Study On Large Eddy Simulation Of Air Pollutant Diffusion In Building Near Field

The prediction of flow field and pollutant diffusion around buildings are the important issue that related to human health and life safety.Thus,numerical simulation technology has been more and more applied in the field of air pollutant diffusion prediction in building environment.Among them,Navier Stokes equations（RANS）model is the most widely used,but RANS is a qualitative analysis of the stable solution,which makes the reattachment length and pollutant concentration behind the building overestimated,and the pollutant distribution is also quite different from the actual situation.With the development of computational fluid dynamics（CFD）,large eddy simulation method has also been applied,which makes it possible to predict the flow field and diffusion around buildings with high accuracy.Due to different research purposes,structures,boundary conditions and modeling methods,then it is difficult to evaluate the optimal model parameters and limitations of LES in evaluating the diffusion of pollutants around buildings.In this paper,WALE model which can highlight the superiority of the prediction effect of flow and diffusion around buildings,and the synthetic turbulence inlet method with better performance-VPDSRFG method are used for simulation calculation,and based on Open FOAM open source software,its effectiveness was verified and the optimal parameters were selected.Then,the wind tunnel was used to predict the diffusion of air pollutants around local buildings in a factory.The main research contents and achievements are as follows:（1）The application of large eddy simulation technology in the near field diffusion of airborne pollutants in buildings,and briefly described the characteristics of atmospheric boundary layer,point to the necessity of using LES simulation technology.Introduced the basic theory and boundary treatment of large eddy simulation,especially the boundary construction method of LES turbulent inlet.（2）Introduced the basic architecture and numerical solution application method of Open FOAM platform,and constructed a three-dimensional numerical wind tunnel for large eddy simulation of incompressible atmospheric pollutant diffusion on Open FOAM platform,including the call of VPDSRFG turbulence inlet boundary method.Development of LES solver for pollutant diffusion in atmospheric boundary layer and treatment of boundary conditions,especially near wall surface.（3）The effectiveness of LES numerical wind tunnel was calibrated by using the wind tunnel experimental data and evaluation indexes in TPU database,and the flow field area division and characteristics around the building were qualitatively defined.Then,the influences of grid resolution⁺,WALE model coefficientand sublattice Schmidt number(8on the simulation results are analyzed,and the optimal parameters of LES numerical wind tunnel in the application of atmospheric pollutants diffusion around buildings are obtained.The results show that the LES numerical wind tunnel constructed in this paper meets the evaluation criteria.The WALE model coefficient in RA region is closer to the optimal value whenis larger,while other regions show an opposite trend.And it is suggested that=0.4005 and(8=0.3 are used to solve the diffusion of things around buildings with coarse resolution grid（⁺30～50）.The time-mean field results obtained have no significant difference with fine-resolution grid（⁺1～2）,and the computational resources consumed are only 1/16 of fine-resolution grid.（4）The numerical wind tunnel constructed in this paper is used to predict the pollutant diffusion in the local street valley of a factory,and the wind environment and pollutant diffusion distribution around the building are analyzed.The results show that the air quality at the air inlet is not affected by pollutant emissions,but the actual average wind speed at the air inlet is greater than the designed wind speed,resulting in excessive air intake.At the pedestrian height near the ground,a vortex with wind speed greater than 3m/s appears in the street valley surrounded by the building.When 250m downwind from the pollution source,the pollutant concentration starts to be greater than 1%of the emissions.Concentrations greater than 1%at 80m above vertical height.