Gap flow dynamics and air pollutant dispersion mechanism behind building clusters.

This article investigates the influence of multi-scale building gaps on wind flow dynamics and air pollutant dispersion within urban building clusters using large-eddy simulation (LES) and bidirectional extended proper orthogonal decomposition (EPOD) analysis. It reveals that accelerated jets formed by building gaps exhibit distinct sweeping motions that interact with wake vortices, producing complex and nonlinear pollutant dispersion patterns; notably, small-scale gaps induce high-frequency jet fluctuations that enhance pollutant removal through harmonic oscillations along the spanwise direction. While traditional proper orthogonal decomposition (POD) captures dominant flow structures, it fails to fully elucidate the intrinsic coupling between velocity and concentration fields, a limitation addressed by the EPOD method which effectively characterizes turbulent transport mechanisms under gap flow effects. The study's findings highlight that mixed-scale gap configurations can optimize urban ventilation by balancing space efficiency and pollutant dispersion, offering valuable insights for urban planning and air quality management in densely built megacities.