Urban Wind Environment

In this chapter, a high-resolution frontal area density (FAD) map that evaluates urban permeability was produced using an empirical model, which takes into account the heterogeneous urban morphology and local wind availability. Using the MM5/CALMET model, the wind data of Hong Kong was simulated, the FAD map of three urban zones were calculated: podium (0–15 m), building (15–60 m), and urban canopy (0–60 m). Wind tunnel test data was used to correlate the FAD understanding of the three zones with pedestrian-level wind environment. Linear regression analysis indicated that a lower urban podium zone yielded the best correlation with the experimental data, and 200 × 200 m was the reasonable resolution for the FAD map. This study further established that the simpler two-dimensional ground coverage ratio (GCR) that is readily available in the planning circle can be used to predict the area’s average pedestrian-level urban ventilation performance of the city. Working with their in-house GIS team using available data, the GCR will provide the planners a way to understand the urban ventilation of the city for decisions related to air paths, urban permeability, and site porosity.

Urban environment has been changed and deteriorated, due to rapid urbanization, as well as other factors including a lack of implementation of environmental information and knowledge in the urban planning practice. Therefore, there is a need to develop a systematic and user-friendly method for city planners and policymakers to make scientific and evidence-based decisions in order to address urban environmental issues. This chapter takes Wuhan, China, as an example, applies the understandings of frontal area density in Chap. 2, and provides a practical tool to identify planning goals and guidelines for master and district planning. In the chapter, specific meteorological information and 3D urban morphology data were comprehensively integrated into Geographical Information System (GIS) to map and evaluate wind permeability at the urban scale. The resultant urban planning strategies at various spatial scales can complement one another, and be interwoven into the whole urban planning process with a collaborative effort from local town planners and policymakers of the Planning Bureau of Government.

The aerodynamic property of urban areas is a necessary component in current urban planning and design. Rather than the morphological models introduced in Part II, which provides modeling results in several hundred meters resolution, this chapter introduces a semiempirical approach to model the pedestrian-level wind speed at the neighborhood scale. The balance between momentum transfer and drag force in both an averaged sense over an area and a moving air particle is discussed in order to extend conventional frontal area density \( \left( {\lambda_{\text{f}} } \right) \) to a point-specific parameter \( \left( {\lambda_{{{\text{f\_point}}}} } \right) \). Through correlation with data from wind tunnel experiments, \( \left( {\lambda_{{{\text{f\_point}}}} } \right) \) was considered a good index to assess the pedestrian-level wind speed at a test point with multiple input wind directions. Regression equations were developed to map the pedestrian-level wind environment at 1 × 1 m pixel resolution. This modeling–mapping approach requires less computational time and supportive technology than CFD simulations. Meanwhile, the modeling method provides accurate results at high resolution from a practical point of view. Therefore, the modeling results for urban wind environment can be well integrated into the neighborhood-scale design. Using this approach, urban designers can estimate the neighborhood-scale pedestrian-level wind speed and optimize proposed planning at the onset of the planning process.

Given that building typology is one of the key elements in architectural design and that the drag force of surface roughness on airflow directly depends on building typologies, it is important to expand our understandings on wind environment from urban and neighborhood scale to the building scale. In this chapter, CFD simulation was conducted to provide the detailed understandings on pedestrian-level wind environment and building typologies. First, \( \kappa - \omega \) SST turbulence model was validated by comparing modeling results with data from the wind tunnel experiment. Second, the impact of various building typologies on pedestrian-level airflow were investigated in a parametric study, in which various parametric cases with different wind porosities were designed, and wind speed was classified based on PET to evaluate outdoor thermal comfort. Subsequently, critical design issues were identified, and the corresponding mitigation strategies were developed. From both the accuracy and practical point of view, this chapter introduces a study that allows architects to improve building porosity efficiently for better pedestrian-level urban ventilation.

Due to complicated building geometries and heterogeneous urban morphologies, project-specific wind analysis is needed in addition to the general understandings of wind environment in order to develop appropriate mitigation strategies for any particular projects that may have outdoor natural ventilation issues. Over the past few decades, rapid development in computational fluid dynamics (CFD) has resulted in itself widespread application in environmental research and design. This chapter introduces a systematic wind analysis to accurately model wind environment and efficiently analyze modeling results to support evidence-based decision-making during design procedure. By conducting a case study in Hong Kong, this chapter addresses issues from input boundary condition setting, modeling settings, modeling verification, to data collection and analysis, which may be encountered by wind consultants and architects when conducting wind analysis. Various statistical analysis methods are applied to compare simulation results, diagnose the designs, and develop corresponding mitigation strategies in terms of natural ventilation performance.

The impact of air pollution on public health is substantial, especially in high-density megacities. Frequent reports of high air pollution concentrations at roadside stations in these cities (e.g., Hong Kong) are the result of both higher anthropogenic pollution emissions in densely populated street canyons and stagnated airflow due to closely packed tall buildings, which result in lower dispersion potential and leads to increased population exposure to air pollutants. Thus, it is critical to design appropriate high-density urban morphologies to lessen the negative impacts of high-density urban living. This chapter addresses the knowledge gap between planning and design principles, and air pollution dispersion potentials in high-density cities, so that air ventilation assessment can appropriately take into account the air pollutant dispersion issues. CFD simulation and parametric study are conducted, and SST κ–ω model is adopted upon balancing the accuracy and computational cost through comparative study. Neighborhood-scale parametric studies are conducted to clarify the effects of urban permeability and building geometries on air pollution dispersion, for both outdoor pedestrian environment and indoor environment of the roadside buildings. Given the limited land resources in high-density cities and the numerous planning and design restrictions for developmental projects, the effectiveness of mitigation strategies is evaluated to optimize the benefits. At the end of the chapter, an actual urban case study is presented to demonstrate how the suggested design principles from the parametric study can be applicable in high-density urban design.

High-density urban areas are often associated with poor outdoor natural ventilation. Given the growing call for more vegetation in the cities, it is important to study wind resistant of urban trees in order to address outdoor natural ventilation problem in landscape planning. Currently, Computational Fluid Dynamics (CFD) simulation and wind tunnel experiment can only model the simplified street canyon with roadside trees, at the expense of intensive technical support and high computational cost. Thus, their application is often restricted to research purpose only, and the impact of their research outputs on the landscape planning remains low due to the impracticality. In this chapter, we developed a semiempirical model to provide scientific understandings and practical tools for landscape planning practice. This new model was based on the balance between momentum flux and the drag force of both buildings and trees on airflow. Friction velocity \( \left( {u_{ *} } \right) \) was modeled and validated by existing CFD and wind tunnel data; effective frontal area density \( \left( {\lambda_{{{\text{f\_tree}}}} } \right) \) was estimated by the measured leaf area index. The impact of urban context density and trees (i.e., plant canopy density and typology) on wind environment was clarified. This research correlated the urban density and tree geometry indices with wind speed, thereby enabling planners to calculate trees’ effects on airflow using their in-house data. With such new practical tool and understandings, knowledge-based landscape planning can be established to introduce more trees into urban areas, and prevent from having negative effects of trees on outdoor wind environment at the same time.