Multiscale Turbulence Characteristics over an Idealized Urban Street Canyon using Large-Eddy Simulation

With rapid urban expansion and population growth, enhancing urban sustainability has become increasingly critical—particularly in addressing environmental challenges. A key aspect is to understand the urban airflow dynamics, especially at the pedestrian level. This study employs Empirical Mode Decomposition (EMD) and the Hilbert–Huang Transform (HHT) to investigate the scale-dependent characteristics of turbulence across the Urban Canopy Layer (UCL) and Roughness Sublayer (RSL). Results show that the recirculation zone is predominantly influenced by the shear layer, characterized by cube-scale turbulence in streamwise velocity fluctuations (u′′). The entrainment zone demonstrates the strongest u′′ intensity, dominated by large-scale turbulent motions, while medium-scale structures prevail in the detrainment zone. For vertical velocity fluctuations (w′′), all three wake zones are mainly governed by small-scale turbulence within the UCL. Additionally, amplitude modulation (AM) analysis reveals that the entrainment zone exhibits the strongest top-down modulation between large scale and large – scale envelop of the small scale, whereas the recirculation zone shows higher AM coefficients near the roof level. The detrainment zone presents a broader vertical extent of high AM coefficients across both UCL and RSL, indicating enhanced vertical mixing capacity. These insights offer valuable guidance for optimizing urban ventilation and pollutant dispersion strategies.