Urban Wind Environment Research Articles

Modelling and optimizing tree planning for urban climate in a subtropical high-density city

Urban trees affect urban climate via three processes: shading effect, evapotranspiration and wind resistance. However, large differences existed in the performance of different tree species and planting patterns. To quantify the effects of trees, we extended a heat-moisture coupled model by adding a new vegetation sub-model. The results showed that tree species with a higher leaf area index (LAI) and larger crown had a better capability of transferring sensible heat to latent heat. Sparsely planted trees provided a better cooling effect than densely planting pattern. The effect of trees was examined in urban areas with different densities. The effect of trees on urban wind environment depends on the building density of urban area. The large drag force posed by a high building density weakened the drag force of trees; thus, the negative effect of trees on urban ventilation was diminished in the high-density urban area. The negative effect of trees was also discussed from the perspective of urban moisture. This study demonstrates the need of strategic tree planting in terms of urban climate and provides insights into the creation of an urban greenery plan.

Development and application of a street-level meteorology and pollutant tracking system (S-TRACK)

Abstract. A multi-model simulation system for street-level circulation and pollutant tracking (S-TRACK) has been developed by integrating the Weather Research and Forecasting (WRF), the STAR-CCM+ (computational fluid dynamics model – CFD), and the Flexible Particle (FLEXPART) models. The winter wind environmental characteristics and the potential contribution of traffic sources to nearby receptor sites in a city district of China are analysed with the system for January 2019. It is found that complex building layouts change the structure of the wind field and thus have an impact on the transport of pollutants. The wind speed inside the building block is lower than the background wind speed due to the dragging effect of dense buildings. Ventilation is better when the dominant airflow is in the same direction as the building layout. Influenced by the building layout, the local circulations show that the windward side of the building is mostly the divergence zone, and the leeward side is mostly the convergence zone, which is more obvious for high buildings. With the hypothesis that the traffic sources are uniformly distributed on each road and with identical traffic intensity, the potential contribution ratios (PCRs) of four traffic sources to certain specific sites under the influence of the street-level circulations are estimated with the method of residence time analysis. It is found that the contribution ratio varies with the height of the receptor site. As a result of the generally upward motion in the airflow, the position with the greatest PCR from the four road traffic sources is located at a certain height which is commonly influenced by the distance of this location from the traffic source and the background wind field (about 15 m in this study). The potential contribution of a road to one of the receptor sites is also investigated under different wind directions. The established system and the results can be used to understand the characteristics of urban wind environment and to help the air pollution control planning in urban areas.

Wind environment assessment and planning of urban natural ventilation corridors using GIS: Shenzhen as a case study

The development of high-density cities has aggravated the harshness of the urban microclimate in face of global warming. However, an adequate urban ventilation system can introduce clean and cool air into high-temperature areas of a city, which can alleviate the urban heat island (UHI) effect and increase residents' thermal comfort. In this study, we devise an integrated air ventilation assessment (IAVA) method to assess the urban wind environment using multi-source data, with Shenzhen as an example. The IAVA map illustrates that developed areas in the south and west of Shenzhen form a “wind wall” that reduces wind speeds and prevents wind flow into inland areas. In light of the results of the IAVA, potential ventilation corridors across Shenzhen are then created using the least cost path method. Subsequently, the optimized ventilation corridors are generated via super-imposition over a satellite image and the identification of functioning and compensating areas. Notably, the IAVA introduces the effects of terrain, vegetation, and open space, thus providing a more pragmatic evaluation of the wind environment than frontal area density results. Furthermore, it provides planning guidance for local governments that considers the quality of the urban wind environment in the low-cost and efficient construction of ecological and livable cities.

Review of Research Advances in CFD Techniques for the Simulation of Urban Wind Environments

Computational fluid dynamics (CFD) has become the main method for the prediction of the properties of the external wind environment in cities and other urban contexts. A review is presented of the existing literature in terms of boundary conditions, building models, computational domains, computational grids, and turbulence models. Some specific issues, such as the accuracy/computational cost ratio and the exploitation of existing empirical correlations, are also examined.

Parameterised drag model for the underlying surface roughness of buildings in urban wind environment simulation

Knowledge of the flow field in the urban canopy layer (UCL) is of great significance in guiding the layout of buildings in blocks and mitigating local air pollution. Large eddy simulation (LES) with relatively high precision can simulate the flow field in the UCL. Considering the limitations of computational power, building resistance source parameterisation is an effective method of solving the average blocking effect of buildings on airflow and reducing the number of grids. Belcher's drag model has been widely used for calculating the average flow field in the UCL. However, the model does not consider the influence of the horizontal recirculation region and the variation of the canopy-drag length scale in the height direction. Therefore, a parameterised drag model for the resistance source of building(s) was improved by considering the horizontal recirculation region and effective roughness density. The simulation results suggest that the horizontal recirculation area of an isolated building is mainly affected by the spanwise width of the building at a certain height and presents a quadratic function distribution in the height direction, with a maximum at approximately z/H = 0.45. The validation of the isolated building and building arrays with different layouts demonstrated that the parameterised drag model could reveal the average flow field more accurately than Belcher's drag model.

Reduced-order modelling of urban wind environment and gaseous pollutants dispersion in an urban-scale street canyon

Hazardous chemical gases will spread rapidly after leakage. For emergency response to that, people need to obtain the information of wind and pollutant in time. However, it takes a lot of time to calculate the flow in large-scale urban areas by numerical simulation. Therefore, reduced-order model(ROM) is developed to improve the efficiency. In this paper, we propose a model based on proper orthogonal decomposition(POD) and radial basis function(RBF) interpolation. We validate the model by calculating the wind field and the pollutant propagation process in a 144 square kilometer area of Beijing. The results show that ROM can reduce CPU times by more than 99% at the cost of only 0.1% information loss, comparing with the traditional approach of computational fluid dynamics(CFD).

Hazardous flight region prediction for a small UAV operated in an urban area using a deep neural network

With an increase of UAVs in logistics and transportation, the safety of UAVs operated in the urban wind environment becomes an important issue. Small UAVs are more sensitive to the wind environment because of their small size, slow cruising speed, and limited endurance. In the unmanned aircraft system traffic management (UTM), a safety risk assessment under bad weather conditions is an important component. In this study, a hazardous flight region prediction system for small UAVs operated in urban areas is developed using a deep neural network (DNN) to support a risk assessment and safe trajectory planning. A large eddy simulation (LES) is applied to reflect the terrain-driven wind environment in the urban area. The result of a weather research and forecasting (WRF) model is used as an initial and boundary condition of the LES to generate a realistic complicated wind environment in an urban area. Furthermore, an iterative nesting algorithm is applied to the LES to obtain a sufficient resolution of the wind environment, which is suitable for the small UAV scale. The deviation distance from the original flight path due to the wind environment is considered as a flight hazard criterion in this study. The proposed system is able to predict deviation distance due to the wind environment over the entire flight space over time by using the DNN model. The training data for the DNN is obtained using the multicopter flight dynamics simulator, which can take into account the influence of a specific wind environment. With the indexes considering this deviation distance and the local topography (distribution of buildings) in the urban area, the hazardous flight region is predicted. The information supplied by the proposed hazardous flight region prediction model can be used for the flight risk assessment and safe flight trajectory planning to increase the flight safety of small UAVs.

Automated Simulation Framework for Urban Wind Environments Based on Aerial Point Clouds and Deep Learning

Computational fluid dynamics (CFD) simulation is a core component of wind engineering assessment for urban planning and architecture. CFD simulations require clean and low-complexity models. Existing modeling methods rely on static data from geographic information systems along with manual efforts. They are extraordinarily time-consuming and have difficulties accurately incorporating the up-to-date information of a target area into the flow model. This paper proposes an automated simulation framework with superior modeling efficiency and accuracy. The framework adopts aerial point clouds and an integrated two-dimensional and three-dimensional (3D) deep learning technique, with four operational modules: data acquisition and preprocessing, point cloud segmentation based on deep learning, geometric 3D reconstruction, and CFD simulation. The advantages of the framework are demonstrated through a case study of a local area in Shenzhen, China.

An Experiment-Based Simplified Method for the Model of Building Groups in CFD Simulation

Computational Fluid Dynamics (CFD) has been widely used in the simulation and analysis of community or urban wind environments. However, the CFD-based wind simulation of large-scale building groups usually consumes a lot of computing resources with high computing costs. To improve the efficiency of CFD-based wind simulation, this paper presents an experiment-based simplified method for the model of building groups. Two rectangular buildings are adopted as the basic unit and four control parameters (B/L, W/L, H/L, and D/L) are selected as the experiment factors to analyze the geometrical relationship of the two buildings. Note that L, W, and H, respectively, represent the windward edge length, width, and height of a building, B is building interval distance, and D is the distance between two building center axes. Then, a single factor experiment and an orthogonal experiment are designed and performed to determine the reasonable value range of each factor. Based on the experiment results and actual situation, the value ranges of four factors for the simplification of building group models are determined as follows: B/L∈{0, 1.5}, W/L∈{0, 2}, D/L∈{0, 0.25}, and H/L∈{0, 1}. Furthermore, a real case is presented to evaluate the performance of the proposed simplified method. The results indicate that the simplified method is able to improve the efficiency of CFD-based wind simulation of building groups, with the number of buildings decreasing from 620 to 395 (by 36.3%), and the number of tetrahedral grids decreasing from 8,832,199 to 7,766,778 (by 12.1%). Thus, this research contributes to the CFD-based wind simulation method of large-scale building groups and the analysis of the urban wind environment.

A Case Study on Urban Ventilation Assessment with Local Climate Zone (LCZ) Parameters

The local climate zone (LCZ) partitions are widely used in urban climate investigations. Parameters characterizing LCZ types could be used in investigations about urban heat island (UHI) intensity and air ventilation modelling and assessment. In the current work, modelling methods of UHI intensity and wind environment are investigated based on the LCZ parameters. A drag source term with a porous media model of the urban canopy layer was proposed to simulate the urban scale wind environment with the CFD method. Vertical and horizontal airflow in the urban canopy layer was analysed. The upward and downward wind was found in the urban canopy model influenced by the variations of building height and density. On the other hand, the heat balance models could also be conducted on LCZ. Results showed the potential capabilities in urban wind assessment. A summary of models and the requirement on LCZ parameters were proposed.

An Investigation of the Quantitative Correlation between Urban Spatial Morphology Indicators and Block Wind Environment

The research purpose of this work is guiding the spatial morphological design of blocks via relevant indicators to realize suitable wind environments. In doing so, it is necessary to find the most suitable indicator types and value ranges for each urban spatial morphology. At present, most of the relevant research has been based on the numerical simulation of ideal block shapes and rarely proposes results based on actual block types, which often tend to be complex environments. Therefore, this paper firstly presents a theoretical speculation on the main factors influencing indicator effectiveness via analyzing physical significance and formulating principles for each indicator. These speculations are verified via wind environment measurement and statistical analysis, indicating that porosity (P0) can be used as an important indicator to guide the design of block wind environments in the case of deep street canyons, while frontal area density (λF) can be used as a supplement in shallow street canyons with no height differences. Finally, computational fluid dynamics (CFD) is used to quantify the impact of block height difference and street canyon depth on λF and P0, thereby finding suitable types of urban form and value ranges for λF and P0. This paper provides a feasible wind environment index system for urban designers.

A Novel Canopy Drag Coefficient Model for Analyzing Urban Wind Environments Based on the Large Eddy Simulation

It is very challenging to capture the drag effects for the computational fluid dynamics numerical simulations of the urban canopy wind environment. This study proposed a novel canopy drag coefficient model for accurate analysis of the urban wind environment based on a large eddy simulation, where the drag coefficients varied with quantitatively identified canopy parameters along with the height. Four computational parameters, namely the average kinetic energy, turbulent kinetic energy, sub-grid scale turbulent kinetic energy, and sub-grid scale dissipation, were incorporated into the conventional drag coefficient. The Meixi Lake International Community in Changsha, China, was considered as a case study. The inlet boundary conditions were provided by the Weather Research and Forecasting model, and the proposed drag coefficient model was utilized to simulate the wind field characteristics. The results showed that the drag coefficient was relatively large near the ground, and it decreased with the increase of height overall. The decay rate of the drag coefficient below 0.4 times the building was significantly higher than the other areas. Finally, compared with the field measurement data, the proposed model had good accuracy of the simulated wind field compared to previous approaches, thus offering a reliable model for analyzing the urban wind environment.

Mapping the urban natural ventilation potential by hydrological simulation

Urban wind environments are closely related to air pollution and outdoor human comfort. The urban natural ventilation potential (NVP) is an important factor in urban planning and design. However, for ventilation studies on urban scales, neither macroscale numerical simulations (i.e., WRF, MM5, etc.) nor microscale computational fluid dynamics (CFD) simulations can conduct efficient analyses. Based on the similarity between water flows and airflows, an efficient approach is proposed in this paper to map the urban NVP. Through integrating the urban terrain model, urban form model, and prevailing wind pressure model, an airflow digital elevation model (AF-DEM), which represents the resistance to airflow and can be used for a hydrological simulation, is generated and applied to evaluate the urban airflow patterns under different terrain, urban form and ambient wind conditions. The objective was to develop a simulation platform that can efficiently predict the distribution of natural ventilation corridor and NVP. The stream network calculated through the simulation is regarded as potential ventilation corridors within the city, and an index calculated from the coverage rate of wind corridors (CRW) is proposed for evaluating the relative NVP. Taking Nanjing as a case study, 8 AF-DEMs based on different wind directions and wind speed conditions are generated, and their corresponding ventilation corridor maps are constructed. The results are in good agreement with the empirical evidence, indicating that the hydrological model, though a rudimentary approximation of the actual airflows, was effective in revealing the natural ventilation corridor and characterize the relative NVP. Moreover, the implementation of this novel method is simple and convenient, and it has great application potential and value in urban design and management.

Impact and Assessment of Urban Wind Environment Change on Architectural Heritage Protection

In view of the contradiction between urban construction and protection of architectural heritage, this paper analyzes the hidden dangers of wind and fire protection of architectural heritage from the perspective of wind environment impact of high-rise buildings. The wind environment simulation and fire simulation based on computational fluid dynamics are used to study the typical building heritage cases in Guangzhou. The wind environment indicators that affect the comfort, safety and fire development of the wind environment of the building heritage are explained, so as to form a strategy for evaluating the impact of wind environment of high-rise buildings on the protection of building heritage.

城市主城区立体模型的构建与风环境模拟——以广州主城区为例

以22 km×21 km的广州主城区为例，以40 m建筑间距作为风道宽度低限、用容积高度对建筑高度赋值和垂向拔高为建模特色，概括构建了城市尺度的立体模型。在此基础上，分别模拟了广州主城区中性流条件下，弱风（2 m/s）和强风（5 m/s）、近地面10、25、50 m三个高度的风环境。模拟表明：来流5 m/s下主城区存在不同风速等级、不同平面形态以及不同高度面的风道；白云山、珠江新城等面积较大的地形或建筑高地形成了减速明显的背风区条带，并相互组合形成了重要的风口和具有较高基面的强风道；在不同高度上主城区风速均由周边向城市中心降低；风道风速强烈地依赖于风道走向，风道风速与走向夹角呈三次函数递减，两者拟合优度R²为0.512。模拟结果很好地呈现了城市尺度宏观地形白云山和建筑密集区相互间的作用，反映了城市尺度模型构建与模拟的优势。;An urban-scale three-dimensional model is constructed based on the workstation platform, and the method of CFD is used to simulate the urban wind environment under the condition of neutral current and identify ventilation channels on different spatial scales, which will help to enrich the methods and theories of ventilation channels planning. Through combining the generalized construction of the urban-scale three-dimensional shape with the identification of ventilation channels organically, this paper takes 22×21 km of main urban area of Guangzhou as an example, takes 40 m of building spacing as the low limit of wind tunnel width, uses the volume height to assign the building height and vertical elevation as the modeling characteristics, and builds a simplified and generalized three-dimensional model of urban scale. The wind environment near the ground at the height of 10 m and 25 m under the condition of neutral current in Guangzhou central urban districts is simulated under different combinations of weak wind (2 m/s) and prevailing wind (5 m/s) and southeast and northwest wind directions through the process of urban-scale three-dimensional modeling, grid division, calculation and solution. The simulation shows that:whether it is weak wind or prevailing wind, there exist ventilation channels with different wind speed levels, different plane shapes and different height surfaces in central urban districts; large terrains or building highlands such as Baiyun Mountain and Pearl River New Town form obviously leeward area with obvious decreasing speed, which combine with each other to form important vents and strong ventilation channels with higher bottom surfaces. The wind speed in Guangzhou decreases from the periphery to the city center at different heights averagely. The wind speed and direction of the downwind ventilation channel under the northwest wind and north wind decrease in cubic function. The simulation results show the interaction between Baiyun Mountain and dense building areas on a macro-scale and reflect the advantages of urban-scale model construction and simulation. Meanwhile, it shows that the technical route of constructing urban-scale three-dimensional model and simulation for the method of CFD based on workstation is simple and feasible.

Convective heat loss from computational thermal manikin subject to outdoor wind environments

Urban residents are increasingly encouraged to go outside for recreational and relaxing purposes, which may improve personal health and reduce building energy consumption. Therefore, it is important to understand the heat transfer between human body and surrounding urban outdoor environments. This study aims to predict the convective heat loss from a human body subject to urban outdoor wind environments. Firstly, the effects of the wind velocity and turbulent conditions on the convective heat loss from human body are investigated through a computational thermal manikin model, which is validated against published experimental data. Subsequently, wind data from onsite measurements in the city of Sydney, Australia is used to predict human body's convective heat loss using numerically obtained empirical correlations. The present result shows that the convective heat loss of most body segments increases with increasing wind velocity and turbulent intensity and decreasing turbulence length scale. Empirical correlations for predicting the convective heat transfer coefficients as a function of the wind velocity, turbulent intensity and turbulence length scale are derived based on simple-geometry assumptions. It is found that, at a given wind velocity with the ranges of the turbulence conditions from the field measurements, the variations between the high and low values of the convective heat transfer coefficient can be up to 67%. The results of this study demonstrate the significance of capturing the turbulent wind conditions for accurately predicting heat loss for outdoor thermal comfort studies.

Study on urban thermal environmental factors in a water network area based on CFD simulation

Abstract Cities on the Jianghan plain with a dense water network have a hot and humid climate and a long period of calm wind in the summer. Urban design guidance of a water network with a thermal environment as the starting point is a design control path that takes into account geographical features, climatic conditions and urban construction under complex constraints. Based on the current block division and development intensity, this study of the thermal environment of Xiantao adopts the methods of multiple linear regression and wind–thermal environment simulation. The results show that regional agglomeration shows the spatial distribution of the urban thermal environment in the water network area. The distribution of hot sensitive areas is closely related to the pattern of clusters. The business district, high-density urban area, basic energy facility area and transportation hub are the thermal environment areas. The experimental results show that residential and industrial land, proportion of water area, average elevation and urban development intensity are the main factors influencing the thermal environment of the water network, and the strengths of their effects are in the order of development intensity > water area proportion > industrial land proportion > residential land proportion > average elevation. The urban thermal environment of the dynamic construction area can be inferred. The improvement of urban wind and thermal environment quality is implemented in the urban space level by guiding the construction intensity, ventilation, greening and water, and paving and shading of the thermal environment control area. The exploration of the thermal environment improvement method in the urban design of a water network region provides the guidance for similar cities seeking to cope with climate change.

Evaluating the Influence of Urban Morphology on Urban Wind Environment Based on Computational Fluid Dynamics Simulation

Due to urbanization around the world, people living in urban areas have been suffering from a series of negative effects caused by changes in urban microclimate, especially when it comes to urban heat islands (UHIs). To mitigate UHIs, management of urban wind environments is increasingly considered as a crucial part of the process. Computational fluid dynamics (CFD) simulation of wind fields has become a prevailing method to explore the relationship between morphological factors and wind environment. However, most studies are focused on building scale and fail to reflect the effects of comprehensive planning. In addition, the combined influence of different morphological factors on wind environment is rarely discussed. Therefore, this study tries to explore the relationship between urban morphology and wind environment in a new-town area. CFD method was applied to simulate the wind field, and 11 scenarios based on criteria according to existing literature, planning regulations and local characteristics were developed. The simulation results from different scenarios show that the impact of the five selected factors on wind speeds was non-linear, and the impact varied significantly among different areas of the study region. Simulation of the differences in regional wind speeds among different planning scenarios can provide strong decision-making support.

Group Layout Pattern and Outdoor Wind Environment of Enclosed Office Buildings in Hangzhou

This paper presents a study of the effects of wind-induced airflow through the urban built layout pattern using statistical analysis. This study investigates the association between typically enclosed office building layout patterns and the wind environment. First of all, this study establishes an ideal site model of 200 m × 200 m and obtains four typical multi-story enclosed office building group layouts, namely the multi-yard parallel opening, the multi-yard returning shape opening, the overall courtyard parallel opening, and the overall courtyard returning shape opening. Then, the natural ventilation performance of different building morphologies is further evaluated via the computational fluid dynamics (CFD) simulation software Phoenics. This study compares wind speed distribution at an outdoor pedestrian height (1.5 m). Finally, the natural ventilation performance corresponding to the four layout forms is obtained, which showed that the outdoor wind environment of the multi-yard type is more comfortable than the overall courtyard type, and the degree of enclosure of the building group is related to the advantages and disadvantages of the outdoor wind environment. The quantitative relevance between building layout and wind environment is examined, according to which the results of an ameliorated layout proposal are presented and assessed by Phoenics. This research could provide a method to create a livable urban wind environment.

Urban form and air pollution disperse: Key indexes and mitigation strategies

Modern cities are experiencing increasingly severe air pollution issues due to massive emissions from industrial waste and vehicles. Additionally, urban spatial patterns and architectural layouts make near-surface urban air pollutants difficult to disperse. Thus, they accumulate at human height level, affecting people’s health. This study performs a three-dimensional simulation of features of spatial forms such as the site coverage, average height, plot ratio or floor area ratio, degree of enclosure, and height difference. The study makes use of a typical high-density built-up area as an example for empirical analysis. The research results show that the factors of three-dimensional forms of urban space, and the urban near-surface wind environment affected by it, have apparent effects on the flow of near-surface urban air pollutants. In particular, site coverage, average height, and degree of enclosure are the key indexes affecting the flow of near-surface air pollutants, while average height shows the most significant impact. Based on these findings, the study proposes mitigation strategies to optimize main urban form indexes from the above three aspects, in order to promote near-surface urban air pollutant dispersion and reduce the impact of air pollution on human health.