A Practical Approach to Time-Varying Inflow Simulation and the Influence on Intermittent Airflow Within Urban Street Canyons

Based on large eddy simulations, intermittent airflow within an urban street canyon was simulated. The practice of time-varying inflow conditions (TVIC) required a time series of inflow wind velocity, which could be collected on a varying curve of the moving averaged measured data. The influences of the time interval of the wind series and the varying trend (or molded line) between adjacent data on airflow within the street canyon were analyzed. The results showed that TVIC would result in larger average wind velocity and turbulence intensity than that simulated under steady inflow conditions (SIC). The simulated total vertical air exchanges under TVIC would be one order of magnitude higher than that simulated under SIC. Airflow characteristics within street canyons were influenced by the varying trends and the time intervals of the time-series inflow wind. Average vertical wind velocity and turbulent kinetic energy (TKE) simulated under the stepped varying trend was higher than that under the jagged varying trend. The shorter the time interval, the larger the TKE within the street canyon. Vertical air exchanges induced by turbulence (ACH′) at the roof level simulated under the stepped molded lines were twice that of the jagged molded line. Under the time interval of 30 s, the ACH′ was significantly increased, which was 2.558 times that simulated with a time interval of 1 min. Thus, the suggested practical approach for time-varying inflow simulations is to obtain time-series wind data with a time interval of 1 min or less, and the linearly molded line would be critical; for larger time intervals, reasonable molded lines would be required.