Abstract

The air quality in street canyons is closely related to the category of the street canyons and approaching wind conditions. Compared to urban street canyon (USC), flow and dispersion in isolated street canyon (ISC) show more instability. This study conducted wind-tunnel experiments to investigate the influence of wind velocity (U_H), upwind building width (W_B), wind profile (α) and turbulence intensity (I) on flow instability and pollutant dispersion characteristics within an ISC. The flow field visualization and quantitative pollutant detection system were applied to capture the flow and diffusion pattern characteristics. Results show that compared to the stable clockwise main vortex in USC, the flow structure in ISC presents the strong instability, and the average concentration in ISC is about half of that in USC. Due to the flow separation vortex, the flow instability in ISC is manifested by the alternation of upper clockwise vortex and lower counterclockwise vortex. That also cause the pollutant accumulate on the windward side. And this particular flow structure in ISC is determined by both building height Reynolds number Re_H,crit (= 1.6 × 10⁴) and building width Reynolds bumber Re_W,crit (= 2.4 × 10⁴). Moreover, as α increases, the clockwise main vortex will gradually dominate in ISC, meanwhile, the increasing turbulence intensity will be conducive to strengthening the air exchange rate and reducing pollutant concentrations. This study will be helpful to further understand the instability characteristics and formation mechanism of flow and dispersion within ISCs.

中文翻译：

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孤立街道峡谷内流动不稳定性和污染物扩散的风洞研究

摘要

街道峡谷的空气质量与街道峡谷的类别和接近的风况密切相关。与城市街道峡谷（USC）相比，孤立街道峡谷（ISC）的流动和扩散表现出更多的不稳定性。本研究通过风洞实验来研究风速（U _H）、迎风建筑物宽度（W _B）、风廓线（α）和湍流强度（I）对ISC内流动不稳定性和污染物扩散特性的影响。应用流场可视化和定量污染物检测系统来捕获流动和扩散模式特征。结果表明，与USC稳定的顺时针主涡相比，ISC的流动结构呈现出较强的不稳定性，ISC的平均浓度约为USC的一半。由于存在流动分离涡，ISC中的流动不稳定性表现为上顺时针涡和下逆时针涡的交替。这也导致污染物在迎风面积聚。 ISC 中的这种特定流动结构由建筑物高度雷诺数Re _H,crit (= 1.6 × 10 ⁴ ) 和建筑物宽度雷诺数Re _W,crit (= 2.4 × 10 ⁴ ) 决定。而且，随着α的增大，ISC中顺时针主涡将逐渐占主导地位，同时湍流强度的增加将有利于加强空气交换率，降低污染物浓度。本研究将有助于进一步了解ISCs内流动和弥散的不稳定性特征和形成机制。