Flow and Turbulence Characteristics in the Roughness Sublayer over Real Urban Morphology of Hong Kong

2020 ◽  
Author(s):  
Ziwei Mo ◽  
Chun-Ho Liu ◽  
Yat-Kiu Ho
Keyword(s):  
Hong Kong ◽  
Wind Tunnel ◽  
Urban Areas ◽  
Air Entrainment ◽  
Urban Morphology ◽  
Pollutant Removal ◽  
Quadrant Analysis ◽  
Vertical Profiles ◽  
Mean Flow ◽  
Law Of The Wall

<p>Mean flow and turbulence in roughness sublayers (RSLs) over urban areas are complicated because of the diversified building configurations (such as size, shape and orientation, etc.). This study investigates the RSL flows over (part of) the real urban morphology of Hong Kong downtown by wind tunnel measurements. The urban models are fabricated by 3D printing using high-resolution digital maps of building morphology and topography. Vertical profiles of mean and turbulent components in three parallel transects are measured by a constant-temperature hot-wire anemometer (CTA). The wind tunnel results reveal that individual (vertical) profiles of streamwise fluctuating velocity u’’, vertical fluctuating velocity w’’ and vertical momentum flux u’’w’’ show noticeable variations in the RSL. It is largely attributed to the wakes and recirculations after the upstream high-rise buildings. A new analytical solution is proposed to predict the mean wind profiles in the RSL as well as the inertial sublayer (ISL) that is more accurate than the conventional logarithmic law of the wall (log-law). The turbulence in the RSL and ISL are examined in terms of quadrant analysis. The ejection Q2 and the sweep Q4 are stronger in the RSL than those in the ISL, collectively improving street-level air entrainment and pollutant removal.</p>

Wind Speed Parameterization and Turbulence Intermittency in the Roughness Sublayers over Urban Areas: a Wind Tunnel Study

2021 ◽  
Author(s):  
Ziwei Mo ◽  
Chun-Ho Liu
Keyword(s):  
Wind Tunnel ◽  
Analytical Solution ◽  
Urban Areas ◽  
Urban Setting ◽  
Quadrant Analysis ◽  
Law Of The Wall ◽  
Flux Gradient ◽  
Turbulence Intermittency ◽  
The Mean ◽  
Mean Square Errors

<p>The flow in inertial sublayer (ISL) is horizontally homogeneous where the Monin–Obukhov similarity theory (MOST) well describes the flux-gradient relationship.  In contrast, roughness sublayer (RSL) flow is highly inhomogeneous. Its dynamics is influenced by the length scale of individual roughness elements. This study presents an analytical solution to the mean wind profile for both ISL and RSL by adding a new function in the flux-gradient relationship to handle the RSL dynamics. The mean wind speeds measured in the wind tunnel experiments over a range of idealized and real urban geometries are well predicted by the new analytical solution. The root-mean-square errors (RMSE) are reduced over an order of magnitude compared with the conventional logarithmic law of the wall (log-law). Its key parameter, the RSL constant converges asymptotically to μ = 1.7 for urban setting which is different from that (μ = 2.6) for vegetation canopy. The RSL turbulence intermittency is revealed by higher-order moments of velocities, probability density function (PDF), quadrant analysis, and conditional sampling. Ejection Q2 (-u’’, +w”) and sweep Q4 (+u’’, -w”) dominate in both RSL and ISL but with different share. Unlike the ISL, Q2 occurs more frequently (but contributes less to momentum flux) than Q4 in the RSL. It is thus suggested that RSL turbulent transport is driven by occasional, fast motions of accelerating downward flow (Q4) and bulk, slow decelerating upward flow (Q2).</p>

Large-Eddy Simulation for the Roughness Sublayers over Real Urban Surfaces 

2021 ◽  
Author(s):  
Lan Yao ◽  
Chun-Ho Liu
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Momentum Flux ◽  
Local Maximum ◽  
Air Entrainment ◽  
Urban Morphology ◽  
Conditional Sampling ◽  
Law Of The Wall ◽  
Eddy Simulation ◽  
Large Eddy

<p><strong>ABSTRACT: </strong></p><p>    With the continuous spreading of global pandemic, environmental issues have aroused worldwide unprecedented attention. Airflow plays a crucial role in aerosol motions and pollutants removal in dense cities. Large-eddy simulation (LES) is conducted for a typical metropolitan, Hong Kong, to investigate the dynamics in the atmospheric boundary layer (ABL) over real urban surfaces. Full-scale building models (average building height h<sub>m</sub> = 36 m) from Tsim Sha Tsui to Sham Shui Po, Kowloon Peninsula, are digitalized. Southerly wind with speed U<sub>∞</sub> (= 10 m sec<sup>-1</sup>) in neutral stratification is prescribed at the domain inlet. The turbulence statistics extracted from three subdomains in Mong Kok neighborhood, each with size 800 m (streamwise) × 100 m (spanwise) × 500 m (vertical), are analyzed. Linear regression of the wind profile with the logarithmic law of the wall (log-law) show that the interface between inertial sublayer (ISL) and roughness sublayer (RSL) is in the range of 2.5h<sub>m</sub> to 4.5h<sub>m</sub>. In the RSL, the streamwise and vertical velocities are positively (S<sub>u</sub> > 0) and negatively (S<sub>w</sub> < 0) skewed, respectively. Their kurtosis K<sub>u</sub> and K<sub>w</sub> is less than 3. Conditional sampling of vertical momentum, flux u’’w’’ showed that ejection Q2 occurs more frequently than does sweep Q4. On the contrary, the contribution of Q4 exceeds that of Q2. These characteristics switch to the other way round in the ISL. Furthermore, the difference between Q4 and Q2, either in terms of occurrence or contribution, shows a local maximum around 50% of the total momentum flux, suggesting the major energy-carrying scales. Coherent structures depict elongated, (massive,) accelerating (decelerating) and descending (ascending) RSL (ISL) flows. Hence, the fresh (aged) air entrainment (detrainment) are signified by fast and extreme (slow and frequent) flows. These distinct features of RSL flows over real urban morphology provide an inspiration to improve the ground-level air quality by proper urban planning.</p><p><strong>KEYWORDS:</strong> Large-eddy simulation (LES), real urban morphology, turbulent boundary layer (TBL), conditional sampling, hole filtering</p><p> </p><p> </p>

Flood alleviation in a heavily urbanized area – Kai Tak Nullah, Hong Kong

2012 ◽  
Vol 7 (4) ◽  
Author(s):  
Alan K. L. Chan ◽  
Colin K. C. Wong ◽  
Robin H. N. Lee ◽  
Mike W. H. Cho
Keyword(s):  
Hong Kong ◽  
Public Engagement ◽  
Urban Areas ◽  
Hydraulic Modelling ◽  
The Public ◽  
Design Considerations ◽  
Urbanized Area ◽  
Development Area ◽  
The Government ◽  
Exercise Design

The existing Kai Tak Nullah flows from Po Kong Village Road along Choi Hung Road and Tung Tau Estate into Kai Tak Development Area before discharging into the Victoria Harbour. Historically its upstream has been subject to flooding under storm conditions and this has had serious repercussions for the adjacent urban areas. A study has been commissioned by the Drainage Services Department of the Government of the Hong Kong Special Administrative Region (HKSAR), China to investigate the flood mechanisms and to provide flood alleviation measures by improving the capacity of the Kai Tak Nullah. In addition to flood alleviation, there is a strong public aspiration to rehabilitate the Kai Tak Nullah by a comparatively natural river design. Since the Kai Tak Nullah is located within a heavily urbanized area, traffic and environmental impacts are also highly concerned. The final flood alleviation scheme has thus had to strike a balance among the aforesaid factors with assistance from the hydraulic modelling utilizing InfoWorks Collection Systems (CS) software. This paper presents the public engagement exercise, design considerations, methodologies, and recommendations regarding the reconstruction and rehabilitation of the Kai Tak Nullah.

scholarly journals Review on pollutant dispersion in urban areas-part A: Effects of mechanical factors and urban morphology

2021 ◽  
Vol 190 ◽  
pp. 107534
Author(s):  
Zhengtong Li ◽  
Tingzhen Ming ◽  
Shurong Liu ◽  
Chong Peng ◽  
Renaud de Richter ◽  
...  
Keyword(s):  
Urban Areas ◽  
Pollutant Dispersion ◽  
Urban Morphology ◽  
Mechanical Factors

Wind tunnel investigation on the retention of air pollutants in three-dimensional recirculation zones in urban areas

2007 ◽  
Vol 41 (23) ◽  
pp. 4949-4961 ◽  
Author(s):  
Marcos Sebastião de Paula Gomes ◽  
André Augusto Isnard ◽  
José Maurício do Carmo Pinto
Keyword(s):  
Wind Tunnel ◽  
Urban Areas ◽  
Air Pollutants ◽  
Three Dimensional ◽  
Recirculation Zones

Exploring the relationships between urban form metrics and the vegetation biomass loss under urban expansion in China

2018 ◽  
Vol 47 (3) ◽  
pp. 363-380
Author(s):  
Tong Zhang ◽  
Sophia Shuang Chen ◽  
Guangyu Li
Keyword(s):  
Urban Form ◽  
Urban Areas ◽  
Yangtze River ◽  
Green Space ◽  
Yangtze River Delta ◽  
Urban Morphology ◽  
Total Biomass ◽  
Urban Metrics ◽  
Biomass Loss ◽  
Loss Index

Compact urban form has been applied as a strategy to reduce the loss of green space that occurs from development, but the impact of this policy on the provision of green space still presents many uncertainties. This research investigated the statistical relationship between urban form indicators and the loss of vegetation biomass to understand the response of quality green space provision to changes in urban morphology. A methodology combining multi-source data assimilation, statistical analysis, and spatial analysis was adopted for the Yangtze River Delta cities of China. First, six urban metrics were selected to describe the shape and layout of urban patches in each city, and the total biomass loss index was then introduced as a parameter. The values of urban metrics and total biomass loss index were calculated for the 50 Yangtze River Delta cities. Second, ordinary least squares regression and geographically weighted regression analyses were then used to establish a quantitative relationship between total biomass loss index and urban form indicators. The results revealed an extremely negative correlation between total biomass loss index and the three urban variables of Richard compactness, density gradient, and the Gini coefficient; moreover, the parameter estimates for the three variables in the geographically weighted regression model were local and varied over space. Third, the mechanisms by which the urban form influences biomass loss were discussed and different urban form planning strategies for particular urban areas were suggested. In conclusion, compact urban form in a clustered layout of urban areas with a dense central agglomeration was verified to be ecologically superior and conducive to green space protection. For the physical interpretation of the statistical relationship between urban morphology and vegetation loss, the interface effect of urban agglomeration on vegetation merits further study.

A method for energy efficiency assessment during urban energy planning

2014 ◽  
Vol 3 (2) ◽  
pp. 132-152 ◽  
Author(s):  
Karin Regina de Casas Castro Marins
Keyword(s):  
Energy Efficiency ◽  
Urban Areas ◽  
Energy Use ◽  
Energy Performance ◽  
Urban Morphology ◽  
Pollutant Emissions ◽  
Energy Planning ◽  
Bottom Up ◽  
Content Type ◽  
Urban Energy

Purpose – Energy use in urban areas has turned a subject of local and worldwide interest over the last few years, especially emphasized by the correlated greenhouse gases emissions. The purpose of this paper is to analyse the overall energy efficiency potential and emissions resulting from integrated solutions in urban energy planning, in the scale of districts and neighbourhoods in Brazil. Design/methodology/approach – The approach is based on the description and the application of a method to analyse energy performance of urban areas and support their planning. It is a quantitative bottom-up method and involves urban morphology, urban mobility, buildings and energy supply systems. Procedures are applied to the case study of Agua Branca urban development area, located in Sao Paulo, Brazil. Findings – In the case of Agua Branca area, energy efficiency measures in buildings have shown to be very important mostly for the buildings economies themselves. For the area as a whole, strategies in promoting public transport are more effective in terms of energy efficiency and also to decrease pollutant emissions. Originality/value – Literature review has shown there is a lack of approaches and procedures able to support urban energy planning at a community scale. The bottom-up method presented in this paper integrates a plenty of disaggregated and multisectoral parameters at the same stage in urban planning and shows that is possible to identify the most promising actions by building overall performance indexes.

scholarly journals Topological analysis of contemporary morphologies under conflict: The urban transformation of Dobrinja in Sarajevo and the Central District of Beirut

2017 ◽  
Author(s):  
Inés Aquilué ◽  
Estanislao Roca ◽  
Javier Ruiz
Keyword(s):  
Civil War ◽  
Urban Form ◽  
Urban Areas ◽  
Urban Morphology ◽  
Specific Method ◽  
Evolutionary Analysis ◽  
Lebanese Civil War ◽  
Post Conflict ◽  
Bosnian War ◽  
Central District

Regarding topological interpretation of space, this research aims to identify urban morphologies, whose topology becomes increasingly determining under high uncertainty. This topological approach has been applied in an evolutionary analysis of urban spaces under siege, fear and conflict, which conducted to the construction of a specific method. This method analyses the transformation of urban areas in five consecutive phases: urban form [1], increase of uncertainty [2], application of the apparatus [3], change in urban form [4], information flows [5]. These five phases were applied to different empirical studies, analysed through specific morphological and topological models. In the light of this method, two selected urban morphologies Dobrinja –a suburb in Sarajevo– and the Beirut Central District have been examined. The urban morphology of both areas was dramatically transformed after both civil conflicts –the Bosnian War and the Lebanese Civil War–. Dobrinja suffered severe modifications, first provoked by the violence of the siege during the Bosnian War [1992-1995], and then by the Inter-Entity Boundary Line as a result of the Dayton Peace Agreement [December 1995], which divided the neighbourhood and caused serious alterations in its ethno-demographic and spatial structure. The Beirut Central District was first destroyed by the violence experienced in the Lebanese Civil War [1975-1990] and then by the process of subsequent reconstruction [since 1992], which led to a simplification of its structure. The two morphological and topological analyses enable us to determine the initial causes and their spatial consequences in both urban areas, regarding their conflict and post-conflict stage.

scholarly journals Using citizen-science reports to document range expansion of the introduced Chinese Water Dragon (Physignathus cocincinus) in Hong Kong

2019 ◽  
Vol 26 (2) ◽  
pp. 128-131
Author(s):  
Matthew Mo
Keyword(s):  
Hong Kong ◽  
Citizen Science ◽  
Range Expansion ◽  
Urban Areas ◽  
Natural Areas ◽  
Chinese Water ◽  
Males And Females

The first incursion of Chinese Water Gragons (Physignathus cocincinus) in Hong Kong occurred in October 2004 on Tsing Yi Island. This study identified a further 54 citizen-science reports of dragons in different locations of Hong Kong. Although there were no subsequent reports from Tsing Yi Island, there were recurring reports in five other locations. In Aberdeen, reports started in 2010, concentrating on the grounds of Ocean Park. There were also numerous reports from the Kam Shan Country Park since 2010, the Pok Fu Lam Reservoir since 2011, the Lion Rock Country Park since 2012, and the Sha Tin District since 2014. Dragons were mostly observed in natural areas, but also in urban areas. The highest number of sightings were reported in April. Combinations of males and females were identified in four of the locations with numerous reports, suggesting that these populations are self-sustaining.

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