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Improving Residential Wind Environments by Understanding the Relationship between Building Arrangements and Outdoor Regional Ventilation
Open AccessArticle

Street-Level Ventilation in Hypothetical Urban Areas

by Yat-Kiu Ho and Chun-Ho Liu *,†
Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, China
*
Author to whom correspondence should be addressed.
These authors contributed equally to this work.
Atmosphere 2017, 8(7), 124; https://doi.org/10.3390/atmos8070124
Received: 11 May 2017 / Revised: 5 July 2017 / Accepted: 10 July 2017 / Published: 16 July 2017
(This article belongs to the Special Issue Recent Advances in Urban Ventilation Assessment and Flow Modelling)
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. View Full-Text
Keywords: air change rate (ACH); flow and turbulence profiles; hypothetical urban areas; street-level ventilation; ventilation assessment; wind-tunnel dataset air change rate (ACH); flow and turbulence profiles; hypothetical urban areas; street-level ventilation; ventilation assessment; wind-tunnel dataset
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Ho, Y.-K.; Liu, C.-H. Street-Level Ventilation in Hypothetical Urban Areas. Atmosphere 2017, 8, 124.

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