Air Pollution within a Street Canyon, CFD Simulation, ANSYS Fluent Training

Description

The present problem simulates pollution diffusion 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 effectively distribute urban heat or the distribution of different gases in the air. 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. The meshing of the model has been done using ANSYS Meshing software. The element number is 1938659.

Air Pollution Methodology

This work investigates the amount and distribution of pollutants in the space of these canyons. Therefore, the Species Transport model has been used to perform this simulation.

The model has two types of gas, including air and pollutants; the pollutant species has a specific heat capacity of 1100 j/kg.K and a molecular weight of 77.49064 kg/kmol, and air has a particular capacity of the heat of 1006.43 j/kg.K and the molecular weight is equal to 28.966 kg/kmol.

It is assumed that all pollutants are generated within the street canyon space; 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/m3.s 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 velocity is defined as a function of the input section’s location, and hence, a profile in UDF format is used.

Also, the air temperature is set to 300K. Moreover, the RNG k-epsilon model and energy equation are enabled to simulate the turbulent fluid equations and calculate the temperature change within the domain.

Air Pollution Conclusion

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 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.