Urban Planning: Air Pollution within a Street Canyon

Description

This project simulates pollution diffusion within a street canyon using ANSYS Fluent. In the model, an urban district is defined as the computational domain: two parallel rows of building blocks are created, and the space enclosed between them forms what is known as a street canyon, or urban canyon. Understanding airflow and pollutant behavior in these canyons is a central concern of urban planning, since the form and dimensions of a canyon strongly influence how urban heat and airborne gases are dispersed through the city.

The model was built in 3D using Design Modeler. The computational domain measures 36 m long, 24 m wide, and 8 m high, and contains two parallel rows of simplified geometric building blocks. To reduce computational cost, the domain is kept limited in extent, with symmetry boundary conditions applied around the perimeter of the urban area. Meshing was performed in ANSYS Meshing, producing 1,938,659 elements.

Methodology

The study examines the quantity and distribution of pollutants within the canyon space, and the Species Transport model was used to carry out the simulation.

Two gaseous species are defined: air and a pollutant. The pollutant has a specific heat capacity of 1100 J/kg·K and a molecular weight of 77.49064 kg/kmol, while air has a specific heat capacity of 1006.43 J/kg·K and a molecular weight of 28.966 kg/kmol.

All pollutants are assumed to originate within the street canyon. To represent this, two grooves are modeled on the canyon floor to act as pollution sources, and a source term of 0.011 kg/m³·s is applied for the pollutant species in this contamination region. Initially the urban area contains only air, after which pollutant generation begins.

At the inlet boundary, only pure air enters under a velocity inlet condition. The inlet velocity is defined as a function of position across the inlet section, so a velocity profile is supplied through a UDF. The air temperature is set to 300 K. The RNG k-epsilon model, together with the energy equation, was enabled to resolve the turbulent flow field and compute the temperature variation throughout the domain.

Conclusion

On completion of the solution, three-dimensional contours of pressure gradient, velocity, temperature gradient, air mass fraction, and pollutant mass fraction were obtained, along with two-dimensional contours of velocity, air mass fraction, and pollutant mass fraction.

As the results show, air pollution builds up from within the street canyon. Two- and three-dimensional velocity vectors were also extracted, and depending on the flow behavior inside the canyon, a vortex or recirculating rotation of the flow appears between the building rows.