Publication date: September 2014
Source:Building and Environment, Volume 79
Author(s): Man Lin , Jian Hang , Yuguo Li , Zhiwen Luo , Mats Sandberg
This paper investigates urban canopy layers (UCLs) ventilation under neutral atmospheric condition with the same building area density (λ p = 0.25) and frontal area density (λ f = 0.25) but various urban sizes, building height variations, overall urban forms and wind directions. Turbulent airflows are first predicted by CFD simulations with standard k -ε model evaluated by wind tunnel data. Then air change rates per hour (ACH ) and canopy purging flow rate (PFR ) are numerically analyzed to quantify the rate of air exchange and the net ventilation capacity induced by mean flows and turbulence. With a parallel approaching wind (θ = 0°), the velocity ratio first decreases in the adjustment region, followed by the fully-developed region where the flow reaches a balance. Although the flow quantities macroscopically keep constant, however ACH decreases and overall UCL ventilation becomes worse if urban size rises from 390 m to 5 km. Theoretically if urban size is infinite, ACH may reach a minimum value depending on local roof ventilation, and it rises from 1.7 to 7.5 if the standard deviation of building height variations increases (0%–83.3%). Overall UCL ventilation capacity (PFR ) with a square overall urban form (Lx = Ly = 390 m) is better as θ = 0° than oblique winds (θ = 15°, 30°, 45°), and it exceeds that of a staggered urban form under all wind directions (θ = 0°–45°), but is less than that of a rectangular urban form (Lx = 570 m, Ly = 270 m) under most wind directions (θ = 30°–90°). Further investigations are still required to quantify the net ventilation efficiency induced by mean flows and turbulence.
Source:Building and Environment, Volume 79
Author(s): Man Lin , Jian Hang , Yuguo Li , Zhiwen Luo , Mats Sandberg