Urban Planning: Pollution in a Real Urban Zone

Computational Simulation of Carbon Dioxide Dispersion in an Urban Street Canyon: Implications for Urban Planning Engineering

The dispersion of vehicular emissions within densely built environments represents a central concern of contemporary urban planning, particularly in developing regions where air quality continues to deteriorate despite advances in emission-control technology. This study addresses that concern through a computational fluid dynamics (CFD) simulation of carbon dioxide transport along an urban street, performed in ANSYS Fluent. The objective is to quantify the extent to which free airflow disperses the CO₂ generated by vehicular exhaust within a representative city block, thereby providing a physically grounded basis for evaluating urban ventilation.

The model is three dimensional and reproduces a configuration of building blocks bordering a city street, enclosed within a rectangular computational domain measuring 9 m × 13 m × 4 m. A continuous source region of 0.1 m height is defined along the street to represent the integrated production of carbon dioxide from traffic, with a generation rate of 4 kg·m⁻³. Free airflow enters through three lateral faces of the domain at a velocity of 0.2 m·s⁻¹ and a temperature of 300 K. Because two gaseous constituents — air and CO₂ — are considered, the Species Transport model is employed, solving a separate transport equation for each component of the mixture; the energy equation is activated to account for thermal effects. Turbulence is represented using the standard k–ε model with standard wall functions, and the governing equations are advanced using a transient, pressure-based solver, consistent with the aim of resolving the temporal evolution of pollutant concentration. The domain is discretised with an unstructured mesh of approximately 4.14 million elements, refined in the vicinity of the internal boundaries to enhance resolution where concentration gradients are steepest.

The solution yields two- and three-dimensional contours of pressure, temperature, velocity, and the mass fractions of air and carbon dioxide throughout the domain, with particular attention to the region surrounding the source term. These fields characterise how the imposed airflow transports and dilutes the emitted CO₂ across the street canyon and around the surrounding structures.

The findings bear directly on several aspects of urban planning engineering. First, the predicted distribution of pollutant mass fraction reveals zones of accumulation and stagnation, information that supports the siting of pedestrian areas, building entrances, and ground-level activities away from regions of elevated concentration. Second, the dependence of dispersion on the prevailing wind field underscores the role of street orientation, building height, and block spacing in promoting or impeding natural ventilation — design variables over which the planner exercises control. Third, the methodology provides a transferable framework for assessing the air-quality consequences of proposed developments prior to construction, enabling the evaluation of alternative urban geometries with respect to their capacity to disperse traffic-derived emissions. Collectively, the study demonstrates how CFD-based species transport modelling can inform the design of healthier and better-ventilated urban environments.