| Author: | Li, Wenxin |
| Title: | Wind and thermal comfort design for urban open and semi-open structures : wind tunnel and numerical studies |
| Advisors: | Mak, Cheuk Ming (BEEE) |
| Degree: | Ph.D. |
| Year: | 2026 |
| Department: | Department of Building Environment and Energy Engineering |
| Pages: | xxx, 234 pages : color illustrations |
| Language: | English |
| Abstract: | Due to rapid global urbanization and heatwaves intensified by the Urban Heat Island (UHI) effect, densely populated cities face inadequate pedestrian-level ventilation, degraded air quality, and severe thermal discomfort. As a result, urbanites increasingly stay indoors and avoid outdoor activities, leading to a more sedentary lifestyle that contributes to higher obesity rates and negatively impacts human health. Open and semi-open spaces distributed in individual precincts are widely recognized as heat resilience hubs that are conducive to wind flow and improve thermal comfort. Exploring the characteristics of urban open and semi-open designs improves our understanding of building-wind interactions to support sustainable environment development. Besides building-induced flow changes, atmospheric boundary layer (ABL) conditions significantly influence pedestrian-level wind environments (PLWEs). Urban wind direction is often altered by hilly terrain. In cities with such topography, like Hong Kong and Chongqing, wind direction varies significantly with height, a factor overlooked in prior studies on urban open designs. This vertical wind direction twist creates a twisted wind profile (TWP), distinct from the conventional wind profile (CWP), which assumes constant wind direction with height. Thus, understanding how urban open and semi-open designs influence PLWEs and how twisted wind impacts both PLWEs and surrounding thermal comfort is essential. This thesis provides a comprehensive analysis of the impacts of open or semi-open building designs on pedestrian-level wind and thermal comfort. In addition to time-averaged velocity, turbulent statistics that influence wind comfort are also considered. The impact of twisted wind profiles (TWPs) on PLWEs and thermal comfort is further included. For buildings featuring open or semi-open designs, two representative layouts are the elevated (open) and arcade (semi-open) designs. The elevated design refers to the structures that are raised on pillars, creating activity and recreational areas for residents beneath. The arcade's design adopts a semi-enclosed structural arrangement, characterized by horizontally recessed spaces positioned beneath the building body. Although the effectiveness of improving urban wind comfort using a single elevated building has been verified, previous studies have primarily focused on mean wind velocities; there is a lack of knowledge regarding both time-averaged quantities and gust wind characteristics around irregular elevated buildings with urban arrays. Moreover, while open and semi-open designs have been shown to improve wind and thermal comfort, their impact on airflow under twisted wind conditions remains unclear and requires further study. Based on the identified research gaps, this thesis conducted four sub-studies as follows: The first and second sub-work involve wind tunnel experiments investigating three-dimensional turbulent flow structures around a building group, which includes a high-rise unconventional elevated building located centrally. Additionally, the Computational fluid dynamics (CFD) simulation, including the Steady Reynolds-averaged Navier-Stokes (RANS) and Large eddy simulation (LES), is conducted on a single Arc-shaped elevated building to predict mean and gust wind effects, validated against the results of the wind tunnel experiments. The results indicate that a region of high mean and gust wind velocity was observed at the lateral and rear side of the central elevated building, indicating that elevated buildings can enhance PLWEs even in the presence of surrounding buildings. The Arc-shaped elevated building tends to generate a larger "Intolerable" area compared with V- and Rectangular-shaped buildings due to its curved surface. The wind impacts the convex portion of the building, leading to airflow divergence and facilitating downstream ventilation. The RANS models substantially underestimated the mean wind velocity in the side and wake regions of the building when compared to the wind tunnel experiment results, while these models agreed well with wind tunnel data on mean wind velocity in the upstream region. The LES simulation can accurately reproduce the mean wind velocity and reasonably estimate the rmse velocity components around the elevated building. The vortex shedding frequency around an elevated building exhibits a wider range compared to that around a non-elevated building, which indicates the vortex shedding phenomenon is more complex around elevated buildings. The third sub-work investigated two tandem buildings with varying relative height differences (HUB/HDB) ranging from 3:1 to 0.33:1 and different elevated structure locations under CWP and TWP by wind tunnel experiment. The two twist vanes were mounted upstream of the center of the turntable to simulate the twisted wind profile (TWP) with two different angles (15° and 30°). The results show that the step-up building configurations (HUB<HDB) exhibit heightened sensitivity to twisted wind profiles, resulting in larger deviation angles ( β ) of downstream low wind velocity (DSLWV) zones. Step-down building configurations (HUB>HDB) are relatively insensitive to the twisted wind profile. The presence of elevated structures enhances sensitivity to the twisted wind effect, resulting in increased displacement of DFLWV regions relative to the building's central axis. The twisted wind reorients the "Sitting short" zone in alignment with the direction of incoming twist flow and concurrently enlarges the "Strolling" area on the opposing side. The twisted wind can diminish the thermal comfort surrounding the two tandem structures during summer while enhancing the thermal environment in winter. Based on the third sub-work, the fourth sub-work further explores the influence of twisted wind on semi-open structures and influential factors under solar heating. The steady RANS model with Standard k - ε models with discrete ordinate (DO) radiation was applied. The results showed that weakened downwash flow caused by the twisted wind within the street canyon was insufficient to counteract the buoyancy-driven flow generated by solar radiation. This condition facilitated the escape of heated air from the lower levels of the street canyon. Increasing the arcade width may negatively impact thermal comfort under TWP. The cooling effect of twisted winds was more pronounced in shorter street canyons. This thesis examines how irregular elevated buildings influence turbulent flow structures and interact with surrounding buildings, and evaluates the effectiveness of elevated and arcade designs in improving wind and thermal comfort in areas with frequent twisted flows. The findings of this study can help city planners create local comfortable outdoor spaces by minimizing thermal risk and enhancing wind and thermal comfort. Predicting reduced high-wind areas under TWPs may also help avoid unnecessary costs from corrective measures based on overestimated wind speeds near building corners under the conventional wind profile, thus supporting better urban design for sustainable and liveable environments. |
| Rights: | All rights reserved |
| Access: | open access |
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