Southgate-Ash Cameron, Mishra Abhishek, Grimmond Sue, Robins Alan, Placidi Marco
Department of Mechanical Engineering Sciences, Environmental Flow Research Centre (EnFlo), University of Surrey, Guildford, UK.
Department of Meteorology, University of Reading, Reading, UK.
Boundary Layer Meteorol. 2025;191(5):20. doi: 10.1007/s10546-025-00910-3. Epub 2025 May 5.
Urban forms characterised by multi-scale roughness can drastically modify the wind structure within cities affecting both pedestrian comfort and air quality at street level. For simplicity, most urban flow studies focus on cuboid buildings with a single length scale. We consider six forms to assess how additional length scales impact urban flow: two reference cuboids that differ in aspect ratio (mean building height to width) cases (Standard, 1; Tall, 3) plus two additional fractal iterations of each. The six models have the same mean building width, height, and frontal area but their length scale characteristics differ. These are used in wind tunnel experiments within a deep turbulent boundary layer. The length scale differences are found to affect the drag force exerted by the buildings in a non-negligible way (up to 5 and 13% for Standard and Tall buildings, respectively). The added length scales also modify the wake lateral spread and intensity of the turbulence fluctuations, with smaller the length scales having the lower (higher) intensity of fluctuations in the near (far) wake. Additionally, the strength of the vortex shedding emanating from the buildings is reduced by introducing systematically smaller length scales. This work suggests that omission of additional length scales can lead to inaccuracies in drag and wake recovery estimations. The reduction in intensity of vortex shedding found with each fractal iteration could have engineering applications (e.g. reducing vibration).
具有多尺度粗糙度的城市形态会极大地改变城市内部的风结构，影响行人舒适度和街道层面的空气质量。为简单起见，大多数城市气流研究聚焦于具有单一长度尺度的长方体建筑。我们考虑六种形态来评估额外的长度尺度如何影响城市气流：两种长宽比（平均建筑高度与宽度之比）不同的参考长方体案例（标准型，1；高瘦型，3），每种再加上两种额外的分形迭代。这六种模型具有相同的平均建筑宽度、高度和正面面积，但它们的长度尺度特征不同。这些模型用于深湍流边界层内的风洞实验。发现长度尺度差异会以不可忽略的方式影响建筑物施加的阻力（标准型和高瘦型建筑分别高达5%和13%）。额外的长度尺度还会改变尾流的横向扩展和湍流波动强度，长度尺度越小，近尾流（远尾流）中的波动强度越低（越高）。此外，通过系统地引入更小的长度尺度，建筑物产生的涡旋脱落强度会降低。这项工作表明，忽略额外的长度尺度可能会导致阻力和尾流恢复估计不准确。每次分形迭代中发现的涡旋脱落强度降低可能具有工程应用（例如减少振动）。
