Roughness sublayer flows over real urban morphology: A wind tunnel study

Abstract

Atmospheric surface layer (ASL) consists of the inertial sublayer (ISL) and the roughness sublayer (RSL) with different dynamics. This study characterizes the ASL flows over real urban morphology using wind tunnel experiments. The building models of downtown Kowloon Peninsula, Hong Kong are fabricated by 3D printing. The vertical profiles (3 × 45 numbers) on three vertical planes in the streamwise direction are selected for velocity measurements by constant-temperature (CT) hot-wire anemometry (HWA). There exist noticeable variations of mean wind speed u‾, fluctuating velocities u’’ and w’’ together with vertical momentum flux u’’w’’ in the ISL and RSL over urban areas. On the plane with maximum drag coefficient Cd (25% higher than that on the other two), the RSL is thicker by 10% in which the winds evolve in response to individual buildings. A new analytical solution is derived to improve the prediction of ASL mean wind speeds whose root-mean-square error (RMSE) is reduced by 10 times. Its key variable, the RSL constant, converges asymptotically μ = 1.7 for urban setting. Unlike the ISL, turbulence statistics show that the RSL velocities are positively skewed (Su > 0) with high kurtosis (Ku > 3). The occurrence of ejection Q2 (detrainment; 32%–37%) is more frequent than sweep Q4 (entrainment; 30%–32%). Whereas, high-speed Q4 contributes most (0.2u’’w’’ to 0.4u’’w’’) to the RSL vertical momentum flux among the four quadrants. It is thus suggested that fresh air entry is driven by spurlike downdrafts while street-level aged air is removed by massive uprising current.