Abstract:
The effects of temperature stratification on the wind and turbulence characteristics of different local climate zones were numerically investigated using high-resolution large-eddy simulation methods. Four local climate zones (open highrise climate zone, compact highrise climate zone, compact midrise climate zone, and sparsely built climate zone) in Mentougou, Beijing, were selected as research objects. The simulation results on 7 Nov 2019, under light windy and sunny conditions, reveal the following results: 1) Temperature stratification remarkably impacts the shape and extent of turbulent eddies. In the near-surface horizontal profile, the number of vortices decreases under stable stratification compared with that under neutral stratification, but the longitudinal extension of vortices can increase by 67%. However, under unstable stratification, the number of vortices increases compared with that under neutral stratification, and the longitudinal extension of vortices can be reduced by 60%. In the vertical profile, the circulation structure is weakened, and the longitudinal extent of vortices can be reduced by 40% under stable stratification compared with that under neutral stratification; in the case of unstable stratification, the circulation structure is enhanced, and the longitudinal extent of vortices can be increased by 20%. This phenomenon is most pronounced in the compact highrise climate zone. 2) The high wind speeds in the four local climate zones are predominantly located on either side of the buildings that run parallel to the prevailing wind direction and near the rooftops. The thermal effect amplifies the total wind speed, resulting in the wind speed near the ground increasing to 1.27–2.18 times in comparison with the incoming wind speed. 3) The high turbulence energies in the four local climate zones are primarily located at the bottom corners of buildings and roofs. The turbulence energy near the ground under unstable stratification is 1.2–1.5 times higher than that under neutral stratification, while that under stable stratification is 0.5–0.8 times higher than that under neutral stratification. The buoyancy-induced thermal turbulence enhances the mixing efficiency under unstable stratification, while the turbulent motion is suppressed under stable stratification. 4) Compared with other local climate zones, the wind speeds at the bottom of the buildings in highrise-dense areas are larger. A stronger narrow-tube effect is easily formed under unstable stratification compared to other stratification. The maximum wind speed in its neighborhood canyon is 1.5 times higher than that in the compact midrise climate zone.