Pollution in a Real Urban Zone

Description

This project applies computational fluid dynamics to study how carbon dioxide from vehicle exhaust disperses through an urban street canyon, a question that matters increasingly for urban planning as air quality pressures mount in fast-growing cities. Built in ANSYS Fluent, the simulation models a three-dimensional block of city buildings framing a street, contained in a 9 m × 13 m × 4 m domain. Traffic emissions are represented as a continuous 0.1 m-high source strip running along the street, generating CO2 at 4 kg·m⁻³. Ambient air enters through three sides of the domain at 0.2 m/s and 300 K, allowing the surrounding wind field to carry the pollutant away from its source and through the canyon.

Methodology

Because the flow involves two distinct gas species, air and CO2, the Species Transport model is used to solve separate transport equations for each, with the energy equation active to capture thermal effects on the mixture. Turbulence is closed with the standard k–ε model and standard wall functions, and the equations are solved transiently with a pressure-based solver, so the concentration field can be tracked as it evolves over time rather than only at steady state. The domain is meshed with roughly 4.14 million unstructured elements, with local refinement near solid boundaries to resolve the sharp concentration gradients expected close to the source and building walls.

Analysis

Results are reported as 2D and 3D contours of pressure, temperature, velocity, and species mass fraction, with the region around the emission source examined most closely to show how incoming airflow dilutes and redistributes the CO2 across the canyon. These fields point to concrete conclusions for urban planning: they identify pockets of stagnation and accumulation that should steer where pedestrian zones, entrances, and street-level activity are placed; they illustrate how street orientation, building height, and block spacing govern natural ventilation, giving planners a physical basis for these design choices; and, more broadly, they establish a workflow that can be reused to screen the ventilation performance of proposed developments before construction, comparing candidate urban layouts on their ability to clear traffic-derived pollutants.