Influence of thermal stability on the ventilation of a 3-D building array. (October 2020) - Duan, G.
Abstract: The influence of thermal stability on the ventilation of a cubic building array is examined via large-eddy simulation. For unstable stratification (with bulk Richardson numbers in the range − 0.5 ≲ Rb < 0 ), ventilation timescales decrease as the ground-level heating increases; in particular, the mean tracer age and pollutant retention timescale scale linearly with the mean circulation timescale. For stable stratification ( 0 < Rb ≲ 0.3 ), however, the ventilation timescales increase with stronger ground-level cooling even though the mean streamwise wind speed and turbulence kinetic energy remain approximately constant. The ventilation pathway changes for stable stratification because pollutants preferentially escape through the lateral openings rather than the roof level. It is argued that conventional time-averaged turbulence statistics (e.g. Reynolds stresses) provide an incomplete picture of the ventilation mechanism under stable stratification. Highlights: The ventilation mechanism undergoes a transition at bulk Richardson number Rb = 0. Ventilation through the roof level and lateral boundaries are linked for Rb < 0 but decoupled for Rb > 0. The mean tracer age and retention timescales scale linearly with the mean circulation timescale for unstable stratification. Conventional time-averaged turbulence statistics provide an incomplete picture of the ventilation mechanism for stable stratification.