Ventilation Of Street Level Urban Blocks And Far-field Airborne Pollutant Dispersion

Ventilation and air quality in urban areas directly affect the life and health of residents.In order to ensure urban air quality,it is necessary not only to carry out urban planning and design in a scientific and reasonable manner,but also to have a detailed understanding of the flow and diffusion of gaseous pollutants under various complex weather conditions.In order to better predict the ventilation of urban blocks,this thesis uses the computational fluid dynamics(CFD)method to simulate the wind environment and the migration process of air pollutants in an idealized urban block,and quantitatively analyzes the ventilation and diffusion conditions of the block through the corresponding evaluation indicators.Firstly,the basic theory of numerical calculation of urban block ventilation is introduced and deduced in detail,which is the key content of this work.Specifically,it includes fluid flow governing equations,far-field gaseous pollutant transport equations,turbulent model equations and wall function methods.At the same time,the evaluation indicators of ventilation and short-distance diffusion of pollutants in urban block-scale buildings are also summarized.Then,an idealized urban block physical model is established,and the computational fluid dynamics(CFD)method is used to investigate the air flow and pollutant dispersion in the urban block under the background of strong wind and weak wind,respectively.Under the background of strong wind,the effects of six street valley height-to-width ratios and six block shapes on the flow characteristics and pollutant distribution in the urban canopy were considered.In this case,the interaction of the weather background wind with the surrounding building conditions determines the ventilation in the urban environment.As important geometric parameters affecting urban ventilation,both the height-to-width ratio of the street valley and the block shape have a significant impact on the flow structure in an idealized urban block.In a uniform street valley,the ventilation performance of the street and the street valley and the change of building height show an opposite trend.At low street valley aspect ratio(H/W ≤ 1.0),the increase of building height will weaken street ventilation and improve street valley ventilation;while at high street valley aspect ratio(H/W > 1.0),the increase of building height will enhance Street ventilation reduces street valley ventilation.In uneven street valleys,there will be relatively different ventilation conditions in adjacent street valleys.In general,the ventilation conditions of the upper stepped street valley are better than that of the lower stepped street valley.Under the weak wind background,unstable thermal stratification has a significant impact on ventilation and diffusion in different street valleys.The increase of heating intensity at the bottom of the street canyon significantly affects the airflow structure and pollutant distribution in the down-step street canyon.And with the strengthening of the ground thermal effect,the ventilation capacity in the street valley is gradually enhanced.In addition,compared with when no wall is heated,it is concluded that the thermal effect of the building wall can better enhance the air flow in the street valley and promote the ventilation of the street valley.For different wall heating,it is found that the ventilation in the street valley is the best when the floor is heated.Changes in the symmetry of street canyons can cause large differences in ventilation in adjacent canyons.In a down-step canyon,the worse the building symmetry,the better the ventilation.In the upper stepped canyon,the excessive street canyon asymmetry will significantly reduce the internal ventilation capacity.When σH = 16.7%,the ventilation performance in the stepped street canyon is the best.For the migration and diffusion of far-field gaseous pollutants in urban blocks,the ventilation performance of street canyons is not the only factor that determines the dilution and removal of pollutants.In a uniform street canyon,when the height-to-width ratio of the street canyon is H/W=1.0,the canopy airflow has the strongest ability to dilute and diffuse pollutants.In non-uniform street canyons,the influence of the far-field gaseous pollutant diffusion path is greater than that of the airflow characteristics of the street valley.Through the research work of this thesis,it provides a theoretical basis for in-depth understanding and objective evaluation of block-scale ventilation performance,block penetration and diffusion characteristics of far-field pollutants,block planning and design(weak wind environment),etc.