Inertial and roughness sublayer flows over real urban morphology: A comparison of wind tunnel experiment and large-eddy simulation

Turbulent flows in the inertial sublayer (ISL) and the roughness sublayer (RSL) are highly complicated by real urban surfaces with various building heights and scales. This study used wind tunnel (WT) modeling and large-eddy simulation (LES) to examine the flow structure in the ISL and RSL over a real urban district of Hong Kong. Although random vertical variations are found in the RSL, the results show that the vertical profiles of mean wind speed, streamwise and vertical fluctuating velocity together with momentum flux are comparable between WT and LES. Unlike the linear decrease of momentum flux with increasing height over idealized urban surfaces, significant vertical changes of momentum flux are found in the RSL in both WT and LES, indicating that modification of momentum flux parameterization over real urban surfaces is needed. High-speed, downward flows are observed in the RSL compared with low-speed, upward flows in ISL. Quadrant-hole analysis shows that the wakes after high-rise buildings could significantly affect Q2 and Q4 events that enlarge the turbulence motion scales. These results could help in-depth understanding of the difference in flow structure of ISL and RSL, as well as formulate the implications to CFD validation.