Numerical Simulation Of PM2.5 Diffusion Based On The Street Canyons Model

In this thesis, computational fluid dynamics, atmospheric sciences, urban planning, traffic engineering disciplines were used to simulate how the exhaust distribute and diffuse within the urban street canyons discharged by vehicles. It plays an important role in urban planning and design, air pollution and traffic control, and other fields for the comprehensive study of atmospheric environment in the microscope street canyons in the future.The combination of standard k-ε turbulence model with spatial component transport model was used to explore the role of air flow field and the distribution of fine particulate matter in typical street canyons. Finite volume method is an effective way to discrete the fluid differential equations. Considering the precision of the calculation and the stability of difference scheme, the predictor corrector method was applied to solve the differential equation of fluid control.The unstructured grid is translated into a structured grid in the computational domain which can effectively improve the efficiency of programming. According to knowledge of meteorology and fluid mechanics, the flow boundaries and the street wall boundary of valley model were set under the premise calculation accuracy.On the basis of symmetric street canyons model, considering the special geometry factors, we aim to promote it to the construction of ascending and descending type street canyons model; considering the factor of roof shape, we aim to build the peaked roof and streamlined street canyons. The main conclusions can be summarized as follows:(1)The internal flow field of and PM2.5 concentration for different aspect ratio H/ W of street canyons model were simulated and analyzed. In the narrow canyon, when aspect ratio is large, two vortexes whose directions on the contrary and turbulent kinetic energy were widely different in size vertically appear. As buildings have wind speed corner, it causes PM2.5 difficult diffusion discharged by the motor vehicle at the bottom of the street.(2)According to different roof structure(ceiling, spires and streamline roof) model of street canyons, the flow field in the streets and PM2.5 concentration field were analyzed. In the roof above a wider area, a turbulent kinetic vortex with decreased energy was created through the spire structure model. However, the small turbulence kinetic energy at the bottom of the street limited internal transport of pollutants diffusion. Streamlined street canyon had more advantages for transport and dilution of pollutants by wind flow in the bottom of street.(3)According to the order of PM2.5 diffusion, the effect from strong to weak in all kinds of street canyon was explored: ascending type street canyon > declining type street canyon > symmetrical street canyon.The diffusion process of ground source pollutant from street canyons has a lot to do with the geometric shape of streets buildings, as the distribution and diffusion mainly depended on the wind field momentum. Those conclusions can be used for guiding to establish street canyons model, improving the atmospheric quality of city.