Effects of low boundary walls under dynamic inflow on flow field and pollutant dispersion in an idealized street canyon

In this study, we examine the magnitude of the boundary layer inflow that gradually changes with time at the top of a building. A standard k-ε model was used to study how low boundary walls (LBWs) under dynamic wind field and dynamic vehicle flow affect the dispersion of pollutants in an idealized street canyon. The results show that the flow field under dynamic inflow inside the idealized street canyon within LBWs is composed of a major eddy and three corner eddies at the street level. When the external wind speed drops, the air is expanded, regarding which the canyon from air expansion is approximately 11 m high at time t = 25 min and 3 m at time t = 33 min. The air at the road corners is correspondingly expanded. The presence of LBWs increases the peak of pollutant concentrations in the canyon and alters the pollutant distribution at the canyon bottom under the dynamic flow at different time points. In the LBWs street canyon compared with the non-LBWs street canyon, the pollutant concentration peaks at time t = 25, 29, and 33 min increase by 12.74%, 30.90% and 22.80%, respectively. The presence of LBWs significantly decreases the maximum x-direction wind speeds at 1-m height in the bottom canyon at different time points but alters the wind directions at the corners and changes the value and distribution of turbulent kinetic energy in the canyon. The largest leeward and windward y-direction wind speeds under dynamic inflow are 27.41% and 32.75% compared with under steady inflow.