Study on thermal environment and pollutant dispersion in real urban area by Large Eddy Simulation

Abstract

Modern urban districts are mainly composed of buildings and roads. The buildings are usually tall and various in shapes and moisture flux on the surfaces is of low level. Atmospheric Environment in modern urban districts gets wide attention because of those landmark buildings and the large population in it. In this paper, the central business district (CBD) is set as an example to study thermal environment and pollutant dispersion in urban area. Atmospheric flows in typical summer sunny days are simulated by Large Eddy Simulation (LES) and wind field, temperature field and dispersion process of point pollutant are analysed. The momentum, mass and thermal transport equations are discretized by the finite volume method with non-staggered grids and a third-order Runge-Kutta intergration in time. The filtered Navier-Stocks equations are solved using SIMPLE algorithm. Lagrangiam dynamic model, which is suitable for complex turbulence in urban districts, is applied to close sub-grid stress. Temperature distribution is calculated according to surface energy balance relation. The results reveal that temperature on the wall surfaces is mainly determined by solar radiation while that in air is mainly determined by convection. Near field distribution of pollutant is considerably influenced by wind direction, building distribution and partial flow field. Meanwhile bouyancy enhances vertical convection and expands vertical plume width, while has no considerable influence on spanwise plume width.