Assessing the Effects of Urban Canyon's Open Space and CO Dispersion with Using CFD (A Case Study of Tehran)

- Introduction: Metropolitans are increasingly facing the problem of air pollution due to the widespread presence of vehicles. Air pollution at the street level is a challenging issue of urban sustainable development. In addition to its sources of production, air pollution deals with a large number of factors such as urban morphology and ventilation, and urban wind. The latter can be considered as an important one since the long-term stability of air in an urban area can quickly stabilize pollutants and increase their volume in urban space. In addition, urban morphology can play a role in transfer pollution from one place to another by creating specified paths for wind.
Thus, triple relationships are created between urban morphology, air flow and air pollution. Urban morphology as an independent variable directly affects the accumulation and dispersion of pollutants (as a dependent variable) and indirectly affects the air flow.
In recent years, computational fluid dynamics (CFD) has been employed for assessment of a wide variety of variables and indices including: wind angle with respect to the street canyon, aspect ratio of the streets, the average height, different heights, street continuity ratio and street spatial closure ratio, neighborhood form (rectangular and square), length of the urban canyon, size of neighborhood, street architecture (roof configuration), degree of enclosure, plot ratio or floor area ratio (FAR).
This study is intended to prove the existence or non-existence of a relationship between air quality (CO pollutants) and mineralization index in the neighborhood and open space index in the street canyon in Tehran (where the wind is perpendicular to the main street) with the help of CFD, which is known as a more reliable than statistical studies, due to better computational accuracy.
- Materials and Methods: The CFD simulations have been performed using Ansys Fluent. The validation of the all CFD settings (including mesh arrangement and turbulence model etc.) is based on experimental analysis (wind tunnel -reduced scale (. The case study is located in the residential areas of Tehran, Iran. The GIS software and satellite images have been applied to select the case study. The dimensions of the neighborhood are 300 m wide, 300 m long, and 16 m high. The street width equals 12m. In the models, tetrahedral meshing for the inner region and hexahedral meshing for the outer region have been used (Hybrid mesh). The aspect ratio equal to 1.1 in inner region and is 1.15 in the whole geometry. The number of cells in the F1, F2, F3, and F4 is 7.3, 7.4, 7.4 and 7.5 million cells respectively for the simulation of one half of the geometry.
The turbulence is simulated using RANS models, which are formed based on the temporal averaging of parameters. Due to high speed, low computational cost and acceptable accuracy of RANS models, RANS equations have been used in this research. Among the RANS models, the Realizable k-epsilon turbulence model has been selected, which has achieved better in validation part. The model is three-dimensional, isothermal, steady, and incompressible. Carbon monoxide is considered as the pollutant which is injection from two lines source (with 5cm wide and 40cm high) along the main street. The pollutant emission modeling method is the species transfer model (mixed-species).
- Discussion of Results: Based on the CFD output, the maximum velocity at the pedestrian height in F1, F2, F3and F4 respectively equals 4.27, 5.31, 5.31, and 5.35 (m/s), which has been created in the corners of windward blocks. In the other forms except for F1 (it lacks an East-West street), the maximum velocity is blown at the entrances of the streets which are parallel wind.
By increasing the OS index in F1, F2, and F3 (0, 0.04, 0.27), the mean velocity at the main street increases (0.73, 0.75, 0.78), but in F4, where the index equals F3, we see a decrease in velocity (0.59) due to the difference in the shape and size of the open space in the neighborhood. The longer length of this space in F4 has minimized the canalization effect of the west-east street and consequently the wind velocity in the middle of the open space (where the main street passes).
With the decrease of the MI index, the average velocity in the whole domain decreases. But F1 is exception. although it has the highest index, it also has the lowest velocity, which is due to the lack of East-the West street in this form.
Based on the maximum mass fraction, F4 is the worst form (0.0136). After that, F1, F3, and F2 are in the next ranks in terms of CO mass fraction with 0.0116, 0.0104, and 0.0103, respectively. The concentration of pollutants in all forms can be seen in the vicinity of the leeward wall. In F1, the accumulation of pollutants is in the middle of the street, in F2, it is inclined to the intersection, in F3, it is inclined in the vicinity of the open space, and in F4, it is in the middle of the enclosed sections of the street.
Considering the average mass fraction at the height of the pedestrian in the main street and comparing it between the forms, it should be said that the F3 has the best conditions. It is 10% less than F4, 20% less than F2, and 30% lower than F1.
Based on the OS index, it can be said that with the increase of the index, the amount of pollutant in the main street decreases and there is a negative correlation between them. But in F4, due to the lower wind velocity, the amount of pollutants is slightly higher than the F3.
The street roof (16 meters) in the F1, F2, F3, and F4 has the highest amount of pollutant respectively and their mass fraction average equals to 0.00066, 0.00052, 0.00041, and 0.00036. So, increasing in the OS index and decreasing in MI index (F1 to F4) cause a reduction in vertical ventilation (by the street roof) as well as an increase in horizontal ventilation (through lateral openings).
The amount of CO mass fraction in the longitudinal profile in the sidewalk axis in the main street (near the western wall), in F1 at the beginning and end of the street is the minimum and in the center of the street, this amount has reached its maximum value of 0.0072. In F2, at the intersection of the East-West street and the main street, CO mass fraction is drastically reduced to zero. In the F3 and F4 at the open space, the amount of co is very small. Based on the graph and contour outputs, F1 has the worst form and F4 has the best form. The average mass fraction in F4, F3, and F2 is 56.52%, 65.22%, and 82.61% of F1, respectively.
- Conclusions: Findings show that in the forms in which wind direction is perpendicular to the street, ventilation is mostly done through the street's roof and by increasing the open space index and decreasing the mineralization index, the vertical ventilation decreases, and the horizontal ventilation via lateral openings increases. On the other hand, increasing the figures of the open space index, leads to a decrease in the amount of mass fraction at pedestrian height (2 meters) in the main street. Thus, a negative correlation is reported between them. In addition, also the results show a relationship between the increase in both wind velocity and ventilation rate with the decrease in the amount of CO, but the relationship could not be considered a direct relationship. The reason is that the ventilation is not only by horizontal movement of pollutants, but there are other vertical and turbulent flows too which causes ventilation. Finally, regarding the mineralization and open space indices, the third form is evaluated as the most suitable form, which should be considered in the future developments of Tehran.