Eliminating Risk of Air Pollution in Big Cities Through Sustainable Urban Planning

1. Introduction

Urban Heat Island (UHI) is a critical problem for metropolises around the world. UHI is defined as an area in a city that is warmer than its neighboring countryside, making it a direct cause of Urban Air Pollution and thermal discomfort in the cities (Koohirostami & Abukhalaf, 2021). There are individual and urban strategies for reducing the UHI effect. Individual strategies are related to residential complexes and houses, such as using green roofs, reducing waste, vegetating courtyards, using sustainable building materials and energy-efficient applications, installing cool roofs, and increasing shades around homes (Abukhalaf & Koohirostami, 2021; Abukhalaf, 2021). Urban strategies are related to comprehensive urban planning, such as the Urban Ventilation Corridor (UVC), where highways and roads are considered the main paths of airflow in the city. Research shows that road direction and intersection angle significantly impact wind flow and velocity (Koohirostami & Abukhalaf, 2021). So, finding the most efficient direction and pattern to design roads and highways would increase the function of those paths. The study of the airflow in urban areas is a complicated process and needs highly technical tools such as Computational Fluid Dynamics (CFD).

2. Computational Fluid Dynamics

Computational Fluid Dynamics (CFD) software simulates and predicts the behavior of all fluids, including wind and water. CFD is a branch of fluid mechanics that deals with flow issues. This act is like a prediction of fluid behavior in different conditions with surfaces defined by a boundary condition. Naiver-Stokes equation has been used to simulate fluid or gas behavior. Almost all CFD problems are built on this foundation. The Euler equations can be obtained by omitting variables that describe viscous actions from these equations (Perén et al., 2015). Further simplification obtains the entire potential equations by omitting components indicating vorticity.

Aerodynamics and aerospace analysis, natural science and environmental engineering, weather simulation, industrial system design and analysis, fluid flows, biological engineering, engine and combustion analysis, and heat transfer are just a few of the research and engineering problems that CFD is used to solve in a variety of fields and industries. In 1922, CFD was used as a 2D model for weather prediction by Richardson (Hunt et al., 1997). From 1922 to 1940, the most formula was similar to Richardon’s idea. After that, computer power was used to solve the 2D formula from the 1950s to the 1960s.

CFD simulation in buildings started by simulating airflow rate in a single room. In 1990, Mokhtarzadeh-Dehghan simulated airflow in single-sided rooms by CFD (Mokhtarzadeh-Dehghan et al., 1990). Papakonstantinou has used CFD for single-sided rooms, similar to Mokhtarzadeh-Dehghan (Papakonstantinou et al., 1990). Later in 1996, Dascalaki used his prediction with different mathematical methods to show that the accuracy of the result is strongly dependent on the accuracy of input (Dascalaki & Santamouris, 1996). Recently, many researchers focused on comparing variables with different configurations, angels, geometries, or locations. Some researchers, such as Eftekhari in 2003 and Chandrashekaran in 2010, tried to compare experimental methods with CFD simulation to find the difference between results (Chandrashekaran et al., 2010; Eftekhari et al., 2003). Kouhirostami attempted to understand the effect of louvers’ geometry on natural ventilation (Kouhirostami, 2018; Kouhirostami et al., 2020). All these studies have the same numerical prediction method for their results.

CFD simulation in urban context started in 1998. Bruse and Fleer simulated microscale numerical of the surface plant-air interactions inside urban structures and the feedback between artificial surfaces like buildings and vegetation inside street canyons, backyard or green using ENVI-met (Burse and Fleer, 1998). Baik has used the CFD model to taste for three different building configurations. In each case, the CFD model has been shown to simulate urban street-canyon flow and pollutant dispersion well (Baik et al., 2003). Zhao has compared the influence of the most common variables related to the environment in China’s urban residential quarters by using a dynamic model of street canyons numerical simulation of temperature field for typical street canyon (Zhao et al., 2008). Allegrini studied urban heat flux using both CFD and building energy simulation. The results showed that the importance of buoyancy for law wind speed cases and the strong influence of buildings upstream on the heat flux and temperature further downstream (Allegrini et al. 2015).

3. Urban Ventilation Corridor

Ng stated that connecting low-density and low-roughness areas in a city would create a path to direct wind through the city. He believed that this phenomenon, called UVC, has a significant effect on reducing urban temperature (Ng, 2009). Air Ventilation Assessment is the rule for sustainable urban planning, which was established in 2005 by the Office of the Chief Secretary for Administration. Ng reported the scientific process of the AVA system in his research to clarify this technical method for urban planners (Ng, 2009).

After proving UVC is a substantial strategy for urban cooling, researchers focused on finding a methodology for simulating and analyzing UVC. For example, in 2008, Tominaga presented a significant and practical study of the CFD simulation principles and guidelines for airflow outside of the buildings and in urban areas (Tominaga et al., 2008). This study is a reliable reference for researchers who work on the urban ventilation simulation by CFD. Mirzaei used CFD basic strategies as his study’s foundation and then developed a CFD model for evaluating urban ventilation. He compared different mitigation techniques on the pedestrian level and suggested some alternative techniques (Mirzaei & Haghighat, 2011).

In 2009, Maio studied UHI through the Weather Research and Forecasting (WRF) and observation methods. He used the WRF model with a single-layer urban canopy model to simulate the urban weather and compared it with the observation. He presents that the mountain-valley airflow has a significant effect on Beijing airflow. He shows that urban circulation and the urban surface have a considerable impact on the formation of horizontal convection (Miao et al., 2009). Another city in China that is being considered for urban ventilation corridor is Hong Kong. In 2010, Wong used the “Building Frontal Area Index” to map the Urban Wind Ventilation. He chose a high-density neighborhood in Hong Kong with a sub-tropical environment for his study. He suggests ventilation would be the best answer to reduce UHI in high-density urban neighborhoods (Figure 1) (Wong et al., 2010). Many other metropolitans in China are used as the case studies for UHI mitigation, such as Shanghai, Xi’an, Chongqing, and Shenzhen. For example, in 2011, Zhang studied the effect of high-density construction and energy usage on the summer UHI by regional boundary layer modeling. He tried to analyze the influence of UVC on reducing the temperature in the metropolis of Shenzhen, China. The results show that the area of a city with high-density construction has a higher temperature. He demonstrates that UVC has a significant effect on reducing UHI intensity, and it can be an effective method to mitigate the negative climate effect caused by UHI (Zhang et al., 2011).

Guan stated that UVC is one of the most effective ways for cooling Chinese cities. He concludes that three levels of UVC can be applied in Guiyang, China, to provide fresh air and a healthy environment for residents. His planning suggestion would increase the average wind speed in winter by 8% and in summer by 6% (Guan et al., 2011). In 2016, another study was released in China to mitigate UHI by UVC. Hsieh chose wind paths along low-density urban paths based on Frontal Area Index (FAI) and the least-cost path methodology for his CFD simulation study area. Results of this study identified potential cooling routes in the study area (Figure 2) (Hsieh & Huang, 2016). The main point in this study is simplifying urban areas to simulate the most accurate results.

A few researchers believe that CFD simulation is not accurate enough for making a decision. Thus, they prefer to compare CFD simulation results with other methods such as GIS spatial analysis and FAI. For example, Chang used CFD simulation and GIS spatial analysis to identify the effect of UVC on reducing UHI in Changchun, China. This study presents that water paths, main roads, and green spaces would create an effective UVC. Furthermore, air flowing would be beneficial for cooling cities at the height of 100 feet above the pedestrian level of the urban area (Shouzhi et al., 2018). The literature review on UVC shows that most studies related to this topic are focused on Chinese cities to find practical and efficient city planning. A study by Ren reviewed UVC applications in Chinese cities since 2000 to find the successes and failures (Figure 3) (Ren et al., 2018).

4. Summary

UHI is a direct cause of thermal discomfort in cities worldwide. UVC is an urban strategy used to mitigate UHI, where the main paths of airflow in any city are controlled through the design of highways and roads. The study of the airflow in urban areas is a complicated process and needs highly technical tools such as CFD. However, this is still a relatively new field of study and has many gaps that need to be researched, especially when considering culturally different urban contexts.

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