Street-Level Ventilation in Hypothetical Urban Areas

Street-level ventilation is often weakened by the surrounding high-rise buildings. A thorough understanding of the flows and turbulence over urban areas assists in improving urban air quality as well as effectuating environmental management. In this paper, reduced-scale physical modeling in a wind tunnel is employed to examine the dynamics in hypothetical urban areas in the form of identical surface-mounted ribs in crossflows (two-dimensional scenarios) to enrich our fundamental understanding of the street-level ventilation mechanism. We critically compare the flow behaviors over rough surfaces with different aerodynamic resistance. It is found that the friction velocity u(tau) is appropriate for scaling the dynamics in the near-wall region but not the outer layer. The different freestream wind speeds (U-infinity) over rough surfaces suggest that the drag coefficient C-d (= 2u(tau)(2)/U-infinity(2)) is able to characterize the turbulent transport processes over hypothetical urban areas. Linear regression shows that street-level ventilation, which is dominated by the turbulent component of the air change rate (ACH), is proportional to the square root of drag coefficient ACH '' proportional to C-d(1/2). This conceptual framework is then extended to formulate a new indicator, the vertical fluctuating velocity scale in the roughness sublayer (RSL) (w) over cap ''(RSL), for breathability assessment over urban areas with diversified building height. Quadrant analyses and frequency spectra demonstrate that the turbulence is more inhomogeneous and the scales of vertical turbulence intensity (w '' w '') over bar (1/2) are larger over rougher surfaces, resulting in more efficient street-level ventilation.