Thorough investigations of urban-canopy drag primarily stemming from pressure drag on building surfaces are necessary given the turbulent flows within complex urban areas. Moreover, a gap persists regarding the relationships between canopy drag and breathability. Therefore, this work delves into the canopy-layer airflow dynamics for generic urban neighborhoods by performing three-dimensional Reynolds-Averaged Navier-Stokes simulations. A total of 32 subcases are examined, encompassing uniform- and varying-height and diverse plan area densities (, categorized into groups of sparse: 0.0625/0.067, medium: 0.23/0.25, and dense: 0.53/0.56). Results for the drag distribution highlight the windward-row shelter effect for the medium and the dense, local shelter by taller buildings, and distinct shapes of sectional drag forces (). Local velocity and mean age of air are found strongly positively and negatively correlated to , respectively, with distinct slopes in relation to . For the uniform-height, the normalized bulk drag (, referred to as drag coefficient in literature) peaks for the medium with wake-interference regime; demonstrates a maximum increase of over two times with height variation; moreover, for varying-height groups exhibits a marked increase from the sparse to the medium, while remaining comparable values for the dense. The frontal area averaged drag () exhibits a decreasing trend against across all cases. Further, exhibits strong correlations with and porosity, and with bulk ventilation indices such as spatially averaged velocity, air change rate, and normalized net escape velocity. Throughout the ‘suburban-urban-suburban’ canopy, medium neighborhoods exerting larger drag cause greater streamwise outdoor pressure drops and flow reductions compared to the sparse. However, dense neighborhoods with lower drag exhibit even larger pressure losses, which should be carefully scrutinized. The findings can inform urban planners in designing more aerodynamically efficient neighborhoods and guide strategies for improving air quality within urban environments.

中文翻译：

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城市冠层气流动力学：对一般社区阻力和分布及其与透气性关系的数值研究

考虑到复杂城市地区的湍流，有必要对主要源于建筑物表面压力阻力的城市冠层阻力进行彻底研究。此外，关于顶篷阻力和透气性之间的关系仍然存在差距。因此，这项工作通过执行三维雷诺平均纳维斯托克斯模拟，深入研究一般城市社区的冠层气流动力学。总共检查了 32 个子案例，包括均匀高度和不同高度以及不同的平面区域密度（分为稀疏组：0.0625/0.067、中等组：0.23/0.25 和密集组：0.53/0.56）。阻力分布的结果突出了中等和密集的迎风排遮挡效应，较高建筑物的局部遮挡，以及截面阻力的不同形状（）。发现空气的局部速度和平均年龄分别与 呈强烈正相关和呈负相关，并且与 相关的斜率明显。对于均匀高度，归一化体积阻力（文献中称为阻力系数）对于具有尾流干扰状态的介质达到峰值；表现出随高度变化最大增加两倍以上；此外，对于不同高度的组，表现出从稀疏到中等的显着增加，同时保持密集的可比值。在所有情况下，正面区域平均阻力 () 均呈现下降趋势。此外，与孔隙率以及体积通风指数（例如空间平均速度、换气率和归一化净逃逸速度）表现出很强的相关性。在整个“郊区-城市-郊区”的树冠中，与稀疏社区相比，中等社区施加更大的阻力会导致更大的流向室外压降和流量减少。然而，阻力较低的密集社区表现出更大的压力损失，应仔细检查。研究结果可以为城市规划者设计空气动力学效率更高的社区提供信息，并指导改善城市环境空气质量的策略。