Techniques for improving the removal of pollution from urban canyons are crucial for air quality control in cities. The removal mainly occurs at the building roof level, where it is supported by turbulent mixing and hampered by roof shear, which tends to isolate the internal canyon region from the atmospheric flow. Here, a modification of roof infrastructures is proposed with the aim of increasing the former and reducing the latter, overall enhancing the removal mechanisms. The topic is investigated by numerical experiment, using large-eddy simulation to study the paradigmatic case of a periodic square urban canyon at \( Re=2 \times 10^4\). Two geometries are analyzed: one with a smooth building roof, the other having a series of solid obstacles atop the upwind building roof. The pollutant is released at the street level. The simulations are successfully validated against laboratory and numerical datasets, and the primary vortex displacement detected in some laboratory experiments is discussed. The turbulence triggered by the obstacles destroys the sharp shear layer that separates the canyon and the surrounding flow, increasing the mixing. Greater vertical turbulent mass fluxes and more frequent ejection events near the upwind building (where pollution accumulates) are detected. Overall, the obstacles lead to a reduction in the pollution concentration within the canyon of about \(34\%\).

改善从城市峡谷中清除污染的技术对于控制城市的空气质量至关重要。清除主要发生在建筑物的屋顶，由湍流混合支撑并受到屋顶剪切的阻碍，这往往使内部峡谷区域与大气流隔离开来。在此，提出了对屋顶基础设施的修改，其目的是增加前者并减少后者，从而总体上增强了拆除机制。通过数值实验研究该主题，使用大涡模拟研究\（Re = 2 \ times 10 ^ 4 \）上一个周期性方形城市峡谷的典型案例。。分析了两种几何形状：一种具有光滑的建筑物屋顶，另一种具有在上风建筑物屋顶上的一系列坚固障碍物。污染物在街道上释放。针对实验室和数值数据集对仿真进行了成功验证，并讨论了在某些实验室实验中检测到的主要涡旋位移。障碍物引发的湍流破坏了将峡谷和周围水流分隔开的尖锐剪切层，从而增加了混合度。在上风建筑物附近（污染物积聚的地方）检测到更大的垂直湍流质量通量和更频繁的喷射事件。总体而言，障碍导致峡谷内污染浓度降低约\（34 \％\）。