Modelling the transfer of road transport emissions in a street canyon
The study is focused on developing and testing a microscale mathematical model for the analysis of turbulent flow and passive gaseous admixture transfer in street canyons. Mathematical model is based on Reynolds-averaged Navier-Stokes and continuity equations. Boussinesq approximation and two-parameter k-epsilon turbulence model is used to close the equations. Numerical solution of the system of differential equations is obtained with the finite volume method on a staggered mesh. Convective terms of the Navier-Stokes equations are approximated with MLU numerical scheme. SIMPLE computational algorithm is used to couple velocity and pressure fields. Laminar flow on the inlet section of the 2D channel was modelled to test the computational algorithm. Turbulent flow and emission transport in a wind tunnel was modelled to verify the mathematical model. Series of computations of the flow influenced by natural convection in an ideal model of street canyon were performed using the presented mathematical model. Computations were performed for a 24 m height and 20 m wide street canyon. The constant emission source was placed in the center of the canyon near the floor. Analysis of the results has showed that in cases of heating the upwind side or the bottom of the canyon emissions are transported out of the canyon more intensively and maximal concentrations decrease by 10-15% from the isothermal case. In case of heating the downwind side the structure of the flow changes significantly and maximal concentrations increase by 3-3.5 times. The structure of the flow in the street canyon was investigated depending on the ratio of the street width to the height of the buildings. Both width of the street and height of the building varied from 5 to 40 m. The results show that increase in the height of the canyon decrease ventilation of the street canyon and increases local maximal concentrations of adverse emissions.