微尺度城市冠层模型下的通风与放射性核素扩散特性

Analysis of Ventilation and Radionuclide Diffusion Features Under the Micro-Scale Urban Canopy Layer Model

采用数值模拟方法研究了城市冠层形态对放射性核素迁移扩散特性的影响,并引入速度比(VR)、空气龄(τ_(p))、滞留时间(τ_(c))和大气弥散因子(ADF)4个指标评价城市冠层行人高度处放射性核素的流动和扩散状况。数值结果表明,城市冠层形态的改变对主街道上的空气龄影响较小,但对街道峡谷内的空气龄影响较大;在较高建筑迎风侧下洗风的现象较强,通风状况较好,而在其背风侧存在气流停滞现象,通风状况较差。放射性核素的迁移扩散主要受城市冠层形态的影响,通风良好的街道峡谷不一定有较小的滞留时间,而通风较差的街道峡谷也不一定有较大的滞留时间。受建筑群的机械扰流影响,大气弥散因子明显降低,当城市冠层为凸型和递增型时,放射性核素在下游距离450~900 m区域内扩散效果最好。

A research has been carried out on the influence of urban canopy morphology on the migration and diffusion features of radionuclides by using the method of numerical simulation,with an introduction of such four indicators as the velocity ratio(VR),age of air(τ_(p)),retention time(τ_(c))and atmospheric dispersion factor(ADF),for an evaluation of the flow and diffusion of radionuclides at pedestrian altitude in urban canopy.The numerical results indicate that changes in urban canopy morphology have a relatively small impact on the air age on the main streets,but have a greater impact on the air age within the street canyons.There is a stronger washing air on the windward side of higher buildings with a better ventilation,while there is airflow stagnation on the leeward side accompanied by a poor ventilation.The migration and diffusion of radioactive isotopes are mainly under the influence of the morphology of urban canopies.Street canyons with a good ventilation may not have a shorter residence time,while poorly ventilated street canyons may not necessarily have a longer residence time.Due to the influence of mechanical turbulence of buildings,the atmospheric dispersion factor is obviously reduced.When the urban canopy shows a protuberant and ascending tendency,the radionuclides are characterized with the best diffusion effect within an area of 450~900 m downstream.