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 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, "Large-eddy Simulation of Transport of Inert and Chemically Reactive Pollutants Over 2D Idealized Street Canyons" by Nga-hang, Chung, 鍾雅行, 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 view of the worsening air quality in the world, more concerns are focused on the environment. This thesis uses the technique of CFD and develops the computer model to investigate the wind and pollutant transport, as well as the chemistry of reactive pollutants in idealized two-dimensional (2D) street canyons. Three scientific questions are raised in this thesis. The first task is to find out the po- sition with the most favorable pollutant removal along the ground level over 2D idealized street canyon of different building-height-to-street-width (aspect) ratios (ARs). The di- mensionless parameter, C, represents the pollutant removal performance. In the isolated roughness regime, the two local maximum C locate at the reattachment point and the windward corner. In the wake interference regime, C is peaked on the windward side. The number of vertically aligned recirculations depends on the street depth in the skimming flow regime. The sizes of the secondary recirculation upstream and downstream deter- mine how the maximum C shifts from the street centre. After identifying the position of peaked pollutant removal rate at the ground level, the emission source should be placed with the highest constant C in order to remove the pollutants upward more quickly to safeguard the street-level air quality. After understanding the best pollutant removal in the street canyon of different ARs, the second task is to find out what AR is the most favorable for the ventilation and pollutant removal across the roof level. The three parameters, namely friction factor, air exchange rate (ACH) and pollutant exchange rate (PCH), are introduced to quantify the pressure difference to sustain the mean flow, the ventilation and pollutant removal, respectively. The turbulence contributes more than 70% to the total ACH and PCH in all the three flow regimes. By increasing the atmospheric turbulence in building geometry as well as the surface roughness, the ventilation and pollutant removal performance can be improved. The linear relation between the friction factor and ACH demonstrates the larger resistance that in turn promotes the air exchange over the roof level. The physical dispersion is studied; however atmospheric pollutants are seldom in- ert but chemically reactive instead. The last task is to include the three common air pollutants, NO, NO2 and O3, in the simple NOx ?O3 mechanism in terms of the photo- stationary state and reaction rates. The Damkohler numbers of NO and O3, DaNO and DaO3, are parameterized by the concentrations of the sources NO and O3. The normalized mean and fluctuation NO, NO2 and O3 are separately considered. The integrated pho- tostationary state (PSS) in the first canyon increases with DaO3 under the same DaNO. The integrated PSS of the second to the twelveth street canyons are compared with each case, the monotonic increase in the PSS from the second to twelveth canyon is perceived in DaNO/DaO3 1, 0.03, 0.02, 0.001 and 0.000333. Further decreases the DaNO/DaO3 to 0.000143, 0.000125, 0.000118, 0.000111 and 0.0001, the PSS is found to be non-linear and the trough appears in the fourth and fif

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