1200, 1600 and 2000 to study the changes in the wind field and pollutant dilution patterns. Comparison cases with constant kinematic viscosity are performed to investigate the characteristics of the localised varying eddy viscosity. For the standard configuration of h/w = h /h = 1.0, 2 1 further investigations with higher Re values (2400, 2800, 3000 and 3500) are conducted. In canyon aspect ratio analysis, regimes of flow shift from isolated roughness flow to wake interference flow, and then to skimming flow, as the canyon deepens. At increasing Re values, lifts and enlargements of vortices promote mixing, and consequently smear the threshold values of the three regimes. The lowest retention values are normally found when a complete mixing of vortices is obtained in a wake interference regime. A gradual increase in retention values is found when stable vortices are formed. For canyon height ratios, two main regimes can be found. Wake interference flow is obtained when the downstream canopy is lower. Increasing Re thus lifts and enlarges the vortices and a merging of canyon and downstream vortices generates a single circulation surmounting the corresponding region. When h /h >1.0, vortices 2 1 appear with skimming flow characteristics. The vortices formed are stable and fairly uniform. The rise in Re values generates larger vortices, and no merging occurs. Dilution of canyon pollutants is particularly encouraged when h /h 1.0. A gradual decrease in retention 2 1 values is found when Re values increase. It is shown that the canyon configuration is an important parameter in determining the wind field and pollutant dispersion in canyons when Reynolds number values are fairly low. At higher Re values, those patterns become less changeable as lifting of vortices promotes their merger into single circulat

This dissertation, "Flow and Pollutant Dispersion Over Idealized Urban Street Canyons Using Large-eddy Simulation" by Ching-chi, Wong, 黃精治, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Flows and pollutant dispersion over flat rural terrain have been investigated for decades. However, our understanding of their behaviours over urban areas is rather limited. Most cases have either focused on street level or in the roughness sub-layer (RSL) of urban boundary layer (UBL). Whereas, only a handful of studies have looked into the coupling between street-level and UBL-core dynamics, and their effects on pollutant dispersion. In this thesis, computational fluid dynamics (CFD) is employed to examine the flows and pollutant transport in and over urban roughness. Idealised two-dimensional (2D) street canyons are used as the basic units fabricating hypothetical urban surfaces. A ground-level passive and chemically inert pollutant source is applied to simulate the flows and pollutant dispersion over rough surfaces in isothermal condition. Large-eddy simulation (LES) with the one-equation subgrid-scale model is used to solve explicitly the broad range of scales in turbulent flows. Arrays of idealized street canyons of both uniform and non-uniform building height are used to formulate a unified theory for the flows and pollutant dispersion over urban areas of different morphology. The geometry of roughness elements is controlled by the building-height-to-street-width (aspect) ratio (0.083 A detailed analysis on the roof-level turbulence structure reveals parcels of low-speed air masses in the streamwise flows and narrow high-speed down-drafts in the urban canopy layer, signifying the momentum entrainment into the street canyons. The decelerating streamwise flows in turn initiate up-drafts carrying pollutants away from the street canyons, illustrating the basic pollutant removal mechanism in 2D street canyons. Turbulent transport processes, in the form of ejection and sweep, are the key events governing the exchanges of air and pollutant of street canyon. Air exchange rate (ACH) along the roof level is dominated by turbulent transport, in particular over narrow street canyons. The LES results show that both the turbulence level and ACH increase with increasing aerodynamic resistance defined in term of the Fanning friction factor. At the same AR, BHV greatly increases the friction factor and the ACH in dense built areas (AR DOI: 10.5353/th_b5270547 Subjects: Air - Pollution - Mathematical models Eddies - Mathematical models

This book contains twenty-one original papers and one review paper published by internationally recognized experts in the Atmosphere Special Issue "Recent Advances in Urban Ventilation Assessment and Flow Modelling", years 2017–2019. The Special Issue includes contributions on recent experimental and modelling works, techniques, and developments mainly tailored to the assessment of urban ventilation on flow and pollutant dispersion in cities. The study of ventilation is of critical importance, as it addresses the capacity with which a built urban structure is capable of replacing the polluted air with ambient fresh air. Here, ventilation is recognized as a transport process that improves local microclimate and air quality and closely relates to the term “breathability”. The efficiency with which street canyon ventilation occurs depends on the complex interaction between the atmospheric boundary layer flow and the local urban morphology. The individual contributions to this Issue are summarized and categorized into four broad topics: (1) outdoor ventilation efficiency and application/development of ventilation indices, (2) relationship between indoor and outdoor ventilation, (3) effects of urban morphology and obstacles to ventilation, and (4) ventilation modelling in realistic urban districts. The results and approaches presented and proposed will be of great interest to experimentalists and modelers, and may constitute a starting point for the improvement of numerical simulations of flow and pollutant dispersion in the urban environment, for the development of simulation tools, and for the implementation of mitigation strategies.

This volume contains a selection of the papers presented at the Eighth Symposium on Turbulent Shear Flows held at the Technical University of Munich, 9-11 September 1991. The first of these biennial international symposia was held at the Pennsylvania State Uni versity, USA, in 1977; subsequent symposia have been held at Imperial College, London, England; the University of California, Davis, USA; the University of Karlsruhe, Ger many; Cornell University, Ithaca, USA; the Paul Sabatier University, Toulouse, France; and Stanford University, California, USA. The purpose of this series of symposia is to provide a forum for the presentation and discussion of new developments in the field of turbulence, especially as related to shear flows of importance in engineering and geo physics. From the 330 extended abstracts submitted for this symposium, 145 papers were presented orally and 60 as posters. Out of these, we have selected twenty-four papers for inclusion in this volume, each of which has been revised and extended in accordance with the editors' recommendations. The following four theme areas were selected after consideration of the quality of the contributions, the importance of the area, and the selection made in earlier volumes: - wall flows, - separated flows, - compressibility effects, - buoyancy, rotation, and curvature effects. As in the past, each section corresponding to the above areas begins with an introduction by an authority in the field that places the individual contributions in context with one another and with related research.

This dissertation, "On the Study of Ventilation and Pollutant Removal Over Idealized Two-dimensional Urban Street Canyons" by Ka-kit, Pieta, Leung, 梁家杰, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: In the last century, there has been a rapid growth and development in economy and modern technology around the world. This phenomenon helped improving wealth and living standard but also brought pollutions to the society and the environment. Among various kinds of pollution, air pollution takes a larger proportion. Therefore, there is increasing concern about the ventilation and pollution removal behavior in the urban environment. Among different academic studies performed, the use of computational fluid dynamics (CFD) had become more popular. Since wind tunnel experiments serve as validations for CFD results, this thesis developed the technique required for wind tunnels experiments and to investigate the pollutant removal related to urban geometry, as well as the technique for gas sampling to examine the distribution of pollutants in urban boundary layer over idealized two-dimensional (2D) street canyons. Three specific tasks are archived to accomplish the above objectives. The first task was to extend the wind tunnel in the Department of Mechanical Engineering, the University of Hong Kong. An extension duct was designed to increase the length of the test section in which the reduced-scale model could be installed. The dimensions of the test section were specified according to the required length for fully developed flow inside the test section, the environment in the laboratory and the original wind tunnel conditions. The extension duct was then constructed and mounted, with the wind profile inside the test section obtained afterwards. After construction of the extended test section for experimental purposes, the second task was to examine the pollutant transport behaviors from the ground level of idealized 2D urban street canyons to the urban atmospheric boundary layer (ABL) using both laboratory wind tunnel measurements and CFD. Movable rectangular aluminum blocks were placed in the wind tunnel in cross-flow to construct street canyons of different building-height-to-street-width (aspect) ratios. Wetted filter papers were applied on the surface of the blocks inside the street region, modeling the source of pollutant emission inside the street canyons. The wind tunnel and CFD results complemented each other to elucidate the pollutant removal mechanism that is in line with other results available in literature. From the experimental results obtained, scaling effect was observed in the mass transfer behaviors even the flows had fulfilled kinematic similarity. A new indicator, the scaled overall pollutant removal coefficient, was formulated for the comparison of pollutant removal performance. The improved agreement in the comparison with the CFD results showed that the scaled overall pollutant removal coefficient could be used to account for the scaling effects occurred in laboratory experiments at finite Reynolds number (〖10〗 DEGREES(3 ) to 〖10〗 DEGREES(5 ) in this study) for comparison of pollutant removal performance. The behavior of pollutants inside the street canyons was studied; however, the pollutant concentration inside a street could be affected by the pollutant source in another street, even there were several streets away from it. The pollutant escaped from the source street could act as air entrainment into other streets, affecting the air quality. The concentration profile correlated to the street geometry was thus stud