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Title: Investigation of wind and thermal comfort in high-density urban environment
Authors: Du, Yaxing
Degree: Ph.D.
Issue Date: 2018
Abstract: City residents wish to have pleasant experience in outdoor space, which are often impeded by uncomfortable wind and thermal conditions, particularly in hot and humid summer. With the rapid progress of urban development, modern mega-cities are teeming with tall and closely spaced buildings, which subsequently results in congested airflow at pedestrian level. The issues are more serious in the subtropical urban area, like Hong Kong. Previous studies regarding wind comfort were focused on dealing with nuisance caused by strong wind conditions. Given that the existing wind comfort criteria cannot fully represent weak wind conditions, a wind comfort criterion for Hong Kong is in urgent need. Moreover, most studies investigated the pedestrian level wind environment in urban areas were based on generic urban arrays. Obviously, an accurate reproduction of wind flow in real urban environment is the prerequisite of the successful evaluation of wind and thermal comfort. Therefore, this thesis provides a systematic investigation on the improvement of wind and thermal comfort, and accurate computational fluid dynamics (CFD) simulation of wind flow in a complex urban environment. To achieve the above research objectives, the following three sub-works are conducted: (a) the wind comfort criterion for low wind environment in Hong Kong is established. (b) Against the background of rapid urbanization and urban heat island (UHI) effects, the improvement of outdoor wind and thermal comfort in a local precinct are investigated. (c) A computational model to predict the pedestrian level wind environment in a real university campus is developed. The choice of proper wind comfort criterion is considered to be crucial to reliable assessment of pedestrian level wind comfort. By thoroughly reviewing and comparing existing wind comfort criteria, the parameters in Lawson (1978) criterion are adopted for acceptable, tolerable and intolerable category and the parameters in NEN8100 (2006) criterion are adopted for danger category in the proposed new wind comfort criteria. Besides, a low wind parameter suggested by AVA scheme (2005) is adopted for unfavourable category in summer criterion. The adopted parameters are carefully chosen to adapt the weak wind conditions. The prominent features of the criteria are proposed seasonally (summer and winter, respectively). The outcomes of the proposed wind comfort criteria provide scientific basis for future policy-making.
The implementation of innovative building design to create a local wind and thermal comfort region under the background of rapid urbanization and UHI effects is a promising way for sustainable urbanization. Thus, the lift-up design is used to enhance the low wind condition and provide local cooling spot. The effects of lift-up design in four common building configurations, including the "-", "L", "U" and "□" shaped buildings, on the wind comfort under different wind directions are studied via CFD simulations. Besides, the effects of lift-up design on outdoor thermal comfort are assessed by using a proposed integrated method, which combines wind tunnel tests and on-site monitoring to calculate Physiologically Equivalent Temperature (PET) values. Moreover, the optimum wind comfort around an isolated building with lift-up design is obtained by using a proposed multi-variable optimization method, which couples CFD technique and response surface methodology (RSM). In particular, the Detached Eddy Simulation (DES) model is employed throughout the whole CFD simulation process. Based on the optimization method, the optimum wind and thermal comfort in an idealized urban street canyon are determined. These findings provide solid bases to city planners and architects of available options for creating pleasant outdoor microclimate in precinct planning. Quality and efficiency of CFD simulation in a complex urban area are often compromised by many influencing factors, particularly mesh quality. Thus, a systematic and efficient mesh generation method is proposed and the geometrically complex HKPolyU campus is utilized to demonstrate and evaluate the proposed mesh generation method. Based on the high-quality mesh, the influence of three important computational parameters, namely, turbulence model, near-wall mesh density and computational domain size, on CFD prediction of pedestrian level wind environment are quantitatively evaluated. Validation of CFD models are carried out against a wind tunnel test of the HKPolyU campus model, where the CFD results and wind tunnel test results are agreed well. To reveal complex turbulent mechanisms and turbulent uncertainties, the unsteady RANS model and DES model are used to predict wind flow in the campus model based on high-quality mesh. Moreover, the influence of important issues, like the inflow fluctuating algorithm of DES approach, are investigated. These findings can improve quality and efficiency of CFD simulation in predicting pedestrian level wind environment in a complex urban region.
Subjects: Hong Kong Polytechnic University -- Dissertations
Winds and architecture
Architecture and climate
Urban climatology
Pages: xxii, 163 pages : color illustrations
Appears in Collections:Thesis

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