A wind tunnel study of ventilation mechanism over hypothetical urban roughness: The role of intermittent motion scales

Urban morphology is a major factor governing the dynamics in atmospheric surface layers (ASLs) of which our understanding is rather limited. In this paper, wind tunnel experiments are conducted to characterize the flows over different types of urban roughness in attempt to demystify the mechanism of street-level ventilation in isothermal conditions. Hypothetical urban areas are assembled by idealized street canyons using aluminum square tubes (ribs) and plastic LEGO® bricks (cubes). The velocity components are sampled by hot-wire anemometry (HWA) with X-wire probes. The drag coefficient Cd (= 2uτ2/U∞2; where uτ is the friction velocity and U∞ the freestream wind speed) is used to measure the aerodynamic resistance (3.588 × 10−3 ≤ Cd ≤ 10.799 × 10−3) and parameterize the street-level ventilation of urban areas. The results show that the air exchange rate ACH, as a measure of the aged air removal, is proportional to the root of drag coefficient (ACH ∝ Cd1/2), implying that rougher urban surfaces favor street-level ventilation. Quadrant analyses illustrate that ejection (Q2) and sweep (Q4) are enhanced by aerodynamic resistance so are the transport processes. Frequency spectra further demonstrate that the dynamics is dominated by large-scale motions (f × δ/uτ ≤ 10; where f is the spatial frequency and δ the thickness of turbulent boundary layer) which are more energetic with increasing drag coefficient. The above findings collectively suggest the importance of ASL large scales to street-level ventilation. In addition to promoting ground-level mean wind speed, increasing urban roughness could be a solution to the air quality problems nowadays.