Street-Level Ventilation in Hypothetical Urban Areas

Yat-Kiu Ho,Chun-Ho Liu

doi:10.3390/atmos8070124

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 (= 2 u τ 2 / 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 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 ^ 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.

Highlights

Cities are growing [1], with over 50% of the global population currently residing in these areas [2]
We focus on the surface layer of the urban atmospheric boundary layer (ABL) in neighborhood scales [44] in attempt to examine how building morphology modifies the dynamics together with the implication to street-level ventilation
While the roughness sublayer (RSL) dynamics were examined in details in our previous study [65], such as the velocity profiles and length scale in RSL, this paper focuses on the practical significance of RSL related to the street-level ventilation estimate

Summary

Introduction

Cities are growing [1], with over 50% of the global population currently residing in these areas [2]. Typical applications include wind engineering for the built environment [7,8], particulate matter (PM) in street canyons [9], city breathability [10,11], and pedestrian wind comfort/safety [12] as well as guideline formulation [13]. Unlike their smooth-surface counterparts, the aerodynamic resistance induced by rough surfaces on turbulent boundary layers (TBLs) is less sensitive to the Reynolds number Re (= Uh/ν; where U is the characteristic velocity scale of flows, h the characteristic length scale of roughness elements and ν the kinematic viscosity). It is largely influenced by the roughness geometries of surfaces that are commonly measured by blockage ratio h/δ (where δ is the TBL thickness), friction velocity

Methods

Results

Conclusion