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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

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