Species Transport: Pollution in a Real Urban Zone

This project simulates the dispersion of carbon dioxide from vehicle exhaust along an urban street using ANSYS Fluent, with species transport as the central theme. Air pollution remains a worsening problem in many developing cities, driven by ever-increasing transport demand even as emission technology improves. Tracking how a pollutant mixes and spreads through the air requires a model that resolves the concentration of each gaseous constituent separately, and that is exactly what the Species Transport model does — solving a dedicated transport equation for every component of the mixture. The core objective here is to quantify how much CO₂ is dissipated across an urban zone and how free airflow influences it.

The problem captures the change in carbon-dioxide mass fraction on a city street. A thin source region 0.1 m high is defined along the street to represent the integrated production of CO₂ from car exhaust, acting as a mass source within the domain at a generation rate of 4 kg/m³. Free airflow enters the surrounding urban environment at 0.2 m/s and 300 K, and the simulation examines how this airflow transports and dilutes the emitted CO₂. Because two gaseous species — air and CO₂ — are modelled, the Species Transport model is the heart of the setup.

The geometry is three-dimensional, created in Design Modeler, and represents a city block comprising several buildings and a street, enclosed within a rectangular domain measuring 9 m × 13 m × 4 m. Airflow enters through three lateral faces, and the 0.1 m source region sits on one of the streets. Meshing was carried out in ANSYS Meshing with an unstructured grid of 4,137,570 elements, refined near the internal boundaries where concentration gradients are steepest.

The simulation uses a pressure-based, transient solver, since the goal is to follow the change in CO₂ concentration over time. Turbulence is represented with the standard k-ε model and standard wall functions, and the energy equation is enabled to account for thermal effects. Within the species-transport framework, air and CO₂ are the modelled species: the inlet supplies clean air at 0.2 m/s and 300 K with zero CO₂ mass fraction, the outlet is a pressure outlet at atmospheric pressure, and the walls are treated as stationary with zero heat flux and zero diffusive flux of CO₂. Second-order discretisation is used for pressure, momentum, energy and the CO₂ transport equation to sharpen the resolution of the concentration field.

The solution yields two-dimensional contours of pressure, temperature, velocity and the mass fractions of air and CO₂ on XY and YZ planes, together with three-dimensional contours of the same quantities in the region of the CO₂ source. As a study in species transport modelling, the project demonstrates how a separate transport equation for each gaseous component, combined with a defined mass source, can reproduce the generation and wind-driven spread of a pollutant — showing how free airflow disperses traffic-derived CO₂ through a built urban environment.

A Geometry & Mesh file and a comprehensive Training Movie demonstrating how to set up the problem and extract all the desired results are available.