Simulation Study Of Downwash Effects On Pollutant Dispersion From A Stack On The Rooftop

With the rapid development of global and China's economic and urbanization,population explosion and expansile traffic scale lead to the increasingly obvious conflicts between humans and the environment.Due to the diminishing land resources in the city and new construction technology skills applied,the tall buildings are springing up and their shape and structure are diversified.Specific emissions inevitably produced by human activities and are emitted through the stack on the roof into the atmosphere.A variety of harmful pollutants will cause the air pollution.When blocked by tall buildings,parts of the wind goes upward and across both sides and some sinks to ground.This is called downwash effect,which makes the city building roof with chimney polluted air brought to the building near the ground,and it will produce some effects on human health.In this paper,a method of combining wind tunnel experiments and CFD simulations is used to explore the factors such as different building configurations,layouts of buildings,various pollution source locations,different roof shapes and heights of the upstream building and various heights of downstream building that how they affect the airflow and pollutant dispersion between the buildings in oder to reveal the influence of roof pollutant exhausts by building downwash effects.A three-dimensional CFD model for evaluating the flow and pollutant dispersion was built up,which was based on the incompressible N-S equations,realizable k-? turbulence model and the pollutant transportation equation.The numerical simulations of the model were conducted on ANSYS FLUENT 14.0 with SIMPLE scheme applied.In this study,16 cases are composed of two effect factors.They are different configurations of buildings(four kinds of upstream building's height)and four kinds of pollution source location.Another 20 cases are composed of there factors which are two layouts of building,six kinds of roof shape and three kinds of roof height of upstream buildings.Finally 5 cases of different heights of downstream building under three buildings' layout situation.The results showed that the air flow,vortex structure,the pollutant distribution are closely related to the configuration and layout of buildings,roof shape and height of upstream building and the building height:(1)When the upstream building height is lower than the downstream's,the pollution location will not affect the pollutant distribution inside the upstream canyon and pollutants will spread with wind direction to downstream.When the upstreambuilding height is taller,the distribution will be affected.When the location moves downward,the pollutant is hard to dilute in upper air and the pollutant concentration level gradually reduces down inside canyon.When the pollution location is fixed and the roof shape is steeple,it will do a great harm to environment and hazard to body health in the street canyon.When roof shape is dome,it is friendly to environment.When the upstream building is higher,the concentration distribution law is very different for the layout of two buildings or three buildings' case.(2)Different downstream building(B3)heights make significant influence to the air flow and pollutant diffusion in the entire computing area.The distribution law in two canyons on both longitudinal and transversal surface is same.The distribution is affected by the downstream building height: the concentration is first increased and then decreased with the increase of building height.A point source distribution attached to the large environmental wind tunnel were applied,and a light-monitoring system in the transient laser sheet for pollutant concentration was formed in the wind tunnel experiment for researching the distribution of pollutant between the buildings affected by downwash effect.The concentration distribution figures of some cases are obtained followed the designed experiment process.On this comparison and analysis of simulation and wind tunnel results,the numerical simulation results were in conformity with those of wind-tunnel experiment in a variety of conditions.Meanwhile we also find that the numerical simulation results simulated by realizable k-? turbulence model were feasible and reliable.This research is believed that it has certain guidance and reference for design on layout,structure,height and configuration of future urban tall buildings group and scientific selection of the test points in the environmental monitoring.