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The configuration of an urban street canyon plays an important role in determining local microclimate including pedestrian level wind environment (PLWE). The semi-closed U-type street canyon has been widely used in high-density trans-oriented-development urban design for providing private space. However, a quantitative evaluation on PLWE of the U-type street canyon is still absent to date. This study quantitatively assesses PLWE of the U-type canyon via particle image velocimetry tests in a wind tunnel. A range of canyon aspect ratios (the ratio of the canyon height to width) and length ratios (the ratio of the canyon length to height) were investigated. The wind flow inside and in the vicinity of the street canyons was measured and analysed under the perpendicular, oblique and parallel approaching wind directions. It was found that the U-type canyon shows a higher PLWE inside canyon on the open side while a lower PLWE inside canyon on the closed side. Compared to parallel canyon, there is a significant enhancement of PLWE in the centre of the U-type canyon and in some spots on the leeward side of the canyon. This is further strengthened with increasing the canyon aspect ratio and length ratio. Nevertheless, the U-type canyon exhibits a lower wind speed at the pedestrian level than that of the parallel canyon both inside and in the vicinity of the street canyons in general especially under parallel wind direction. It is believed that these findings are greatly beneficial to build sustainable cities with a high quality PLWE.

Large eddy simulation (LES) is increasingly recognized as an important approach in the assessment of pedestrian level wind (PLW) conditions, while its numerical performance is sensitive to a few key parameter settings. This study aims to further identify the sensitivities to these main influencing parameters in modeling wind flow around a building array using LES. This is achieved by quantitatively comparing mean wind velocities from the LES predictions and those from a benchmark wind tunnel experiment. The parameters investigated are the mesh resolution, discretization time step and sampling period, vortices’ number of the inlet flow, the upstream distance of the computational domain, and the sub-grid scale (SGS) models. Results are quantified using four validation indices. Specifically, the correlation coefficient R between the predicted results using SGS model with the dynamic Smagorinsky-Lilly method and the experimental results is 0.89, which is higher than those between the experiment results and other three SGS models. Based on the sensitivity tests, it is recommended that the normalized discretization time step be set below 0.09, and that the normalized sampling period and vortices’ number be above 219 and 190, respectively, in the LES simulation of PLW flows around a building array.

The large-scale practical application of Large-Eddy Simulation (LES) for predicting long-term wind flow and pollutant dispersion in urban areas is inhibited mainly by the associated very large computational costs. To overcome this difficulty, the present study, for the first time, applies transport-based recurrence Computational Fluid Dynamics (rCFD) to simulate atmospheric pollutant dispersion around a building. A novel diffusion model is proposed to accurately predict pollutant transport with rCFD. To illustrate the feasibility and advantages of rCFD, pollutant dispersion around an isolated cubical building with a rooftop vent, immersed in neutral atmospheric boundary layer flow is used as a case study and both LES and rCFD simulations are conducted. It is shown that rCFD simulation results agree well with those from LES both in terms of mean and fluctuating concentrations while the simulation wall-clock time drops from 222 h to 16 min. The application of four evaluation metrics (FAC2, FB, NMSE and R) indicates very good agreement between LES and rCFD results. Another major advantage of rCFD is that different pollutant events can be simulated promptly once the database has been stored for a given flow configuration, as shown by the comparison of LES and rCFD results for two other cases with different release locations. This study extends the application of transport-based rCFD to pollutant dispersion in the built environment and indicates that rCFD is a promising approach to facilitate the large-scale practical application of LES for this type of applications.

Highlights • A general design framework is developed for improving low pedestrian level wind environment. • The improvement measures and evaluation criteria in current studies are reviewed and summarized. • A university campus is used as case study to demonstrate the design framework. • The outcomes of this paper can assist policy-makers to establish sustainable urban planning policies., An acceptable pedestrian level wind environment is essential to maintain an enjoyable outdoor space for city residents. Low wind velocity environment can lead to uncomfortable outdoor thermal experience in hot and humid summer, and it is unable to remove the pollutants out of city canyons. However, the average wind velocity at pedestrian level is significantly lowered by closely spaced tall buildings in modern megacities. To improve the low wind velocity environment at pedestrian level in high-density cities, a general framework and detailed guidelines are needed. This study is the first time to develop such a framework, and provide detailed guidelines for improving pedestrian level low wind velocity environment in high-density cities. Additionally, a detailed review and summarisation of evaluation criteria and improvement measures are presented in this paper, which provide additional options for urban planners. To investigate the performance of the framework, the Hong Kong Polytechnic University campus was utilised as a case study. Results showed that pedestrian level wind comfort was greatly improved with the developed framework. The outcomes of this study can assist city planners to improve the low wind velocity environment, and can help policy makers to establish sustainable urban planning policies.

Pedestrian level wind environment is affected by stagnated airflow in high-density cities. This study provides an understanding of the effects of building height and porosity size on pedestrian level wind comfort. The computational fluid dynamics (CFD) technique is utilized to reproduce wind flow around buildings at pedestrian level, and new wind comfort criteria for a low wind environment are adopted to evaluate wind comfort. More specifically, the Steady Reynolds Averaged Navier–Stokes (RANS) renormalization group (RNG) k–e turbulence model is employed in this study, and the accuracy of the simulation results are assured by validation against the wind tunnel test data. The influence of different building heights and porosity sizes on wind comfort around an isolated building and a group of buildings are subsequently examined. It is shown that an increase building height could improve wind comfort inside the site boundary for both the isolated building and group of buildings. Furthermore, the wind comfort benefits increased when porosity is on the first floor compared to when it is on the second floor. Moreover, larger porosity size generally results in better wind comfort than smaller porosity size. From a practical view point, this study provides information for city planners and architects to use in the improvement of pedestrian level wind comfort, without losing land use efficacy.

Quality and efficiency of computational fluid dynamics (CFD) simulation of pedestrian level wind environment in a complex urban area are often compromised by many influencing factors, particularly mesh quality. This paper first proposes a systematic and efficient mesh generation method and then performs detailed sensitivity analysis of some important computational parameters. The geometrically complex Hong Kong Polytechnic University (HKPolyU) campus is taken as a case study. Based on the high-quality mesh system, the influences of three important computational parameters, namely, turbulence model, near-wall mesh density and computational domain size, on the CFD predicted results of pedestrian level wind environment are quantitatively evaluated. Validation of CFD models is conducted against wind tunnel experimental data, where a good agreement is achieved. It is found that the proposed mesh generation method can effectively provide a high-quality and high-resolution structural grid for CFD simulation of wind environment in a complex urban area.

The lift-up building design has been demonstrated to provide favorable wind comfort, but there is a lack of investigation on optimum wind comfort condition. This study coupled computational fluid dynamics (CFD) technique and response surface methodology (RSM) to determine the most desirable wind comfort around an isolated building with lift-up design. A multi-variable optimization method is proposed to determine optimum wind comfort and the corresponding lift-up design variables, namely, lift-up height ( H L ), core aspect ratio ( A R ) and core number ( N ). To better illustrate wind comfort around the building, the wind comfort in the lift-up area and the podium area are investigated separately. The Detached Eddy Simulation (DES) approach is employed throughout the whole CFD simulation process. The quality and goodness of the established RSM models are examined by analysis of variance and genetic algorithm is applied to generate optimal design solution. The generated results illustrate good performance of the established RSM model. Results show that the optimum wind comfort is obtained when H L is 8 m, A R is 10%, and N is 6. The lift-up core aspect ratio is subsequently found to have greatest effect on wind comfort among the three design variables in both the lift-up area and the podium area. In addition, the proposed method is applicable to other similar environmental design conditions and the outcomes of study can also be of great value in the improvement of wind comfort in compact urban cities.

City residents wish to have pleasant experience in outdoor space, which are often impeded by thermally uncomfortable conditions, particularly in hot and humid summer. Lift-up design can provide comfortable microclimate in summer but the effects of lift-up design on thermal comfort in a built-up environment have not been systematically studied. This paper aims to investigate the effects of lift-up design on outdoor thermal comfort comprehensively, as well as the effects on pedestrian level wind environment. The thermal comfort assessments are carried out by using a proposed integrated method, which combines wind tunnel tests and on-site monitoring to calculate Physiologically Equivalent Temperature (PET) values. The Hong Kong Polytechnic University (HKPolyU) campus is selected as study area. The investigation mainly focuses on summer and winter seasons. Four typical days in a year were chosen to carry out on-site monitoring for obtaining environmental parameters. This study demonstrates that the proposed integrated method can be used to predict outdoor thermal comfort. Results also show that lift-up design can effectively improve pedestrian level wind environment and thermal comfort. Moreover, lift-up design can provide a comfortable microclimate in summer while not cause strong cold stress in winter. These findings provide solid evidence bases to city planners and architects of available options for creating pleasant outdoor microclimate in precinct planning.

The pedestrian level wind environment is seriously deteriorated by moderated local wind flow in a densely built-up subtropical city like Hong Kong. In order to improve the weak wind condition, the lift-up design has been used for some time. However, there is a lack of understanding and quantitative assessment of its modification on the pedestrian level wind comfort around different building configurations under different wind directions. This paper aims to study the effects of lift-up design in four common building configurations on the wind comfort via computational fluid dynamics (CFD) simulations. The turbulence model and numerical method are firstly validated by comparing the simulated wind flow data with the wind tunnel test results. The validated model is then utilized to simulate the four building configurations, including the “─”, “L”, “U” and “□” shaped buildings. The mean wind velocity ratio ( MVR ) and mean wind velocity change ratio ( Δ MVR ) are employed to identify the wind comfort and to quantitatively evaluate the improvements due to the lift-up design. Results show that the lift-up design can improve the wind comfort in building surroundings and its influence is highly dependent on the incident wind direction. Specifically, the wind comfort is better under the oblique wind direction than the other two wind directions. These findings can provide us a better understanding of the lift-up design and will be helpful in better precinct planning.

Traffic pollution has posed a serious threat to the health of near-road city residents and pedestrians, especially in high-density cities. However, the influences of envelope features, including balconies, overhangs and wing walls, on pollutant exposure to near road residents and pedestrians have not been fully understood. This paper investigates the effects of three commonly-used envelope features on wind flow and pollutant exposure to residents in street canyon with three different aspect ratios. The evaluation metrics of personal intake fraction and daily pollutant exposure are used to quantitatively assess the influences caused by different envelope features on healthy risk of near-road residents and pedestrians, alongside with wind flow pattern and pollutant distribution. The results show that these envelope features have increased the risk of pollutant exposure for the leeward side residents, while the risk of pollutant exposure for the windward side residents is reduced for most cases, in particular for the first floor. This observation is especially prominent when the canyon has the highest aspect ratio among the tested ratios, with the increased ratio of personal intake fraction reaching up to 540%. Moreover, the pollutant concentration is overall higher on leeward side of upstream building than that of windward side of downstream building. These findings can help urban planners and architects to build healthy and sustainable urban environment.

Improvements for the pedestrian level wind environment and outdoor thermal comfort have become increasingly important in urban planning in light of concerns about global warming and urban heat island effects. Therefore, the goal of this study is to determine the optimum wind environment and outdoor thermal comfort for an ideal urban canyon in which the buildings have lift-up designs. A multi-stage optimization method is proposed consisting of three stages for the optimization process, e.g., surrogate model development, multi-objective optimization, and decision-making. An area weighted wind velocity parameter ( M V R ¯ ) and an outdoor thermal comfort parameter ( P E T ¯ ) are chosen as the design objectives, and four design variables are selected. The response surface methodology combining computational fluid dynamics simulation results are used to fit surrogate models. The non-dominated sorting genetic algorithm is employed to find Pareto optimal solutions, and three decision-making strategies are adopted to determine the final optimum design solution in parallel. The optimization process of the ideal urban canyon confirms that the proposed method is highly effective to determine optimum building design in urban areas. The findings in this study are valuable for city-planners and policy-makers to build a sustainable urban living environment.