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Analysis of Pollutant Entrainment from Localized Sources in a Street Network
Boundary-Layer Meteorology ( IF 4.3 ) Pub Date : 2021-01-21 , DOI: 10.1007/s10546-020-00598-7
F. C. Cezana , E. V. Goulart , N. C. Reis , O. Coceal

The propagation of a pollutant emitted from localized sources both within and above a regular street network is studied by analyzing data from direct numerical simulations of passive scalar dispersion. Two wind directions are considered, corresponding to aligned and oblique flow with respect to the street axes. Particular attention is paid to the role of entrainment of the scalar into the urban canopy from an elevated source and re-entrainment of material originally released further upstream from a ground source. The variation of concentration differences and vertical fluxes between the streets and the air above as a function of distance reveals important differences between the rate of lateral and vertical mixing for the two sources. Detrainment and entrainment need a longer fetch to equilibrate for the elevated source than for the ground source. There are large differences between the advection and detrainment velocities for the aligned and oblique cases, so that a change in wind direction could affect ventilation efficiency considerably. Time scales associated with different dispersion processes are computed and the time of first appearance of the scalar from the onset of release in different streets is mapped. It is shown that re-entrainment can provide a shortcut dispersion pathway for reaching certain parts of the network. This is particularly striking in the case of oblique flow, when material can be transferred by entrainment up to twice as rapidly as by advection. Taken together, these results highlight the overall message that vertical exchange is a two-way process and that entrainment needs to be considered in the context of emergency response as well as urban ventilation.

中文翻译:

街道网络局部源污染物夹带分析

通过分析来自被动标量扩散的直接数值模拟的数据,研究了从常规街道网络内部和上方的局部源排放的污染物的传播。考虑了两个风向,对应于相对于街道轴线的对齐和倾斜流。特别注意标量从高架源夹带到城市树冠中的作用,以及最初从地源更上游释放的材料的再夹带作用。街道和上方空气之间的浓度差异和垂直通量随距离的变化揭示了两个源的横向和纵向混合速率之间的重要差异。与地面源相比,高架源的牵引和夹带需要更长的提取才能达到平衡。对齐和倾斜情况下的平流和滞留速度之间存在很大差异,因此风向的变化会显着影响通风效率。计算了与不同分散过程相关的时间尺度,并绘制了从不同街道释放开始标量首次出现的时间。结果表明,再夹带可以为到达网络的某些部分提供一条捷径分散途径。这在斜流的情况下尤其显着,因为材料通过夹带传输的速度是平流传输速度的两倍。综上所述,这些结果强调了垂直交换是一个双向过程的总体信息,需要在应急响应和城市通风的背景下考虑夹带。
更新日期:2021-01-21
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