Numerical Simulation Of Diffusion Characteristics Of Gas Pollutants Near The Surface Under The Terrain Condition Of Mountainous Towns

With the acceleration of urbanization and the rapid development of population and economy in China,air pollution and air quality degradation caused by near-surface pollutants have become the main environmental problems that puzzle the urban development in China.As the world's largest and most populous"hill city",the geographical pattern of hill basins in Chongqing's main urban area has formed the special topographic conditions of hillous towns in Chongqing,and the weather conditions resulting from the low annual average wind speed and high atmospheric stability have led to serious air pollution in some parts of the main urban area of Chongqing.Therefore,it is necessary to study the flow and diffusion of near-surface pollutants in hillous cities and towns of Chongqing.In this paper,the component transport model of numerical simulation is used to study the flow state and pollutant diffusion distribution of two typical hill buildings in Chongqing:one is the building group on slopes,and the other is the small building group near hills and hills.The effects of wind speed,height of hill and hill,distance from hill and hill to buildings and location of pollution sources on the diffusion characteristics of pollutants near the ground were studied.The results show that for buildings on slopes,when the transverse spacing B_w between buildings is much larger than the width S_w of the external wake,the flow state in the canopy of buildings is isolated rough flow,which is characterized by the full development of both the external wake and the internal wake.When the transverse distance B_w between buildings is about equal to the width S_w of the external wake,the flow state in the canopy of the complex is the external wake interference flow,that is,the external wake can not fully develop.When the longitudinal spacing B_l is the same as the length R_l of the internal wake,the flow state in the canopy of the complex is the internal wake interference flow,and the development of the internal wake is limited.When the transverse distance B_w between buildings is less than R_l,the flow state in the canopy of the complex is skimming flow.The development of the outer and inner wakes is further restricted.The air flow above the top of the building has little effect on the air flow in the street.When the transverse distance between B_w buildings is about equal to the width of the inner wake R_w,the outer wake and the inner wake will disappear,and the flow state in the canopy is street network flow.When the transverse distance B_w between buildings is smaller than the wake width R_w,the flow state in the canopy is the standing eddy flow,the flow in the pipeline parallel to the flow direction is more stable,and the flow state in the street perpendicular to the flow direction becomes the standing eddy.In the six flow states,with the decrease of building spacing,the concentration of pollutants in front of the building group becomes more serious,the amount of pollutants entering the building group is less,the attenuation speed along the flow direction is slower,the attenuation distance is shorter,and the transverse distribution of pollutants changes from single external wake effect to double wake effect,and finally to no wake effect.In the same flow state,the larger the slope is,the smaller the spacing between the buildings forming the typical flow state is due to the influence of the updraft on the slope.Generally speaking,the larger the distance between buildings,the higher the terrain gradient,the smaller the diffusion of pollutants.Under the condition of neutral atmospheric stability,there is a stable stratification effect near small buildings with hilly and mountainous obstacles.The stratification effect near buildings is not affected by the wind speed.The wind speed will not affect the flow around the complex,but will affect the wind speed in the corresponding layers.Wind speed will directly affect the ventilation of buildings.The ventilation volume in buildings is proportional to the reference wind speed.At the same level,the average wind speed inside buildings is much lower than the average wind speed outside buildings.The bigger the wind speed,the more conducive to the dilution of pollutants.The change of geometric characteristics of buildings and surrounding buildings will affect the change of fluid flow state,and then affect the distribution of pollutants.There is a negative correlation between the height of hills and the ventilation volume of buildings.The higher the height of hills,the lower the ventilation volume.At the same time,the less pollutants that accompany the fluid entering the buildings,the less the distribution of pollutants in the buildings.The distance between hills and buildings has little effect on ventilation and pollutant diffusion.The height and distance of pollution sources have little effect on the flow state of the group.In terms of the height of pollution sources,the height of pollution sources has little effect on the total amount of pollutants entering the complex.With the increase of the height of pollution sources,when the height of pollution sources reaches 0.25H₂,the degree of pollutants near the ground is the greatest.When the height of pollution sources reaches 0.5H₂,the exchange between pollutants entering the complex and the top of the complex is the best,so the higher the height of pollution sources,the better the pollution sources.When the distance to the building complex is 1 times H₂,pollutants are mainly transferred through the lateral effect?horseshoe vortex?,which is very conducive to the diffusion of pollutants.