Gap flow dynamics and air pollutant dispersion mechanism behind building clusters

Multi-scale gaps within the urban building configuration intricate the wind flow dynamics surrounding the building clusters and hinder the understanding of pollutant removal mechanisms. Consequently, the impact of fluid dynamics caused by gap flow on air pollutant dispersion has not been fully revealed. This work examines such gap flow dynamics and the fundamental air pollutant dispersion mechanisms in three representative building cluster layouts via the large-eddy simulation and bi-directional extended proper orthogonal decomposition (EPOD) analysis framework. Results show that the accelerated jets generated by building gaps exhibit alternating low/high frequency sweeping motions, which interact with the recirculation vortices in the wake, leading to pronounced differences in air pollutant dispersion patterns. Bi-directional EPOD modes capture the turbulent transport effect of pollutant under the combined influence of jet and recirculation flows, as well as the relevant turbulent coherent structures across various scales. The fluctuation of jet flow dominates the turbulent structure and flow dynamics. A small gap size triggers a high-frequency jet fluctuation that undergoes harmonic swing in the crosswind direction, driving the dynamic pollutant transport behind the buildings. Additionally, the continuous harmonic oscillations in dispersion are found along the spanwise direction, favoring the removal of air pollutants.