Street-Level Ventilation in Hypothetical Urban Areas

Abstract

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 τ is appropriate for scaling the dynamics in the near-wall region but not the outer layer. The different freestream wind speeds (U ∞ ) over rough surfaces suggest that the drag coefficient C d (= 2u 2 τ /U 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 ∝ C 1/2 d . This conceptual framework is then extended to formulate a new indicator, the vertical fluctuating velocity scale in the roughness sublayer (RSL) w 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 1/2 are larger over rougher surfaces, resulting in more efficient street-level ventilation.

The Ventilation Efficiency of Urban Built Intensity and Ventilation Path Identification: A Case Study of Wuhan

“…The urban built intensity determines the ventilation efficiency of the ventilation path [ 35 , 36 ]. As a morphological characteristic of the built environment, the urban built intensity has an obvious correlation with ventilation potential [ 37 , 38 ].…”

Numerical Analysis of the Effects of Different Window-Opening Strategies on the Indoor Pollutant Dispersion in Street-Facing Buildings

“…But, the natural ventilation effect is greatly affected by external climate conditions (such as wind speed, wind direction, and temperature differences), and it is difficult to ensure a stable ventilation effect. To examine the dispersion patterns of pollutants in urban settings under natural ventilation, researchers' studies have focused on analyzing pollutant dispersion in street canyons through CFD simulations [5][6][7][8][9], wind/water tunnel experiments [2,[10][11][12], and field measurements [13][14][15]. The primary source of pollutant exposure within street canyons stems from vehicle emissions, which vary across vehicle models and emission levels under distinct driving conditions.…”

CFD Analysis of Urban Canopy Flows Employing the V2F Model: Impact of Different Aspect Ratios and Relative Heights

“…Much effort has been done in recent years to analyze urban canopy flows by means of CFD, often using Reynolds-averaged NavierStokes (RANS) simulations of two-dimensional (2D) arrays of buildings. e interest of the scientific community for such a simplified building arrangement is justified by the fact that the 2D array can be considered as an archetype for more complex geometries [10][11][12][13]. Huang et al [14] carried out 2D simulations to investigate the effect of wedge-shaped roofs on the flow in an urban street canyon and found that they have significant influence on the vortex structure and pollutant distribution pattern.…”