We investigate the flow characteristics around step-up street canyons with various building aspect ratios (ratio of along-canyon building length to street-canyon width, and upwind building height to downwind building height) using a computational fluid dynamics (CFD) model. Simulated results are validated against experimental wind-tunnel results, with the CFD simulations conducted under the same building configurations as those in the wind-tunnel experiments. The CFD model reproduces the measured in-canyon vortex, rooftop recirculation zone above the downwind building, and stagnation point position reasonably well. We analyze the flow characteristics, focusing on the structural change of the in-canyon flows and the interaction between the in- and around-canyon flows with the increase of building-length ratio. The in-canyon flows undergo development and mature stages as the building-length ratio increases. In the development stage (i.e., small building-length ratios), the position of the primary vortex wanders, and the incoming flow closely follows both the upstream and downstream building sidewalls. As a result, increasing momentum transfer from the upper layer contributes to a momentum increase in the in-canyon region, and the vorticity in the in-canyon region also increases. In the mature stage (i.e., large building-length ratios), the primary vortex stabilizes in position, and the incoming flow no longer follows the building sidewalls. This causes momentum loss through the street-canyon lateral boundaries. As the building-length ratio increases, momentum transfer from the upper layer slightly decreases, and the reverse flow, updraft, and streamwise flow in the in-canyon region also slightly decrease, resulting in vorticity reduction.

中文翻译：

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不同建筑纵横比的阶梯式街道峡谷周围的流动特征

我们使用计算流体动力学 (CFD) 模型研究了具有各种建筑物纵横比（沿峡谷建筑物长度与街道峡谷宽度的比率，以及迎风建筑物高度与顺风建筑物高度的比率）的阶梯街道峡谷周围的流动特性。模拟结果根据风洞实验结果进行验证，CFD 模拟在与风洞实验相同的建筑配置下进行。CFD 模型合理地再现了测量的峡谷内涡流、顺风建筑物上方的屋顶再循环区和驻点位置。我们分析了流动特征，重点研究了峡谷内流动的结构变化以及随着建筑物长度比的增加峡谷内外流动之间的相互作用。随着建筑长度比的增加，峡谷内水流经历了发展和成熟阶段。在发展阶段（即小建筑长度比），主涡的位置发生漂移，流入的气流紧随上游和下游建筑侧壁。因此，来自上层的动量传递的增加有助于峡谷内区域的动量增加，并且峡谷内区域的涡度也增加。在成熟阶段（即大的建筑物长度比），主涡在位置上稳定，进入的水流不再跟随建筑物侧壁。这会导致通过街道峡谷横向边界的动量损失。随着建筑长比的增加，来自上层的动量传递略有减少，逆流、上升气流、