Wind tunnel study of turbulence flows over real urban surface with scale analysis

Abstract

Roughness sublayer (RSL) and inertial sublayer (ISL) in the atmospheric surface layer (ASL) over roughness elements exhibit dissimilar dynamics and turbulence structures. In this study, wind tunnel experiments are carried out to measure the ASL flows over the reduced-scale model of real urban morphology. Quadrant and frequency analyses are used to examine the ASL momentum transport and how the processes are influenced by different motion scales. The downtown Kowloon Peninsula, Hong Kong is fabricated by 3D-printing that is adopted as the model. Notable variations of mean wind speed u, fluctuating streamwise u' and vertical w' velocities together with vertical momentum flux u'w' exist in the ISL and RSL. With increasing motion scales, the efficiency of momentum transfer increases in the RSL top and the ISL. The transfer efficiency is peaked in the ISL regardless of the strength of motions. Moreover, the strengthening motion scales result in an increasing and decreasing contributions from ejection Q2 and sweep Q4, respectively, to momentum transport. Whereas, the phenomenon is opposite in the ISL. Additionally, the contribution from eddies in the frequency range of 0.1 Hz to 10 Hz is least then increases thereafter with increasing frequency in the RSL. On the other hand, the contribution increases monotonically with increasing frequency in the ISL. It is thus suggested that large turbulence scales dominate aged air removal of urban areas.