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Title: Parameterization and computational simulation of turbulent transfer in urban atmospheric boundary layers
Other Titles: Cheng shi da qi bian jie ceng tuan liu shu song de can shu hua yu shu zhi mo ni
Authors: Yang, Yan (楊艷)
Department: Dept. of Physics and Materials Science
Degree: Doctor of Philosophy
Issue Date: 2006
Publisher: City University of Hong Kong
Subjects: Air -- Pollution -- Meteorological aspects
Atmospheric turbulence
Boundary layer (Meteorology)
Urban climatology
Notes: CityU Call Number: QC880.4.T8 Y36 2006
Includes bibliographical references (leaves 169-180)
Thesis (Ph.D.)--City University of Hong Kong, 2006
xx, 180 leaves : ill. (some col.), col. map ; 30 cm.
Type: Thesis
Abstract: The aim of this thesis is to investigate pollution transfer in urban atmospheric boundary layers. The thesis consists of two main components: a scalar transfer scheme and a computational fluid dynamics simulation. The two components are inter-related. In the first component, a parameterization scheme is proposed for computing scalar transfer in urban canyon flows. The urban boundary layer is divided into three layers: an inertial layer, a shear layer and a canyon layer. The mechanisms for the scalar transfers in these layers are different. In the inertial layer turbulent diffusion dominates, which depends on the background atmospheric flow and the canyon macroscopic features. In the shear and canyon layers, vortex advection and vortex-generated turbulent diffusion dominate. Our scheme accounts for these mechanisms. An aerodynamic resistance network is proposed to represent the scalar transfer processes, using two resistances for the shear layer and three resistances for the canyon layer. A drag partition theory is developed for rough surfaces, in which the total drag is divided into a pressure drag, a surface drag and a skin drag. The concept of effective frontal area index is introduced and the expressions for drag partition functions are derived. The drag partition theory is valid for surfaces of arbitrary roughness densities. The aerodynamic resistances required for the scalar transfer scheme are then estimated by relating them to the drag partition theory. The scheme developed in this thesis is simple and requires only a few parameters. To validate the scheme, the estimated resistances are compared with the wind tunnel data. The comparisons show that the theory fits well to the observations. In the second component, the flow characteristics and turbulent transfer in 2-D, 3-D and real urban canyons are investigated using a computational fluid dynamics (CFD) model. The CFD model is used in this thesis as a tool to generate data for theory development and validation. The CFD simulations with progressively more complex canyon configurations enable the identification of the flow and dispersion characteristics and of the problems (e.g. transfer in complex urban canyons) which require further research. The results of the 2-D simulations, carried out for canyons with various aspect ratios, are consistent with the existing theory. The canyon flow patterns and pollution distribution characteristics are found to depend strongly on the canyon aspect ratio. The scalar transfer resistances estimated from the CFD results are in general agreement with the estimates using the scalar transfer scheme. The 3-D simulations show that flow and pollution transfer in 3-D canyons are profoundly different from those in 2-D canyons. Due to the limited length of the 3-D canyons, flows inside the canyon and along the aisle are closely inter-related. Nevertheless, on the cross-section along the 3-D canyon axis, the flow shows similar features as known from 2-D canyon studies. In the aisle, the flow is characterized by a double vortex roll elongating along the street, which generates complex patterns of vertical motion and affects the circulations inside the canyon. The patterns of pollution concentration in the 3-D street canyons are largely explained in terms of vortex circulations. In general, pollutants accumulate in the leeward sides of buildings where convergence and upward motion dominate. The profiles of flow velocity, pollution concentration and flux vary greatly in space. And in the canyon layer, a linear flux-gradient relationship does not exist event on average, although it does almost immediately above the canyon. Flow in a real urban canyon is very complex and difficult to describe in general terms. However, the CFD simulation for the Hong Kong Central region suggests that it is possible to divide the complex urban canyon flow fields into channel-flow regimes and wake-flow regimes. In a channel-flow regime, horizontal flow is strong which carries pollutants downstream, while in a wake-flow regime, horizontal flow is weak, which allows pollutants to accumulate. As the vertical motion in the wake-flow regime is relatively strong, pollutants are transported in the vertical. For the case of the Hong Kong Central region, our transfer scheme is found to be too simplistic, because the urban structures are mostly asymmetric with building heights varying from several meters to several hundred meters. For such complex urban surfaces, it remains a challenge to develop an adequate methodology for the determination of pollution transfer in the urban boundary layer.
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