Author: Chen, Lan
Title: A study of pedestrian-level wind and outdoor thermal comfort under urban elevated designs
Advisors: Mak, Cheuk Ming (BEEE)
Degree: Ph.D.
Year: 2023
Subject: Buildings -- Aerodynamics
Air flow
Human comfort
Architectural design -- Environmental aspects
Hong Kong Polytechnic University -- Dissertations
Department: Department of Building Environment and Energy Engineering
Pages: xxxiii, 206 pages : color illustrations
Language: English
Abstract: In the context of continuous urbanization and global warming, many high-density cities face pressing issues from weak wind and intense heat, especially in hot-humid climates. The removal of pedestrian-level heat and pollutants is impeded; citizens are vulnerable to severe heat stress and thermal discomfort; the development of a livable and sustainable urban environment is challenged. Therefore, it is imperative to reveal the interaction of urban designs with pedestrian-level wind and outdoor thermal comfort, subsequently exploring mitigation strategies.
This thesis aims to systematically investigate pedestrian-level wind and outdoor thermal comfort under elevated designs in high-density urban environments with a hot-humid climate. Elevated designs, such as lift-up buildings and elevated walkways, prevail in hot-humid and high-density cities like Hong Kong. Lift-up buildings, with the main body elevated off the ground through supporting structures, have proven to improve pedestrian-level wind velocity and thermal comfort. However, few studies are centered on unconventional building configurations, and the impacts of building height on wind and thermal comfort have not been revealed thoroughly. Elevated walkways may alter local wind environments by suppressing air movements and improve outdoor thermal comfort through shading, yet scientific evidence remains insufficient.
Based on these research gaps, this thesis conducted four sub-works centering on the following research questions: (1) how do building configurations influence pedestrian-level wind comfort around a lift-up building? (2) how does building height affect pedestrian wind and thermal comfort around a lift-up building with and without the influence of an upstream building? (3) how do elevated walkway dimensions and street aspect ratio impact pedestrian-level wind environments on the ground sidewalk? (4) what are the differences in outdoor thermal comfort between the elevated walkway and the ground sidewalk, and how do walkway dimensions and street aspect ratio affect ground-level and walkway-level thermal comfort?
The first and second sub-works revolve around pedestrian wind and thermal comfort of various lift-up buildings by computational fluid dynamics (CFD) simulations. The steady Reynolds-averaged Navier-Stokes method with a realizable k-­ε turbulence model was employed to predict mean flows. The wind comfort assessment primarily referred to published wind comfort criteria for weak wind conditions. The physiological equivalent temperature (PET), calculated by RayMan Pro, was adopted for evaluating thermal comfort.
To evaluate wind comfort around lift-up buildings with different configurations, the first sub-work examined twenty-two building configurations, grouped into “polygonal,”“slab-like,”“cruciform,”“trident,” and “assembled.” All configurations were derived from existing buildings in Hong Kong. The results indicate that wind comfort around an isolated building is sensitive to incident wind direction, building configuration, and interest precinct size. The lift-up design significantly improves near-field wind comfort; however, this improvement diminishes as the interest precinct expands. The number of sides, projected width, building depth, included angle, converging and diverging flows, surface curvature, and surface discontinuity were identified as configuration parameters influencing wind comfort around a building and the lift-up design’s performance for wind comfort improvement.
To reveal the integrated impacts of building height and upstream building, the second sub-work designed fifteen scenarios by varying incident wind direction and heights of the lift-up building and upstream building. PET was computed based on wind velocity data from CFD simulations and thermal-related parameters from published field surveys. The findings show that increasing building height, being under a diverging flow, removing upstream buildings, and making the lift-up building taller or shorter than the upstream building can improve wind comfort in the lift-up area; however, their effects on thermal comfort vary seasonally. For improving wind and thermal comfort in the podium, making the lift-up building taller or shorter than the upstream building or under a diverging flow is beneficial; nevertheless, increasing building height and removing upstream buildings are not necessarily favorable.
The third sub-work investigated pedestrian-level wind environments around elevated walkways under different street aspect ratios (H/W), walkway widths (Wew), and sidewall types using large eddy simulations (LES). Besides mean wind velocity fields, gust wind velocity fields were analyzed. The following results are based on the premise that the more wind, the better in weak wind conditions. The elevated walkway worsens the pedestrian-level wind environment. Adding an elevated walkway decreases the target street’s overall mean and gust wind velocities, with a maximum reduction of over 20% and 30%. The overall mean and gust wind velocities decrease first and then increase with H/W rising from 1 to 3. Pedestrian-level wind environments worsen with increased Wew. Compared to open sidewalls, semi-hermetic or hermetic sidewalls slightly improve pedestrian-level wind environments.
Based on the third sub-work, the fourth sub-work further explored the influence of elevated walkways on outdoor thermal comfort in ideal urban street canyons by combining LES and RayMan modeling. Moreover, on-site measurements were conducted on the elevated walkway and ground sidewalk in Hong Kong during summer and winter. Results indicate that the elevated walkway has a minor cooling effect on air temperature but cools mean radiant temperature significantly. The elevated walkway increases the ground-level PET value slightly but causes a notably lower walkway-level PET value, benefiting thermal comfort on hot days. Furthermore, PET exhibits an initial increase and sequential decrease with rising H/W but positively correlates with Wew, which applies to ground sidewalks and elevated walkways. Compared to open sidewalls, semi-hermetic sidewalls decrease the ground-level PET value slightly but raise the walkway-level PET value.
This thesis provides insight into how elevated designs influence pedestrian-level wind and outdoor thermal comfort and confirms the efficacy of elevated designs in improving thermal comfort in hot weather. The research outcome can guide urban planning and building design to utilize elevated designs to create local thermally comfortable outdoor places against the unfavorable wind and thermal context. For lift-up buildings, it is recommended to determine the appropriate configuration and orientation based on the local wind condition and avoid uniform height to the upstream building. The elevated walkway width, sidewall, and cover type should be adaptive to the local climate and street morphology, avoiding excessive shade and wind attenuation issues.
Rights: All rights reserved
Access: open access

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