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Air Pollution within a Street Canyon, CFD Simulation

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The present problem simulates the diffusion of pollution in a street canyon using ANSYS Fluent software.

This product includes a Mesh file and a comprehensive Training Movie.

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Air Pollution in Street Canyon Project Description

The present problem simulates the diffusion of pollution in a street canyon using ANSYS Fluent software. In this modeling, an urban area is defined as the computational area of ​​the model; Thus, two rows of building blocks are created parallel to each other, and as a result, space is created between these two rows of building blocks, which is called a street canyon or an urban canyon. The type of canyons and their constituents’ geometric dimensions can be effective in discussing the distribution of urban heat or the distribution of different gases in the air. This work aims to investigate the amount and distribution of pollutants in the space of these canyons. Therefore, to perform this simulation, the Species Transport model has been used.

The model has two types of gas, including air and pollutants; So that the pollutant species has a specific heat capacity of 1100 and a molecular weight of 77.49064 kg.kmol-1 and air has a particular capacity of the heat of 1006.43 -1, and the molecular weight is equal to 28.966 kg.kmol-1. It is assumed that all pollutants are generated within the street canyon space, and therefore, two grooves are created on the ground in the canyon space, which acts as a source of pollution. Therefore, a source term for the pollutant species of 0.011 kg.m-3.s-1 is defined in the contamination area from the model’s computational area. Initially, there is only air inside the urban area, and then pollutants begin to be produced.

At the model inlet boundary, only pure airflow enters, and the velocity inlet condition is used. The value of input speed is defined as a function of the input section’s location, and hence, a profile in UDF format is used.

Geometry & Mesh

The present model is designed in three dimensions using Design Modeler software. The model’s computational area has a length of 36 m, a width of 24 m, and a height of 8 m. Within this computational area, two rows of simple geometric building blocks are designed to parallel each other. Also, to reduce the computational cost, the model is designed in a limited way, and the symmetry boundary condition is used in the surrounding areas of this urban area.

air pollution

We carry out the model’s meshing using ANSYS Meshing software, and the mesh type is unstructured. The element number is 1938659. The following figure shows the mesh.

air pollution

Air Pollution in Street Canyon CFD Simulation

We consider several assumptions to simulate the present model:

  • We perform a pressure-based solver.
  • The simulation is steady.
  • The gravity effect on the fluid is ignored.

The following table represents a summary of the defining steps of the problem and its solution:

Models (Air Pollution)
Viscous k-epsilon
k-epsilon model RNG
near wall treatment standard wall functions
Species Model Species Transport
formulation implicit
number of volumetric species 2 (air & pollutant)
Energy On
Boundary conditions (Air Pollution)
Inlet Velocity Inlet
velocity magnitude UDF
temperature 300 K
pollutant mass fraction 0
air mass fraction 1
Outlet Pressure Outlet
gauge pressure 0 pascal
Walls Wall
wall motion stationary wall
heat flux 0 W.m-2
Symmetry Wall Symmetry
Methods (Air Pollution)
Pressure-Velocity Coupling SIMPLE
Pressure second order
momentum second order upwind
turbulent kinetic energy first order upwind
turbulent dissipation rate first order upwind
pollutant second order upwind
energy second order upwind
Initialization (Air Pollution)
Initialization methods Standard
gauge pressure 0 pascal
x-veloity 6.57939 m.s-1
y-veloity & z-veloity 0 m.s-1
pollutant 0
temperature 300 K

Results & Discussions

At the end of the solution process, three-dimensional contours related to pressure gradient, velocity, temperature gradient, air mass fraction, mass fraction of pollutants, and two-dimensional contours related to velocity, air mass fraction, and mass fraction of pollutants were obtained. As can be seen from the pictures, air pollution occurs from the interior of the street canyon. Two-dimensional and three-dimensional velocity vectors have also been obtained. Depending on the behavior of the velocity vectors inside the canyon, a vortex or flow rotation appears.

There are a Mesh file and a comprehensive Training Movie that presents how to solve the problem and extract all desired results.


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