Large-eddy simulations of nine idealized heterogeneous urban morphologies with identical building density and frontal area index are used to explore the impact of heterogeneity on urban airflow. The fractal-like urban morphologies were generated with a new open-source Urban Landscape Generator tool (doi:10.5281/zenodo.3747475). The vertical structure of mean flow and the dispersive vertical momentum transport within the roughness sublayer are shown to be strongly influenced by the building morphologies. The friction velocity and displacement height show high correlations with the maximum building height rather than the average height. Well-known roughness parametrizations of the logarithmic layer cannot adequately capture the large spread observed in the large-eddy simulation data. A generalized frontal area index Λf\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\Lambda }_f$$\end{document} is introduced that characterizes the vertical distribution of the frontal area in the urban canopy. The vertically distributed stress profiles, which differ significantly per simulation, are shown to roughly collapse upon plotting them against Λf\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\Lambda }_f$$\end{document}. The stress distribution representing urban drag can be fitted with a third degree polynomial. The results can be used for more detailed and robust representations of building effects in the development of urban canopy models.

中文翻译：

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理想化异构城市环境中的阻力分布

九个具有相同建筑密度和锋面指数的理想化异质城市形态的大涡模拟被用来探索异质性对城市气流的影响。类似分形的城市形态是使用新的开源城市景观生成器工具 (doi:10.5281/zenodo.3747475) 生成的。平均流的垂直结构和粗糙度子层内的分散垂直动量传输受建筑物形态的强烈影响。摩擦速度和位移高度与最大建筑物高度而非平均高度显示出高度相关性。众所周知的对数层粗糙度参数化不能充分捕捉在大涡模拟数据中观察到的大范围。广义正面面积指数Λf\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \ setlength{\oddsidemargin}{-69pt} \begin{document}$${\Lambda }_f$$\end{document} 被引入，它表征了城市冠层中锋区的垂直分布。每个模拟显着不同的垂直分布的应力分布在将它们与 Λf\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} 作图时显示大致崩溃\usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\Lambda }_f$$\end{document}。代表城市阻力的应力分布可以用三次多项式拟合。结果可用于在城市冠层模型开发中更详细、更可靠地表示建筑效果。