Effect of building form on wind velocity and pollutant concentration in urban residential context (case Study: District 1 of Shiraz city)

Subject Areas : Sustainable urban development

mozhgan kamali ¹ , ali akbar heidari ² , Yaghowb Peyvastehgar ³

1 - yasooj islamic azad university
2 - Assistant professor of Architecture, Technical and Engineering Department, Yasouj University, Yasouj, Iran
3 - Associate Professor of Architecture and Urban Planning, Yasuj Branch , Islamic Azad University, Yasuj, Iran

Received: 2023-09-30 Accepted : 2023-11-25 Published : 2024-06-20

Keywords: building form, air quality, air pollutant, air velocity, computational fluid dynamics (CFD),

Abstract :

With the increasing use of motor vehicles in cities, the amount of traffic-related pollutants is growing and affects indoor air quality. One of the influencing factors on the pollutants penetration into the urban context and the change of its diffusion speed (depending on the air velocity) is the building form. Therefore, in the current research, the effect of building form on pollutant concentration and air velocity inside the building has been investigated. For this purpose, 3 common building forms have been investigated in district 1 of Shiraz city. These forms create 12 different urban contexts with four 90 degree rotations in a regular pavilion-shaped context. Each context is placed in the vicinity of an urban highway as a pollutant source. Examining the cases has been done through CFD simulation. Steady 3-dimensional flow using the SST-Kω turbulence model has been used to simulate the cases, which has been numerically solved based on the Reynolds-averaged Navier-Stokes (RANS) equations. The validation of the CFD software used in this research has been done in comparison with the wind tunnel tests and has yielded acceptable results. The results showed that the building form has a significant effect on the air quality inside the building. Also, based on the results of the TOPSIS multi-criteria decision-making method, the best and the worst building forms in order to increase the air velocity and reduce the concentration of pollutants inside the building, respectively, related to the form with an overhang, facing the windward, and an overhang, facing the leeward. Extended Abstract Introduction According to estimates, about 91% of the world's urban population is exposed to various types of pollution (WHO, 2016; Pepe et al., 2019). Air pollution in cities is caused by various factors such as the activities of industries and factories, traffic of cars and the use of fossil fuels for cooling and heating of buildings. Meanwhile, the traffic of cars in cities is one of the most important factors in the production and spread of pollution in urban spaces (Zhao et al., 2020). The spread of pollution in urban spaces depends on various factors, including meteorological factors (like wind speed and its direction), urban morphological characteristics (such as the geometry and arrangement of the buildings), urban density and the location of the pollution source (Blocken et al., 2013; Di Sabatino et al., 2018; Hang et al., 2015; He et al., 2020a, 2020b; Miao et al., 2020). In this regard, the role of urban morphology in the spread of pollution is one of the fields that has received less attention in urban research and environmental design. In most of this research, the relationship between urban form and air ventilation has been investigated and in a small number of them the relationship between urban form and air quality and the pollutant abundance have been studied. (Yang, Shi, Shi, et al., 2020). As mentioned above, in this research the impact of urban morphology on pollution spread caused by vehicular traffic is analyzed. The purpose of this research is to analyze the typological form of buildings in an urban context in terms of indoor air quality indicators. In order to measure the air quality inside the building, two parameters of the pollutant concentration and the air velocity will be examined. According to the objectives of the research, the following questions are answered in this research: 1. What effect does the change in the pattern of the urban context (affected by the change in the form and arrangement of the building) have on the penetration of pollutants in the building? 2. What are the characteristics of the optimal building form in relation to the pattern of wind behavior to increase air quality? Methodology Based on what was mentioned, in this research, the impact of urban morphology on pollution spread caused by vehicular traffic is analyzed. The pattern of urban morphology is selected based on the form of existing residential context in Shiraz city. Among the pollutants affected by urban traffic, , which has the highest concentration and abundance (73 ) in the annual concentration chart of Shiraz city (PlumeLabs, 2022), will be investigated. The research process is carried out in such a way that, in the first step, using Grasshopper software, different alternatives for the residential building form are generated. In this step, form generation is performed based on two variables, volume and relative compactness. In the next step, among the generated forms, the most similar options to the existing buildings in the residential context of district one in Shiraz City are selected. In the third step, the types of context that can be extracted from the rotation of the selected building patterns are determined. The determined contexts are analyzed in the fourth step using numerical simulation in CFD, and the wind velocity and concentration of pollutants within the target building are measured. Finding the best building configuration in the context in order to reduce the concentration of pollutants and increase the air velocity (in the target building) is the last step in this research which is done using the TOPSIS MCDM. Results and discussion In this section, the results of air velocity and concentration analyses inside the target building are presented. For the reference case, the increase of concentration along the longitudinal walls and its decrease in the middle part of the target building can be seen. For Configuration A, in cases A-01 and A-03 depending on the direction of the overhang, the pollution diffusion pattern has been transferred to the passage in front of the overhang. In A-02, in addition to the north-south streets, the wind flow containing the pollutants has also been drawn into the east-west streets, which has caused the accumulation of pollutants in this area and its entry into the interior of the target building. In A-04, the negative pressure area reduces the concentration of pollutants in the south face of the buildings (due to the increase in the wind shadow and the decrease in the wind flow containing pollutants). Therefore, the entry of pollutants into the target building has been reduced. For Configuration B, cases B-01 and B-03 have similar behavior in pollution diffusion patterns in the context. In case B-02 in the target building, the concentration has gradually increased from the sides of the building towards the middle. In Case B-04, in the target building, the pollutant concentration is low, and the particle dispersion is uniform. For Configuration C, in cases C-01 and C-03, Pollutant accumulation is on the front facing the facade overhang. In case C-02, the middle part of the target building has the highest concentration of pollutants. In case C-04 in the target building, the dispersion of particles is low and uniform. Conclusion The main purpose of this research is to investigate the effect of different building configurations in an urban context on IAQ. In this paper, reducing the amount of pollutants and increasing the air velocity inside the building are considered as two research criteria. 12 urban contexts consisting of the most common forms of low-rise buildings in Shiraz city in a regular pavilion-shaped urban form are considered case studies and are simulated in CFD. Based on this, the most important results obtained are as follows: The rotation of building blocks in an urban context causes a change in the pollution diffusion pattern. In this regard, the form of blocks and their filled and empty spaces play an important role in the diffusion pattern. The average concentration in the target building in configuration C and configuration A are the lowest and the highest, respectively. configuration A with an overhang and configuration C with a façade overhang have the lowest and highest air velocity in the target building, respectively. The results of using TOPSIS MCDM showed that A-04 and A-02 are the best and the worst examined cases in terms of IAQ. The results of this study indicate that the dilution of polluted air in the interior of the building is strongly related to the form of the blocks, which depends on both the form pattern and the way the buildings are arranged and rotated in the surrounding context.

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