Air Pollution within a Street Canyon

Air Pollution within a Street Canyon — ANSYS Fluent CFD Simulation

Description

This project simulates pollution diffusion in a street canyon using ANSYS Fluent. When two rows of building blocks stand parallel to each other, the space between them forms what is known as a street canyon (or urban canyon) — a configuration whose geometry strongly influences how urban heat and airborne gases are distributed. This project investigates the amount and distribution of pollutants within that canyon space. Within the Urban Microclimate: All Levels CFD Training Package, this project introduces pollutant dispersion through the classic street-canyon geometry, applying the Species Transport model to the fundamental unit problem of urban air quality.

Methodology

The three-dimensional model is designed in Design Modeler, with a computational area 36 m long, 24 m wide, and 8 m high containing two rows of simple building blocks parallel to each other. To reduce computational cost, the model is limited in extent and symmetry boundary conditions are applied around the urban area. Meshing in ANSYS Meshing produces 1,938,659 elements. The Species Transport model is used to represent the two gaseous species — air and pollutant — where the pollutant has a specific heat capacity of 1100 J/kg·K and a molecular weight of 77.49064 kg/kmol, and air has 1006.43 J/kg·K and 28.966 kg/kmol. All pollutants are assumed to be generated within the canyon: two grooves in the ground act as the pollution source, with a source term of 0.011 kg/m³·s. Initially only air fills the domain, and pollutants then begin to be produced. At the inlet, pure airflow enters through a velocity-inlet condition whose magnitude varies with the inlet location — implemented as a profile in UDF format — with the air temperature set to 300 K. The RNG k-epsilon model and the energy equation are enabled to resolve the turbulent flow and the temperature field.

Analysis

At the end of the solution, three-dimensional contours of pressure gradient, velocity, temperature gradient, air mass fraction, and pollutant mass fraction are obtained, along with two-dimensional contours of velocity, air mass fraction, and pollutant mass fraction. The results show air pollution originating from the interior of the street canyon, and the two- and three-dimensional velocity vectors reveal a vortex — a rotation of the flow — forming inside the canyon, which governs how the pollutant is trapped or cleared. By the end of this project, you'll be able to set up a Species Transport simulation with a ground-level pollution source, apply a UDF velocity profile at the inlet, and interpret the concentration and velocity fields that characterize air quality in a street canyon.