Large eddy simulation of the effect of unstable thermal stratification on airflow and pollutant dispersion around a rectangular building

Unstable thermal stratification, quantified by the Richardson number (Ri) herein, has important effects on pollutant dispersion. In this study, the impacts of unstable thermal stratification on airflow and pollutant dispersion around a rectangular building are investigated through large eddy simulation (LES), in which a wide range of Richardson numbers (−1.5–0) is considered. Increasing instability enhances the removal of pollutant near the ground with strengthened advection and turbulence, and thus results in lower normalized pollutant concentration near the ground and higher concentration around the building height. Meanwhile, the streamwise length of the recirculation region behind the building decreases by 32% when Ri decreases from 0 to −1.5, which further enhances pollutant dispersion behind the building. In terms of pollutant dispersion behind the building where the recirculation region emerges, the advection flux of concentration plays a crucial role, which is nearly four times the turbulent diffusion flux in magnitude when Ri ​= ​−1.5. On the other hand, near the boundaries of the recirculation region, turbulent diffusion flux is non-negligible in transporting the pollutant along the concentration gradient. With lower stability, the proportions of fluctuation energy of velocity and concentration in the low-frequency range decrease behind the building compared to those in the high-frequency range.