Chun-Ho Liu

A review on the generation, determination and mitigation of Urban Heat Island

Urban Heat Island (UHI) is considered as one of the major problems in the 21st century posed to human beings as a result of urbanization and industrialization of human civilization. The large amount of heat generated from urban structures, as they consume and re-radiate solar radiations, and from the anthropogenic heat sources are the main causes of UHI. The two heat sources increase the temperatures of an urban area as compared to its surroundings, which is known as Urban Heat Island Intensity (UHII). The problem is even worse in cities or metropolises with large population and extensive economic activities. The estimated three billion people living in the urban areas in the world are directly exposed to the problem, which will be increased significantly in the near future. Due to the severity of the problem, vast research effort has been dedicated and a wide range of literature is available for the subject. The literature available in this area includes the latest research approaches, concepts, methodologies, latest investigation tools and mitigation measures. This study was carried out to review and summarize this research area through an investigation of the most important feature of UHI. It was concluded that the heat re-radiated by the urban structures plays the most important role which should be investigated in details to study urban heating especially the UHI. It was also concluded that the future research should be focused on design and planning parameters for reducing the effects of urban heat island and ultimately living in a better environment.

Large-Eddy Simulation of Flow and Scalar Transport in a Modeled Street Canyon

This study uses large-eddy simulation (LES) to illustrate the flow and turbulence structure and to investigate the mechanism of passive scalar transport in a street canyon. Calculations for a modeled street canyon with building-height-to-street-width ratio of unity at Reynolds number equal to 12 000 are conducted. When the approaching wind is perpendicular to the street axis, the calculation produces a primary vortex in the street canyon, similar to previous studies. An evaluation of the LES results with wind-tunnel measurements reveals good agreement for both mean and turbulence parameters of the flow and scalar fields. The computed primary vortex is confined to the street canyon and is isolated from the free stream flow such that the removal of a scalar emitted at the street level is accomplished by turbulent diffusion at the roof level. It is determined from the calculations that very little scalar is removed from the street canyon, and 97% of the scalar is retained. The scalar mixing at the roof level occurs primarily on the leeward side of the street canyon. In addition to the primary vortex, three secondary vortices are located in the corners of the street canyon at which scalar mixing is enhanced. An examination of additional simulations shows how the location of the scalar source affects the distribution of the scalar.

Large-Eddy Simulation of Flow and Pollutant Transport in Street Canyons of Different Building-Height-to-Street-Width Ratios

This study employs a large-eddy simulation technique to investigate the flow, turbulence structure, and pollutant transport in street canyons of building-height-to-street-width (aspect) ratios of 0.5, 1.0, and 2.0 at a Reynolds number of 12 000 and a Schmidt number of 0.72. When the approaching wind is perpendicular to the street axis, a single primary recirculation is calculated for the street canyons of aspect ratios 0.5 and 1.0, and two vertically aligned, counterrotating primary recirculations are found for the street canyon of aspect ratio 2.0. Two to three secondary recirculations are also calculated at the corners of the street canyons. A ground-level passive pollutant line source is used to simulate vehicular emission. The turbulence intensities, pollutant concentration variance, and pollutant fluxes are analyzed to show that the pollutant removal by turbulent transport occurs at the leeward roof level for all aspect ratios. Whereas the ground-level pollutant concentration is greatest at the leeward corner of the street canyons of aspect ratios 0.5 and 1.0, the ground-level pollutant concentration in a street canyon of aspect ratio 2.0 occurs at the windward corner and is greater than the peak concentrations of the other two cases. Because of the smaller ground-level wind speed and the domination of turbulent pollutant transport between the vertically aligned recirculations, the ground-level air quality is poor in street canyons of large aspect ratios. The retention of pollutant in the street canyons is calculated to be 95%, 97%, and 99% for aspect ratios of 0.5, 1.0, and 2.0, respectively.

Physical Modeling of Flow Field inside Urban Street Canyons

Abstract The flow characteristics inside urban street canyons were studied in a laboratory water channel. The approaching flow direction was horizontal and perpendicular to the street axis. The street width was adjusted to form street canyons of aspect ratios 0.5, 1.0, and 2.0. The velocity field and turbulent intensity were measured with a laser Doppler anemometer at various locations within the street canyons, which were used to elucidate the flow pattern inside the street canyons. It was found that the previous numerical modeling results are in good agreement with the current experimental results at most locations. For the street canyon of aspect ratio 0.5, which belongs to the wake interference flow regime, the mean and fluctuating velocity components were more difficult to measure as compared with the other two cases because of its more complicated flow pattern. Some guidelines for numerical modeling were developed based on the measurement results. The data presented in this paper can also be used as...

Effects of Urban Vegetation on Urban Air Quality

Abstract 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 increase ozone and particulate matter ambient concentration and hence deteriorate air quality. Moreover, pollen grains and fungal spores from plants are health hazards for allergic or other sensitive members of the population. This paper reviews benefits and limited hazards of urban vegetation on air quality, highlighting useful computer simulations for predicting some of the interaction between urban forestry and the ambient atmosphere. To maximize the beneficial environmental effects of urban forestry, careful design, planning, and cost–benefit analysis would be required.

On the Mechanism of Air Pollutant Removal in Two-Dimensional Idealized Street Canyons: A Large-Eddy Simulation Approach

Flow resistance, ventilation, and pollutant removal for idealized two-dimensional (2D) street canyons of different building-height to street-width (aspect) ratios

Improved estimators for the standard deviations of horizontal wind fluctuations

AR

Parallel computation of atmospheric pollutant dispersion under unstably stratified atmosphere

are examined using the friction factor