Numerical Study On Identification Of Urban Ventilation Corridor And Wind-thermal Dynamics In Urban Blocks

Urban is large human settlements that are constantly developing and expanding,and frequent human activities have led to more anthropogenic heat and pollution sources,leading to serious problems such as decreasing urban ventilation performance,urban heat island phenomenon(UHIs),and environmental pollution.There is a mutual feedback mechanism between ventilation capacity,thermal environment,pollution and atmospheric circulation.The study of urban climate and environment is of great significance for the development of sustainable ecology city and protection of human health.As urbanization advances and expands,its boundaries and internal morphology tend to be complex,so studies on urban climate and environment usually include multiple physical scales.In this study,the identification of urban ventilation corridors at the regional scale(10 ～ 100 km)and the characteristics of atmospheric circulation and its pollutant dispersion capacity at the block scale(100 ～ 1000 m)are carried out based on multi-scale numerical simulation techniques.For regional scale,urban ventilation corridors have the ability to alleviate UHIs and air pollution caused by the continuous development and expansion of cities.Traditional methods for researching RVCs are typically based on Geographic Information Systems(GIS)or Computational Fluid Dynamics(CFD).In this study,a new method based on backward trajectory simulation,which has rarely been employed in RVC identification,is proposed to overcome the limitations of GIS-and CFD-based methods.Taking Beijing as the research area,this study evaluates regional ventilation potential based on wind statistics,surface roughness,and wind field simulated by the Weather Research and Forecasting(WRF)model.Moreover,the backward mode of the coupling FLEXPART-WRF model is used to identify RVCs flowing through the city.The spatiotemporal differences between the RVC boundary layer ventilation(BLV)and urban canopy layer ventilation(UCLV)on synoptic and seasonal scales under meso-scale atmospheric circulation in different seasons are effectively identified.Some RVC characteristics that have not been found in previous studies are revealed: In winter,RVCs affecting the BLV of Beijing include mainly a northeast RVC and a west RVC,and the northeast RVC also affects UCLV.In summer,the southwest corridor contributes the most to ventilation.For block scale: The complex mechanical turbulence(MT)and thermal turbulence(TT)in the urban atmosphere determines the stability and circulation characteristics.Most of the previous studies are based on the assumption of single turbulence dominated,i.e.,MT dominated(neutral,stable)or TT dominated(unstable).However,when the intensity of these two types of turbulence is comparable,that is,the atmosphere is in transition,the characteristics and evolution of the circulation have not received enough attention at present.This study presents a CFD model-based gradient scenario experiments of the inlet wind profile and surface temperature to describe transition state atmosphere.Using the bulk Richardson number as an indication,the transition-state emergence scenario is first explored,i.e.,under which wind-heat conditions the MT and TT are of similar strength.The evolution of multiple steady-state circulations is then explored.The main pattern of transition from MT-dominated to TT-dominated is that the horizontal airflow above the street valley is noticeably lifted under buoyancy and correspondingly forms a sinking airflow behind.Finally,the dispersion capacity under different atmospheric state controls is examined.The key conclusion is that upward TT and strong MT makes a meaningful contribution to pollutant dispersion at the neighborhood scale.