Chun-Ho Liu

Chun-Ho Liu
The University of Hong Kong | HKU · Department of Mechanical Engineering

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130
Publications
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Publications

Publications (130)
Article
Plume chasing is cost-effective, measuring individual, on-road vehicular emissions. Whereas, wake-flow-generated turbulence results in intermittent, rapid pollutant dilution and substantial fluctuating concentrations right behind the vehicle being chased. The sampling duration is therefore one of the important factors for acquiring representative (...
Article
Full-text available
Aerodynamic resistance changes in the atmospheric surface layers (ASLs), such as coastal zones and rural–urban interface, complicate the transport processes. Their effect on ASL flows is investigated by the large-eddy simulation (LES) with the one-equation subgrid-scale model. The roughness elements at the bottom are explicitly resolved by sinusoid...
Article
Full-text available
In conventional modeling of air pollution dispersion, pollutants are treated as passive scalars or inert species even though most of them are chemically reactive [1]. Chemical reactions contribute to pollutant dispersion via the generation and depletion of pollutants, in addition to other two mechanisms: advection and turbulent diffusion. This stud...
Article
Dynamics and organized turbulence structure in the atmospheric boundary layer (ABL) over a real urban area, Mong Kok neighborhood in Kowloon Peninsula, Hong Kong, is investigated using the large-eddy simulation (LES) to enrich the transport mechanism. The urban morphology is explicitly resolved and the LES results are validated by wind tunnel measu...
Article
We report a large-eddy simulation of the turbulent pollutant dispersion in the near wake behind a real, light-duty lorry. The time-averaged mean wake and dominant global flow instabilities are compared with the results of simplified square-back vehicle models in the literature, showing good agreement. In addition to the ring-shape mean wake topolog...
Article
Previous research on university students’ academic performance indicates that students’ seating locations can be an indicator of their academic performance. However, previous research on the relationship between university students’ seating locations and their academic performance is mixed, and there is a lack of research about whether students’ ac...
Article
Full-text available
The massive lockdown of global cities during the COVID-19 pandemic is substantially improving the atmospheric environment, which for the first time, urban mobility is virtually reduced to zero, and it is then possible to establish a baseline for air quality. By comparing these values with pre-COVID-19 data, it is possible to infer the likely effect...
Article
Optimization algorithms and decision-making techniques are major components of multi-objective optimization. This study evaluated the performance of population-based metaheuristic algorithms and decision-making techniques in optimizing an unconventional building design – a lift-up design – to maximize the areas with wind and thermal comfort in a ‘h...
Article
Full-text available
Urban street canyons formed by high-rise buildings restrict the dispersion of vehicle emissions, which pose severe health risks to the public by aggravating roadside air quality. However, this issue is often overlooked in city planning. This paper reviews the mechanisms controlling vehicle emission dispersion in urban street canyons and the strateg...
Experiment Findings
Full-text available
Article
Wind tunnel experiment and steady-state Reynolds-averaged Navier-Stokes (RANS) approaches are used to examine the urban boundary layer (UBL) development above Kowloon Peninsula, Hong Kong Special Administrative Region (HKSAR). The detailed urban morphology is resolved by computational fluid dynamics (CFD) and is fabricated by 3D-printing (reduced s...
Article
Full-text available
Wind tunnel tests and computational fluid dynamics (CFD) simulations remain the main modeling techniques in wind engineering despite being expensive, time-consuming, and requiring special facilities and expert knowledge. There is a clear need for a fast, accurate, but, at the same time, computationally economical substitute. This study proposes a G...
Article
The World Health Organization considered the widespread of COVID-19 over the world as a pandemic. There is still a lack of understanding of its origin, transmission, and treatment methods. Understanding the influencing factors of the COVID-19 can help mitigate its spread, but little research on the spatial factors has been conducted. Therefore, thi...
Article
Atmospheric surface layer (ASL) consists of the inertial sublayer (ISL) and the roughness sublayer (RSL) with different dynamics. This study characterizes the ASL flows over real urban morphology using wind tunnel experiments. The building models of downtown Kowloon Peninsula, Hong Kong are fabricated by 3D printing. The vertical profiles (3 × 45 n...
Article
Apart from the aerodynamic performance (efficiency and safety), the wake after an on-road vehicle substantially influences the tailpipe pollutant dispersion (environment). Remote sensing is the most practicable measures for large-scale emission control. Its reliability, however, is largely dictated by how well the complicated vehicular flows and in...
Article
Full-text available
Pedestrian wind comfort near a 400 m super-tall building in high and low ambient wind speeds, referred to as Windy and Calm climates, is evaluated by conducting computational fluid dynamics (CFD) simulations. The super-tall building has 15 different configurations and is located at the center of 50 m medium-rise buildings in an urban-like setting....
Article
Full-text available
On-road remote sensing technology measures the concentration ratios of pollutants over CO2 in the exhaust plume in half a second when a vehicle passes by a measurement site, providing a rapid, non-intrusive and economic tool for vehicle emissions monitoring and control. A key assumption in such measurement is that the emission ratios are constant f...
Article
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Unconventional configurations of tall buildings are noticeably different from their counterpart of traditional building designs but nevertheless, the unconventional configurations have often been adopted for tall buildings without their impact on the pedestrian-level wind environment (PLWE) fully understood. To fill the existing knowledge gap, this...
Article
Winds (advection) and turbulence (diffusion) are the driving forces for urban plume dispersion whose partitioning is complicated by urban morphology and the conditions of atmospheric surface layer (ASL). Wind tunnel parametric tests are conducted to examine the plume behavior in the turbulent boundary layers (TBLs) over hypothetical buildings (roug...
Article
Full-text available
Understanding the effect of building morphology on the flow aloft is important to the ventilation and pollutant removal in cities. This study examines the dynamics over hypothetical urban areas in isothermal conditions using wind tunnel experiments. Different configurations of rib-type and cube-type arrays are designed to model hypothetical rough u...
Article
Full-text available
Abstract Pedestrian-level air quality in urban areas is largely affected by the pollutants emitted from street canyons to the atmospheric boundary layer (ABL). Most vehicular exhausts are chemically reactive that evolve to their secondary counterparts. Besides, building roughness modifies the flows, which, in turn, influences the chemical reactions...
Article
Full-text available
Urban air quality is an important problem nowadays because of the close proximity of sources and receptors in densely built environment. Gaussian plume models have been commonly employed in the industry to estimate air pollution impact over open terrain for decades. However, they should be applied cautiously to urban environment in view of the comp...
Article
Gaussian plume models have been used to estimate pollutant distribution for decades. In view of the empirically determined dispersion coefficients (largely based on atmospheric stability), their application in urban setting needs to be interpreted cautiously. It is even more complicated if chemically reactive pollutants are considered. In this tech...
Article
Non-computational-fluid-dynamics (Non-CFD) solutions, such as Gaussian plume models, are commonly employed to predict ground-level pollutant concentrations because of their cost-effectiveness. Whilst, they should be applied with caution for pollutant plume dispersion over complicated urban morphology in view of their implicit limitation of empirica...
Article
Urban morphology is a major factor governing the dynamics in atmospheric surface layers (ASLs) of which our understanding is rather limited. In this paper, wind tunnel experiments are conducted to characterize the flows over different types of urban roughness in attempt to demystify the mechanism of street-level ventilation in isothermal conditions...
Chapter
Atmospheric surface layer (ASL) is characterized by its relatively uniform fluxes with height z in which the dimensionless velocity gradient \(\phi _m\) is expressed.
Article
Wind flows and pollutant dispersion over cities are strongly affected by urban morphology. Scientific evidence measuring the effect of surface roughness on transport processes is needed to effectuate air quality strategy. Based on idealised building models assembled by ribs and LEGO ® bricks, a series of wind tunnel experiments are performed to inv...
Article
Full-text available
Dynamics in the roughness (RSLs) and inertial (ISLs) sublayers in the turbulent boundary layers (TBLs) over idealised urban surfaces are investigated analytically and experimentally. In this paper, we derive an analytical solution to the mean velocity profile, which is a continuous function applicable to both RSL and ISL, over rough surfaces in iso...
Article
Full-text available
Street-level ventilation is often weakened by the surrounding high-rise buildings. A thorough understanding of the flows and turbulence over urban areas assists in improving urban air quality as well as effectuating environmental management. In this paper, reduced-scale physical modeling in a wind tunnel is employed to examine the dynamics in hypot...
Article
Urban roughness is a major factor governing the flows and scalar transport in the atmospheric boundary layer (ABL) but our understanding is rather limited. The ventilation and pollutant removal of hypothetical urban areas consisting of various types of street canyons are examined using computational fluid dynamics (CFD). The aerodynamic resistance,...
Article
Dispersion of gas and particulate matter (PM) pollutant in urban environments is a rising concern. Physical and numerical modelling is extremely challenging due to a number of parameters involved including the turbulence nature and complex physical and/or chemical processes. The paper attempts to review the latest advances on this topic. The major...
Article
In this paper, the flows over idealised two-dimensional (2D) urban street canyons of different building-height-to-street-width (aspect) ratios (ARs) and urban boundary layer (UBL) depths are numerically examined. We attempt to utilise the friction factor f and the air-exchange rate (ACH) to parameterise the aerodynamic resistance and the street-lev...
Article
Full-text available
In this study, the dispersion of chemically reactive pollutants is calculated by large-eddy simulation (LES) in a neutrally stratified urban canopy layer (UCL) over urban areas. As a pilot attempt, idealized street canyons of unity building-height-to-street-width (aspect) ratio are used. Nitric oxide (NO) is emitted from the ground surface of the f...
Article
In urban areas, pollutants are emitted from vehicles then disperse from the ground level to the downstream urban canopy layer (UCL) under the effect of the prevailing wind. For a hypothetical urban area in the form of idealized street canyons, the building-height-to-street-width (aspect) ratio (AR) changes the ground roughness which in turn leads t...
Conference Paper
Urban roughness is a major factor governing the flows and pollutant transport in the urban atmospheric boundary layer (UABL) over buildings. Whereas, our understanding of its effects on ventilation and pollutant dispersion in/over urban areas is rather limited. In this paper, the ventilation of and pollutant dispersion over idealized street canyons...
Article
A series of large-eddy simulation (LES) models consisting of two-dimensional (2D) idealised street canyons with building height variability (BHV) are examined. Building blocks with two different heights are placed alternately in the computational domains, constructing repeated street canyons of building-height-to-street-width (aspect) ratio (AR) =...
Article
Flows in the urban boundary layer (UBL) are strongly affected by the inhomogeneous roughness elements at the bottom surface. In particular, in the near-ground region (roughness sublayer), the effect of the surface roughness dominates that complicates the behaviours of mean flow and turbulence and subsequently the near-wall transport processes. To s...
Article
The relationship among air exchange rate (ACH), pollutant exchange rate (PCH) and friction factor (f) is proposed in this study to evaluate the performance of ventilation and pollution removal of hypothetical urban areas. Urban areas were simplified to arrays of idealised, repeated two-dimensional (2D) street canyons of different building-height-to...
Article
Flow resistance, ventilation, and pollutant removal for idealized two-dimensional (2D) street canyons of different building-height to street-width (aspect) ratios $AR$ are examined using the friction factor $f$ , air exchange rate (ACH), and pollutant exchange rate (PCH), respectively, calculated by large-eddy simulation (LES). The flows are ba...
Article
Urban roughness is one of the major factors affecting the flows and turbulence structures in the bottom of the atmospheric boundary layer (ABL). Whereas, our understanding of their relation is limited. In this paper, we attempt to examine the interaction among aerodynamic resistance (friction factor f), ventilation (air exchange rate ACH), and poll...
Article
Most air pollutants are chemically reactive so using inert scalar as the tracer in pollutant dispersion modelling would often overlook their impact on urban inhabitants. In this study, large-eddy simulation (LES) is used to examine the plume dispersion of chemically reactive pollutants in a hypothetical atmospheric boundary layer (ABL) in neutral s...
Article
Recently, with the ever increasing urban areas in developing countries, the problem of air pollution due to vehicular exhaust arouses the concern of different groups of people. Understanding how different factors, such as urban morphology, meteorological conditions and human activities, affect the characteristics of street canyon ventilation, pollu...
Article
Extensive human activities (e.g. increased traffic emissions) emit a wide range of pollutants resulting in poor urban area air quality. Unlike open, flat and homogenous rural terrain, urban surface is complicated by the presence of buildings, obstacles and narrow streets. The irregular urban surfaces thus form a random roughness that further modifi...
Article
Human activities (e.g. vehicular emission) are the primary pollutant sources affecting the health and living quality of stakeholders in modern compact cities. Gaussian plume dispersion model is commonly used for pollutant distribution estimate that works well over rural areas with flat terrain. However, its major parameters, dispersion coefficients...
Article
Ventilation and pollutant transport in urban areas are major research problems nowadays due to the large number of urban inhabitants. However, our understanding of the street-level transport processes is rather limited because of the highly complicated urban morphology. This paper is thus conceived, using both laboratory wind tunnel measurements an...
Article
Flow over transverse ribs is a fundamental problem that has numerous applications in a range of scales from turbine cooling to urban roughness. It can be broadly divided into k-type or d-type flows exhibiting different characteristics. In this study, large-eddy simulation (LES) is used to examine the flows between two ribs at various separation and...
Article
The Gaussian model of plume dispersion is commonly used for pollutant concentration estimates. However, its major parameters, dispersion coefficients, barely account for terrain configuration and surface roughness. Large-scale roughness elements (e.g. buildings in urban areas) can substantially modify the ground features together with the pollutant...
Article
Full-text available
Engineered bacteria in which motility is reduced by local cell density generate periodic stripes of high and low density when spotted on agar plates. We study theoretically the origin and mechanism of this process in a kinetic model that includes growth and density-suppressed motility of the cells. The spreading of a region of immotile cells into a...
Article
Dispersion of chemically reactive pollutants in the urban canopy layer (UCL) over twelve idealised two-dimensional (2D) street canyons of unity aspect ratio in isothermal conditions is examined. The reversible NO<sub align="right"> x </sub>-O<sub align="right"> 3 </sub> mechanism is integrated into a large-eddy simulation (LES) model. The ground-le...
Article
A series of two-dimensional (2D) street canyon models with a wide range of building-height-to-street-width (aspect) ratios are employed in this study to elucidate the pollutant transport over idealised urban areas. The large-eddy simulation (LES) is used to resolve the turbulent flows and pollutant transport in the urban boundary layer (UBL) over t...
Article
In view of the large number of urban inhabitants, ventilation and pollutant transport in urban areas are major research problems for several decades. This paper focuses on the studies of the correlation between the ventilation and pollutant removal over idealised two-dimensional urban street canyons using wind tunnel experiments in the Department o...
Article
Full-text available
Obstructive Sleep Apnea Syndrome (OSAS) is a common sleep disorder. It is characterized by repeated occlusion of upper airway and discontinuation of sleep. The breathing pauses and starts again with a loud snort. There may even be an abrupt interruption of sleep to maintain the patency of the airway. The pressure drop along the pharyngeal pathway s...
Article
Full-text available
Periodic stripe patterns are ubiquitous in living organisms, yet the underlying developmental processes are complex and difficult to disentangle. We describe a synthetic genetic circuit that couples cell density and motility. This system enabled programmed Escherichia coli cells to form periodic stripes of high and low cell densities sequentially a...
Article
The two-dimensional (2D) idealized street canyon, which is the generic unit of a city, is the platform for our fundamental understanding of ventilation and pollutant removal at the neighborhood scale. The building-height-to-street-width aspect ratio h/b is the key geometric parameters affecting the flow structures in a street canyon. In this study,...
Article
Human activities in urban areas is one of the major sources of anthropogenic releases in the atmospheric boundary layer (ABL). The mechanism of urban morphology for the heat and mass transfer in built environment is thus an attractive topic in the research community. In this paper, a series of laboratory measurements is conducted to elucidate the m...
Article
Full-text available
Obstructive Sleep Apnea Syndrome (OSAS) is a common sleep disorder. It is characterized by repeated occlusion of upper airway and discontinuation of sleep. The breathing pauses and starts again with a loud snort. There may even be an abrupt interruption of sleep to maintain the patency of the airway. The pressure drop along the pharyngeal pathway s...
Article
Full-text available
A large-eddy simulation (LES) model, using the one-equation subgrid-scale (SGS) parametrization, was developed to study the flow and pollutant transport in and above urban street canyons. Three identical two-dimensional (2D) street canyons of unity aspect ratio, each consisting of a ground-level area source of constant pollutant concentration, are...
Article
This paper reviews the interrelationship among boundary layer, urbanization, and evolution of ozone, with particular emphasis on how boundary layer dynamics and urbanization affects the evolution of ozone under different meteorological and climatological conditions. The aim of this work is not only to provide an exhaustive investigation of individu...
Article
Natural ventilation, which is in line with the concepts of sustainability and green energy, is widely acknowledged nowadays. Prevailing winds in urban areas are unavoidably modified by the increasing number of closely placed high-rise buildings that significantly modify the natural ventilation behaviour. This paper explores the effects of building...
Article
Five sets of large-eddy simulations (LES) were performed to examine the characteristics of flows and pollutant dispersion in two-dimensional (2D) urban street canyons of unity building-height-to-street-width ratio in neutral, unstable, and stable thermal stratifications. The characteristic flows fall into the skimming flow regime for all the cases...
Article
Vegetation has been well recognized for its ability in improving air quality and alleviating global warming. Tree planting has been used to mitigate urban heat island phenomena, sequester carbon dioxide, and help to trap air pollutants on leaves. Because some plant species emit biogenic volatile organic compounds (BVOCs), planting of some cultivars...
Conference Paper
Urbanization modifies the bottom of the atmospheric boundary layer (ABL) leading to elevated air pollutant concentrations. Over 50% of the world population lives in cities nowadays (United Nations, 2008). Urban air quality is thus a problem of major concern.
Article
Unlike pollutant transport over flat terrain, the mechanism and plume dispersion over urban areas is not well known. This study is therefore conceived to examine how urban morphology modifies the pollutant transport over urban areas. The computational domain and boundary condition used in this study is shown in Figure 1. The LES shows that inside t...
Article
Full-text available
Our study employed large-eddy simulation (LES) based on a one-equation subgrid-scale model to investigate the flow field and pollutant dispersion characteristics inside urban street canyons. Unstable thermal stratification was produced by heating the ground of the street canyon. Using the Boussinesq approximation, thermal buoyancy forces were taken...
Article
In the present study, the interaction effects of different sulfur contents, relative humidities, driving modes and vehicular exhaust tailpipe exit conditions on the three-dimensional exhaust gas-to-nanoparticle conversion and concentration distribution in the wake region of a typical bus-shaped vehicle in urban road microenvironments were comprehen...
Article
Turbulence in the wake generated by wind flow over buildings or obstacles may produce complex flow patterns in downstream areas. Examples include the recirculating flow and wind deficit areas behind an airport terminal building and their potential impacts on the aircraft landing on nearby runways. A computational fluid dynamics (CFD) simulation of...
Article
To investigate the detailed momentum and pollutant transports between urban street canyons and the shear layer, a large-eddy simulation (LES) model was developed to calculate the flow and pollutant dispersion in isothermal conditions. The computational domain consisted of three identical two-dimensional (2D) idealized street canyons of unity aspect...
Article
Anthropogenic emissions are the major sources of air pollutants in urban areas. To improve the air quality in dense and mega cities, a simple but reliable prediction method is necessary. In the last five decades, the Gaussian pollutant plume model has been widely used for the estimation of air pollutant distribution in the atmospheric boundary laye...
Article
This study investigates the urban heat island characteristics of four major areas of Hong Kong. The areas of study include a densely populated and well-developed commercial area (i.e., Tsim Sha Tsui) and three suburban areas (i.e., Cheung Chau, Lau Fau Shan and Sha Tin) with differing degrees of development. The weather station data of respective a...
Article
The objective of this study is to simulate the characteristic role of building aspect ratio (AR) and wind speed on air temperatures during different street canyon heating situations. A two-dimensional Renormalization Group (RNG) k–ɛ turbulence model is employed to solve the Reynolds-averaged Navier–Stokes (RANS) and energy transport equations. A co...
Article
Full-text available
This article develops a parallel large-eddy simulation (LES) with a one-equation subgrid-scale (SGS) model based on the Galerkin finite element method and three-dimensional (3D) brick elements. The governing filtered Navier–Stokes equations were solved by a second-order accurate fractional-step method, which decomposed the implicit velocity–pressur...
Article
The objective of this paper is to evaluate the reliability of urban heat island intensity (UHII) as an indicator of urban heating. The diurnal patterns of air and surface-temperature based UHII and variations in urban and rural area heating were analyzed and discussed. The detailed air-temperature based UHII patterns were determined in one urban an...
Article
Full-text available
Targeted gene repair directed by single-stranded oligodeoxynucleotides (ssODNs) offers a promising tool for biotechnology and gene therapy. However, the methodology is currently limited by its low frequency of repair events, variability, and low viability of "corrected" cells. In this study, we showed that during ssODN-mediated gene repair reaction...
Article
The ventilation and pollutant transport in a two-dimensional (21)) street canyon of building-height-to-street-width (aspect) ratio h/b = 1 under different unstable stratifications were examined. To characterize the combined wind-buoyancy-driven flow and pollutant transport at different Richardson number Ri, a computational fluid dynamics (CFD) mode...