Numerical analysis on the thermal impact of vegetation on air flow in street canyon

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Numerical analysis on the thermal impact of vegetation on air flow in street canyon

 

Author: Li, Guangyin
Title: Numerical analysis on the thermal impact of vegetation on air flow in street canyon
Degree: M.Eng.
Year: 2012
Subject: Plant-atmosphere relationships.
Computational fluid dynamics.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Building Services Engineering
Pages: xi, 77 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2525558
URI: http://theses.lib.polyu.edu.hk/handle/200/6642
Abstract: On account of the express urbanization, microclimate inside street canyons has been in the spotlight. Urban planners attempted to mitigate the problem by introducing vegetation as a kind of compensation to environmental drawbacks. However, the overall ecological effect of vegetation is still not clear. Although the beneficial influence of vegetations on thermal comfort and dynamic field for pedestrians in street canyon has been discussed in both site measurements and wind tunnel experiments, but further developments in the aspect of the computational simulations are still needed to determine the impacts in practical applications. This paper introduces a numerical approach based on the computational fluid dynamics (CFD) models, the code FLUENT. The model considers vegetations in canyon as porous materials, and the Rossland radiation model is employed to combine with thermal conduction using standard and realizable k-ε turbulence models. After validating with the previous experimental data, with-tree models have been built based on without-tree models. In order to identify the optimum arrangement of street trees, a number of parameters were investigated, including of street canyon aspect ratio(H/W=1 and 2), crown types (9m length×9m weigh×9m height and 9m length×4.5m weigh×9m height), green coverage (12.5%, 25% and 50% ) and solar orientations (at 9 a.m., 12 a.m. and 3 p.m.). According to comparisons among the situations of street canyons with tree or without tree, this paper reveals the tendencies of average temperature and velocity in the pedestrian zone. Results from this study reveal the impacts of vegetation in street canyons and determine a reasonable density of vegetation planting for urban thermal environment control.

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