Large-Eddy Simulation for the Roughness Sublayers over Real Urban Surfaces

ABSTRACT:

    With the continuous spreading of global pandemic, environmental issues have aroused worldwide unprecedented attention. Airflow plays a crucial role in aerosol motions and pollutants removal in dense cities. Large-eddy simulation (LES) is conducted for a typical metropolitan, Hong Kong, to investigate the dynamics in the atmospheric boundary layer (ABL) over real urban surfaces. Full-scale building models (average building height h_m = 36 m) from Tsim Sha Tsui to Sham Shui Po, Kowloon Peninsula, are digitalized. Southerly wind with speed U_∞ (= 10 m sec^-1) in neutral stratification is prescribed at the domain inlet. The turbulence statistics extracted from three subdomains in Mong Kok neighborhood, each with size 800 m (streamwise) × 100 m (spanwise) × 500 m (vertical), are analyzed. Linear regression of the wind profile with the logarithmic law of the wall (log-law) show that the interface between inertial sublayer (ISL) and roughness sublayer (RSL) is in the range of 2.5h_m to 4.5h_m. In the RSL, the streamwise and vertical velocities are positively (S_u > 0) and negatively (S_w < 0) skewed, respectively. Their kurtosis K_u and K_w is less than 3. Conditional sampling of vertical momentum, flux u’’w’’ showed that ejection Q2 occurs more frequently than does sweep Q4. On the contrary, the contribution of Q4 exceeds that of Q2. These characteristics switch to the other way round in the ISL. Furthermore, the difference between Q4 and Q2, either in terms of occurrence or contribution, shows a local maximum around 50% of the total momentum flux, suggesting the major energy-carrying scales. Coherent structures depict elongated, (massive,) accelerating (decelerating) and descending (ascending) RSL (ISL) flows. Hence, the fresh (aged) air entrainment (detrainment) are signified by fast and extreme (slow and frequent) flows. These distinct features of RSL flows over real urban morphology provide an inspiration to improve the ground-level air quality by proper urban planning.

KEYWORDS: Large-eddy simulation (LES), real urban morphology, turbulent boundary layer (TBL), conditional sampling, hole filtering