Ventilation Mechanism in an Idealized Street Canyon: A Multiscale Turbulence Approach

The low-altitude atmospheric environment has been receiving increasing attention in recent years due to rising human activities and the emerging growth of the low-altitude economy. Urban wakes generate highly inhomogeneous, multiscale turbulent flows, posing challenges for momentum transport, pedestrian-level ventilation, and low-altitude aerial vehicle operations. Large eddy simulation of a neutral boundary layer over an idealized urban street canyon is conducted, and wavelet transforms combined with quadrant analysis are applied to investigate scale-dependent momentum transport across three wake zones: recirculation, entrainment, and detrainment. The results show that strong momentum transport across a broad range of scales is exhibited at the roof level of the recirculation zone. The momentum transport in the entrainment zone is governed by large-coherent-scale turbulence. On the contrary, the dynamics are governed by small-scale turbulence activities accompanied by distinct quadrant asymmetry in the detrainment zone because of the major energy contribution from Q2 and Q4 events. Furthermore, the multiscale characteristics of turbulent transport produce distinct frequency signatures across different wake zones, underscoring their dynamically heterogeneous nature and potential implications for UAV operation.