Turbulent flows over real heterogeneous urban surfaces: Wind tunnel experiments and Reynolds-averaged Navier-Stokes simulations

Abstract

Wind tunnel experiment and steady-state Reynolds-averaged Navier-Stokes (RANS) approaches are used to examine the urban boundary layer (UBL) development above Kowloon Peninsula, Hong Kong Special Administrative Region (HKSAR). The detailed urban morphology is resolved by computational fluid dynamics (CFD) and is fabricated by 3D-printing (reduced scale) for wind tunnel experiments. Different from the majority existing results based on idealized, homogeneous urban geometries, it was found that the wind and turbulence in the UBL over downtown Kowloon are characterized by the wake behind several high-rise buildings. In particular, local maxima of turbulence kinetic energy (TKE) and shear stress are found at the roof level of those high-rise buildings. In the downstream region where the flows are already adjusted to the urban surfaces, the urban roughness sublayer (URSL) can be further divided into two layers based on the structures of the mixing length lm, effective drag Dx and dispersive stress. In the lower URSL (z ≤ 100 m), lm is rather uniform, and the Reynolds stress and dispersive stress are comparable. In the upper URSL (100 m ≤ z ≤ 300 m), on the contrary, lm is peaked at the mid-height and the magnitude of dispersive stress is smaller than that of the Reynolds stress (< 30%). The effective drag Dx is negligible in the upper URSL.