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Studying the mechanism of pollutant transport around an isolated building is essential to address the environmental issue of air pollution around buildings and to understand the intricate process of pollutant dispersion in urban environments. In this paper, an effective numerical approach is used to study the effects of building heights and widths on airflow and pollutant dispersion around an isolated building. The source of pollutants is behind the building. The pollutant is confined to the lower areas as a result of small vortices behind the low-rise buildings, and transported to the wake area. Whereas high-rise buildings make the pollutant move to an upper region. At the pedestrian level, the wake area of the lower building stores more pollution. Therefore, the environment around the low-rise buildings may be more hazardous. The concentration of pollutants is greater at higher altitudes around wider buildings. Pollutants are transferred to the sides of wider buildings at the pedestrian level, leading to lower concentrations in the wake-central area. A wider building poses a greater lateral dangerous-area, while a narrower building results in a longer central wake area that is hazardous. [ABSTRACT FROM AUTHOR]

The ventilation of nine neutral boundary layers over three representative urban geometries (in-line, staggered, and long-canyon building layouts) under three different geostrophic wind conditions was investigated using large-eddy simulations. Pollutants were injected into a well-developed boundary layer, and the replacement of pollutants with fresh air was simulated in the nine cases. In all cases, the domain-averaged concentration first decreased linearly, following the overall flow, and then decreased slowly owing to the trapping and intermittent drainage of canopy-layer pollutants. During drainage, downstream in-canopy pollutant concentrations were the highest in the long-canyon layout owing to more building-induced vertical recirculations, more isolated flows, and more trapping of pollutants. In contrast, the in-line layout had the lowest downstream in-canopy pollutant concentrations. Pedestrian-level ventilation was investigated by analyzing the changes in 2-m pollutant concentration over normalized time, considering the overall flow and building-induced turbulent friction together. The time series of the pedestrian-level pollutant concentrations in the nine cases were divided into three types based on the building layout rather than geostrophic wind speed. Additionally, the normalized ventilation time, that is, the normalized time for the initial concentration to decrease to 1/ e , was calculated and compared to the simple ratios of the 2-m wind speed and geostrophic wind speed. While the velocity ratios and non-normalized ratios were grouped by geostrophic wind speed, the normalized ventilation times were grouped into three groups according to the building layout, showing that differences in geometry (e.g., long-canyon to in-line) can decrease ventilation time by more than 9.7%. [Display omitted] • Escaping pollutants from urban boundary layers are simulated using an LES model. • Initial ventilation depends on the overall driving flow. • Later ventilation in the canopy layer depends on the building layout. • Ventilation indicators, considering canopy friction and external flow, are needed. • Normalized ventilation time changes by ∼ 10% from long-canyon to in-line geometries. [ABSTRACT FROM AUTHOR]

In this study we introduce a novel extension of an existing Lagrangian particle dispersion model for application over urban areas by explicitly taking into account the urban canopy layer. As commonly done, the original model uses the zero-plane displacement as a lower boundary condition, while the extension reaches to the ground. To achieve this, spatially-averaged parametrizations of flow and turbulence characteristics are created by fitting functions to observational and numerical data. The extended model is verified with respect to basic model assumptions (well-mixed condition) and its behaviour is investigated for unstable/neutral/stable atmospheric stabilities. A sensitivity study shows that the newly introduced model parameters characterizing the canopy turbulence impact the model output less than previously existing model parameters. Comparing concentration predictions to the Basel Urban Boundary Layer Experiment—where concentrations were measured near roof level—shows that the modified model performs slightly better than the original model. More importantly, the extended model can also be used to explicitly treat surface sources (traffic) and assess concentrations within the urban canopy and near the surface (pedestrian level). The small improvement with respect to roof level concentrations suggests that the parametrized canopy profiles for flow and turbulence characteristics realistically represent the dispersion environment on average. [ABSTRACT FROM AUTHOR]

The goal of the study is to identify characteristic features of multilevel public spaces (MPS’s) and perspectives of their development, based on complex analysis of their formation in the late 20thto early 21st centuries.Authors were able to identify architectural planning factors of MPS formation and functioning in North America and Asia by summarizing the experience in creation of multilevel public spaces by architectural planning means.

The geometric dependency of the wind environment at a pedestrian level is an important issue that influences human comfort and safety in urban outdoor spaces. As such, this paper proposes to investigate the statistical features of wind speeds at the pedestrian level by calculating wind speed probability density functions based on flow field data from large-eddy simulations of simplified urban arrays, aiming to clarify the effects of urban geometry on rare velocity events such as strong gusts or extremely weak air flow. Though strong wind events occur infrequently, a positive correlation was demonstrated between percentile and mean wind speeds, indicating that the risk of gusty events increases with the increase of mean wind speeds. Conversely, the frequency of weak wind events shows an inverse correlation with mean wind speeds, showing that better ventilated urban arrays will retain higher wind speeds. Furthermore, these percentiles and occurrence frequencies are clearly expressed by the frontal area indices of urban block arrays. These results imply a trade-off between the following two objectives for urban area wind environments characterized by the urban geometry: enhancing air ventilation in urban areas and preventing strong wind gust events at a pedestrian level. [ABSTRACT FROM AUTHOR]

With continuous urbanisation, an increasing number of post-construction facilities (such as high-rise buildings and green plants) have altered the original layout of the community. Which has resulted in deterioration of the wind environment. However, there is a deficiency of qualitative studies on the pedestrian-height wind environment that focus on the influence of post-construction facilities. The presents research takes Changsha universal time community as a case study. A wind tunnel test at a scale of 1:250 was performed to analyse the wind environment by considering the post-construction facilities. 81 sets of cases with different inflow patterns, wind directions, adjacent high-rise buildings, and plants were examined to reveal the flow field distribution characteristics. Furthermore, the exceedance probability method was introduced to assess the wind environment of the community in different cases. The results indicate that the index distribution coefficient of wind velocity and the turbulence intensity profile inside the practical community is significantly larger than the code values. A roughness distance of 800 m can generate highly effective mean wind profiles. However, it is unable to generate the desired turbulence profiles. Considering the post-construction facilities, high-rise buildings cause acceleration effects on the average wind velocity and the acceleration effects are inversely proportional to the distance of the post-construction facilities. Large, medium, and small trees cause deceleration effects of the average wind speed and improve the wind environment comfort quality of the community. • The impact of post-construction high-rise buildings on the wind environment is revealed. • The influence of plants on the pedestrian-level wind environment (PLWE) is quantitatively obtained. • The comfort degree of the post-construction facilities in the PLWE is quantitatively revealed. [ABSTRACT FROM AUTHOR]

Now little attention has been paid to using field measurements to explore the coupling relationships between urban forms and the outdoor environment in China. A cold northern city of China was chosen as the research city. The coupling relationships of 30 points between the urban morphology and Outdoor Environment Performance (OEP) at the pedestrian level were analyzed at different scales. Then five different function blocks of education, commerce, industry, residence and parks were selected as the ENVI-met simulation models. The results indicate that an area of 500 m range represents the suitable range to analyze the OEP, block size, building density, building height and green space are closely related. The simulation results indicate that air temperature, relative humidity and wind environment differ in land-use categories and urban forms. Urban forms significantly affect human health and comfort at the pedestrian level. This study proposes appropriate urban arrangements and building layouts to lessen the effects of UHIs in various land-use categories. This study contributes to architectural design and urban planning at the 500 m range of the city block. It provides a scientific basis for formulating construction planning principles and revising urban construction codes. • The correlation of urban form with OEP at a pedestrian level was analyzed. • A hilly coastal city in the cold region of northern China was detailed studied. • Field measurements of 30 different positions were carried out in Dalian. • The urban form within a range of 500 m strongly affects the thermal environment. • Appropriate urban morphologies for different categories of land were suggested. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to study the wind environment of parallel courtyard in campus which is affected by the yard aspect ratio and wind directions. Research done so far mainly focused on the thermal comfort of courtyards and wind environment at pedestrian level in urban. This article aims to study the wind environment of courtyards under aspect ratios via the high resolution RANS CFD simulation method. Two performance indicators are identified: 1) wind amplified velocity 2) age of air. The evaluation is based on the validation with field measurement. The results show that for single parallel courtyard, the best aspect ratio for air quality and ventilation locates in the range of 1–2. Furthermore, for double parallel combined courtyard, 0° and 15° is the most recommended approaching wind angle for ventilation. Contrarily, 75°–90° are the worst inlet airflow direction for courtyard flow field because of insufficient abounding wind amplified area. Apart from this, only in the range of 0°–30°, it is possible to enhance the inlet courtyard air quality in double combined yard shape compared to single courtyard. [ABSTRACT FROM AUTHOR]